GIFT OF
MICHAEL REESE

PEINCIPLES OF CHEMISTKY

VOL. I.

PRINTED BY

SPOTTISWOODE AXD CO., NEW-STREET SQUARE
LONDON

THE

PRINCIPLES OF CHEMISTRY

BY

D. MENDELEEFF

TRANSLATED FROM THE RUSSIAN (FIFTH EDITION) BY

GEOEGE KAMENSKY, A.E.S.M.

OF T.HB IMPERIAL MINT, ST PETERSBURU
EDITED BY

A. J. GREENAWAY, F.LC.

SCB-EUITUll OF THE JOURNAL OK TilE CHEMICAL SOCIETY

IN TWO VOLUMES

VOL, L

LONDON
LONGMANS, GEEEN, AND CO.

AND NEW YOBK: 15 EAST 16ih STKEET
1891

A II >•>'/?! /> reti n • <l

3°

o

PBEFACE

TO THE

ENGLISH TRANSLATION

IN presenting to the scientific world an English translation of
the text-book of Chemistry written by the great master of the
Periodic Law, we feel that no apology is necessary, for it was in
preparing the first edition of this book that the author was led to
those considerations which resulted in the discovery of that law,
and, moreover, the book is quite unique in its treatment of its
subject.

In order to convey as nearly and clearly as possible the exact
meaning of the author, it has been our endeavour to give, as far as
the genius of the two languages permits, a literal rendering of the
original work. Some exception may no doubt be taken to some of
the sentences, but it was felt on the whole that it would be better
to have some inelegance of language rather than to risk the loss
of the exact shade of meaning that the author had intended to
convey.

We have not considered ourselves at liberty to make any
alterations in the matter of the work, save the omission of two
notes referring to the meaning of Russian words, and of some
details referring to the waters of the streams near St. Petersburg,
which required local knowledge to be of any utility. It has,
however, been necessary to make a considerable change in the

VI PRINCIPLES OF CHEMISTRY

illustrations, as electro-types of the figures in the original could
not be obtained.

Since the publication of the Russian fifth edition, Professor
Mendeleeff has issued some appendices to the work, which will be
found printed at the end of Volume II. We have to express our
thanks to the Managers of the Royal Institution for permission to
reprint the lecture delivered at the Royal Institution by Professor
Mendeleeff (Appendix I.), and to the Council of the Chemical
Society for permission to reprint the Faraday lecture which forms
Appendix II.

In conclusion, we would express our gratitude to Professor
Kinch for the aid so kindly given in revising the sheets for
the press.

G. K.
A. J. G.

October, 1891.

AUTHOR'S PREFACE

TO

THE FIFTH EDITION

THIS work was written during the years 1868-1870, its object
being to acquaint the student not only with the methods of ob-
servation, the experimental facts, and the laws of chemistry, but
also with the aspect of this science towards the invariable sub-
stance of varying matter. If the facts themselves include the
person who observes them, then how much more inevitable is the
reflection of personality in giving an account of methods and of
philosophical speculations ? For the same reason there will inevi-
tably be much that is subjective in every objective exposition of
science. And as an individual production is only significant in
virtue of that which has preceded and which surrounds it, so it
essentially resembles a mirror which in reflecting exaggerates the
size and clearness of neighbouring objects, and causes a person
near it to see reflected most plainly those objects which are 011 the
side to which it is directed. Although I have endeavoured to make
my book a true mirror directed towards the domains of chemical
transformations, yet involuntarily those influences near to me have
been the most clearly reflected, the most brightly illuminated,
and have tinted the entire work with their colouring. In this
way the chief peculiarity of the book has been determined. Ex-
perimental and practical data occupy their place, but the philo-

Vlll PRINCIPLES OF CHEMISTRY

sophical principles of our science form the chief theme of the work.
In former times sciences, like bridges, could only be built up by
supporting them on a few deep abutments and long girders. In
addition to the exposition of the principles of chemistry, it has
been my desire to show how science has now been built up like
a suspension bridge, supported by the united strength of a number
of slender, but firmly-fixed, threads, which individually are of
little strength, and has thus been carried over difficulties which
before appeared impassable. In comparing the science of the past,
the present, and the future, in placing the particulars of its re-
stricted experiments side by side with its aspirations for unbounded
and infinite truth, and in restraining myself from yielding to a bias
towards following the most attractive representation, I have en-
deavoured to incite in the reader a spirit of inquiry, which, unsatis-
fied with speculative reasonings alone, should subject every idea
to experiment, excite the habit of stubborn woi-k, necessitate a
knowledge of the past, and a search for fresh threads to complete
the bridge over the bottomless unknown. Experience proves that
it is possible by this means to avoid two equally pernicious extremes,
the Utopian — a visionary contemplation which proceeds from a
current of thought only — and the realistic stagnation which is
content with bare facts. In sciences like chemistry, which treat
of ideas as well as of the substances of nature, experience demon-
strates at every step that the work of the past has availed much,
and that without it it would be impossible to advance ' into the
ocean of the unknown/ We are compelled to value their history,
to cast aside classical illusions, and to engage in a work which not
only gives mental satisfaction but is also practically useful.1

1 Chemistry, like every other science, is at once a means and an end. It is a
means of attaining certain practicable aspirations. Thus, by its assistance, the
obtaining of matter in its various forms is facilitated ; it shows new possibilities
of availing ourselves of the forces of nature, indicates the methods of preparing
many substances, points out their properties, etc. In this sense chemistry is
closely connected with the work of the manufacturer and the artisan, its sphere
is active, and is a means of promoting general welfare. Besides this honourable
vocation, chemistry has another. With it, as with every other elaborated science,
there are many lofty aspirations, the contemplation of which serves to inspire its
workers and partisans. This contemplation comprises not only the principal data

I'KKI'ACE ix

Thus the desire to direct those thirsting for truth to the pure
source of the science of the forces acting throughout nature forms

of the science, but also the generally-accepted deductions, and also hypotheses,
which refer to phenomena as yet but imperfectly known. In this latter sense
scientific contemplation varies much with times and persons, it bears the stamp
of creative power, and comprehends the highest branch of scientific progress.
In that pure enjoyment experienced on approaching to the ideal, in that eagerness
to draw aside the veil from the hidden truth, and even in that discord which
exists between the various workers, we ought to see the surest pledges of further
scientific success. Science thus advances, discovering new truths, and at the
same time obtaining practical results. The edifice of science not only requires
material but also a plan, and necessitates the work of preparing the materials,
putting them together, working out the plans and the symmetrical proportions
of the various parts. To conceive, understand, and grasp the whole symmetry of
the scientific edifice, including its unfinished portions, is equivalent to tasting
that enjoyment only conveyed by the highest forms of beauty and truth. Without
the material, the plan alone is but a castle in the air, a mere possibility, whilst
the material without a plan is but useless matter ; all depends on the concordance
of the materials with the plan and execution, and the general harmony thereby
attained, In the work of science, the artisan, architect, and creator are very
often one and the same individual, but sometimes, as in other walks of life,
there is a difference between them ; sometimes the plan is preconceived, some-
times it follows the preparation and accumulation of the raw material. Free
access to the edifice of science is not only allowed to those who devised the plan,
worked out the detailed drawings, prepared the materials, or piled up the brick-
work, but also to all those who are desirous of making a close acquaintance with
the plan, and wish to avoid dwelling in the vaults or in the garrets where the
useless lumber is stored.

Knowing how contented, free, and joyful is life in the realms of science, one
fervently wishes that many would enter their portals. On this account many
pages of this treatise are unwittingly stamped with the earnest desire that the
habits of chemical contemplation which I have endeavoured to instil into the
minds of my readers will incite them to the further study of science. Science
will then flourish in them and by them, on a fuller acquaintance not only with
that little which is enclosed within the narrow limits of my work, but with the
further learning which they must imbibe in order to make themselves masters of
our science and partakers in its further advancement.

Those who enlist in the cause of science have no reason to fear when they
remember the urgent need for practical workers in the spheres of agriculture,
arts, and manufacture. By summoning adherents to the work of theoretical
chemistry, I am confident that I call them to a most useful labour, to the
habit of dealing correctly with nature and its laws, and to the possibility of
becoming truly practical men. In order to become actual chemists, it is
necessary for beginners to be well and closely acquainted with three impor-
tant branches of chemistry— analytical, organic, and theoretical. That part of
chemistry which is dealt with in this treatise is only the ground work of the edifice.
For the learning and development of chemistry in its truest and fullest sense,
be.iri nners ought, in the first place, to turn their attention to the practical work of
analytical chemistry: in the second place, to practical and theoretical urquaiut-

X PRINCIPLES OF CHEMISTRY

the first and most important aim of this book. The time has ar-
rived when a knowledge of physics and chemistry forms as im-
portant a part of education as that of the classics did two centuries
ago. In those days the nations which excelled in classical learning
stood foremost, just as now the most advanced are those which are
superior in the knowledge of the natural sciences. I also wished
to show in an elementary treatise on chemistry the palpable ad-
vantages gained by the application of the periodic law, which I first
saw in its entirety in the year 1869 when I was engaged in writing
the first edition of this book, in which, indeed, the law was first
enunciated. Then, however, this law was not established so firmly
as now, when so many of its consequences have been verified by
the researches of numerous chemists, and especially by Roscoe,
Lecoq de Boisbaudran, Nilson, Brauner, Thorpe, Carnelley, Laurie,
Winkler, and others. As the entire scheme of this work2 is sub-
jected to the law of periodicity, which may be illustrated in a

ance with some special chemical question, studying the original treatises of the
investigators of the subject (at first, under the direction of experienced teachers),
because in working out particular facts the faculty of judgment and of correct
criticism becomes sharpened ; in the third place, to a knowledge of current scien-
tific questions through the special chemical journals and papers, and by inter-
course with other chemists. The time has come to turn aside from visionary
contemplation, from platonic aspirations, and from classical verbosity, and to
enter the regions of actual labour for the common weal, and to prove that the
study of science is not only an excellent education for youth, but that it instils
the virtues of labour and truth, and creates solid national wealth, material and
mental, which without it would be imattainable. Science, which deals with the
infinite, is itself without bounds.

2 I recommend those who are commencing to study chemistry with my book
to first learn only Ballot is printed in the large type, because in that part I have en-
deavoured to concentrate all the fundamental, indispensable knowledge required
for the study of chemistry. In the footnotes, printed in small type (which I advise
being read only after the large text has been mastered), certain details are dis-
cussed ; they are either further examples, or debatable questions on existing ideas
which I thought indispensable to lay before those entering into the sphere of
science, or certain historical and technical details which might be withdrawn
from the fundamental portion of the book. Without intending to attain in my
treatise to the completeness of a work of reference, I have still endeavoured
to express the principal developments of science as they concern the chemical
elements viewed in that aspect in which they appeared to me after long con-
tinued study of the subject and participation in the contemporary advance of
knowledge.

PKEFACE xi

tabular form by placing the elements in series, groups, and
periods, two such tables are given at the end of this preface.

In this fifth edition I have not altered any essential feature of
the original work, but have enlarged it in two directions. First,
the doctrine of chemical equilibria, originally introduced by
Berthollet and Henri Sainte-Claire Deville, is discussed more
fully and minutely than in the earlier editions, as it has during
recent years been established on a much firmer footing; and,
second, the descriptive data referring to the elements have been
increased by many new facts.

D. MENDELEEFF.

Xll

PRINCIPLES OF CHEMISTRY

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TABLE II.

THE ATOMIC WEIGHTS OF THE ELEMENTS
Distribution of the Elements in Periods

Groups

Higher
Salt-
forming
Oxides

Typical or
1st small
Period

Large Periods

1st

2nd

3rd

4th

5th

I.

R.O

Li = 7

K 39

Rb 85

Cs 133

—

—

II.

RO

Be =9

Ca40

S 87

Bal37

—

—

III.

RA

B =11

Sc 44

Y 89

La 138

Ybl73

—

IV.

R02

C =12

Ti 48

Zr 90

Ce 140

—

Th232

V.

RA

N =14

V 51

Nb 94

—

Ta 182

—

VI.

RO,

O =16

Cr 52

Mo 96

—

W 184

Ur 240

VII.

RA

F =19

Mn 55

—

—

—

—

Fe56

Rul03

—

Os 191

—

VIII.

-

Co 58-5

Rhl04

—

Ir 193

—

Ni59

Pdl06

—

Pt 196

—

I.

R20

H-l.Na=23

Cu63

AglOS

—

Aul98

—

II.

RO

Mg = 2*

Zn 65

Cdll2

—

Hg200

—

III.

RA

Al =27

Ga70

In 113

—

Tl 2C4

—

IV.

R02

Si =28

Ge72

Sn 118

—

Pb206

—

V.

RA

P =31

As 75-

Sb 120

—

Bi 208

—

VI.

RO,

S =32

Se 79

Tel25

—

—

—

VII.

RA

Cl =35-5

Br 80

I 127

—

—

—

2nd small
Period

1st

2nd

3rd

4th

6th

Large Periods

CONTENTS

OF
THE FIRST VOLUME

PAGE

TRANSLATORS' PREFACE ....... v

AUTHOR'S PREFACE TO THE FIFTH EDITION . . . . .. vii

TABLE OF THE DISTRIBUTION OF THE ELEMENTS IN GROUPS AND

SERIES . . . . . . . xii

TABLE OF THE ATOMIC WEIGHTS OF THE ELEMENTS. DISTRIBUTION

OF THE ELEMENTS IN PERIODS ..... xiii

INTRODUCTION . . . 1

CHAPTER

I. ON WATER AND ITS COMPOUNDS . . . . .40

II. THE COMPOSITION OF WATER. HYDROGEN . . . 112

III. OXYGEN AND THE CHIEF ASPECTS OF ITS SALINE COMBINATIONS . 151

IV. OZONE AND HYDROGEN PEROXIDE. DALTON'S LAW . . . 197
V. NITROGEN AND AlR ....... 221

VI. THE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN . 243

VII. MOLECULES AND ATOMS. THE LAWS OF GAY-LUSSAC AND

AVOGADRO-GERHARDT . . . . . . 292

VIII. CARBON AND THE HYDROCARBONS . ... 3*2(5

IX COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN . . 307

XVI PRINCIPLES Otf CHEMISTRY

< HAl'TKR

X. SODIUM CHLORIDE. BERTHOLLET'S LAWS. HYDROCHLORIC ACID .

THE HALOGENS I CHLORINE, BROMINE, IODINE, AND FLUORINE . 45
XII. SODIUM . . . . . . . . 5CK

XIII. POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM. SPECTRUM

ANALYSIS. ....... 535

XIV. THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM,

CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM . 572

Erratum.
Page 91, line 4 from foot,/cr Prinsep read Pierson.

•

PEINCIPLES OF CHEMISTRY

INTRODUCTION

CHEMISTRY is concerned with the study l of the homogeneous substances

1 The investigation of a substance or phenomenon of nature consists (a) in determin-
ing the relation of the thing under investigation to that which is already known, either
from former studies, or from experiment, or from the consciousness of the common sur-
roundings of life — that is, in determining and expressing the quality of the unknown by
the aid of that which is known; (6) in measuring all that which can be subjected to
measurement, and thereby denoting the quantitative relation of that under investigation
to that already known and its relation to the categories of time, space, temperature,
ma^s. Are. ; (c) in determining the position held by the thing under investigation in the
system of the things known, guided by both qualitative and quantitative data; (d) in
finding, from the quantities which have been measured, the empirical (visible) depen-
dence (function, or ' law,' as it is sometimes termed) of variable factors — for instance, the
dependence of the composition of the substance on its properties, of temperature on
time, of time on locality, &c. ; (r] in framing hypotheses or propositions as to the actual
cause and true nature of the relation between that studied (measured or observed) and
that which is known or the categories of time, space, Arc.; (f) in verifying the logical
consequences of the hypotheses by experiment ; and (g) in advancing a theory which
shall account for the nature of the properties of that studied in its relations with things
already known and with those conditions or categories among which it exists. It is
certain that it is only possible to thus study, when we have taken as a basis some incon-
testable fact which is self-evident to our understanding ; as, for instance, number, time,
space, movement, or mass. The determination o-f such primary or fundamental concep-
tions (categories), although not excluded from the possibility of investigation, frequently
does not subject itself to our present mode of scientific generalisation. Hence it follows
in the investigation of anything, there always remains something which is recognised
without investigation, or admitted as a known factor. The axioms of geometry may be
taken as an example. Thus in the science of biology it is necessary to admit the faculty
of organisms for multiplying themselves, as a conception whose meaning is yet unknown.
Thus in the study of chemistry the notion of elements must be recognised without
hardly any further analysis. However, by first investigating that which is visible and
subject to direct observation by the organs of the senses, we may hope that, first,
hypotheses will be arrived at, and afterwards theories of that which has now to be placed
at the basis of our investigations. The minds of the ancients strove to at once seize the
very fundamental categories of investigation, whilst all the successes of recent know-
ledge are based on the above-cited method of investigation without the determination of
' the beginning of all beginnings.' By following this inductive method, the exact sciences

VOL. I. B

2 PRINCIPLES OF CHEMISTRY

or material 2 of which all the objects of the universe are made up, with
the transformations of these substances into each other, and with the
phenomena 3 which accompany such transformations. Every chemical

have already succeeded in becoming acquainted with certainty with much of the invi-
sible world, which directly is imperceptible to the organs of sense (for example, the mole-
cular movement of all bodies, the composition of the heavenly luminaries, the paths of
their movement, the necessity for the existence of substances which cannot be subjected
to experiment, &c.), and have verified the knowledge thus obtained, and employed it for
increasing the interests of human life ; and therefore it may be safely said that the induc-
tive method of investigation is a more perfect mode of acquiring knowledge than the
deductive method alone (starting from a little of the -unknown accepted as incontestable
to arrive at the much which is visible and observable) by which the ancients strove to
embrace the universe. By investigating the universe by an inductive method (endeavour-
ing from the much which is observable to arrive at a little which may be verified and
is indubitable) the new science refuses to recognise dogma as truth, but through reason,
by a slow and laborious method of investigation, strives for and attains to true de-
ductions.

2 A substance or material is that which occupies space and has weight. That is,
which presents a mass which is attracted by the earth and by other masses of material,
and of which the objects of nature are composed, and through which the movements and
phenomena of nature are accomplished. It is easy to find out by examining and
investigating, by various methods, the objects met with in nature and in the arts, that
some of them are homogeneous, whilst others are composed of a mixture of several
homogeneous substances. This is most clearly seen in solid substances. The metals
used in the arts (for example, gold, iron, copper) should be distinguished for their
homogeneity, otherwise they are brittle and unfit for many uses. Homogeneous matter
exhibits similar properties in all its parts. By breaking up a homogeneous substance we
obtain parts which, although different in form, resemble each other in their properties.
Glass, the best qualities of sugar, marble, &c., are examples of homogeneous substances.
But examples of non-homogeneous substances ai'e much more frequent in nature and the
arts. Thus the majority of the rocks are not homogeneous. In porphyries bright pieces
of a mineral called ' orthoclase ' are often seen strewn amongst the dark mass of the rock.
In ordinary red granite it is easy to distinguish large pieces of orthoclase mixed with
dark semi-transparent quartz and flexible laminae of mica. Nor are plants and animals
homogeneous. Thus leaves are composed of a skin, fibre, pulp, sap, and a green colouring
matter. This is clearly seen by examining under a microscope a thin slice cut off a leaf.
As an example of those non-homogeneous substances which are produced artificially,
gunpowder may be cited, which is prepared by mixing together known proportions of
sulphur, nitre, and charcoal. Many liquids, also, are not homogeneous, as may be observed
by the aid of the microscope, when drops of blood are seen to consist of a colourless
liquid in which red corpuscules, invisible to the naked eye owing to their small size, are
floating about. It is these corpuscules which give blood its peculiar colour. Milk is also
a transparent liquid, in which microscopical drops of fat are floating, and which rise to the
top when milk is left at rest, forming cream. When the fat is beaten up (churned) the
separate drops collect into one mass. It is possible to extract from every non-
homogeneous substance those homogeneous substances of which it is made up. Thus
orthoclase may be separated from porphyry by breaking it off. So also gold is extracted
from gold-bearing sand by washing away the mixture of clay and sand. Chemistry deals
only with the homogeneous substances met with in nature, or extracted from natural or
artificial non-homogeneous substance. The various mixtures found in nature form the
subjects of other natural sciences — as geognosy, botany, zoology, anatomy, &c.

3 All those events which are accomplished by substances in time, are termed ' pheno-
mena.' Phenomena in themselves form the fundamental subject of the study of physics.
Movement is the primary and most generally understood form of phenomenon, and there-
fore we endeavour to reason about other phenomena as clearly as when dealing with move-

iNTHonrrnox 8

change or reaction,4 as it is called, can only take place under a condi-
tion of most intimate and close contact of the reacting substances,5 and
is determined by the forces proper to the smallest invisible particles
(molecules) of matter. We must distinguish three chief classes of
chemical transformations.

1. Combination is a reaction in which the union of two substances
yields a new one, or in general terms, from a given number of sub-
stances a lesser number is produced. Thus, by heating a mixture of
iron and sulphur6 a single new substance is produced, iron sulphide, in
which the constituent substances cannot be distinguished even by the
highest magnifying power. Before the reaction, the iron could be
separated from the mixture by a magnet, and the sulphur by dissolving
it in certain oily liquids ; 7 in general, before combination they might
be mechanically separated from each other, but after combination both
substances penetrate into each other, and are then neither mechanically
separable nor individually distinguishable. As a rule, reactions of
direct combination are accompanied by an evolution of heat, and the
common case of combustion, evolving heat, consists in the combination
of combustible substances with a portion (oxygen) of the atmosphere,

ment. Therefore, mechanics, which treats of movement, forms the fundamental science
of natural philosophy, and all other sciences endeavour to reduce the phenomena with
which they are concerned to mechanical principles. Astronomy was the first to take
to this path of reasoning, and succeeded in many cases in reducing astronomical to
purely mechanical phenomena. Chemistry and physics, physiology and biology are
proceeding in the same direction.

4 The verb ' to react ' means to act or change chemically.

5 If a phenomenon proceeds at visible or measurable distances (as, for instance,
magnetic attraction or gravity) it cannot be ascribed to chemical phenomena, which are
only accomplished at distances immeasurably small and undistinguishable to the eye or
the microscope ; that is to say, which belong to the number of purely molecular pheno-
mena. When a change of material is accomplished within a substance without visible
motion or the interference of foreign matters (for instance, when new wine ' ages ' by
keeping, and acquires a peculiar aroma), it may be classed as a chemical phenomenon ; but
the ordinary cases of chemical reaction are accomplished by the mutual action of different
substances which, previously free, on reaction mutually permeate each other.

f> For this purpose a piece of iron may be made red hot in a smith's furnace, and then
placed in contact with a lump of sulphur, when iron sulphide will be obtained as a
molten liquid, the combination being accompanied by a visible increase in the glow of
the iron. Or else iron filings are mixed with powdered sulphur in the proportion of
5 parts of iron to 3 parts of sulphur, and the mixture placed in a glass tube, which is
then partially heated. Combination does not commence without the aid of external
heat, but when once started in any portion of the mixture it extends throughout the
entire mass, because the portion first heated evolves sufficient heat in forming iron
sulphide to raise the adjacent parts of the mixture to the temperature required for
starting the reaction. The rise in temperature thus obtained is so high as to soften the
glass tube.

7 Sulphur is slightly soluble in many thin oils; it is very soluble in carbon bisulphide
and in some other liquids. Iron is insoluble in carbon bisulphide, and therefore the
sulphur can be dissolved away from the iron.

B 2

4 PWNCIPLES OF CHEMISTRY

the gases and vapours contained in the smoke being the products of
combination.

2. Reactions of decomposition are cases the reverse to those of
combination, that is, in which one substance gives two — or, in general, a
given number of substances a greater number. Thus, by heating wood
(and also coal and many animal or vegetable substances) without access
to air, a combustible gas, a watery liquid, tar, and carbon are obtained.
It is in this way that tar, lighting gas, and charcoal are prepared on a
large scale.8 All limestones, for example, flagstones, chalk, or marble,
are decomposed by heating to redness into lime and a peculiar gas
called carbonic anhydride. A similar decomposition, taking place,
however, at a much lower temperature, proceeds with the green copper
carbonate which enters into the composition of malachite. This ex-
ample will be studied more in detail presently. Whilst heat is evolved
in the ordinary reactions of combination, it is, on the contrary, con-
sumed in the reactions of decomposition.

3. The third class of chemical reactions — where the number of acting
substances is equal to the number of substances formed— consists, as it
were, of an association of decomposition and combination. If, for
instance, two compounds A and B are taken and they react on each
other to form the substances C and D, then supposing that A is de-
composed into D and E, and that E combines with B to form C, we
have a reaction in which two substances A, or D E, and B were taken
and two others C, or E B, and D were produced. Such reactions ought
to be placed under the general term of reactions of 'rearrangement,'
and the particular case where two substances give two fresh ones,
reactions of * substitution.' 9 Thus, if a piece of iron be immersed in a
solution of blue vitriol (copper sulphate), copper is formed — or, rather,

8 Decomposition of this kind is termed ' dry distillation ' because, as in distillation,
the substance is heated and vapours are given off which, on cooling, condense into
liquids. In general, decomposition, in absorbing heat, presents much in common to a
physical change of state — such as, for example, that of a liquid into a gas. Deville
likened complete decomposition to boiling, and compared partial decomposition, when a
portion of a substance is not decomposed in the presence of its products of decomposition
(or dissociation), to evaporation.

9 A reaction of rearrangement may in certain cases take place with one substance
only ; that is to say, a substance may by itself change into a new isomeric form. Thus,
for example, if hard yellow sulphur be heated to a temperature of 250° and then poured
into cold water it gives, on cooling, a soft, brown variety. Ordinary phosphorus, which
is transparent, poisonous, and phosphorescent in the dark (in air), gives, after being
heated at 270° (in an atmosphere incapable of supporting combustion, such as steam), an
opaque, red, and non-poisonous isomeric variety, which is not phosphorescent. Cases of
isomerism point out the possibility of an internal rearrangement in a substance, and are
the result of an alteration in the grouping of the same elements, just as a certain number
of balls may be grouped in figures and forms of different shapes and of various properties.

INTRODUCTION 5

separated out, and green vitriol (iron sulphate, which only differs from
the blue vitriol in that the iron has replaced the copper) is obtained in
solution. In this manner iron may be coated with copper-, so also copper
with silver ; such reactions are frequently made use of in practice.

The majority of the chemical changes accomplished in nature and
the arts are very complicated, as they consist of an association of many
separate and simultaneous combinations, decompositions, and replace-
ments. In this natural complexity of chemical phenomena is discovered
the chief reason why for so many centuries chemistry did not exist as
an exact science ; that is to say, that although many chemical changes
were known and made use of,10 yet their real nature was unknown, nor
could they be foreseen or directed at will. Another reason for the
tardy progress of chemical knowledge is the participation of gaseous
substances, especially air, in many reactions. The true comprehension
of air as a ponderable substance, and of gases in general as peculiar elastic
and dispersive states of matter, was only arrived at in the sixteenth and
seventeenth centuries, and it was only after this that the transformations
of substances could form a science. Up to that time, without under-
standing the invisible and yet ponderable gaseous and vaporous states
of substances, it was impossible to form any fundamental chemical
evidence, because gases escaped from notice between the acting and
resultant substances. It is easy from the impression conveyed to us by
the phenomena we observe to form the opinion that matter is created
and destroyed : a whole mass of trees burn, and there only remains a
little charcoal and ash, whilst from one small seed there grows little
by little a majestic tree. In one case matter seems to be destroyed, and
in the other to be created. This conclusion is arrived at because the
formation or consumption of gases, being under the circumstances
invisible to the eye, is not noted. When wood burns it undergoes a
chemical change into gaseous products, which escape as smoke. A very
simple experiment will prove this. By collecting the smoke it may be
observed that it contains gases which differ entirely from air, being
incapable of supporting combustion or respiration. These gases may
be weighed, and it will then be seen that their weight exceeds that of
the wood taken. This increase in weight arises from the fact that, in
burning, the component parts of the wood combine with a portion of
the air ; in like manner iron increases in weight by rusting. In burn-
ing gunpowder its substance is not destroyed, but only converted into
gases and smoke. So also in the growth of a tree ; the seed does not

10 Thus tin- ancients knew how toconvert the juice of grapes containing the saccharine
principle (glucose) into wine or vinegar, or how to extract metals from the ores which
are found in the earth's crust, and how to prepare glass from earthy substances.

6 PRINCIPLES OF CHEMISTRY

increase in mass of itself and from itself, but it grows because it absorbs
gases from the atmosphere and sucks water and substances dissolved
therein from the earth through its roots. The sap and solid substances
which give plants their form are produced from these absorbed gases
and liquids by complicated chemical processes. The gases and liquids
are converted into solid substances by the plants themselves. Plants
not only do not increase in size, but die, in a gas which does not contain
the constituents of air. When moist substances dry they decrease in
weight ; when water evaporates we know that it does riot disappear,
but will return from the atmosphere as rain, dew, and snow. When
water is absorbed by the earth, it does not disappear there for ever, but
accumulates somewhere underground, from whence it afterwards flows
forth as a spring. Thus matter does not disappear and is not created,
but only undergoes various physical and chemical transformations — that
is to say, changes its locality and form. Matter remains on the earth
in the same quantity as before ; in a word it is, as far as we are con-
cerned, everlasting. It was difficult to submit this simple and primary
truth of chemistry to investigation, but when once made clear it rapidly
spread, and now seems as natural and simple as many truths which
have been acknowledged for ages. Mario tte and other savants of the
seventeenth century already suspected the existence of the law of the
indestructibility of matter, but they made no efforts to express it or to
apply it to the ends of science. The experiments by means of which
this simple law was arrived at were made during the latter half of the
last century by the founder of contemporary chemistry, LAVOISIER, the
French Academician and mayor. The numerous experiments of this
savant were conducted with the aid of the balance, which is the only
means of directly and accurately determining the quantity of matter.

Lavoisier found, by weighing all the substances, and even the
apparatus, used in every experiment, and then weighing the substances
obtained after the chemical change, that the sum of the weights of the
substances formed was always equal to the sum of the weights of the
substances taken ; or, in other words : MATTER is NOT CREATED AND
DOES NOT DISAPPEAR, or that, matter is everlastiny. This expression
naturally includes a hypothesis, but our only aim in using it is to con-
cisely express the following lengthy period — That in all experiments,
and in all the investigated phenomena of nature, it has never been
observed that the weight of the substances formed was less or greater
(as far as accuracy of weighing permits) than the weight of the sub-,
stances originally taken, and as weight is proportional to mass11 or

11 The idea of the mass of matter was first shaped into an exact form by Galileo (died
1642), and more especially by Newton (born 1643, died 1727), in the glorious epoch of the

BPTRODUCnOH 7

quantity of matter, it follows that no one has ever succeeded in observ-
ing a disappearance of matter or its appearance in fresh quantities.
The law of the indestructibility of matter endows all chemical investi-
gations with exactitude, as, on its basis, an equation may be formed for
every chemical reaction. If in any reaction the weights of the sub-
stances taken be designated by the letters A, B, C, &c., and the
weights of the substances formed by the letters M, N, 0, &c., then

A + B + C + = M + N + O +

Therefore, should the weight of one of the acting or resultant sub-
stances be unknown, it may be determined by solving the equation.
The chemist, in applying the law of the indestructibility of matter,
must never lose sight of any one of the acting or resultant substances.
Should such an oversight be made, it will at once be remarked from
the sum of the weights of the substances taken being unequal to the
sum of the weights of the substances formed. All the progress made
by chemistry during the end of the last, and in the present, century is
entirely and immovably founded on the law of the indestructibility of
matter. It is absolutely necessary in beginning the study of chemistry
to become familiar with the simple truth which is expressed by this
law, and for this purpose several examples elucidating its application
will now be cited.

1. It is well known that iron rusts in damp air,1'2 and that when
heated to redness in air it becomes coated with scoria (oxide), having,
like rust, the appearance of an earthy substance resembling some of the
iron ores from which metallic iron is extracted. If the iron is weighed
before and after the formation of the scoria or rust, it will be found
that the metal has increased in weight during the operation.13 It

development of the principles of inductive reasoning enunciated by Bacon and Descartes
in their philosophical treatises. Shortly after the death of Newton, Lavoisier, whose
fame in natural philosophy should rank with that of Galileo and Newton, was born on
August 20, 1743. The death of Lavoisier occurred during the Reign of Terror of the
French Revolution, when he, together with twenty-six other chief farmers of the revenue,
was guillotined on May 8, 1794, at Paris, but his works and thoughts have made him
immortal.

12 By covering iron with an enamel, or varnish, or with unrustable metals (such as
nickel), or a coating of paraffin, or other similar substances, it is protected from the air
ami moisture, and so kept from rusting.

1 • Such an experiment may easily be made by taking the finest (unrusted) iron filings
(ordinary tilings must be first washed in ether, dried, and passed through a very fine
sieve). The filings thus obtained are capable of burning directly in air (by oxidising or
forming rust), especially when they hang (are attracted) on a magnet. A compact piece
of iron does not burn in air, but spongy iron glows and smoulders like tinder. In
making the experiment, a horse-shoe magnet is fixed, with the poles downwards, on one
arm of a rather sensitive balance, and the iron filings are applied to the magnet (on a

8 PRINCIPLES OF CHEMISTRY

can easily be proved that this increase in weight and formation of
earthy substances from the metal is accomplished at the expense of
the atmosphere, and mainly, as Lavoisier proved, at the expense of
that portion which is called oxygen, and, as will afterwards be
explained, supports combustion. In fact, in a vacuum, or in gases
which do not contain oxygen, for instance, in hydrogen or nitrogen,
the iron neither rusts nor becomes coated with scoria. Had the iron
not been weighed, the participation of the oxygen of the atmosphere in
its transformation into an earthy substance might have easily passed
unnoticed, as was formerly the case, when phenomena like the
above were, for this reason, misunderstood. It is evident from the
law of the indestructibility of matter that as the iron increases in
weight in its conversion into rust, the latter must be a more complex
substance than the iron itself, and its formation is due to a reaction of
combination. Were not this chemical change studied in regard to
mass, and did we not know of the ponderability of air, and of its
capacity to take part in the phenomena of combustion, we might form
an entirely wrong opinion about it, and might, for instance, consider
rust to be a simpler substance than iron, and explain the formation of
rust as the removal of something from the iron. Such, indeed, was
the general opinion prior to Lavoisier, when it was held that iron con-
tained a certain unknown substance called * phlogiston,' and that rust
was iron deprived of this supposed substance.

2. Copper carbonate (in the form of a powder, or as the well-known
green mineral called ' malachite,' which is used for making ornaments,
or as an ore for the extraction of copper) changes into a black sub-
stance called 'copper oxide' when heated to redness.14 This black

sheet of paper) so as to form a beard about the poles. The balance pan should be exactly
under the filings on the magnet, in order that any which might fall from it should not
alter the weight. The filings, having been weighed, are set light to by applying the flame
of a candle; they easily take fire, and go on burning by themselves, forming rust.
When the combustion is ended, it will be clear that the iron has increased in weight ;
from 5£ parts by weight of iron filings taken, there are obtained, by complete com-
bustion, 7£ parts by weight of rust. Consequently, if about 5 grams of filings be
applied to the magnet, the increase in weight will be clearly seen by the weights that are
required to restore equilibrium. This experiment proceeds so easily and quickly that it
may be conveniently demonstrated, as a proof of the increase of weight at the expense of
air and of its transformation into the solid iron-rust.

14 For the purpose of experiment, it is most convenient to take copper carbonate, pre-
pared by the experimenter himself, by adding a solution of sodium carbonate to a solution
of copper sulphate. The precipitate (deposit) so formed is collected on a filter, washed,
and dried. The decomposition of copper carbonate into copper oxide is effected by so
moderate a heat that it may be accompished in a glass vessel heated by a lamp. For
this purpose a thin glass tube, closed at one end, and called a ' test tube," may be em-
ployed, or else a vessel called a ' retort.' The experiment is carried on, as described in the
third example above, by collecting the carbonic anhydride over a water bath, as will be
afterwards explained.

UJTBODUCWON (J

substance is also obtain* 'd by heating copper to redness in air — that is,
it is the scoria or oxidation product of copper. The weight of the
black oxide of copper left is less than that of the copper carbonate
originally taken, and therefore we consider the reaction which occurred
to have been one of decomposition, and that by it something was sepa-
rated from the green copper carbonate, and in fact by closing the orifice
of the vessel in which the copper carbonate is heated with a well-
litting cork, through which a gas delivery tube15 passes whose end is
immersed under water, it will be observed that on heating, a gas is
formed which bubbles through the water. This gas can be easily
collected, as will presently be described, and it will be found to essen-
tially differ from air in many respects ; for instance, a burning taper
is extinguished in it as if it had been plunged into water. If weighing
had not proved to us that some substance had been separated, the
formation of the gas might easily have escaped our notice, for it is
colourless and transparent like air, and is therefore evolved without
any striking feature. The carbonic acid gas evolved may be weighed 16
and it will be seen that the sum of the weights of the black copper

lo Gas delivery tubes are usually made of glass tubing as prepared at glass works. It
is made of various diameters and thicknesses. If of small diameter and thickness, a glass
tube is easily bent by heating in a gas jet or the flame of a spirit lamp, and may also be
easily divided at a given point by making a deep scratch with a file and then breaking the
tube at this point with a sharp jerk. These properties, together with their impermea-
bility, transparency, hardness, and regularity of bore, makes glass tubes most useful in
experiments with gases. Naturally they might be replaced by straws, india-rubber,
metallic, or other tubes, but these are more difficult to fix on to a vessel, and are not
entirely impervious to gases. A glass gas delivery tube may be hermetically fixed into
a vessel by fitting it into a perforated cork, which should be soft and free from flaws, and
fixing the coi'k into the orifice of the vessel. Sometimes the cork is previously soaked in
paraffin, or it is replaced by an india-rubber cork.

16 Gases, like all other substances, may be weighed, but, owing to their extreme light-
ness and the difficulty of dealing with them in large masses, they can only be weighed by
very sensitive balances ; that is, in such as, with a considerable load, indicate a very small
difference in weight — for example, a centigram or milligram with a load of 1,000 grams.
In order to weigh a gas, a glass globe furnished with a stop- cock (which must not leak in
any part, and therefore must be kept well lubricated) is first of all exhausted of air by an
air-pump la Sprengel pump ia the best). The stop-cock is then closed, and the exhausted
globe weighed. As the pressure of the atmosphere acts on the walls of the globes, they
should be thick. Glass is found to bear the strain of the inequality of the exterior and
interior pressures best. If the gas to be weighed is then let into the globe, its weight
can be determined from the increase in the weight of the globe. It is necessary, how-
ever, that the temperature and pressure of the air about the balance should remain
constant for both weighings, as the weight of the globe in air will (according to the laws
of hydrostatics) vary with its density. The volume of the air displaced, and its weight,
must therefore be determined by observing the temperature, density, and moisture of the
atmosphere during the time of experiment. This will be partly explained later, but may be
studied more in detail by physics. Owing to the complexity of all these operations, the
mass of a gas is usually determined from its volume and density, or the weight of one
volume.

10

PRINCIPLES OF CHEMISTRY

oxide and carbonic acid gas is equal to the weight of the copper car-
bonate17 originally taken, and thus by carefully following out the
various stages of all chemical reactions we arrive at a continuation of
the law of the indestructibility of matter.

3. Red mercury oxide (which is formed as mercury scoria by heat-
ing mercury in air) is decomposed like copper carbonate (only by
heating more slowly and at a somewhat higher temperature), with the
formation of the peculiar gas, oxygen. For this purpose the mercury
oxide is placed in a glass tube or retort,18 to which, by means of a cork,
a gas delivery tube is attached. This tube is bent downwards, as shown

FIG. 1.— Apparatus for the decomposition of red mercury oxide.

in the drawing (Fig. 1). The open end of the gas delivery tube is im-
mersed in a vessel filled with water, called a pneumatic trough.19 When

17 The copper carbonate should be dried before weighing, as otherwise — besides copper
oxide, and carbonic anhydride — water will be obtained in the decomposition. Water
forms a part of the composition of malachite, and has therefore to be taken into considera-
tion. The water produced in the decomposition may be all collected by absorbing it in
sulphuric acid or calcium chloride, as will be described further on. In order to dry a
salt it must be heated at about 100° until its weight remains constant, or be placed under
an air pump over sulphuric acid, as will also be presently described. A* water is met
with almost everywhere, and as it is absorbed by many substances, the possibility of its
presence should never be lost sight of.

18 As the decomposition of red oxide of mercury requires so high a temperature, near
redness, as to soften ordinary glass, it is necessary for the experiment to take a retort
(or test tube) made of infusible (German) glass, which is able to stand high temperatures
without softening. For the same reason, the lamp used must give a strong heat and a
large flame, capable of embracing the whole bottom of the retort, which should be as
small as possible for the convenience of the experiment.

19 The pneumatic trough may naturally be made of any material (china, earthenware)
or metal, &c.), but usually a glass one, as shown in the drawing, is used, as it allows the
progress of experiment being better observed. For this reason, as well as the ease with
which they are kept clean, and from the fact also that glass is not acted on by many sub-

INTRODUCTION

11

the gas begins to be evolved in the retort it is obliged, having no other
outlet, to escape through the gas delivery tube into the water in the
pneumatic trough, and therefore its evolution will be rendered
\ i.sible by the bubbles coming from this tube. In heating the retort
containing the mercury oxide, the air contained in the apparatus is
first partly expelled, owing to its expansion by heat, and then the
peculiar gas called 'oxygen' is evolved, and may be easily collected as it
comes off. For this purpose a vessel (an ordinary cylinder, as in the
drawing) is filled quite full with water and its mouth closed ; it is then
inverted and placed in this position under the water in the trough ;
the mouth is then opened. The cylinder will remain full of water-
that is, the water will remain at a higher level in it than in the sur-

stances which affect other materials (for instance, metals), glass vessels of all kinds —
such a> retorts, test tubes, cylinders, beakers, flasks, globes, &c. — are preferred to any
other for chemical experiments. Glass vessels may be heated without any danger if the
following precautions be observed : 1st, they should be made of thin glass, as otherwise
they are liable to crack from the bad heat-conducting power of glass ; 2nd, they should be
surrounded by a liquid or with sand (Fig. 2), or sand bath as it is called ; or else should

Fiu. 2.- --Apparatus for distillinsr under a diminished pressure liquids which decoiui>ose at their
boiling poim •; under the ordinary pressure. The apparatus in. which the liquid is distilled is con-
oeoted with a large jilobe from which the air is pumped out; the liquid is heated, and the receiver

cr.nl,., I

stand in a current of hot gases without touching the fuel from which they proceed, or in
the flame of a smokeless lamp. A common candle or lamp forms a deposit of soot on a
cold object placed in their flames. The soot interferes with the transmission of heat, and
so a glass vessel when covered with soot often cracks. And for this reason spirit lamps,
which burn with a smokeless flame, or gas burners of a peculiar construction, are used.
In the Bunsen burner the gas is mixed with air, and burns with a non-luminous and
smokeless flame. On the other hand, if an ordinary lamp (petroleum or benzine) does
not smoke it may be used for heating a glass vessel without danger, provided the glass is
placed well above the flame in the current of hot gases. In all cases, the heating should
be begun very carefully by raising the temperature by degrees, and not all at once, or the
glass will break.

12 PKINCIPLES OF CHE3I1STKY

rounding vessel, owing to the atmospheric pressure. The atmosphere
presses on the surface of the water in the trough, and prevents the
water from flowing out of the cylinder. The mouth of the cylinder is
placed over the end of the gas delivery tube,20 and the bubbles
issuing from it will rise into the cylinder and displace the water con-
tained in it. Gases are generally collected in this manner. When a
sufficient quantity of gas has accumulated in the cylinder it can be
clearly shown that it is not air, but another gas which is distinguished
by its capacity for vigorously supporting combustion. In order to show
this, the cylinder is closed, under water, and removed from the bath ;
its mouth is then turned upwards, and a smouldering taper plunged
into it. As is well known, a smouldering taper will be extinguished in
air, but in the gas which is given off from red mercury oxide it burns
clearly and vigorously, showing the capacity this gas has for vigorously
supporting combustion, and thus enabling it to be distinguished from
air. It may be observed in this experiment that, besides the forma-
tion of oxygen, metallic mercury is formed, and, being volatilised at the
high temperature required for the reaction, condenses on the cooler parts
of the retort as a mirror or in globules. Thus two substances, mer-
cury and oxygen, are obtained by heating red mercury oxide. In this
reaction, from one substance two are produced — that is, decomposition
ensues. The means of collecting and investigating gases were already
known before Lavoisier's time, but he first sho.wed the real part they
played in the processes of many chemical changes which before his era
were either wrongly understood (as will be afterwards explained) or were
not explained at all, but only observed in their superficial aspects. This
experiment on red mercury oxide has a special significance in the
history of chemistry contemporary with Lavoisier, because the oxygen
gas which is here evolved is contained in the atmosphere, and plays a
most important part in nature, especially in the respiration of animals,
in combustion in air, and in the formation of rusts or scorise (earths, as
they were then called) from metals — that is, of earthy substances, like the
ores from which metals are extracted. The law of the indestructibility
of matter could not be discovered or confirmed by the balance until the
part played by the atmosphere as regards the participation of its oxygen
in the numerous chemical phenomena, known either from the everyday
experiences of life (combustion, respiration) or from the researches of

20 In order to avoid the necessity of holding the cylinder, its open end is widened (and
also ground so that it may be closely covered with a ground-glass plate when needful), and
placed on a stand below the level of the water in the bath. This stand is called ' the bridge.'
It has several circular openings cut through it, and the gas delivery tube is placed under
one of these, and the cylinder for collecting the gas over it.

INTKoDHTION 13

previous observers (the transformations of the metals into their earths
or oxides), had been explained.

4. In order to illustrate by experiment one more example of
chemical change and the application of the law of the indestructi-
bility of matter, we will take some common table salt and lunar
caustic, which is well known from its use in cauterising wounds. By
taking a clear solution of each and mixing them together, it will at
once be remarked that a solid white substance is formed, which settles
to the bottom of the vessel, and is insoluble in water. This substance
may be separated from the solution by filtering ; it is then found to be
an entirely different substance from either of those taken originally
in the solutions. This is evident from the fact that it does not
dissolve in water. On evaporating the liquid which passed through
the filter, it will be found to contain a new substance unlike either
table salt or lunar caustic, but, like them, soluble in water. Thus
table salt and lunar caustic, two substances soluble in water, being
taken, by their mutual chemical action produced two new substances,
one insoluble in water, and the other remaining in solution. Here,
from two substances two others are obtained, consequently there
occurred a reaction of substitution. The water served only to convert
the acting substances into a liquid and mobile state. If the lunar caustic
and salt be dried 21 and weighed, and if about 58.^ parts by weight — for
instance, grams2'2 — of salt and 170 grams of lunar caustic be taken,
then 143^ grams of insoluble silver chloride and 85 grams of sodium
nitrate will be obtained. The sum of the weights of the acting and
resultant substances are seen to be similar and equal to 228^ grams,
as necessarily follows from the law of the indestructibility of
matter.

21 Drying is necessary in order to remove any water which may be held in the salts
(see Note 17). If the original and resultant substances be dried, then the water
employed for solution, and which is removed in drying, may be taken in indefinite
quantities.

— The exact weights of the acting and resulting substances are determined with the
greatest difficulty, not only from the possible inexactitude of the balance (every weighing
is only correct within the limits of the sensitiveness of the balance) and weights used
in weighing, not only from the difficulty in making corrections for the weight of air dis-
placed by the vessels holding the substances weighed and by the weights themselves,
but also from the hygroscopic nature of many substances (and vessels) causing absorption
of moisture from the atmosphere, and from the difficulty in not losing any of the substance
to be weighed in the many operations (filtering, evaporating, and drying, &c.) which have to
be gone through before arriving at a final result. All these circumstances have to be
taken into consideration in exact ivs.-uivlu's, and their elimination requires very many
special precautions which are impracticable in preliminary experiments ; these arrive-
within only a certain comparatively rough proximity to those weights (expressed by
chemical formulae) which (all with a certain, definite, and inevitable error) correspond
with reality.

14 PRINCIPLES OF CHEMISTRY

Having accepted the truth of the above law, the question in-
voluntarily arises whether there is any limit to the various chemical
transformations, or are they unrestricted in number — that is to say, is
it possible from a given substance to obtain an equivalent quantity of
all other substances ? In other words, does there exist a perpetual and
infinite change of one kind of material into all other kinds, or is the
cycle of these transformations limited ? This is the second essential
problem of Chemistry, a question of quality of matter, and one, it is
-evident, which is more complicated than the question of quantity. It
cannot be resolved by a mere superficial glance at the subject. Indeed,
on seeing how all the varied forms and colours of plants are built up from
air and the elements of the soil, and how metallic iron can be transformed
into dyes, such as inks and Prussian blue, we might be led to think
that there is no end to the qualitative changes to which matter is
susceptible. But, on the other hand, the everyday experiences of life
compel us to acknowledge that food cannot be made out of a stone, or
gold out of copper. Thus a definite answer can only be looked for in
a close and diligent study of the subject, and the problem has been re-
solved in different ways at different times. In ancient times the
opinion most generally held was that everything visible was composed
of four elements — Air, Water, Earth, and Fire. The origin of this
doctrine can be traced far back into the confines of Asia, whence
it was handed down to the Greeks, and most fully expounded by
Empeclocles, who lived before 460 B.C. By accepting so small a
number of elements it was easy to arrive at the conclusion that the
cycle of chemical changes was, if not infinite, at all events most exten-
sive. This doctrine was not arrived at by the results of exact research,
but was only founded on the speculations of philosophers. It appa-
rently owes its origin to the clear division of bodies into gases (like
air), liquids (like water), and solids (like the earth). It seems that
the Arabs were the first who tried to solve the question by means of
experiment, and they introduced, through Spain, the taste for the
study of similar problems into Europe, where from that time there
appear many adepts in chemistry, which was considered as an unholy
art, and called * alchemy.' As the alchemists were ignorant of any
exact or strict law which could guide them in their researches, they re-
solved the question of the transformation of substances in a most varied
manner. Their chief service to chemistry was that they made a
number of experiments, and discovered many new chemical trans-
formations ; but it is well known how they solved the fundamental
problem of chemistry. Their view may be taken as a positive acknow-
ledgment of the infinite transmutability of matter, for they aimed at

INTRODUCTION 15

discovering the Philosopher's Stone, capable of converting everything
into i^'old and diamonds, and of making the old young again. This
solution of the question was afterwards most decidedly refuted, but it
must not, for this reason, be thought that the hopes held by the
alchemists were only the fruit of their imaginations. On the contrary,
the first chemical experiments might well lead them to their conclusion.
They took, for instance, the bright metallic mineral galena, and they
extracted metallic lead from it. Thus they saw that from a metallic
substance which is unfitted for use they could obtain another metallic
substance which is ductile and valuable for many uses in the arts.
Furthermore, they took this lead and obtained silver, a still more
valuable metal, from it. Thus they might easily conclude that it was
possible to ennoble metals by means of a whole series of transmutations
— that is to say, to obtain from them those which are more and more
precious. Having got silver from lead, they only aimed at getting gold
from silver. The mistake they made was that they never weighed or
measured the substances used or produced in their experiments. Had
they done so, they would have learnt that the weight of the lead was
much less than that of the galena from which it was obtained, and the
weight of the silver infinitesimal compared with that of the lead. Had
they looked more closely into the process of the extraction of the silver
from lead (and now silver is chiefly obtained from the lead ores) they
would have seen that the lead does not change into silver, but that it
only contains a certain small quantity of it, and this amount having
once been separated from the lead it cannot by any further operation
give more. The silver which the alchemists extracted from the lead
was in the lead, and was not obtained by a chemical change of the lead
itself. This is now well known from experiment, but the first view of
the nature of the process was very likely to be erroneous.23 The
methods of research adopted by the alchemists could not but give little

23 Besides which, in the majority of cases, the first judgment on most subjects which
do not repeat themselves in everyday experience under various aspects, but always in one
form, or only at intervals and infrequently, is usually untrue. Thus the daily evidence
of the rising of the sun and stars evokes the erroneous idea that the heavens move and
the earth stands f^fcill. This apparent truth is far from being the real truth, and is even
contradictory to it. Similarly, an ordinary mind and everyday experience concludes that
iron is incombustible, whereas it burns not only as filings, but even as wire, as we shall
afterwards see. With the progress of knowledge very many primitive prejudices have
been obliged to give way to true ideas which have been verified by experiment. In ordi-
nary life we often reason at first sight with perfect truth, only because we are taught a
right judgment by our daily experience. It is a necessary consequence of the nature of
our minds to reach the attainment of truth through elementary and often erroneous
reasoning and through experiment, and it would be very wrong to expect a knowledge of
truth from a simple mental effort. Naturally, experiment itself cannot give truth, but it
gives the means of destroying erroneous representations whilst confirming those which
are true in all their consequences.

16 PRINCIPLES OF CHEMISTRY

success, for they groped in the dark, making all kinds of mixtures and
experiments, without setting themselves clear and simple questions
whose answers would aid them to make further pm^ros. Thus they
did not form one exact law, but, nevertheless, they left numerous and
useful experimental data as an inheritance to chemistry ; they studied,
in particular, the transformations proper to metals, and for this reason
chemistry was for long afterwards entirely confined to the study of
metallic substances.

In their researches, the alchemists frequently made use of two
chemical processes which are now termed 'reduction ' and 'oxidation/
The rusting of metals, and in general their conversion from a metallic
into an earthy form, is called ' oxidation,' whilst the extraction of a
metal from an earthy substance is called * reduction.' A large number
of metals — for instance, iron, lead, and tin — are oxidised by heating in
air alone, and may be again reduced by heating with carbon. Such oxi-
dised metals are found in the earth, and form the majority of metallic
ores. The metals, such as tin, iron, and copper, may be extracted from
these ores by heating them together with carbon. All these processes
were well studied by the alchemists. It was afterwards shown that
all earths and minerals are formed of similar metallic rusts or oxides,
or of their combinations. Thus the alchemists knew of two forms of
chemical changes : the oxidation of metals and the reduction of the
oxides so formed into metals. The explanation of the nature of these
two classes of chemical phenomena was the means for the discovery of
the most important chemical laws. The first hypothesis on. their
nature is due to Becker, and more particularly to Stahl, a surgeon to
the King of Prussia. Stahl writes in his * Fundamenta Chymise,'
1723, that all substances consist of an imponderable fiery substance
called ' phlogiston ' (materia aut principium ignis 11011 ipse ignis) and of
another element having particular properties for each substance. The
greater the capacity of a body for oxidation, or the more combustible it
is, the richer it is in phlogiston. Carbon contains it in great abundance.
In oxidation or combustion phlogiston is emitted, and in reduction it
is consumed or enters into combination. Carbon reduces earthy sub-
stances because it is rich in phlogiston, and gives up a portion of its
phlogiston to the substance reduced. Thus Stahl supposed metals to
be compound substances consisting of phlogiston and an earthy sub-
stance or oxide. This hypothesis is distinguished for its very great
simplicity, and for this and other reasons it acquired many supporters.24

a4 It is true that Stahl was acquainted with a fact which directly disproved Ins
hypothesis. It was already known (from the experiments of Geber, and more especially
of Ray, in 1630) that metals increase in weight by oxidation, whilst, according to Stahl's

17

Lavoisier proved by means of tlir balance that every case of rusting
of metals or oxidation, or of combustion, is accompanied by an increase
in -\vi-iirht at the expense of the atmosphere. He formed, therefore, the
natural opinion that the heavier substance is more complex than the
li^hter one.25 The following remarkable experiment wa> madt> by
Lavoisier in 1774, and gave indubitable support to his opinion, which
was iii many respects contradictory to Stahl's doctrine. Lavoisier

hypothesis, they should den-case in weight, because phlogiston is separated l>y oxidation.
Stahl speaks on this point as follows: — 'I know well that metals, in their transformation
into earths, increase in weight. But not only does this fact not disprove my theory, but,
on the contrary, confirms it, for phlogiston is lighter than air, and, in combining with
substances, strives to lift them, and so decreases their weight ; consequently, a substance
which has lost phlogiston must be heavier.' This argument, it will be seen, is founded
on an improper understanding of the properties of gases, regarding them as having no
weight and as not being attracted by the earth, or else on a confused idea of phlogiston
itself, as it was first defined as imponderable. The conception of imponderable phlogiston
tallies well with the habit and methods of the last century, when recourse was often had
to imponderable fluids for explaining a large number of phenomena. Heat, light,
magnetism, and electricity were explained as being peculiar imponderable fluids. In this
sense the doctrine of Stahl corresponds entirely with the spirit of his age. If heat be
now regarded as movement or energy, then phlogiston also should be considered in this
light. In fact, in combustion, of coals, for instance, heat and energy are evolved, and
not combined in the coal, although the oxygen and coal do combine. Consequently, the
doctrine of Stahl contains the essence of a true representation of the evolution of energy,
but naturally this evolution is only a consequence of the combination going on between
the coal and oxygen. As regards the history of chemistry prior to Lavoisier, besides
Stahl's work (to which reference has been made above), Priestley's Experiments and
(>l>nrrr«{t(iitfi an ])i/cri'/it Kin<7s of Air, London, 1790, and also Scheele's Opuscula
Chiinicfi et Phi/sic<i, Lips., 17NS-s(.». '2 vols., must be recommended as the two leading
works of the English and Scandinavian chemists showing the condition of chemical
learning before the propagation of Lavoisier's views. A most interesting memoir on the
history of phlogiston is that of Rodwell, in the Philosophical Magazine, 1868, in which
it is shown that the idea of phlogiston dates very far back, that Basil Valentine (1894-
in. "i, in the Cnrsiis Tn'mii/iJtaJin A/itimonii Paracelsus (1498-1541), in his work, De
Rerun Xnttmt, Glauber (1604-1668), and especially John Joachim Becher (1625-1682), in
his Phi/Nt'cn Siiltfi-nini'd, all referred to phlogiston, but under different names.

25 An Englishman, named Mayow, who lived a whole century before Lavoisier (in 1666),
understood certain phenomena of oxidation in their true aspect, but was not able to
develop his views with clearness, or make his doctrine a universal inheritance, or express
it by instructive experiments ; he, therefore, cannot be considered, like Lavoisier, as
the founder of contemporary chemical learning. Science is a universal heritage, and
therefore it is only just to give the highest honour in science, not to those who first
enunciate a certain truth, but to those who are first able to convince others of its
authenticity and establish it for the general welfare. It should be observed, with refer-
ence to scientific discoveries, that they are rarely made all at once, but, as a rule, the
first teachers do not succeed in convincing others of the truth they have discovered ; with
time, however, the store of materials for its demonstration increases, and other teachers
come forward, possessing every means for making the truth apparent to all. They are
rightly considered as the founders; but it must not be forgotten they are entirely indebted
to the labours and mass of data accumulated by many others. Such was Lavoisier, and
such an- all the great founders of science. They are the enunciators of all past and
• lit learning, and their names will always be revered by posterity.

VOL. I. C

18 PRINCIPLES OF CHKMISTKY

poured four ounces of pure mercury into a glass retort (fig. 3), whose
neck was bent as shown in the drawing and dipped into the vessel R s,
also full of mercury. The projecting end of the neck was covered
with a glass bell jar P. The weight of all the mercury taken, and the
volume of air remaining in the apparatus, namely, that in the upper
portion of the retort, and under the bell-jar, were determined before
beginning the experiment. In this experiment it was most important
to know the volume of air in order to learn what part it played in the
oxidation of the mercury, because, according to Stahl, phlogiston is
emitted into the air, whilst, according to Lavoisier, the mercury in

FIG. 3. — Lavoisier's apparatus for determining the composition of air and the
reason of metals increasing in weight when they are calcined in air.

oxidising absorbs a portion of the air ; and consequently it wras abso-
lutely necessary to determine whether the amount of air increased or
decreased in the oxidation of the metal. It was, therefore, most import-
ant to measure the volume of the air in the apparatus both before and
after the experiment. For this purpose it was necessary to know the
total capacity of the retort, the volume of the mercury poured into it,
the volume of the bell-jar above the level of the mercury, and also
the temperature and pressure of the air at the time of its measure-
ment. The volume of air held in the apparatus and isolated from the
surrounding atmosphere could be determined from these data. Having
arranged his apparatus in this manner, Lavoisier heated the retort
holding the mercury for a period of twelve days at a temperature near
the boiling point of mercury. The mercury became covered with a
quantity of small red scales ; that is, it was oxidised or converted into
an earth. This substance is the same mercury oxide which has already
been mentioned (example 3). After the lapse of twelve days the
apparatus was cooled, and it was then seen that the volume of the air
in the apparatus had diminished during the time of the experiment.
This result was in exact contradiction to Stahl's hypothesis. Out
of 50 cubic inches of air originally taken, there only remained 42.

INTKnIMVTION 19

Lavoisier's experiment led to other no less important results. The
weight of the air taken decreased by as much as the weight of the
mercury increased in oxidising ; that is, the portion of the air was not
destroyed, but only combined with mercury. This portion of the air
may be again separated from the mercury oxide, and has, as we saw
(example 3), properties different from those of air. That portion of
the air which remained in the apparatus and did not combine with the
mercury does not oxidise metals, and cannot support either combus-
tion or respiration, so that a lighted taper is immediately extinguished
if it be dipped into the gas which remains in the bell-jar. * It is ex-
tinguished in the remaining gas as if it had been plunged into water/
writes Lavoisier in his memoirs. This gas is called ' nitrogen.5 Thus
air is not a simple substance, but consists of two gases, oxygen and
nitrogen, and therefore the opinion that air is an elementary substance
is erroneous. The oxygen of the air is absorbed in combustion and the
oxidation of metals, and the earths produced by the oxidation of
metals are substances composed of oxygen and a metal. By mixing
the oxygen with the nitrogen the same air as was originally taken is
re-formed. The existence of compound substances was incontestably
proved by these experiments. It has also been shown by direct experi-
ment that on reducing an oxide with carbon, the oxygen contained
in the oxide is transferred to the carbon, and gives the same gas as is
obtained by the combustion of carbon in air. Therefore this gas is
a compound of carbon and oxygen, just as the earthy oxides are com-
posed of metals and oxygen.

The many examples of the formation and decomposition of sub-
stances which are met with convince us that the majority of substances
with which we have to deal are compounds made up of several other
substances. By heating chalk (or else copper carbonate, as in the
second example) we obtain lime and the same carbonic acid gas which is
produced by the combustion of carbon. On bringing lime into contact
with this gas and water, at the ordinary temperature, we again obtain the
compound carbonate of lime, or chalk. Therefore chalk is a compound.
So also are those substances from which it may be built up. Car-
bonic anhydride is formed by the combination of carbon and oxygen ;
and lime is produced by the oxidation of a certain metal called ' cal-
cium.' By breaking up substances in this manner into their component
parts, we arrive at last at such as are indivisible into two or more sub-
stances by any means whatever, and which cannot be formed from other
substances. All we can do is to make such substances combine together
or act on other substances. Substances which cannot be formed from or
decomposed into others are termed simple substances (elements). Thus

c 2

20 PRINCIPLES OF CHEMISTRY

all homogeneous substances maybe classified into simple and compound
substances. This view was introduced and established as a scientific
fact during the lifetime of Lavoisier. The number of these elements
is very small in comparison with the number of compound substances
which are formed by them. At the present time, only seventy elements
are known with certainty to exist. Some of them are very rarely met
with in nature, or are found in very small quantities, whilst others
are yet doubtful. The number of elements with whose compounds we
commonly deal in everyday life is very small. Elements cannot be
transmuted into one another — at least up to now not a single ,-ase of
such a transformation has been met with ; it may therefore be said
that, as yet, it is impossible to transmute one metal into another. And
as yet, notwithstanding the number of assays which have been made in
this direction, no fact has been discovered which could in any way
support the idea of the complexity of those indubitably-known ele-
ments 26 — such as oxygen, iron, sulphur, &c. Therefore, from its con-
ception, an element is not susceptible to reactions of decomposition.-7

-fi Many ancient philosophei's admitted the existence of one elementary form of
matter. This idea still appears in our times, in the constant efforts \\hicli are made to
reduce the number of the elements; to prove, for instance, that bromine contains chlorine
or that chlorine contains oxygen. Many methods, founded both on experiment and
theory, have been tried to prove the compound nature of the elements. All labour" in
this direction has as yet been in vain, and the assurance that elementary matter is not
so homogeneous (single) as the mind would desire in its first transport of rapid generali-
sation is strengthened from year to year. At all events, there are as yet no experimental
or theoretical evidences of the compound nature of our elements. With the methods
and evidence now at our disposal it is impossible to even imagine the possibility of a
method by which the different elements could be formed from one elementary material.
Cases of isomerism and of polymerism of compound substances certainly show the pos-
sibility of the formation, from one and the same elements, of substances with different
properties, but every change of this kind is completely levelled and nullified by a certain
rise in temperature by which every isomeride and polymeride is converted into one
variety and changes its original properties All our knowledge Allows that iron and
other elements remain, even at such a high temperature as there exists in the sun. as
different substances, and are not converted into one common material. Admitting, even
mentally, the possibility of one elementary form of matter, a method must lie imagined
by which it could give rise to the various elements, as also the )nt><ln* o/it'r<ni(li of their
formation from one material. If it be said that this diversitude only takes place at low
temperatures, as is observed with isomerides, then there would be reason to expect, if not
the transition of the various elements into one particular and more stable form, at least
the mutual transformation of some into others. But nothing of the kind has yet been
observed, and the alchemist's hope to manufacture (as Berthollet puts it) elements has no
foundation of fact or theory.

27 The weakest point in the idea of elements is the negative character of the determi-
native signs given them by Lavoisier, and from that time ruling in chemistry. They do
no^decompose, they do not change into one another. But it must be remarked that
elements form the limiting horizon of our knowledge of matter, and it is always difficult
to determine a positive side on the borderland of what is known. But all the same, if
not for all, at all events for the majority, of those having the properties of metals, there
is a series of positive common signs (they possess a particular appearance and lustre,

21

The quantity, therefore, <»f each clement remains constant in all
chemical changes ; which fact may be deduced as a consequence of the
la\\ of the indestructibility of matter, and uf the conception of elements
themselves. Thus the equation expressing the law of the indestructi-
bility of matter acquires a new and still more important signilication.
If we know the quantities of the elements which occur in the acting,
it may be compound, substances, and if from these substances there
proceed, by means of chemical changes, a series of new compound sub-
stances, then the latter will together contain the same quantity of each
of the elements as there originally existed in the reacting substances.
The essence of chemical change is embraced in the study of how,
and with what substances, each element is combined before and after
change.

In order to be able to express various chemical changes by equations,
it has been agreed to represent each element by the first or some two
letters of its (Latin) name. Thus, for example, oxygen is represented by
the letter O ; nitrogen by N ; mercury (hydrargyrum) by Hg ; iron
(ferrum) by Fe ; and so on for all the elements, as is seen in the tables
on page 24. A compound substance fe represented by placing the
symbols representing the elements of which it is made up side by side.
For example, red mercury oxide is represented by HgO, which shows
that it is composed of oxygen and mercury. Besides this, the symbol
of every element corresponds with a certain relative quantity of it by
weight, called its ' combining ' weight, or the weight of an atom; so that
the chemical formula of a compound substance not only designates the
nature of the elements of which it is composed, but also their quantita-
tive proportion. Every chemical process may be expressed by an equa-
tion composed of the formulae corresponding with those substances
which take part in it and are produced by it. The amount by weight
of the elements in every chemical equation must be equal on both sides
of the equation, because no element is either formed or destroyed in a
chemical change.

On pages 24, 25, and 26 a list of the elements, with their symbols
and combining or atomic weights, is given, and we shall see afterwards
on what basis the atomic weights of elements are determined. At
present we will only point out that a compound containing the elements
A and B is designated by the formula AMB"1, where m and n are the
coefficients or multiples in which the combining weights of the

they conduct an electric current without decomposing) which allow them to be distin-
guished at a glance from other kinds of matter. Besides, there is no doubt (from the
results of spectrum analysis) that the elements are distributed as far as the most
distant stars, and -that they support the highest attainable temperatures without
decomposing.

22 PRINCIPLES OF CHEMISTRY

elements enter into the composition of the substance. If we repre-
sent the combining weight of the substance A by a and that of the
substance B by 6, then the composition of the substance A"B'" will be
expressed thus : it contains na parts by weight of the substance A and
nib parts by weight of the substance B, and consequently in 100 parts

of our compound there is contained n percentage parts by weight

of the substance A and ^— of the substance B. It is evident that
na-}- mo

as a formula shows the relative amounts of all the elements contained
in a compound, the actual weights of the elements contained in a given
weight of a compound may be calculated from its formula. For example,
the formula NaCl of table salt shows (as Na=23 and Cl = 35'5), that 58'5
Ibs. of salt contain 23 Ibs. of sodium and 35'5 Ibs. of chlorine, and that 100
parts of it contain 39 -3 per cent, of sodium and 60*7 per cent, of chlorine.

What has been said above clearly limits the province of chemical
changes, because from substances of a given kind there can be obtained
only such as contain the same elements. But, notwithstanding this
primary limitation, the number of possible combinations is infinitely
great. Only a comparatively small number of compounds have yet
been described or subjected to research, and any one working in this
direction may easily discover new compounds which had not before
been obtained. It often happens, however, that such newly -discovered
compounds were foreseen by chemistry, whose object is the apprehension
of that uniformity which rules over the multitude of compound sub-
stances, and whose aim is the comprehension of those laws which govern
their formation and properties. When once the conception of ele-
ments had been established, the most intimate object of chemistry
was the determination of the properties of compound substances on the
basis of the determination of the quantity and kind of elements of
which they are composed ; the investigation of the elements themselves;
the determination of what compound substances can be formed from
each element and the properties which these compounds show ; and the
apprehension of the nature of the connection between the elements in
different compounds. An element thus serves as the starting point,
and is taken as the primary conception under which all other bodies
are embraced.

When we state that a certain element enters into the composition
of a given compound (when we say, for instance, that mercury oxide
contains oxygen) we do not mean that it contains oxygen as a gaseous
substance, but only desire to express those transformations which
mercury oxide is capable of making ; that is, we wish to say that it is

INTRODUCTION 23

possible to obtain oxyi^-ii from mercury oxide, and that it can -i\«-
up oxygen to various other substances ; in a word, we desire only to
express those transformations of which mercury oxide is capable. Or,
more concisely, it may be said that the mmjHtxifion of a compound is
the expression of those transformations of which it is capable. It is
useful in this sense to make a clear distinction between the conception
of an element as a x'//'//v/v homogeneous substance, and as a material,
but invisible part of a compound. Mercury oxide does not contain
two simple bodies, a gas and a metal, but two elements, mercury and
oxygen, which, when free, are a gas and a metal. Xeither mercury as a
metal nor oxygen as a gas is contained in mercury oxide ; it only contains
the substance of these elements, just as steam only contains the sub-
stance of ice, but not ice itself, or as corn contains the substance of the
seed but not the seed itself. The existence of an element may be recog
nised without knowing it in the uncombined state, but only from an in-
vestigation of its combinations, and from the knowledge that it gives,
under all possible conditions, substances which are unlike other known
combinations of substances. Fluorine is an example of this kind. It
was for a long time unknown in a free state, and was, nevertheless, recog-
nised as an element because its combinations with other elements were
known, and their difference from all other similar compound substances
was determined. In order to grasp the difference between the con-
ception of the visible form of an element as we know it in the free
state, and of the intrinsic element (or * radicle,' as Lavoisier called it)
contained in the visible form, it should be remarked that compound
substances also combine together forming yet more complex compounds,
and that they evolve heat in the process of combination. The original
compound may often be extracted from these new compounds by exactly
the same methods as elements are extracted from their corresponding
combinations. Besides, many elements exist under various visible forms
whilst the intrinsic element contained in these various forms is some-
thing which is not subject to change. Thus carbon appears as charcoal,
graphite, and diamond, but yet the element carbon alone contained in
each is one and the same. Carbonic anhydride contains carbon, and
not charcoal, or graphite, or the diamond.

Elements alone, although not all of them, have the peculiar lustre,
opacity, malleability, and the great heat and electrical conductivity
which are proper to metals and their mutual combinations. But
elements are far from all being metals. Those which do not possess
the physical properties of metals are called in>n-ntcft(fi< (or metalloids).
It is, however, impossible to draw a strict line of demarcation between
metals and non-metals, there being many intermediary substances.

•24

PRINCIPLES OF CHEMISTRY

Thus graphite, from which pencils are manufactured, is an element
with the lustre and other properties of a metal ; but charcoal and the
diamond, which are composed of the same substance as graphite, do
not show any metallic properties. Both classes of elements are clearly
distinguished in definite examples, but in particular cases the distinc-
tion is not clear and cannot serve as a basis for the exact division of
the elements into two groups.

At all events, the conception of elements forms the basis of chemical
knowledge, and if we give a list of them at the very beginning of our
work, it is that we wish to symbolise the condition of the contemporary
information on the subject. Altogether about seventy elements are
now authentically known, but many of them are so rarely met with in
nature, and have been obtained in such small quantities, that we possess
but a very insufficient knowledge of them. The substances most widely
distributed in nature contain a very small number of elements. These
elements have been more completely studied than the others because a
greater number of investigators have been able to carry on experiments
and observations on them. The elements most widely distributed in
nature are : —

Hydrogen, H =1. In water, and animal and vegetable or-
ganisms.

Carbon, C =12. In organisms, coal, limestones.
Nitrogen, N =14. In air and in organisms.
Oxygen, O =16. In air, water, earth. It forms the greater

part of the mass of the earth. "
In common salt and in many minerals.
In sea-water and in many minerals.
In minerals and clay.
In sand, minerals, and clay.
In bones, ashes of plants, and soil.
In pyrites, gypsum, and in sea- water.
In common salt, and in the salts of MM

water.

K =39. In minerals, ashes of plants, and in nitre.
Ca = 40. In limestones, gypsum, and in organisms.
Fe =56. In the earth, iron ores, and in organisms.
Beside these, the following elements, although not very largely dis-
tributed in nature, are all more or less well known from their applicati< >ns
to the requirements of everyday life or the arts, either in a free state
or in their compounds : —

Lithium, Li =7. In medicine (Li.2C03), and in photography (LiBr).
Boron, B=l 1. As Borax, B4Na2O7, and as boric anhydride, B2O3.

Sodium, Na=23.
Magnesium, Mg = 24.
Aluminium, Al =27.
Silicon, Si =28.
Phosphorus,? =31.

Sulphur,
Chlorine,

Potassium,

Calcium,

Iron,

S
Cl

= 32.
=35-5.

= 39.

[INTRODUCTION

25

Fluorine, F =19.
Chromium, Cr =-r>2.
Maiiu-anc.se, M M=")").
Co=i>9.

Cobalt,

Nickel,

Copper,

Zinc,

Arsenic,

Bromine,

Cu=
Zn=

AS:

Br =

Strontium, Si-
Silver, AO
Cadmium, Cd
Tin, Sn
Antimony, Sb
Iodine, I

=63.

:<;:>.

:7">.

= 80.

= 87.

= 112.

= 118.
= 122.
= 127.

Barium, Ba = 137.

Platinum, Pt =196.

Gold, Au=197.

Mercury, Hg=200.

Lead, ' Pb=207.

Bismuth, Bi =208.

Uranium, U =240.

As fluor spar. Cal%, and as hydrofluoric
,u id, HF.

As chromic anhydride, CrO3, and potas-
sium dichromate, K2Cr2O7.

As manganese peroxide, Mn02, and po-
tassium permanganate, MnKO4.

In smalt and blue glass.

For electro-plating other metals.

The well-known red metal.

Used for the plates of batteries, roofing, &c.

White arsenic, As203.

A browTii volatile liquid ; sodium bromide,
NaBr.

In coloured tires (SrN,O6).

The well-known white metal.

In alloys. Yellow paint (CdS).

The well-known metal.

In alloys such as type metal.

In medicine and photography ; free, and as
KI.

" Permanent white," and as an adulterant
in white lead, and in heavy spar, BaS04.

^Well-known metals.

)

In medicine and fusible alloys.
In green fluorescent glass.

The compounds of the following metals and semi-metals have fewer
applications, but are well known, and are somewhat frequently met
with in nature, although in small quantities : —

Palladium, Pd=106.
Cerium, Ce=140.
Tungsten, W =184.
Osmium, Os=193.
Iridium, Ir=195.
Thallium, Tl=204.

Beryllium,

Be =9.

Titanium,

Ti =48.

Vanadium,

V =51.

Selenium,

Se =78.

Zirconium,

Zr =90.

Molybdenum, Mo =.96.

The following rare metals are still more seldom met with in nature
and are not yet applied to the arts, but have been studied somewhat
fully :—

26 PRINCIPLES OF CHEMISTRY

Scandium, Sc =44. Indium, In =11 3,

Gallium, Ga=68. Tellurium, Te = 12.r>.

Germanium, Ge= 72. Caesium, Cs=132.

Rubidium, Rb=S5. Lanthanum, La]=138.

Yttrium, Y =89. Didymium, Di =143.

Niobium, Nb=94. Ytterbium, Yb=173.

Ruthenium, Ru=104. Tantalum, Ta =182.

Rhodium, Rh= 1 04. Thorium, Th = 234.

Besides these 66 elements there have been discovered : — Erbium,
Terbium, Samarium, Thallium, Holmium, Mosandrium, Phillipium,
Vesbium, Actinium, and several others. But their properties and com-
binations, owing to their extreme rarity, are very little known, and even
their existence as independent substances 28 is doubtful.

It has been incontestably proved from observations on the spectra
of the heavenly bodies that many of the most common elements (such
as H, Na, Mg, Fe) occur on the far distant stars. This fact confirms
the belief that those forms of matter which appear on the earth as
elements are widely distributed over the entire universe. But why,
in nature, the mass of some elements should be greater than that of
others we do not yet know.

The capacity of each element to combine with one or another
element, and to form compounds with them which are in a greater or
less degree prone to give new and yet more complex substances, forms
the fundamental character of each element. Thus sulphur easily com-
bines with the metals, oxygen, chlorine, or carbon, forming stable sub-
stances, whilst gold and silver enter into combinations with difficulty,
and form unstable compounds, which are easily decomposed by heat.
Compounds, and also elements, may be divided into two classes — those
which easily enter into many different chemical changes, and those which
enter into but few combinations, which are characterised by their small
capacity for the direct formation of new, more complex substances.
The cause or force which induces substances to enter into chemical
change must be considered, as also the cause which holds different
substances in combination — that is, which endues the substances
formed with their particular degree of stability. This cause or force
is called affinity (affinitns, affinite, verwandtsckaft), or chemical affinity.29

28 It may be that some of them are compounds of other already-known elements.
Pure and incontestably independent compounds of these substances are unknown, and
some of them have not even been separated but are only supposed to exist from the
results of spectroscopic researches. There can be no mention of such contestalilc and
doubtful elements in a short general handbook of chemistry.

29 This word, first introduced, if I mistake not, into chemistry by Glauber, is based on
the idea of the ancient philosophers that combination can only take place when the sub-

iNTi;oi»rcTiON 27

As this t'< >ivc must be regarded as exclusively an Attractive force,
like gravity, many writers (for instance, Berginanii at the end of the
last, and Berthollet at the beginning of this, century) supposed affinity
to be essentially similar to the universal force of gravity, from which
it only differs in that the latter acts at observable distances whilst
affinity only evinces itself at the smallest possible distances. But
chemical affinity cannot be entirely identified with the universal
at traction of gravity, which acts at observable distances and which
is dependent only on mass and distance, and not on the quality of the
material on which it acts, whilst it is by the quality of matter that
affinity is most forcibly influenced. Neither can it be entirely identi-
fied with cohesion, which gives to homogeneous solid substances their
crystalline form, elasticity, hardness, ductility, and other properties,
and to liquids their surface, drop formation, capillarity, and other
properties, because affinity acts between the component parts of a
substance and cohesion on a substance in its homogeneity, although
both act at imperceptible distances (by contact) and have much in
common. Chemical force, which makes one substance penetrate into
another, cannot be entirely identified with even those attracting
forces which make different substances adhere to each other, or hold
together (as when two plane-polished surfaces of solid substances are
brought into close contact), or which cause liquids to soak into solids,
or adhere to their surfaces, or gases and vapours to condense on the sur-
faces of solids. These forces must not be confounded with chemical
forces, which cause one substance to .penetrate into the substance of
another and to form a new substance, which is not the case with
cohesion. But it is evident that the forces which determine cohesion
form a connecting-link between mechanical and chemical forces, be-
cause they only act by intimate contact and between different kinds of
matter. For a long time, and especially during the first half of this
century, chemical attraction and chemical forces were identified with
electrical forces. There is certainly an intimate relation between them,
for electricity is evolved in chemical reactions, and it, in its turn, has
a powerful influence on chemical processes — for instance, compounds
are decomposed by the action of an electrical current. But the exactly
similar relation which exists between chemical phenomena and the
phenomena of heat (heat being developed by chemical phenomena, and
heat being able to decompose compounds) only proves the unity of the
forces of nature, the capability of one force to produce and to be trans-
stances combining have something in common — a medium. As is generally the case,
another idea evolved itself in antiquity, and has lived until now, side by side with the
first, to which it is exactly contradictory ; this considers union as dependent on con-
trast, on polar difference, on an effort to fill up a want.

28 PRINCIPLES OF CHEMISTRY

formed into others. Therefore the identification of clu inical force with
electricity will not bear experimental proof.30 As of all the (mole-
cular) phenomena of nature which act on substances at immeasurably
small distances, the phenomena of heat are at present the best (com-
paratively) known, having been reduced to the simplest fundamental
principles of mechanics (of energy, equilibrium, and movement), which,
since Newton, have been subjected to strict mathematical analysis,
it is quite natural that an effort, which has been particularly
pronounced during recent years, should have been made to bring
chemical phenomena into strict correlation with, and under the theory
founded on, the already investigated phenomena of heat, without, how-
ever, aiming at any identification of chemical with heat phenomena.
The true nature of chemical force is still a secret to us, just as is the
nature of the universal force of gravity, and yet without knowing what
gravity really is, by applying mechanical conceptions, astronomical
phenomena have been subjected not only to exact generalisation but to
the detailed prediction of a number of particular facts ; and so, also,
although the true nature of chemical affinity may be unknown, there
is reason to hope for considerable progress in chemical science by
applying the laws of mechanics to chemical phenomena by means of
the mechanical theory of heat. But as yet this portion of chemistry
has been but little worked at, and therefore, while forming a current
problem of the science, it is treated more fully in that particular

50 Especially conclusive are those cases of so-called metalepsis (Dumas, Laurent).
Chlorine, in combining with hydrogen, forms a very stable substance, called ' hydrochloric
acid,' which is split up by the action of an electrical current into chlorine and hydrogen,
the chlorine appearing at the positive and the hydrogen at the negative pole. From this
electro-chemists considered hydrogen to be an electro-positive and chlorine an electro-
negative element, and that they are held together in virtue of their opposite electric
charges. It appears, however, from metalepsis, that chlorine can replace hydrogen (and
reversely hydrogen replaces chlorine) in its compounds without in any way changing the
grouping of the other elements, or altering their chief chemical properties. Thus the
capacity of acetic acid to form salts is not altered by replacing its hydrogen by chlorine.
Here an electro-positive element is replaced by an electro-negative element, which is
quite contrary to the electrical theory of the origin of chemical attraction, which has thus
been entirely overthrown by the facts of metalepsis. We must remark, whilst consider-
ing this subject, that the explanation suggesting electricity as the origin of chemical
phenomena is unsound in that it strives to explain one class of phenomena whose nature
is almost unknown by another class which is no better known. It is most instructive to
remark that together with the electrical theory of chemical attraction there arose and
survives a view which explains the galvanic current as being a transference of chemical
action through the circuit — i.e., regards the origin of electricity as being a chemical one. It
is evident that the connection is very intimate, but both kinds of phenomena are indepen-
dent and represent different forms of molecular (atomic) movement, whose real nature is
not yet understood. Nevertheless, the connection between the phenomena of both cate-
gories is not only in itself very instructive, but it extends the applicability of the general
idea of the unity of the forces of nature, conviction of the truth of which has held so
important a place in the science of the last ten years.

province which is termed either 'theoretical' or 'physical' chemistry, or,
better still, flo'ni'n-nl m^-hnnics. As this province of chemistry re-
quires a knowledge not only of the various homogeneous substances
which have yet been obtained and of the chemical transformations which
they undergo, but also of the phenomena (of heat and other kinds) by
which these transformations are accompanied, it is only possible to
• •nter on the study of chemical mechanics after an acquaintance with
the fundamental chemical conceptions and substances which form the
subject of this book.31

r>1 I consider that in an elementary textbook of chemistry, like the present, it is only
possible and advisable to mention, in reference to chemical mechanics, a few general
ideas and some particular examples referring more especially to gases, whose mechanical
theory must be regarded as the most complete. The molecular mechanics of liquids and
solids is as yet in embryo, and contains much that is disputable; for this reason,
chemical mechanics has made less progress in relation to these substances. It may not
be superfluous to here remark, with respect to the conception of chemical affinity, that up
to the present time gravity, electricity, and heat have been respectively applied to its
elucidation. Efforts have also been made to introduce the luminiferous ether into
theoretical chemistry, and should that connection between the phenomena of light and
electricity which was established by Maxwell be worked out more in detail, doubtless
these efforts to elucidate all or a great deal by the aid of luminiferous ether will yet again
appear in theoretical chemistry. An independent chemical mechanics of the material
particles of matter, and of their internal (atomic) changes, would, in my opinion, arise a-
the result of these efforts. Just as the progress made in chemistry in the time of
Lavoisier was reflected over all natural science, so there is reason to think that an in-
dependent chemical mechanics would shed a new light on all molecular mechanics, which
must be considered as the fundamental problem of the exact sciences in our times. Two
hundred years ago Newton laid the foundation of a truly scientific theoretical mechanics
of extemal visible movement, and erected the edifice of celestial mechanics on this
foundation. One hundred years ago Lavoisier arrived at the first fundamental law of the
internal mechanics of invisible particles of matter. This subject is far from having been
developed into a harmonious whole, because it is much more difficult, and, although many
details have been completely investigated, it does not possess any starting points.
Newton was possible only after Copernicus and Kepler, who had discovered the exte-
rior empirical simplicity of celestial phenomena. Lavoisier and Dalton may, in respect
to the chemical mechanics of the molecular world, be compared to Copernicus and
Kepler. But a Newton has not yet appeared in the molecular world ; when he does, I
think that he will find the fundamental laws of the mechanics of the invisible movements
of matter more easily and more quickly in the chemical structure of matter than in
physical phenomena (of electricity, heat, and light), for these latter are accomplished by
already-disposed particles of matter, whilst it is now clear that the problem of chemical
mechanics mainly lies in the apprehension of those movements which are invisibly ac-
complished by the smallest atoms of matter. The general laws of mechanics, established
by Newton, will probably serve as starting points for molecular mechanics, but the
independence of its range becomes more evident when chemical molecules are com-
pared with the celestial systems, such as the solar system. Chemical atoms may be
regarded as separate members of such systems (as, for instance, the sun, planets, comets,
and other heavenly bodies), whilst the ether of light may be likened to the cosmic dust
which without doubt is distributed throughout space. The present condition of molecular
mechanics is, to a certain extent, copied from celestial mechanics, but there is nothing to
prove the entire similarity of both worlds, although it appears to the mind that, starting
from the primary elements of the unity of creation, such a representation is the most

likelv.

30 PRINCIPLES OF CHEMISTUY

As the chemical changes to which substances are liable proceed
from internal forces proper to these substances, as chemical phenomena
certainly consist of movements of material parts (from the laws of the
indestructibility of matter and of elements), and as the investigation
of mechanical and physical phenomena proves the law of the indestruc-
tibility of forces, or the conservation of energy — that is, the possibility
of the transformation of one kind of movement into another (of visible
or mechanical into invisible or physical) — we are inevitably obliged to
acknowledge the presence in substances (and especially in. the elements
of which all others are composed) of a store of chemical energy or in-
visible movement inducing them to enter into combinations. If heat be
evolved in a reaction, it means that a portion of chemical energy is
transformed into heat ; 32 if heat be absorbed in a reaction,33 that it is

32 The theory of heat gave the idea of a store of internal movement or energy, and
therefore with it, it became necessary to acknowledge chemical energy, but there is no
foundation whatever for identifying heat energy with chemical energy. It may be sup-
posed, but not positively affirmed, that heat movement is proper to molecules and
chemical movements to atoms, but that as molecules are made up of atoms, the movement
of the one passes to the other, and that for this reason heat strongly influences reaction
and appears or disappears (is absorbed) in reactions. These relations, which are,
apparent and hardly subject to doubt on general lines, still present much that is doubtful
in detail, because all forms of molecular and atomic movement are able to pass into
each other. On broad general lines it must be acknowledged that as mechanical energy
can entirely pass into heat energy (but the reverse transition is accomplished only
partially, according to the second law of heat), so also heat energy may pass into
chemical energy, but it is doubtful, and even unlikely, that chemical energy passes
altogether into heat energy. Therefore, the heat evolved in chemical reactions cannot
serve as the total measure of chemical energy, more especially as there are a number of
reactions of combination in which heat is absorbed ; for instance, the combination of
charcoal with sulphur is accompanied by an absorption of heat — probably because the
molecules of charcoal are complex, and those of carbon bisulphide less so, and the break-
ing up of the complex molecules of charcoal requires a large absorption of heat (whose
measure we do not know) — and whilst the combination of charcoal with sulphur is accom-
panied by an evolution of heat, yet we only observe the difference of these two heat
effects.

33 The reactions which take place (at the ordinary or at a high temperature) directly
between substances may be clearly divided into exothermal, which are accompanied by
an evolution of heat, and endothermal, which are accompanied by an absorption of heat.
It is evident that the latter require a source of heat. They are determined either by the
directly surrounding medium (as in the formation of carbon bisulphide from charcoal and
sulphur, or in decompositions which take place at high temperatures), or else by a
simultaneously proceeding secondary reaction. So, for instance, hydrogen sulphide is
decomposed by iodine in the presence of water at the expense of the heat which is
evolved by the solution in water of the hydrogen iodide produced. This is the reason why
this reaction, as exothermal, only takes place in the presence of water ; otherwise it would
be accompanied by a cooling effect. As in the combination of dissimilar substances, the
bonds existing between the molecules and atoms of the homogeneous substances have to
be broken asunder, whilst in reactions of rearrangement the formation of any one sub-
stance proceeds parallel with the formation of another, and, as in reactions, a series of
physical and mechanical changes take place, it is impossible to separate the heat directly
depending on a given reaction from the total sum of the observed heat effect. For this

nN 31

partly transformed (rendered latent) into chemical energy. The store
of force or energy going to the formation of new compounds may, after
several combinations, accomplished with an absorption of heat, at last
diminish to such a degree that indifferent compounds will be obtained,
although these sometimes, by combining with energetic elements or
compounds, give more complex compounds, which may be capable of
entering into chemical combination. Among elements gold, platinum,
and nitrogen have but little energy, whilst potassium, oxygen, and
chlorine have a very marked degree of energy. When dissimilar sub-
stances enter into combination they often form substances of diminished
energy. Thus sulphur and potassium when heated easily burn in air,
but when combined together their compound is neither inflammable nor
burns in air like its component parts. Part of the energy of the
potassium and of the sulphur was evolved in their combination in the
form of heat. Just as in the passage of substances from one physical
state into another a portion of their store of heat is absorbed or
evolved, so in combinations or decompositions and in every chemical
process, there occurs a change in the store of chemical energy, and at
the same time an evolution or absorption of heat.34

For the comprehension of chemical phenomena in a mechanical
sense — i.e., in the study of the modus operandi of chemical phenomena-
it is at the present time most important to consider : (1) the facts
gathered from stoichiometry, or that part of chemistry which treats of
the quantitative relation, by weight or volume, of the. reacting sub-
stances ; (2) the distinction between the different forms and classes of
chemical reactions ; (3) the study of the changes in properties produced
by alteration in composition ; (4) the study of the phenomena which
accompany chemical transformation ; (5) a generalisation of the con-
ditions under which reactions occur. As regards stoichiometry, this
branch of chemistry has been worked out most thoroughly, and embraces
laws (of Dalton, A vogadro- Gerhard t, and others) which bear so deeply
on all parts of chemistry that its entire contemporary standing may be

reason, thermo-chemical data are very complex, and cannot by themselves give the key
to many chemical problems, as it was at first supposed they might. They ought to form
a part of chemical mechanics, but alone they do not constitute it.

3* As chemical reactions are effected by heating, so the heat absorbed by substances
before decomposition or change of state, and called ' specific heat,' goes in many cases to the
preparation, if it may be so expressed, of reaction, even when the limit of the temperature
of reaction is not attained. The molecules of a substance A, which is able to react on a
substance B below a temperature t by being heated from a somewhat lower temperature to
/, undergoes that change which had to be arrived at for the formation of A B. This
idea is often extended ; for instance, it is supposed that a given sul>-tance in its passage
from a liquid to a gaseous state gives chemically or materially new, lighter, and simpler
molecules (is depolymerised, according to De Haen).

32

characterised as the epoch of their circumstantial application to par-
ticular cases. The expression of the quantitative (volumetric or gravi-
metric) composition of substances now forms the most important pro-
blem of chemical research, and therefore the entire further exposition
of the subject is subordinate to stoichiometrical laws. All other
branches of chemistry are clearly subordinate to this most important
portion of chemical knowledge. Even the very signification of re-
actions of combination, decomposition, and rearrangement, acquired, as
we shall see, a particular and new character under the influence of the
progress of exact ideas concerning the quantitative relations of sub-
stances entering into chemical changes. Furthermore, in this sense
there arose a new — and, up to then, unknown --division of compound
substances into definite and indefinite compounds. Even at the beginning
of this century, Berthollet had not made this distinction. But Prout
showed that a number of compounds contain the substances of which
they are composed and into which they break up, in exact definite pro-
portions by weight, which are unalterable under any conditions. Thus,
for example, red mercury oxide contains sixteen parts by weight of
oxygen for every 200 parts by weight of mercury, which is expressed
by the formula HgO. But in an alloy of copper and silver one or the
other metal may be added at will, and in an aqueous solution of sugar,
the relative proportion of the sugar and water may be altered and
nevertheless a homogeneous whole with the sum of the independent
properties will be obtained — i.e., in these cases there was indefinite
chemical combination. Although in nature and chemical practice the
formation of indefinite compounds (such as alloys and solutions) plays
as essential a part as the formation of definite chemical compounds, yet,
as the stoichiometrical laws at present apply chiefly to the latter, all
facts concerning indefinite compounds suffer from inexactitude, and it
is only during recent years that the attention of chemists has been
directed to this province of chemistry.

In chemical mechanics it is, from a qualitative point of view, very im-
portant to clearly distinguish at the very beginning bet ween reversible and
non-reversible reactions. One or several substances capable of reacting on
each other at a certain temperature produce substances which at the same
temperature either can or cannot give back the original substances. For
example, salt dissolves in water at the ordinary temperature, and the
solution so obtained is capable of breaking up at the same temperature,
leaving salt and separating the water by evaporation. Carbon bisul-
phide is formed from sulphur and carbon at the same temperature at
which it can be resolved into sulphur and carbon. Iron, at a certain
temperature, separates hydrogen from water, forming iron oxide, which,

I NTRODUCTION 33

in contact with hydrogen at the same temperature, is able to produce
iron and water. It is evident that if two substances, A and B, give
two others C and D, and the reaction be reversible, then C and D will
form A and B, and, consequently, by taking a definite mass of A,
and B, or a corresponding mass of C and D, we shall obtain, in each
case, all four substances — that is to say, there will be a state of chemical
equilibrium between the reacting substances. By increasing the mass
of one of the substances we obtain a new condition of equilibrium, so
that reversible reactions present a means of studying the influence of
mass on the imnJnx operand* of chemical changes. Many of those
reactions which occur with very complicated compounds or mixtures
may serve as examples of non-reversible reactions. Thus many of the
compound substances of animal and vegetable organisms are broken
up by heat, but cannot be re-formed from their products of decomposi-
tion at any temperature. Gunpowder, as a mixture of sulphur, nitre,
and carbon, on burning, forms gases from which the original substances
cannot be re-formed at any temperature. In order to obtain them, re-
course must be had to an indirect method of combination at the moment
of separation. If A does not under any circumstances combine directly
with B, it does not imply that it cannot give a compound A B. For
A can often combine with C and B with D, and if C has a great
affinity for D, then the reaction of A C on B D produces not only C D,
but also A B. As on the formation of C D, the substances A and B
(previously in A C and B D) are left in a peculiar state of separation,
it is supposed that their mutual combination occurs because they meet
together in this nascent state at the moment of separation (in statu
nascendi). Thus chlorine does not directly combine with charcoal,
graphite, or the diamond, nevertheless there are compounds of chlorine
with carbon and many of them are distinguished by their stability.
They are obtained during the action of chlorine on hydrocarbons, as
the separation products from the direct action of chlorine on hydrogen.
Chlorine takes up the hydrogen, and the freed carbon at the moment
of its separation enters into combination with another portion of the
chlorine, so that in the end the chlorine is combined with both the
hydrogen and the carbon.35

•"•"' Itis possible to imagine that the cause of a great many of such reactions is, that sub-
stances taken in a separate state, for instance, charcoal, present a complex molecule
composed of separate atoms of carbon which are fastened together (united, as is usually
said) by a considerably affinity ; for atoms of the same kind, just like atoms of different
kinds, possess a mutual affinity. The affinity of chlorine for carbon, although unable
to break this bond asunder, may be sufficient to form a stable compound with already
separate atoms of carbon. Such a view of the subject presents a hypothesis which,
although dominant at present, is without sufficiently firm foundation. Were the matter

VOL. I. D

34 PRINCIPLES OF CHEMISTRY

As regards those phenomena which accompany chemical action, the
most important circumstance in reference to chemical mechanics is that
not only do chemical processes produce a mechanical displacement (a
visible disturbance), heat, light, electrical potential and current ; but
that all these agents are themselves capable of changing and governing
chemical transformations. This reciprocity or reversibility naturally
depends on the fact that all the phenomena of nature are only different
kinds and forms of visible and invisible (molecular) movement. First
sound, and then light, was shown to consist of vibratory movements, as
the laws of physics have proved and developed beyond a doubt. Then,
the connection between heat and mechanical motion and work has
ceased to be a supposition, but has become a known fact, and the
mechanical equivalent of heat (424 kilogrammetres of mechanical work
correspond with one kilogram unit of heat or Calorie) gives a mecha.-
nical measure for heat phenomena. Although the mechanical theory
of electrical phenomena cannot be considered so fully developed as the
theory of heat, nevertheless there can be no doubt but that the elec-
trical state of substances, and electric or galvanic currents, represent a
peculiar form of motion ; more especially as both statical and dyna-
mical electricity are produced by mechanical means (in common elec-
trical machines or in Gramme or other dynamos), and, as conversely, a
current (in electric motors) can produce mechanical motion, as heat
produces motion in heat (steam, gas, or air) engines. Thus by passing
a current through the poles of a Gramme dynamo it may be made
to revolve, and, conversely, by revolving it an electrical current is
produced, which demonstrates tlje reversibility of electricity into
mechanical motion. Therefore, chemical mechanics must look for the
fundamental lines of its advancement in the correlation of chemical
with physical and mechanical phenomena. But this subject, owing to
its complexity and comparative novelty, has not yet been subjected to
a harmonious theory, or even to a satisfactory hypothesis, and there-
fore we shall avoid lingering over it.

A chemical change in a certain direction is accomplished not only

as simple as it appears to be, according to this hypothesis, one would expert, for
instance, that the compounds of carbon with chlorine would be easily decomposable by
reason of the supposed considerable affinity of the separate atoms of carbon, which should
therefore tend to mutual combination and the formation of charcoal. It is evident, how-
ever, that not only does reaction itself consist of movements, but that in the compound
formed (in the molecules) the elements (atoms) forming it are in harmonious stable move-
ment (like the planets in the solar system), and this movement will affect the stability
and capacity for reaction, and therefore these depend not only on the affinity of the
participating substances, but also on the conditions of reaction which change the state of
movement of the elements in the molecules, as well as on the nature, form, and inten-
sity of those movements which the elements have in their given state. In a word, the
mechanical side of chemical action must be exceedingly complex.

INTRODUCTION • 35

by reason of the difference of masses, the composition of the sub-
stances concerned, the distribution of their parts, and their affinity or
chemical energy, but also by reason of the conditions under which the
substances occur, and these conditions differ for every particular reac-
tion. In order that a certain chemical reaction may take place between
substances which are capable of reacting on each other, it is often
necessary to have recourse to conditions which are sometimes very
different from those in which the substances usually occur in nature.
For example, not only is the presence of air (oxygen) necessary for the
combustion of charcoal, but the latter must also be heated to redness.
The red-hot portion of the charcoal burns — i.e., combines with the
oxygen of the atmosphere— and in doing so evolves heat, which heats
the adjacent parts of charcoal, which are thus able to burn. Just as
the combustion of charcoal is dependent on its being heated to red-
ness, so also every chemical reaction only takes place under certain
physical, mechanical, or other conditions. The following are the
chief conditions which exert an influence on the progress of chemical
reactions.

(a) Temperature. — Chemical reactions of combination only take
place within certain definite limits of temperature, and cannot be
accomplished outside these limits. As examples we may cite, not only
that the combustion of charcoal begins at a red heat, but also that
chlorine and salt only combine with water at a temperature below 0°.
These compounds cannot be formed at a higher temperature, for they
are then wholly or partially broken up into their component parts.
A certain rise in temperature is necessary to start, combustion. In
certain cases the effect of this rise may be explained as causing one
of the reacting bodies to change from a solid into a liquid or gaseous
form. The transference into a fluid form facilitates the progress of
the reaction, because it aids the intimate contact of the particles acting
on each other. Another reason, to which must be ascribed the chief
influence of heat in exciting chemical action, is that the physical cohe-
sion, or the internal chemical union, of homogeneous particles is thereby
weakened, and therefore the separation of the particles of the sub-
stances taken, and their transference into new compounds, is rendered
easier. When a reaction absorbs heat — as in decomposition, where the
heat is transformed into latent chemical energy — the reason why heat
is necessary is self-evident.

It is most important to observe the effect of an elevation of tem-
perature on all compounds, as there is reason to believe that they are
all decomposed at a more or less high temperature. We have already
seen examples of this in describing the decomposition of mercury oxide

D 2

36 PRINCIPLES OF CHEMISTRY

into mercury and oxygen, and the decomposition of wood under the
influence of heat. Many substances are decomposed at a very mode-
rate temperature ; for instance, the fulminating salt which is employed
in cartridges is decomposed at a little above 120°. The majority of
those compounds which make up the mass of animal and vegetable
matters are decomposed at 250°. On the other hand, there is reason
to think that at a very low temperature no reaction whatever can
take place. Thus plants cease to carry on their chemical processes
during the winter. Every chemical reaction requires certain limits
of temperature for its accomplishment, and, doubtless, many of the
chemical changes observed by us cannot take place in the sun, where
the temperature is very high, or on the moon, where it is very low.

The influence of heat on reversible reactions is particularly instruc-
tive. If, for instance, a compound which is capable of being reproduced
from its products of decomposition be heated up to the temperature at
which decomposition begins, the decomposition of a mass of the sub-
stance contained in a definite volume is not immediately completed.
Only a certain fraction of the substance is decomposed, the other por-
tion remaining unchanged, and if the temperature be raised, the quan-
tity of the substance decomposed increases ; furthermore, for a given
volume the ratio between the part decomposed and the part unaltered
corresponds with each definite rise in temperature until it reaches that
at which the compound is entirely decomposed. This partial decom-
position under the influence of heat is called dissociation. It is pos-
sible to distinguish between the temperatures at which dissociation
begins and ends. Should dissociation proceed at a certain temperature,
yet should the product or products of decomposition not remain in
contact with the still undecomposed portion of the compound, then
decomposition will go on to the end. Thus limestone is decomposed
in a limekiln into lime and carbonic anhydride, because the latter is
carried off by the draught of the furnace. But if a certain mass of
limestone be enclosed in a definite volume — for instance, in a gun
barrel — which is then sealed up, and heated to redness, then, as the
carbonic anhydride cannot escape, a certain proportion only of the
limestone will be decomposed for every increment of heat (rise in tem-
perature) higher than that at which dissociation begins. Decomposition
will cease when the carbonic anhydride evolved presents a maximum
dissociation pressure corresponding with each rise in temperature. If
the pressure be increased by increasing the quantity of gas, then com-"
bination begins afresh ; if the pressure be diminished decomposition
will recommence. Decomposition in this case is exactly similar to
evaporation ; if the steam given off by evaporation cannot escape, its

INTKoIUVTION 37

pressure will reach a maximum corresponding with the given tempera-
ture, and then evaporation will cease. Should steam be added it will
be condensed in the liquid ; if its quantity be diminished — i.e., if the
pressure be lessened, the temperature being constant — then evaporation
will go on. We shall afterwards discuss more fully these phenomena of
dissociation, which were first discovered by Henri St. Claire Deville.
We will only remark that the products of decomposition re-cornbine
with greater facility the nearer their temperature is to that at which
dissociation begins, or, in other words, that the initial temperature of
dissociation is near to the initial temperature of combination.

(b) The influence of an electric current, and of electricity in general,
on the progress of chemical transformations is very similar to the
influence of heat. The majority of compounds which conduct elec-
tricity are decomposed by the action of a galvanic current, and there
being great similarity in the conditions under which decomposition and
combination proceed, combination often proceeds under the influence
of electricity. Electricity, like heat, must be regarded as a peculiar
form of molecular motion, and all that which refers to the influence of
heat also refers to the phenomena produced by the action of an electrical
current, only with this difference, that a substance can be separated
into its component parts with much greater ease by electricity, as the
process goes on at the ordinary temperature. The most stable com-
pounds may be decomposed by this means, and a most important fact
is then observed — namely, that the component parts appear at the
different poles or electrodes by which the current passes through the
substance. Those substances which appear at the positive pole (anode)
-are called ' electro-negative,' and those which appear at the negative
pole (cathode, that in connection with the zinc of an ordinary galvanic
battery) are called 'electro-positive.' The majority of non-metallic
elements, such as chlorine, oxygen, etc., and also acids and substances
analogous to them, belong to the first group, whilst the metals, hydro-
gen, and analogous products of decomposition appear at the negative
pole. Chemistry is indebted to the decomposition of compounds by the
electric current for many most important discoveries. Many elements
have been discovered by this method, the most important being potas-
sium and sodium. Lavoisier and the chemists of his time were not
able to decompose the oxygen compounds of these metals, but Davy
showed that they might be decomposed by an electric current, the
metals sodium and potassium appearing at the negative pole.

(c) Certain unstable compounds are also decomposed by the action of
light. Photography is based on this property in certain substances (for
instance, in the salts of silver). The mechanical energy of those vibra-

38 PRINCIPLES OF CHEMISTRY

tions which determine the phenomena' of light is very small, and there-
fore only certain, and these generally unstable, compounds can be decom-
posed by light — at least under ordinary circumstances. But there is
one class of chemical phenomena dependent on the action of light
which forms as yet an unsolved problem in chemistry — these are the
processes accomplished in plants under the influence of light. Here
there take place most unexpected decompositions and combinations,
which are often unattainable by artificial means. For instance, carbonic
anhydride, which is so stable under the influence of heat and electricity,
is decomposed, and evolves oxygen in plants under the influence of
light. In other cases, light decomposes unstable compounds, such as
are usually easily decomposed by heat and other agents. Chlorine
combines with hydrogen under the influence of light, which shows that
combination, as well as decomposition, can be determined by its action,
as was likewise the case with heat and electricity.

(d) Mechanical effects exert, like the foregoing agents, an action
both on the process of chemical combination and of decomposition.
Many substances are decomposed by friction or by a blow — as, for
example, the compound called iodide of nitrogen (winch is composed of
iodine, nitrogen, and hydrogen), and silver fulminate. Mechanical
friction causes sulphur to burn at the expense of the oxygen contained
in potassium chlorate.

(e) Besides the various conditions which have been enumerated
above, the progress of chemical reactions is accelerated or retarded by
the condition of contact in which the reacting bodies occur. Other
conditions remaining constant, the rate of progress of a chemical re-
action is accelerated by increasing the number of points of contact. It
will be enough to point out the fact that sulphuric acid does not absorb
ethylene under ordinary conditions of contact, but only after con-
tinued shaking, by which means the number of points of contact is
greatly increased. To ensure full action between solids, it is necessary
to reduce them to very fine powder and to mix them as thoroughly as
possible, as by this means their reaction is greatly accelerated. M.
Spring, the Belgian chemist, has shown that finely-powdered solids
which do not react on each other at the ordinary temperature may
undergo reaction under an increased pressure. Thus, under a pressure
of 6,000 atmospheres, sulphur combines with many metals at the ordinary
temperature, and the powders of many inetals form alloys. It is evident
that an increase in the number of points or surfaces must be regarded
as the chief cause producing reaction, which is doubtless accomplished
in solids, as in liquids and gases, in virtue of an internal movement or
mobility of the particles, which movement, although in different degrees

INTKolHXTInN 39

and ton us, must exist in all the states of matter. It is very important
to direct attention to the fact that the internal movement or condition
of the parts of the particles of matter must be different on the surface
of a substance from what it is inside ; because in the interior of a sub-
stance similar particles are acting on all sides of every particle, whilst
at the surface they only act on one side. Therefore, the condition of
a substance at its surfaces of contact with other substances must be
more or less modified by them — it may be in a manner similar to that
caused by an elevation of temperature. These considerations throw
some light on the action in the large class of contact reactions ; that
is, such as seem to proceed from the mere presence (contact) of certain
special substances. Porous or powdery substances are very prone to
act in this way, especially spongy platinum and charcoal. For example,
sulphurous anhydride does not combine directly with oxygen, but this
reaction takes place in the presence of spongy platinum. 36

The above general and introductory chemical conceptions cannot be
thoroughly grasped in their true sense without a knowledge of the
particular facts of chemistry to which we shall now turn our attention.
It was, however, absolutely necessary to become acquainted on the
very threshold with such fundamental principles as the laws of the
indestructibility of matter and of the conservation of energy, as it is
only by their acceptance, and under their direction and influence, that
the examination of particular facts can give practical and fruitful results.

56 Contact phenomena are separately considered in detail in the work of Professor
Konovaloff (1884). In my opinion, one must consider that the state of the internal move-
ments of the atoms in molecules is modified at the points of contact of substances, and
this state determines chemical reactions, and therefore, that reactions of combination,
decomposition, and rearrangement are accomplished by contact. Professor Konovaloff
showed that a number of substances under certain conditions of their surfaces act by con-
tact ; for instance, powdery silica (from the hydrate) acts just like platinum, decom-
posing certain compound ethers. As reactions are only accomplished under close contact,
it is probable that those modifications in the distribution of the atoms in molecules which
come about by contact phenomena prepare the way for them. By this the role of con-
tact phenomena is considerably extended. By such phenomena the fact should be
explained why a mixture of hydrogen and oxygen yields water (explodes) at different
temperatures according to the kind of heated substance which transmits this tempera-
ture. In chemical mechanics, phenomena of this kind have great importance, but as yet
they have been but little studied.

40 PRINCIPLES OF CHEMISTRY

CHAPTER I

ON WATER AND ITS COMPOUNDS

WATER is found almost everywhere in nature, and in all three physical
states. As vapour, water occurs in the atmosphere, and in this form
it is distributed over the entire surface of the earth. The vapour of
water in condensing, by cooling, forms snow, rain, hail, dew, and fog.
One cubic metre (or 1,000,000 cubic centimetres, or 1,000 litres, or
35'316 cubic feet) of air can contain at 0° only 4-8 grams of water, at
20° about 17'0 grams, at 40° about 50*7 grams ; but ordinary air only
contains about 60 per cent, of the possible moisture. Air containing
less than 40 per cent, of the possible moisture is felt to be dry, and air
which contains more than 80 per cent, of the possible moisture is con-
sidered as already damp.1 Water in the liquid state, in falling as rain

1 In practice, the chemist has to continually deal with gases, and gases are often
collected over water; in which case a certain amount of water passes into vapour.
and this vapour mingles with the gases. It is therefore most important that he
should be able to calculate the amount of water or of moisture in <dr and other gasen.
Let us consider the relations in volume and weight which exist in this case. Let us
imagine a cylinder standing in a mercury bath, and filled with a dry gas whose volume
equals u, temperature t°, and pressure or tension li mm. (h millimetres of the column of
.mercury at 0°). We will introduce water into the cylinder in such a quantity that a -mall
part remains in the liquid state, and consequently that the gas will be saturated with
aqueous vapour ; the volume of the gas will then increase (if a larger quantity of water be
taken some of the gas will be dissolved in it, and the volume may therefore be diminished).
We will further suppose that the temperature remains constant after the addition of
the water; then the pressure (as the volume increases the mercury in the cylinder
falls, consequently the pressure is increased) and the volume is increased. In order to
investigate the phenomenon we will artificially increase the pressure, and reduce the
volume to the original volume v. Then the pressure or tension will prove greater than
h, namely h+f, which means that by the introduction of aqueous vapour the tension
of the gas is increased. The researches of Dalton, Gay-Lussac. and Regnatilt showed
that this increase is equal to the maximum pressure which is proper to the aqueous
vapour at the temperature at which the observation is made. The maximum pressure
for all temperatures may be found in the tables made from observations on the tension
of aqueous vapour. The quantity/ will be equal to this maximum pressure of aqueous
vapour. This may be expressed thus : the maximum tension of aqueous vapour land of
all other vapours) saturating a space in a vacuum or in any ,uras U the same. This
rule is known as Dalian's law. Thus we have a volume of dry gas v, under a pressure
h, and a volume of moist gas, saturated with vapour, under a pressure // +/. The volume
v of the dry gas under a pressure h+f occupies, according to the law of Mariotte, a

<»N AVATKK AND ITS COMPOUNDS 41

,-iiul snow, soaks into the soil and collects together into springs, lakes,
livers, seas, and oceans. It is absorbed from the soil by the roots of

volume . ; consequently the volume occupied by the aqueous vapour under the

pre-sure // +/ equals v — -_ , or v* . Thus the volumes of the dry gas and of the

h +f k +/

moisture which occurs in it, at a pressure /*•+/, are in the ratio /: h. And, therefore, if
the aqueous vapour saturates a space at a pressure n, the volumes of the dry air and of
the moisture which is contained in it are in the ratio n—f:f, where / is the pressure of
the vapour according to the tables of vapour tension. Thus, if a volume N of a gas
saturated with moisture be measured at a pressure H, then the volume of the gas, when

TT _ f

dry, will be equal to N • , because the volume N requires to be divided into parts
H

which are in the ratio H— /:/. In fact, the entire volume N must be to the volume of

dry gas x as H is to H-/; therefore, N : x = H : H-/, from which a; = NH~-^. Under

H

TT TT /•

any other pressure — for instance, 760 mm. — the volume of dry gas will be -2:, or ~^

and thus we obtain the following practical rule : If a volume of a gas saturated with
aqueous vapour be measured at a pressure H mm., then the volume of dry gas contained
in it will be obtained by finding the volume corresponding with the pressure H, less the
pressure due to the aqueous vapour at the temperature of observation. For example,
37-5 cubic centimetres of air saturated with aqueous vapour was measured at a tempera-
ture of 15'3°, and under a pressure of 747'3 mm. of mercury (at 0°). What will be the
volume of dry gas at 0° and 760 mm. ? The pressure of aqueous vapour corresponding
witli 15"3C is equal to 12*9 mm., and therefore the volume of dry gas at 15'3° and

747-3 mm. is equal to 37'5 x 747'8~12>y ; at 760 mm. it will be equal to 87'5x Z!£i-
747-3 TOO '

and at 0° the volume of dry gas will be 37'5 x x - — = 34'31 c.c.

760 273-15-3

From this rule may also be calculated what fraction of a volume of gas is occupied by
moisture under the ordinary pressure at different temperatures ; for instance, at 30° C
/=31'5, consequently 100 volumes of a moist gas or air, at 760 mm., contain a volume of

aqueous vapour 100 x >:>1 ;>, or 4'110; also it is found that at 0° there is contained

0'61 p.c. by volume, at 10° 1-21 p.c., at 20° 2'29 p.c.,and at 50° up to 12'11 p.c. From this
it may be judged how great an error might be made in the volumetric determination
of gases were the moisture not taken into consideration. From this it is also evident
how great are the variations in volume of the atmosphere when it loses or gains aqueous
vapour, which again explains a number of atmospheric phenomena (winds, variation of
pressure, precipitations, storms, <fec.).

If aqueous vapour does not saturate a gas, then it is indispensable that the degree of
moisture should be known in order to determine the volume of dry gas from the volume
of moist gas. The preceding ratio gives the maximum quantity of water which can
be held in a gas, and the degree of moisture shows what fraction of this maximum
quantity occurs in a given ease, when the vapour does not saturate the space occupied
by the gas. Consequently, if the degree of moisture equals 50 p.c.— that is, half the
maximum— then the volume of dry gas at 760 mm. is equal to the volume of dry gas

at Till) mm. multiplied by _— -/, or, in general, by ~t' •?, where r is the degree of mois-

ture. It, therefore, it is required to measure the volume of a moist gas, it must either be
entirely dried or quite saturated with moisture, or else the degree of moisture deter-
mined. The first and last methods are inconvenient, and therefore recourse is usually
had to the second. For this purpose water is introduced into the cylinder holding the
gas to be measured ; it is left for a certain time so that the gas may become saturated,

42 PRINCIPLES OF CIIEMJSTRY

plants, which, when fresh, contain from 40 to 80 per cent, of water by
weight. Animals contain about the same amount of water. In a

the precaution being taken that a portion of the water remains in a liquid state; then
the volume of the moist gas is determined, from which that of the dry gas may be
calculated. In order to find the weir/lit <>r' the aq//ca//v ntjixntr in a pis it is necessary
to know the weight of a cubic measure at 0~ and 7(U) mm. Knowing that one cubic
centimetre of air under these circumstances weighs O'OOl'J'.Ki gram, and that the density
of aqueous vapour is 0'62, we find that one cubic centimetre of aqueous vapour at 0° and
760 mm. weighs 0'0008 gram, and at a temperature t and pressure // the weight of one

cubic centimetre will be O'OOOS x — x — ^ -- . We already know that v volumes of a ga^

at a temperature t° pressure h contain v x •-- volumes of aqueous vapour which satu-
rate it, therefore the weight of the aqueous vapour held in v volumes of a gas will bt

VJ1/ x 0-0008 x Ax -™ , or z; x O'OOOS x f x >27l! .
h 7GO 273 + t° 7(50 878 + <

Consequently, the weight of the water which is held in one volume of a gas is only
dependent on the temperature and not on the pressure. This also signifies that evapo-
ration proceeds to an equal extent in air as in a vacuum, or, in general terms (this is
Dalian's law), vapours and gases diffuse into each other as if into a vacuum. In a given
space there enters, at a given temperature, a constant quantity of vapour whatever be
the pressure of the gas filling that space. If the degree of moisture equals r then the

weight of the vapour in v cubic centimetres will be y) = v x O'OOOS x J[ x J ' grams,

7oO

From this it is clear that if the weight of the vapour held in a given volume of a gas
be known, it is easy to determine the degree of moisture r= u-ono X / X •)-•>'
On this is founded the very exact determination of the degree of moisture of air by the
weight of water contained in a given volume. It is easy to calculate from the preceding
formula the number of grams of water contained at all pressures in one cubic metre or
million centimetres of air saturated with vapour at various temperatures ; for example,

at 80° /= 31-5, therefore p = 1000000 x 0*0008 x ^ x 27g + g(j or 2<)\S4 grams.

The laws of "Mariotte, Dalton, and Gay-Lussac, which are here applied to gases and
vapours, are not entirely exact, but are approximately true. Were they unite exact, a mix-
ture of several liquids, having a certain vapour pressure, would be able to give vapours
of a very great pressure, which is not the case. In fact the pressure of aqueous vapour
is slightly less in a gas than in a vacuum, and the weight of aqueous vapour held in a
gas is slightly less than it should be according to Daltoifs law. as was shown by the ex-
periments of Eegnault and others. This means that the tension of the vapour is less
in air than in a vacuum, which also is the reason why the weight of vapour is less than
the theoretical weight. The difference between the pressure of vapours in air and in a
vacuum does not, however, exceed ^ of the total pressure of the vapours, and therefore
in practice the application of Dalton's law may be followed. This i/rcm/trnf in rajtour
tension which occurs in the intermixture of vapours and gases, although small, indicates
that there is then already, so to speak, a beginning of chemical change. The essence of
the matter is that in this case there occurs as on contact (see preceding footnote) an
alteration in the movements of the atoms in the molecules, and therefore also a change
in the movement of the molecules themselves.!

In the uniform intermixture of air and other gases with aqueous vapour, and in tin-
capacity of water to pass into vapour and form a uniform mixture with air, we may
perceive an instance of a physical phenomenon which is analogous to chemical phe-
nomena, forming indeed a transition from one class of phenomena to the other. Between
water and dry air there exists a kind of affinity which obliges the water to saturate the

<)N WATKIi AND ITS COMPOUNDS 43

solid state water appears ;is snow, ice, or in an intermediate form
lit -tween these two, which is seen on mountains covered with perpetual
sn<i\\r. The water of rivers,- springs, oceans and seas, lakes, and wells

air. But such a homogeneous mixture is formed (almost) independently of the nature of
the pis in which evaporation takes place; even in a vacuum the phenomenon occurs in
exactly the same way as in a pis, and therefore it is not the property of the gas, nor its
relation to water, but the property of the water itself, which obliges it to evaporate, and
therefore in this case chemical affinity is not yet acting — at least its action is not clearly
pronounced. That it does, however, play a certain part is seen from the deviation from
Dalton's law.

- In falling through the atmosphere, water dissolves the gases of the atmosphere,
nitric acid, ammonia, organic compounds, salts of sodium, magnesium, and calcium, and
mechanically washes out a mixture of dust and microbes which are suspended in the
atmosphere. The amount of these and certain other constituents is very variable. Even
in the beginning and end of the same rainfall, a variation which is often very considerable
may be remarked. Thus, for example, Bunsen found that rain collected at the begin-
ning of a shower contained 3'7 grams of ammonia per cubic metre, whilst that collected
at the end of the same shower contained only 0'64 gram. The water of the entire
shower contained an average of 1*47 grams of ammonia per cubic metre. In the course
of a year rain supplies an acre of ground with up to 5^ kilos of nitrogen in a combined
form. Marchand found in one cubic metre of snow water 15'03, and in one cubic metre
of rain water 10'07, grams of sodium sulphate. Angus Smith showed that after a thirty-
hours' fall at Manchester the rain still contained 34'3 grams of salts per cubic metre. A
considerable amount of organic matter, namely 25 grams per cubic metre, has been found
in rain water. The total amount of solid matter in rain water reaches 50 grams per
cubic metre. Rain water contains generally very little carbonic acid, whilst stream
water contains a considerable quantity of it. In considering the nourishment of
plants, it is necessary to keep in view the substances which are carried into the soil
by rain.

River ivater, which is accumulated from springs and sources fed by atmospheric
water, contains from 50 to 1,600 parts by weight of salts in 1,000,000 parts. The amount
of solid matter, per 1,000,000 parts by weight, contained in the chief rivers is as
follows :— the Don 124, the Loire 135, the St. Lawrence 170, the Rhone 182, the Dnieper
187, the Danube from 117 to 234, the Rhine from 158 to 317, the Seine from 190 to 432,
the Thames at London from 400 to 450, in its upper parts 387, and in its lower parts up to
1,017, the Nile 1,580, the Jordan 1,052. The Neva is characterised by the remarkably
small amount of solid matter it contains. From the investigations of Prof. G. K. Trapp,
a cubic metre of Neva water contains 32 grams of incombustible and 23 grams of
organic matter, or altogether about 55 grams. This is one of the purest waters which is
known in rivers. The large amount of impurities in river water, and especially of organic
impurity produced by pollution with putrid matter, makes the water of many rivers unfit
for n-.e.

The chief part of the soluble substances in river water consists of the calcium salts.
100 parts of the solid residues contain the following amounts of calcium carbonate —
from the water of the Loire 53, from the Thames about 50, the Elbe 55, the Vistula 65,
the Danube 05, the Rhine from 55 to 75, the Seine 75, the Rhone from 82 to 94. The
Neva contains 40 parts of calcium carbonate per 100 parts of saline matter. The con-
siderable amount of calcium carbonate held by stream water is very easily explained from
the fact that water which contains carbonic acid in solution easily dissolves calcium
carbonate, which occurs all over the earth. Besides calcium carbonate and sulphate,
river water contains magnesium, silica, chlorine, sodium, potassium, aluminium, nitric acid,
and manganese. The presence of salts of phosphoric acid has not yet been determined
with exactitude for all rivers, but the presence of nitrates has been proved with certainty
in almost all kinds of well-investigated river water. The quantity of calcium phosphate
does not exceed 0'4 gram in the river of the Dnieper, and the Don does not contain more

44 PHIXCIL'LES OF CHEMISTRY

contains various substances in solution, mostly salts —that is, sub-
stances resembling common table salt in their physical properties and

than 5 grams. The water of the Seine contains about 15 grams of nitrates, and the Rhone
about 8 grams. The amount of ammonia is much less ; thus in the water of the Rhine
about 0*5 gram in June, and 0'2 gram in October ; the water of * he Seine contains the
same amount. This is less than in rain water. Notwithstanding this insignificant
quantity, the water of the Rhine alone, which is not so very large a river, carries U'>.'_!4.1
kilograms of ammonia into the ocean every day. The difference between the amount oi
ammonia in rain and river water depends on the fact that the soil through which tht
rain water passes is able to withhold the ammonia. (Soil can also absorb many othei
substances, such as phosphoric acid, potassium salts, Arc.)

The water of springs, rivers, wells, and in general of those localities from which it is
taken for drinking purposes, may be very injurious to the health if it contains much
organic pollution — all the more, as in such water the lower organisms (bacteria) maj
rapidly develop, and these organisms often serve as the carriers or causes of infectious-
diseases. Thanks to the work of Pasteur, Koch, and many others, this province of researcl
has made considerable progress during the past ten years, and has shown the possi-
bility of investigating even the number and properties of the germs held by water
because those pathogenic bacteria which produce sickness, such as typhoid fever, Siberiai
plague, &c., have been distinguished. In bacteriological researches, a gelatinous
medium, enabling the germs to develop and multiply, is prepared with gelatin and water
which has previously been heated several times, at intervals, to 100° (it is thus renderec
sterile — that is to say, all the germs in it are killed). The water to be investigated
is added to this prepared medium in a definite and small quantity (it is sometimes
diluted with sterilised water to facilitate the calculation of the number of germs), it is
protected from dust (which contains germs), and is left at rest until whole families o:
lower organisms are developed from each germ. These families (colonies) are visible tc
the naked eye (as spots), they may be counted, and by examining them under th<
microscope and observing the number of organisms they produce, their significance ma>
be determined. The majority of bacteria are harmless, but there decidedly are patho
genie bacteria whose presence is one of the causes of malady, and of the spreading o
certain diseases. The number of bacteria in one cubic centimetre of water sometime!
attains the immense figures of hundreds of thousands and millions. Certain well, spring
and river waters contain very few bacteria, and are free from disease-producing bacterii
under ordinary circumstances. By boiling water, the bacteria in it are killed, but th<
organic matter necessary for their nourishment remains in the water. The best kind:
of water for drinking purposes do not contain more than 800 bacteria in a cnbii
centimetre.

The presence in water of every residue of destroyed organisms may be partly judgc<
from the amount of combined nitrogen, as all organisms contain nitrogen compotmdfl
It is mo'st essential to distinguish and determine nitrogen in the form of organic mattei
and in the form of oxides (nitric acid). The former is not separated, on heating, iron
water by the action of reducing agents, such as sulphurous anyhdride, whilst thi
nitrogen which occurs as oxide is evolved by this means. Thus on adding hydrochlori
"acid and ferrous chloride to water, the nitrogen of the nitric acid gives oxide of nitrogen
which may be determined. The presence of nitric acid indicates that the organr
matter in water has already been oxidised. Water which contains more than 1 par
of nitrogen (in this form) in a million parts is considered as injurious, and should no
be used. Frankland found about r.s parts of nitrogen in an oxidised form, and Iron
0'22 to 0*5 part in organic combinations in the water of the Thames at London.

The amount of gases dissolved in river water is much more constant tha
solid constituents. One litre, or 1,000 c.c., of water contains 40 to 5,1
measured at normal temperature arid pressure. In winter the amount of <ra
than in summer or autumn. Allowing that a litre contains 50 c.c. of gases, it may b
admitted that these consist, on the average, of 20 vols. of nitrogen, 20 vols. of carl ion i

ON AY.V

AND ITS roMI'<TNI).«

45

cliicf

chemical transformations. Further, the quantity and nature of
salts differ in different waters.3 Everybody knows that there

anhydride
of 10 vols.
still in abc
dominates
which .sin
succeeded,
anhydride,
Deville. CO
litre. Fir

M'oeeeding in all likelihood from the soil and not from the atmosphere), and
if oxygen. If the total amount of gases be less, the constituent gases are

h

>ut the same proportion; in many ca>es, however, carbonic anhydride pre-
The water of many deep and rapid rivers contain?-, less carbonic anhydride,
\s their rapid formation from atmospheric water and that they have not
during a long and slow course, in absorbing a greater quantity of carbonic
Thus, for instance, the water of the Khine, near Strasburg, according to
itains M c.c. of carbonic anhydride, 16 c.c. of nitrogen, and 7 c.c. of oxygen per
n the researches of Prof. M. R. Kapoustin and his pupils, it appears that in
determining the quality of a water for drinking purposes, it is most important to investi-
gate the composition of the dissolved gases.

3 Sprinij water is formed from rain water percolating through the soil. Naturally a
part of the rain water is evaporated straightway from the surface of the earth and from
the vegetation on it. It has been shown that out of 100 parts of water falling 011 the
earth only 36 parts flow to the ocean ; the remaining 64 are evaporated, or percolate
far underground. The collection of water by means of ponds, common wells, or artesian
wells is dependent on the presence of subterranean water. After flowing underground
along some impervious strata, water comes out at the surface in many places as springs,
whose temperature is determined by the depth from which the water has flowed.
Springs penetrating to a great depth may become considerably heated, and this is why
hot mineral springs, with a temperature of up to 30° and higher, are often met with. For
instance, there is one Caucasian spring whose temperature is 90°. Most likely in this
Ban the water is heated owing to its penetrating near a rock formation which is heated
by volcanic action. The composition of spring water is most varied. When a spring
water contains substances which endow it with a peculiar taste, and especially if these
substances are such as are only found in minute quantities or not at all in river and
other flowing waters, then the spring water is termed a mineral water. Many such
waters are employed for medicinal purposes. Mineral waters are classed according to
their composition into — (a) saline waters, which often contain a large amount of common
salt; (b) alkaline waters, which contain sodium carbonate; (c) bitter waters, which
contain magnesia ; (d) chalybeate waters, which hold iron carbonate in solution ; (e}
aerated waters, which are rich in carbonic anhydride ; ( f ) sulphuretted waters, which
contain hydrogen sulphide. Sulphuretted waters may be recognised by their smell of
rotten eggs, and by their giving a black precipitate with lead salts, and also by their tar-
nishing silver objects. Aerated waters, which contain an excess of carbonic anhydride,
effervesce in the air, have a sharp taste, and redden litmus paper. Saline waters leave a
large residue of soluble solid matter on evaporation, and have a salt taste. Chalybeate

4

z2

9 G^

j

-_

3

3

•f € 1 g 1 .2

- I

— 5

||

P 03 & 53 <"3 i O 'S

|| "I « "I * ™

§1

'55 «j

5

ii

it

wf =11
°

|g

^ 0

1^

fj S-s "as* ^ §

a

O ™

1 "5 «g~ , H

I.

1,928

152

24

448

152 1,300 80 1 2,609

II.

816

386

1,239

26 i —

43

9

257

46 1,485 I ! 2,812

III.

1,085

1,430

1,105

4

90

—

187

05 1,326 11 j 3,950

IV.

343

3,783

16

3,431 —

14

—

251

112 2,883 —

V. 3,406

15,049

- 2

—

17

1,587

229 76 20,290

VI. 352

3,145

—

€5 35

50

1

260

11 iu — 3,970

VII. 30K

1,036

2,583 1,261 , —

—

4

178

75 ' _ 1 5 451

VIII.1 1.7LV,

9,480

—

— | 40

120

26

208

40 — 11*790

IX. 551

2,040

1,150

999 , —

1

30

209

50 2,740 i 4,070

X. 285

558

279

3,813 1 —

— 7

45

45 2,268 1 5,031

XL 340

910

Iron and aluminium sulphates : | {'ggn

940

190 2 550 ( SulPhuric
'DO/. ] aild hydro-
( chloric acids

46 PRINCIPLES OK CIIK.M1STKY

are salt, fresh, iron, and other waters. The presence of about 3^ per
cent, of salts renders sea-water 4 heavy and bitter to the taste. Fresh
water also contains salts, only in a comparatively small quantity.
Their presence may be easily proved by simply evaporating water in a
vessel. By evaporation the water passes away as vapour, whilst the
salts are left behind. This is why a crust (incrustation), consisting of
salts, previously in solution, is deposited on the insides of kettles or
boilers, and other vessels in which water is boiled. Running water
(rivers, etc.) is charged with salts, owing to its being formed from the
collection of rain water percolating through the soil. While percolating
the water dissolves certain parts of the soil. Thus water which niters
or passes through saline or calcareous soils becomes charged with salts
or contains calcium carbonate (chalk). Rain water and snow are much
purer than river or spring water. This is because snow and rain are
only condensed aqueous vapour, and salts do not pass into the vapour.

waters have an inky taste, and are coloured black by an infusion of galls ; on being
exposed to the air they usually give a brown precipitate. Generally, the character of
mineral waters is mixed. In the table on page 45 are given the analysis of certain
mineral springs which are known for their medicinal properties. The quantity of the
substances is expressed in millionths by weight — that is, in grams per cub. metre or
milligrams per litre.

I. Sergieffsky, a sulphur water, Gov. of Samara (temp. 8° C.), analysis by Clause.
II. Geleznovodskya water source No. 10, near Patigorsk, Caucasus (temp. 22'5°), analysis
by Fritzsche. III. Aleksandroff sky, alkaline-sulphur source, Patigorsk (temp. 46'5°), average
of analyses by Herman Zinin and Fritzsche. IV. Bougountouksky, alkaline source,
No. 17, Essentoukah, Caucasus (temp. 21'6°), analysis by Fritzsche. V. Saline water,
Staro-Russi, Gov. of Novgorod, analysis by Nelubin. VI. Water from artesian well at
the factory of state papers, St. Petersburg, analysis by Struve. VII. Spriidel, Carlsbad
(temp. 83'7°), analysis by Berzelius. VIII. Kriitznach spring (Elisenquelle), Prussia
(temp. 8'8°), analysis by Bauer. IX. Eau de Seltz, Nassau, analysis by Henry. X. Vichy
water, France, analysis by Berthier and Puvy. XI. Paramo de Ruiz, New Granada,
analysis by Levy ; it is distinguished by the amount of free acids.

4 Sea-water contains more non-volatile saline constituents than the usual kinds of
fresh water. This is explained by the fact that the waters flowing into the sea supply
it with salts, and whilst a large quantity of vapour is given off from the surface of the
sea, the salts remain behind. Even the specific gravity of sea-water differs con-
siderably from that of pure water. It is generally about T02, but in this and also in
respect to the amount of salts contained, samples of sea-water from different localities
and from different depths offer rather remarkable variations. It will be sufficient to
point out that one cubic metre of water from the undermentioned localities contains the
following quantity in grams of solid constituents :— Gulf of Venice 19,1^2, L«-gli..rn
Harbour 24,812, Mediterranean, near Cetta, 87,655, the Atlantic Ocean from :j-2.:,sr, t.«,
85 695 the Pacific Ocean from 85,283 to 84,708. In closed s'eas which do not communi-
cate, or are in very distant communication, with the ocean, the difference is often still
greater. Thus the Caspian Sea contains 6,800 grams ; the Black Sea and Baltic 17,700.
Common salt forms the chief constituent of the saline matter of sea- or ocean-water ; thus
in one cubic metre of sea-water there are 25,000-81,000 grams of common salt, '2,C,(>0-
6,000 grams of magnesium chloride, 1,200-7,000 grams of magnesium sulphate, i.:,oo-t;,<H)i>
grams of calcium sulphate, and 10-700 grams of potassium chloride. The small amount
of organic matter and of the salts of phosphoric acid in sea- water is very remarkable.

ON AVATKR AND ITS COMPOUNDS 47

Neverthrlos. in passing through the atmosphere, r;i in and snow succeed
in catcliinu' tin- .lust held in it, and dissolve air, which is found in every
water. The dissolved gases of the atmosphere are partly disengaged,
as bubbles from water on heating, and water after long boiling is quite
freed from them.

In general terms water is called pure when it is clear and free from
insoluble particles held in suspension and visible to the naked eye, from
which it may be freed by nitration through charcoal, sand, or porous
(natural or artificial) stones, and when it possesses a clean fresh taste.
It depends on the absence of any tastable, decomposing organic matter,
on the quantity of air 5 and atmospheric gases in solution, and on the
presence of mineral substances to the amount of about 300 grams per
ton (or cubic metre, or, what is the same, 300 milligrams to a kilo-
gram or litre of water), and of not more than 100 grams of organic
matter.6 Such water is suitable for drinking and every practical

5 The taste of water is greatly dependent on the quantity of dissolved gases it con-
tains. On boiling, these gases are given off, and it is well known that, even when cooled,
boiled water has, until it has succeeded in absorbing gaseous substances from the atmo-
sphere, quite a different taste from fresh water containing a considerable amount of gas.
The dissolved gases, especially oxygen and carbonic anhydride, have an important
influence on the health. The following instance is very instructive in this respect. The
Grenelle artesian well at Paris, at the first period of its opening, supplied a water which
had an injurious effect on animals and people. It appeared that this water did not
contain oxygen, and in general was very poor in gases. As soon as it was made to fall in
a cascade, by which it absorbed air, it proved entirely fit for consumption. In long sea
voyages by steamer sometimes fresh water is not taken or only taken in a small quantity
because it spoils by keeping, and becomes putrid from the organic matter it contains under-
going decomposition. Fresh water may be obtained directly from sea-water by distilla-
tion. The distilled water 116 longer contains sea salts, and is therefore fit for consump-
tion, but it is very tasteless and has the properties of boiled water. In order to render it
palatable certain salts, which are usually held in fresh water, are added to it, and it is
made to flow in thin streams exposed to the air in order that it may become saturated
with the component parts of the atmosphere — that is, absorb gases.

6 Hard icat^r is such as contains much mineral matter, and especially a large pro-
portion of calcium salts. Such water, owing to the amount of lime it contains, does not
form a lather with soap, prevents vegetables boiled in it from softening properly, and
forms a great deal of incrustation on vessels in which it is boiled. Owing to its high
degree of hardness, it is injurious for drinking purposes, which is evident from the fact
that in many large cities the death-rate decreased after introducing a soft water in the
place of a hard water. Putrid water contains a considerable quantity of decomposing
organic matter, chiefly vegetable, but in populated districts, especially in towns, chiefly
animal remains. Such water acquires an unpleasant smell and taste, by which stagnant
bog water and the water of certain wells in inhabited districts are particularly charac-
terised. Such water is especially harmful at a period of epidemic. It may be partially
purified by passing through charcoal, which retains the putrid and certain organic sub-
stances, and also certain mineral substances. Turbid water may be purified to a certain
extent by the addition of alum, which aids, after standing some time, the formation of a
sediment. Condy's fluid (potassium permanganate) is another means for purifying
putrid water. A solution of this substance, even if very diluted, is of a red colour ; on
adding it to a putrid water, the permanganate oxidises and destroys the organic matter.
When added to water in such ;i quantity as to impart to it an almost imperceptible rose

48

PRINCIPLES OF CHEMISTRY

application, but evidently it is not pure in a chemical sense. A
chemically pure water is necessary not only for scientific purposes, as
an independent substance having constant and definite properties, and
as the chief component of all forms of water which play such an impor-
tant part in nature, but also for many practical purposes — for instance,
in photography and in the preparation of medicines — because many
properties of substances in solution are changed by the impurities of
natural waters. Water is usually purified by distillation, because the
solid substances in solution are not transformed into vapours in this
process. Such distilled water is prepared by chemists and in labora-
tories by boiling water in closed metallic boilers or stills, and causing
the steam produced to pass into a condenser — that is, through tubes
(which should be made of tin, or, at all events, tinned, as water and its
impurities do not act on tin) surrounded by cold water, and in which
the steam, being cooled, condenses into water which is collected7 in a

colour it destroys much of the organic substances it contains. It is especially salutary
to add a small quantity of Condy's fluid to impure water in times of epidemic.

The presence in water of one gram per litre, or 1,000 grams per cubic metre, of any
substance whatsoever renders it unfit and even injurious for consumption by animals,
and this whether organic or mineral matter predominate. The presence of 1 p.c. of
chlorides makes water quite salt, and produces thirst instead of assuaging it. The
presence of magnesium salts is most unpleasant ; they have a disagreeable bitter taste,
and in fact impart to sea water its peculiar taste. A large amount of nitrates is only
found in impure water, and is usually injurious, as they may indicate the presence of
decomposing organic matter.

7 Distilled water may be prepared, or distillation in general carried on, either in a

FIG. 4.— Distillation by means of a metallic still. The liquid in C is heated by the fire F. The
vapours rise through the head A and pass by the tube T to the worm S placed in a vessel R,
through which a current of cold water flows by means of the tubes D and P.

o.\ WATKi; AND ITS COMPOUNDS

1!)

receiver. By standing exposed to the atmosphere, however, the water
in time absorbs air, and dust carried in the air, and ceases to be en-
tirely pure. However, the amount of impurities in distilled water is
so small that they have hardly any effect on the properties of the
water, and it is fit for many purposes. Nevertheless, in distillation,
water retains, besides air, a certain quantity of volatile impurities
(especially organic) and the walls of the distillation apparatus are
partly corroded by the water, and a portion, although small, of their
substance renders the water not entirely pure, thus a sediment is ob-
tained on evaporation'.8

Still, for certain physical and chemical researches it is necessary to
have completely pure water. To obtain it a solution of potassium
permanganate is added to distilled water until it all becomes tinted
light rose colour. By this means the organic matter in the water is
destroyed (converted into gases or non-volatile substances). An excess

metal still with worm condenser (fig. 4), or on a small scale in the laboratory in a glass
retort (fig. 5) heated by a lamp (see footnote 19, Introduction). Fig. 5 illustrates
the main parts of the usual glass laboratory apparatus used for distillation. The steam

FIG. 5. — Distillation from a glass retort. The neak of the retort fits into the inner tube of the
Ltebig's condenser. The space between the inner and outer tube of the condenser is filled with
col< I water, which enters by tliu tube g and Hows out at/.

issuing from the retort (on the right-hand side) passes through a glass tube surrounded
by a larger tube, through which a stream of cold water passes, by which the steam is
cuii.lcnsed and trickles into a receiver (on the left-hand side). •

8 One of Lavoisier's first memoirs (1770) referred to this question. He investigated
the formation of the earthy residues in the distillation of water in order to prove whether
it was possible, as was affirmed, to convert water into earth, and he found that the
residue was produced by the action of water on the walls of the vessel holding it, a:id
not from the water itself. He proved this to be the case by direct weighing.

VOL. I. E

50 PRINCIPLES OK CHEMISTRY

of potassium permanganate does no harm, because in the next distilla-
tion it is left behind in the distillation apparatus. The next distilla-
tion should then be from a platinum retort with a platinum receiver.
Platinum is a metal which is not in any way changed either by air or
water, and therefore nothing passes from it into the water. The water
obtained in the receiver still contains air. It must then be boiled for
a long time, and afterwards cooled in a vacuum under the receiver
of an air pump. Pure water on evaporation does not give any sedi-
ment, does not in the least change, however long it be kept, and if air
have no access to it does not putrefy like water only once distilled or
impure ; and it does not give bubbles of gas on heating, nor does it
change the colour of a solution of potassium permanganate. These
are a few signs by which the complete purity of water may be recog-
nised.

Water, purified as above described, has constant physical and
chemical properties. For instance, it is of such water only that one
cubic centimetre weighs one gram at 4° C. — -i.e., it is only such pure
water whose specific gravity equals 1 at 4° C.9 Water in a solid state
forms crystals of the hexagonal system10 which are seen in snow, which

9 Taking the generally-accepted specific gravity of water at its greatest density — i.e.
at 4° as 1 — it has been shown by experiment that the specific gravity of water at different
temperatures is as follows : —

At - 5D . . . 0-99929 At 30° ... G'99577

., 0° ... 0-9JHIS7 „ 40° . . . 0-99230

,,4-10° . . . 0-99974 „ 50° . . . 0'98817

„ 15° ... 0-9991C) „ 80° ... 0-97192

20° 0*99820 100: 0-9.VO4

Water at 4° is taken as the basis for reducing measures of length to measures of
weight and volume. The metric, decimal, si/stem of measures of weights and volumes is
generally employed in science. The starting point of this system is the metre (39'37
inches) divided into decimetres ( = 0'1 metre), centimetres ( = O'Ol metre), millimetres
( = O'OOl metre), and micrometres (- one millionth of a metre). A cubic decimeti'e is
called a litre, and is used for the measurement of volumes. The weight of a litre of
water at 4° in a vacuum, is called a kilogram. One thousandth part of a kilogram, or one
cubic centimetre, of water weighs one yratn. It is divided into decigrams, centigrams,
and milligrams ( = O'OOl gram). An English pound equals 453'59 grams. The great
advantage of this system is that it is a decimal one, and that it is universally adopted in
science and in most international relations. All the mecuures died in thin ii-orl; (in-
metrical. The units most often used in science are : — Of length, the centimetre ; of
weight, the gram ; of time, the second ; of temperature, the degree Celsius or Centigrade.

10 As solid substances appear in independent, regular, crystalline forms which are
dependent, judging from their cleavage or lamination (in virtue of which mica breaks
up into laminae and Iceland spar, &c., into pieces bounded by faces inclined to each other
at angles which are definite for each substance), on an inequality of attraction (cohesion
hardness) in different' .'directions which intersect at definite angles ; therefore, the
determination of crystalline forms offers one of the most important external marks

<>N WATF.U AND ITS COMPOUNDS

51

c<>M>isrs < »f star-like clusters of several crystals, and also in
tin- half-incited scattered ice floating on rivers in spring time. At

characterising separate, definite chemical compounds. The elements of crystallography
\vhi«-h comprise a -)>• •< -i:il science, sh mid therefore be familiar to all who desire to work

Fi<;. r,. — Example <>t rhc form belonging to the FIG. 7. — Rhombic Dodecahedron of the regular
regular system. Combination of an octahedron system. Garnet.

an< I a cube. The former predominates. Alum,
thior spar, suboxide of copper, ami others.

s. Hexagonal prism ti-nninati-d by hexagonal Fio. 9.— Rhombohedron. Ca!c spar,

>. Quaitz. &c.

Fa;. 10.— lihnmbic system. FK;. 11. — Triclinia pyramid.

Desminc.

FIG. 12. — Triclinic sv.-tt in.
AU.ite, «fcc.

in scientific chemistiy. In this work we shall only have occasion to speak of a few
crystalline forms, some of which are shown in Figs. 6 to 12.

E —

52 PRINCIPLES OF CHEMISTRY

this time of the year the ice splits up into spars or prisms, bounded by
angles proper to substances crystallising in the hexagonal system. The
temperatures at which water passes from one state to another are
taken as fixed points on the thermometer scale : namely, the zero
corresponds with the temperature of melting ice, and the temperature
of the steam disengaged from water boiling at the normal barometer
pressure (that is 760 millimetres measured at 0°, at the latitude of 45°,
at the sea level) is taken as 100° of the Celsius scale. Thus, the fact
that water liquefies at 0° and boils at 100° is taken as one of its
properties as a definite chemical compound. The weight of one cubic
metre of water at 4° is 1,000 kilos, at 0° it is 999'8 kilos. The weight
of a cubic metre of ice at 0° is less — namely, 917 kilos ; the weight of a
cubic metre of water vapour at 760 mm. pressure and 100° is only 0'60
kilos ; the density of the vapour compared with air =; 0'62. and com-
pared with hydrogen = 9.

These data briefly enumerate the physical properties of water as a
separate substance. As a supplement to this it may be added that water
is a mobile liquid, colourless, transparent, without taste or smell, £c.
It is unnecessary to dwell on these properties here, as water is familiar
to all ; other properties will also be pointed out in describing less known
substances. Its latent heat of vaporisation is 534 units, of liquefac-
tion 79 units of heat.11 The large amount of heat stored up in water

11 Of all known liquids, water exhibits the greatest cohesion of particles. Indeed, it
ascends to a greater height in capillary tubes than other liquids ; for instance, two and a
half times as high as alcohol, nearly three times as high as ether, and to a much greater
height than oil of vitriol, &c. In a tube of two millimetres diameter, water at 0° ascends
15 '8 millimetres, counting from the level of the liquid to two-thirds of the height of the
meniscus, and at 100° it rises 12'5 millimetres. The cohesion varies very uniformly with
the temperature ; thus at 50° the height of the capillary column equals 13'i) millimetres —
that is, the mean between the columns at 0° and 100°. This uniformity is not destroyed
even on approaching the freezing point, and gives reason to think that at high tempera-
tures cohesion will vary as uniformly as at ordinary temperatures ; that is, the difference
between the columns at 0° and 100° being 2'8 millimetres, the height of the column at
500° should be 15;/- (5 x 2'8) = Vji millimetres. Consequently, at these high temperatures
the cohesion between the particles of water would be almost nil. Only certain solutions
(sal ammoniac r.i-a lithium chloride), and these only with a great excess of water, rise
higher than pure water in capillary tubes. The great cohesion of water doubtless
determines many of both its physical and chemical properties.

The quantity of heat required to raise the temperature of one part by weight of(
water from 0° to 1°, i.e., by 1° C., is called the unit of heat or calorie; the specific
heat of liquid water at 0° is taken as equal to ttnity. The variation of this specific
heat with a rise in temperature is inconsiderable in comparison with the variation
exhibited by the specific heats of other liquids. According to Ettinger, the specific heat
of water at 20° =1'016, at 50° = r039, and at 100° = 1'078. The specific heat of water is
greater than that of all other known liquids ; for example, the specific heat of alcohol at
0° is 0'5475 — i.e., the quantity of heat which raises 55 parts of water 1° raises 100 parts
of alcohol 1°. The specific heat of oil of turpentine at 0° is 0'4106, of ctli.-r <f,V2<), of
acetic acid G'527-4, of mercury 0'038. This means that water is the best condenser or

ON WAT Hi; AND ITS COMPOUNDS 53

vapour and also in liquid water (for its specific heat is greater than
that of other liquids) renders it available in both forms for heating

absorber of heat. This property of water has an important significance in practice and
in nature. Water impedes rapid cooling or heating ; it tempers cold and heat. The
specific heats of ice and aqueous vapour are much less than that of water ; namely
that of ice is 0'504, and of steam 0'48.

With an irerease in pressure equal to one atmosphere, the compressibility of water is
0-000047, of mercury 0-00(KHI:!.VJ, of ether 0'00012 at 0°, of alcohol at 13° O'QO.0095. The
addition of various substances to water generally simultaneously decreases its com-
pressibility and cohesion. The compressibility of other liquids increases with a rise of
temperature, but for water it decreases up to 53° and then increases like other liquids.

The expansion of /rate?- by heat (Note 9) also exhibits many peculiarities which are
not found in other liquids. The expansion of water at low temperatures is very small
compared with other liquids ; at 4° it reaches even 0, and at 100° it is equal to O'OOOS ;
below 4° it is negative — i.e., water on cooling then expands, and does not decrease in
volume. In passing into a solid state, the specific gravity of water decreases ; at 0° one
c.c. of water weighs 0-999888 gram, and one c.c. of ice at the same temperature weighs only
0'9175 gram. The ice formed, however, contracts on cooling like the majority of other
substances. Thus 100 volumes of ice are produced from 92 volumes of water — that is,
water expands considerably on freezing, which fact determines a number of natural
phenomena. The freezing point of water falls with an increase in pressure (0'007J per
atmosphere), because in freezing water expands (Thomson), whilst with substances which
contract in solidifying the melting point rises with an increase in pressure ; thus, for
paraffin it is at one atmosphere 46° and at 100 atmospheres 49°.

When liquid water passes into vapour, the cohesion of its particles must be destroyed,
as the particles are removed to such a distance from each other that their mutual
attraction no longer exhibits any influence. As the cohesion of aqueous particles varies at
different temperatures, the quantity of heat which is expended in overcoming this
cohesion — or the latent heat of evaporation — for this reason alone will be different at
different temperatures. The quantity of heat which is consumed in the transformation
of one part by weight of water, at different temperatures, into vapour was determined by
Regnault with great accuracy. His researches showed that one part by weight of water
taken at 0°, in passing into vapour having a temperature t°, consumes 606'5 + 0'305£ units
of heat, at 50° (521-7, at 100° 637'0, at 150 652'2, and at 200° 667'5. But this
quantity includes also the quantity of heat required for heating the water from 0° to t° —
i.e., besides the latent heat of evaporation, also that heat which is used in heating the water
in a liquid state to a temperature t°. On deducting this amount of heat, we obtain the
latent of evaporation of water as 60t>'5 at 0°, 571 at 50°, 534 at 100°, 494 at 150°, and only
453 at 200°, which shows that the conversion of water at different temperatures into
vapour at a constant temperature requires very different quantities of heat. This is
chiefly dependent on the difference of the cohesion of water at different temperatures ;
the cohesion is greater at low than at high temperatures, and therefore at low tem-
peratures a greater quantity of heat is required to overcome the cohesion. On comparing
these quantities of heat, it will be observed that they decrease rather uniformly,
namely their difference between 0° and 100° is 72, and between 100° and 200 3 is 81 units
of heat. From this we may conclude that -this variation will be approximately the same
for high temperatures also, and therefore that no heat would be required for the con-
version of water into vapour at a temperature of about 400° — 600D. At this temperature,
water passes into vapour whatever be the pressure (see chap. II. The absolute boiling
point of water, according to Dewar, is 370°, the critical pressure 196 atmospheres). It
must here be remarked that water, in presenting a greater cohesion, requires a larger
quantity of heat for its conversion into vapour than other liquids. Thus alcohol consumes
208, ether 90, turpentine 70, units of heat in their conversion into vapour.

The whole amount of heat which is consumed in the conversion of water into vapour
is not used in surmounting the cohesion — that is, in internal work accomplished in the

54

PRINCIPLES OF CHEMISTRY

purposes. The chemical reactions which water undergoes, and by
means of which it is formed, are so numerous, and so closely allied to

liquid. A part of this heat is employed in moving the aquj.ms particles; in fact, aqueous
vapour at 100° occupies a volume 1,650 times greater than that of water (at the ordinary
pressure), consequently a portion of the heat or work is employed in lifting the aqueous
particles, in overcoming pressure, or in external work, which may be usefully employed
and which is so employed in steam engines. In order to determine this work we will
first separately consider all the factors necessary for this calculation, and we will then
make a deduction from the comparison of these factors.

The maximum pressure or tension of aqueous vapour at different temperatures
has been determined with great exactitude by many observers. The observations of
Regnault in this respect, as on those preceding, deserve special attention from their
comprehensiveness and accuracy. The pressure or tension of aqueou* vapour at various
temperatures is given in the adjoining table, and is expressed in millimetres of the
barometric column having a temperature of 0°.

Tr;iiiu'r;tum- Tension

Temperature Trusimi

-20°

0-9

70° 233-3

— 10°

2-1

90° .VJ.V4

0°

4-6

100°

760-0

+ 10°

9-1 105°

90d'4

15°

12'7

110°

1075-4

20°

17-4

115°

1269'4

25°

23-5

120°

L491-8

30°

31-5

150° :j5Hl-o

50°

92-0

200° 110H9-0

The table shows the boiling points of water at different pressures. Thus on the
summit of Mont Blanc, where the average pressure is about 424 mm., water boils at
84*4°. In a rarefied atmosphere water boils at even the ordinary temperature, but in
evaporating it absorbs heat from the neighbouring parts, and therefore it becomes cold
and may even freeze if the pressure does not exceed 4'(5 mm., and especially if the vapour
be rapidly absorbed as it is formed. Oil of vitriol, which absorbs the aqueous vapour, is
Uried for this purpose. Thus ice may be obtained artificial!}' at the ordinary temperature
with the aid of an air-pump. This table of the tension of aqueous vapour also shows the
temperature of water contained in a closed boiler if the pressure of the steam formed l>e
known. Thus at a pressure of five atmospheres (a pressure of five times the ordinary
atmospheric pressure — i.e., 5x760 = 3,800 mm.) the temperature of the water would lie
152 '. The table also shows the pressure produced on a given surface by steam on issuing
from a boiler. Thus steam having a temperature of 152° exerts a pressure of 517 kilos, on a
piston whose surface equals 100 sq. c.m., for the pressure of one atmosphere on one
sq. c.m. equals 1,033 kilos., and steam at 152° has a pressure of five atmospheres. A>
a column of mercury 1 mm. high exerts a pressure of 1'35959 grams on a surface of
1 sq. c.m., therefore the pressure of aqueous vapour at 0° corresponds with a pressure of
6'25 grams per square centimetre. The pressures for all temperatures may be calculated
in a similar way, and it will be found that at 100° it is equal to ].o:;:;--2,s grams. This
means that if a cylinder be taken whose sectional area equals 1 sq. c.m.. and if water be
poured into it and it be closed by a piston weighing 1,0:!:! grams, thfii on heating it in a
vacuum to 100° no steam will be formed, because the steam cannot overcome the pressure
of the piston ; and if at 100° 534 units of heat be transmitted to each unit of weight of
water, {hen the whole of the water will be converted into vapour having the same
temperature ; and so also for every other temperature. The question now arises, To
what height does the piston rise under these circumstances ; that is, in other words, What
is the volume occupied by the steam under a known pressure ? For this we must know

ON WATKK AND ITS COMI'orNDS 55

the reactions of many other substances, that it is impossible to describe
tin- majority of them at this early stage of chemical exposition. After-
wards \vc shall become acquainted with many of them, but at present
w.' shall only cite certain compounds formed by water. In order to
see clearly the nature of the various kinds of compounds formed by

tin- weight df a cubic centimetre of steam at various temperatures. It has been shown by
experiment that the density of steam, which does not saturate a space, varies very
inconsiderably at all possible pressures, and is nine times the density of hydrogen under
similar conditions. Steam which saturates a space varies in density at different tem-
peratures, but this difference is very small, and its average density with reference to air is
OT>4. We will employ this figure in. our calculation, and will calculate what volume the
steam occupies at 100°. One cubic centimetre of air at 0° and 760 mm. weighs

0'00r2'.)3 gram, at 100 and under the same pressure it will weigh — or about

1°368

tr()UO'.»4(> gram, and consequently one cubic centimetre of steam whose density is 0'64
will weigh 0'000605 gram at 100°, and therefore one gram of aqueous vapour will
occupy a volume of about 1,653 c.c. Consequently, the piston in the cylinder of
1 sq. c.m. sectional area, and in which the water occupied a height of 1 c.m., will be
raised l,f>r>3 c.m. on the conversion of this water into steam. This piston, as has been
mentioned, weighs 1,033 grams, therefore the external icork of the steam — that is, that
work which the water does in its conversion into steam at 100° — is equal to lifting a piston
weighing 1,033 grains to a height of 1,653 c.m., or 17'07 kilogram-metres of work — i.e., is
capable of lifting 17 kilograms 1 metre, or 1 kilogram 17 metres. One gram of water
requires for its conversion into steam 534 gram units of heat or 0'534 kilogram units of
heat i.r., the quantity of heat absorbed in the evaporation of one gram of water is equal
to the quantity of heat which is capable of heating 1 kilogram of water 0'534°. Each
unit of heat, as has been shown by accurate experiment, is capable of doing 424 kilogram-
metres of work. Therefore, in evaporating, one gram of water expends 424xO'534 =
(almost) '226 kilogram-metres of work. The external work was found to be only
17 kilogram-metres, therefore 209 kilogram-metres are expended in overcoming the
internal cohesion of the aqueous particles, and consequently about 92 p.c. of the heat or
work consumed goes in overcoming the internal cohesion. The following figures are
thus calculated approximately : —

Total work of External work of T ,

•JVmpeniture evaporation in vapour in , "J " "

Kiln-ram -metres Kiln -ram-metres woikol \apom

0° 255 13 242

50° 242 15 227

100° 226 17 209

150° 209 ly 190

200° l'.)-2 20 172

Thus it will be remarked from this table that the work necessary for overcoming the
internal cohesion of water in its passage into vapour decreases with the rise in tempera-
ture; this is in connection with the decrease of cohesion with a rise in tempera-
ture, and, in fact, the variations which take place in this case are very similar to those
which are observed in the heights to which water rises in capillary tubes at different
t •lup.-ratures. It is evident, therefore, that the amount of external — or, as it is termed,
useful— work which water can supply by its evaporation is very small compared with the
am unit which it expends in its conversion into vapour.

IP. considering certain physico-meclianical properties of water, I had in view not only
their importance for theory and practice, but also their purely chemical significance, for
it is evident from the above considerations that in even a physical change of state the
greatest part of the work accomplished goes in overcoming cohesion, and that chemical
cohesion, or affinity, is an enormous internal energy.

56 I'KINCIPLKS OF CHEMISTRY

water we will begin with the most feeble, which are determined by
purely mechanical superficial properties of the reacting substances.12

Water is mechanically attracted by many substances ; it adheres to
their surfaces just as dust adheres to objects, and one polished glass
adheres to another. Such attraction is termed ' moistening,' ' soaking,' or
* absorption of water.' Thus water moistens clean glass and adheres to
its surface, is absorbed by the soil, sand, and clay, and does not flow
away from them but lodges itself between their particles. Similarly,
water soaks into a sponge, cloth, hair, or paper, etc., but fat and greasy
substances in general are not moistened. Attraction of this kind does
not alter the physical or chemical properties of water. For instance,
under these circumstances water, as is known from everyday experi-
ence, may be expelled from objects by drying. Water which is in any
way held mechanically may be dislodged by mechanical means, by fric-
tion, pressure, centrifugal force, <fcc. Thus water is squeezed from wet
cloth by pressure or centrifugal machines. But objects which in prac-
tice are called dry (because they do not wet people's hands) often still
contain moisture, as may be proved by heating the object in a glass
tube closed at one end. By placing a piece of paper, dry earth, or any
similar object (especially porous substances) in such a glass tube, and
heating that part of the tube where the object is situated, it will be
remarked that water condenses on the cooler portions of the tube. The
presence of such absorbed, or, as it is termed, ' hygroscopic,'' water is
generally best recognised in non- volatile substances by drying at 100°,

12 When it is necessary to heat a considerable mass of liquid in different vessels, it
would be very uneconomical to make use of metallic vessels and to construct a separate
fire grate under each one ; such cases are continually met with in practice. A considerable
mass of water, for instance, may have to be heated for making solutions, or it may be
required to expel volatile liquids from different vessels at intermittent periods ; as, for
instance, alcohol from partially fermented liquors, &c. In such cases one boiler or
vessel containing water is made use of. Steam from this boiler is introduced into the
liquid, or, in general, into the vessel which it is required to heat. The steam, in con-
densing and passing into a liquid state, parts with its latent heat, and as this is very
considerable a small quantity of steam will produce a considerable heating effect. If it
be required, for instance, to heat 1,000 kilos, of water from 20° to 50°, which requires
approximately 30,000 units of heat, steam heated to 100° is passed into the water from
a boiler. Each kilogram of water at 50° contains about 50 units of heat, and each kilo-
gram of steam at 100° contains 637 units of heat ; therefore, each kilogram of steam in
cooling to 50° gives up 587 units of heat, and consequently 52 kilos of steam are capable
of accomplishing the required heating of 1,000 kilos, of water from 20° to 50°. Water is
very often applied for heating in chemical practice. For this purpose metallic vessels
or pans, called ' water-baths,' are made use of. They are closed by a cover formed of
concentric rings lying on each other. The objects — such as beakers, evaporating basins,
retorts, &c. — containing liquids are placed on these rings, and the water in the bath is
heated. The steam given off heats the bottom of the vessels to be heated, and thus
accomplishes the evaporation or distillation or other required process. A water-bath
may also be used for heating a vessel directly immersed in the water.

()N WATKi; AND ITS CoMrol'NDS

57

or under the receiver of an air-pump and over substances which attract
water chemically. By weighing a substance before and after drying, it
is easy to determine the amount of hygroscopic water from the loss in
weight.13 Only in this case the amount of water must be judged with

13 In order t«« dry any substance at about 100°— that is, at the boiling point of water
(hygroscopic- water passes off at this temperature) — an apparatus called a ' drying-oven '
is employed. It consists of a double copper box ; water is poured into the space
between the internal and external boxes, and the oven is then heated over a stove or by
any other means, or else steam from a boiler is passed between the walls of the two
boxes. When the water boils, the temperature inside the inner box will be approximately
100° C. The substance to be dried is placed inside the oven, and the door is closed.
Several holes are cut in the door to allow the free passage of air, which carries off the
aqueous vapour by the chimney on the top of the oven. Often, however, desiccation is
carried on in copper ovens heated directly over a lamp fig. 13). In this case any desired

FIG. 13. — Drying oven, composed of brazc-d copper. It is heated by a lamp. The object to be dried
is placed on the gauze inside the oven. The thermometer indicates the temperature.

temperature may be obtained, which is determined by a thermometer fixed in a special
orifice. There are substances which only part with their water at a much higher
temperature than 100°, and then such air baths are very useful. In order to directly
determine the amount of water in a substance which does not part with anything except
water at a red heat, the substance is placed in a bulb tube. By first weighing the tube
empty and then with the substance to be dried in it, the weight of the substance taken may
be found. The tube is then connected on one side with a gas-holder full of air, which, on
opening a stop-cock, passes first through a flask containing sulphuric acid, and then into
a vessel containing lumps of pumice stone moistened with sulphuric acid. In passing
through these vessels the air is thoroughly dried, having given up all its moisture to the
sulphuric m-id. Thus dry air will pass into the bulb tube, and as hygroscopic water is
entirely given up from a substance in dry air at even the ordinary temperature, and still

58 PRINCIPLES OF CHEMISTRY

care, because the loss in weight may sometimes proceed from the de-
composition of the substance itself, with disengagement of gases or
vapour. In making exact weighings the hygroscopic capacity of sub-
stances— that is, their capacity to absorb moisture — must be continually
kept in view, as otherwise the weight will be untrue from the presence
of moisture. The quantity of moisture absorbed depends on the degree
of moisture of the atmosphere (that is, on the tension of the aqueous
vapour in it) in which a substance is situated. In an entirely dry
atmosphere, or in a vacuum, the hygroscopic water is expelled, being
converted into vapour ; therefore, if we have the means of drying yases
(or a vacuum) — that is, of removing the aqueous vapour from them —
objects impregnated with water may be entirely dried by placing them
in such a desiccated atmosphere. The process is aided by heat, as it
increases the tension of the aqueous vapour. Phosphoric anhydride (a
white powder), liquid sulphuric acid, solid and porous calcium chloride,
or the white powder of ignited copper sulphate are most generally
employed in drying gases. They absorb the moisture contained in air
and all gases to a considerable, but not unlimited, extent. Phosphoric
anhydride and calcium chloride deliquesce, become damp, sulphuric acid
changes from an oily thick liquid into a more mobile liquid, and ignited
copper sulphate becomes blue ; after which changes these substances
partly lose their capacity of holding water, and can, if it be in excess,
even give up their water to the atmosphere. We may remark that the
order in which these substances are placed above corresponds with the
order in which they stand in respect to their capacity for absorbing
moisture. Air dried by calcium chloride still contains a certain amount
of moisture, which it can give up to sulphuric acid. The most com-
plete desiccation takes place with phosphoric anhydride. Water is also
removed from many substances by placing them in a basin over a vessel
containing a substance absorbing water under a glass bell.14 The
bell, like the receiver of an air pump, should be hermetically closed.

more rapidly on heating, the moisture given up by the substance in the tube will be
carrietl off by the air passing through it. This damp air then passes through a U-shaped
tube full of pieces of pumice stone moistened with sulphuric acid, which absorbs all the
moisture given off from the substance in the bulb tube. Thus all the water expelled
from the substance will collect in the U tube, and so, if this be weighed before and after,
the difference will show the quantity of water expelled from the substance. If only water
(and not any gases) come over, the increase of the weight of the U tube will be equal to
the decrease in the weight of the bulb tube.

14 Instead of under a, glass bell, drying over sulphuric acid is often carried on in a
desiccator composed of a wide-mouthed low glass vessel, closed by a well-fitting ground-
glass stopper. Sulphuric acid is poured over the bottom of the desiccator, and the
substance to be dried is placed on a glass stand above the acid. A lateral glass tube with
a stop-cock is often fused into the desiccator in order to connect it with an air pump, and
so allow drying under a diminished pressure, when the moisture evaporates more rapidly.

(>.\ \VATKK AM) ITS m.MI>nrNI>S 59

In this case desiccation takes place ; because sulphuric acid, for instance,
iirst dries the air in the bell by absorbing its moisture, the substance
to he dried then parts with its moisture to the dry air, from which it is
again absorbed by the sulphuric acid, Arc. Desiccation proceeds still
better under the receiver of an air pump, for then the aqueous vapour
is formed more quickly than in a bell full of air.

From what has been said above, it is evident that the transference
of moisture to gases and the absorption of hygroscopic moisture present
un-at resemblance to, but still are not, chemical combinations with
water. Water, when combined as hygroscopic water, does not lose
its properties and does not form new substances.15

The attraction of water for substances which dissolve in it is of a
different character. In the solution of substances in water there pro-
ceeds a peculiar kind of indefinite combination ; there is formed a new
homogeneous substance from the two substances taken. But here also
the bond connecting the substances is very unstable. Water contain-
ing different substances in solution boils at a temperature near to its
usual boiling point, and acquires properties which are closely allied to
the properties of water itself and of the substances dissolved in it.
Thus, from the solution of substances which are lighter than water
itself, there are obtained solutions of a less density than water — as, for
example, in the solution of alcohol in water ; whilst a heavier sub-
stance in dissolving in water gives it a higher specific gravity. Thus
salt water is heavier than fresh.16

We will consider aqueous solutions somewhat fully, because, among
other reasons, solutions are constantly being formed on the earth and
in the waters of the earth, in plants and in animals, in chemical prac-
tice and in the arts, and these solutions play an important part in
the chemical transformations which are everywhere taking place, not
only because water is everywhere met with, but chiefly because a sub-
stance in solution presents the most propitious conditions for the process
of chemical changes, which require a mobility of parts and an intimate

1 ' Chapuy, however, determined that in wetting 1 gram of charcoal with water 7 units
of lieat are evolved, and on pouring carbon bisulphide over 1 gram of charcoal as much
as -Jl units of heat are evolved. Alumina (1 gram), wlien moistened with water, evolves
lories. This indicates that even in respect to evolution of heat moistening already
presents a transition towards exothermal combinations (those evolving heat in their
formation), like solutions.

"'• Strong acetic acid (CoH4O.j, whose specific gravity at 15° is T055, does not become
lighter on the addition of water (a lighter substance, sp. gr. = 0'99!)), but heavier, so that
a solution of so parts of acetic acid and 20 parts of water has a specific gravity of 1'074,
and even a solution of equal parts of acetic acid and water (50 p.c.) has a sp. gr. of T065,
which is still greater than that of acetic acid itself. This shows the high degree of con-
traction which takes place on solution. In fact, solutions — and, in general, liquids — on
mixing with water, decrease in volume.

60

PRINCIPLES OF CHEMISTRY

contact. In dissolving, a solid substance acquires a mobility of parts,
and a gas loses its elasticity, and therefore reactions often take place
in solutions which do not proceed in the undissolved substances. Fur-
ther, a substance, distributed in water, evidently breaks up (or ' disin-
tegrates ') — that is, becomes more like a gas and acquires a greater
mobility of parts. All these considerations require that in describing

Flo. 14. — Method of transferring a gas into a cylinder filled with mercurv and wlm.-e open end is im-
mersed under the mercury in a bath having two glass sides. The apparatus containing the gas is
represented on the right. Its upper extremity is furnished with a tube extending under the
cylinder. The lower part of the vessel communicates with a vertical tube. If mercury be poured
into this tube, the pressure of the gas in the apparatus is increased, and it passes tlm>ui:li the gag-
conducting tube into the cylinder, where it displaces the mercury, and can be measured or subjected
to the action of absorbing agents, such as water.

the properties of substances, particular attention should be paid to their
relation to water as a solvent.

Everybody knows that water dissolves many substances. Salt,
sugar, alcohol, and a number of other substances, by dissolving in water
form with it homogeneous liquids. To clearly show the solubility
of gases in water a gas should be taken which has a high co-efficient
of solubility — for instance, ammonia. This is introduced into a bell
(or cylinder, as in fig. 14), which is previously filled with mercury
and stands in a mercury bath. If water be then introduced into the
cylinder, the mercury will rise, owing to the water dissolving the
ammonia gas. If the column of mercury be less than the barometric

ON WATKK' AND ITS COMPOUNDS 61

column, and if there be sufficient water to dissolve the gas, all the
ammonia \vi\\ be absorbed by the water. The water is introduced into
tin- cylinder by a glass pipette, with a bent end. Its bent end is put
into water, and the air is sucked out from the upper end. When full
of water, its upper end is closed with the finger, and the bent end placed
in the mercury bath under the orifice of the cylinder. The water will
then be forced from the pipette by the atmospheric pressure, and will
i-i.se to the surface of the mercury in the cylinder owing to its lightness.
The solubility of a gas like ammonia may be demonstrated by taking a
flask full of the gas, and closed by a cork with a tube passing through
it. On placing the tube under water, the water will rise into the flask
(this may be accelerated by heating the flask), and begin to play like a
fountain inside it. Both the rising of the mercury and the fountain
clearly show the considerable affinity of water for ammonia gas, and the
force acting in this dissolution is rendered evident. For both the homo-
geneous intermixture of gases (diffusion) and the process of solution a
certain period of time is required, which depends, not only on the sur-
face of the participating substances, but also on their nature. This is
seen from experiment. Prepared solutions of different substances
heavier than water, such as salt or sugar, are poured into tall jars.
Pure water is then most carefully poured into these jars (through a
funnel) on to the top of the solutions, so as not to disturb the lower
stratum, and the jars are then left undisturbed. The line of demarca-
tion between the solution and the pure water will be visible, owing to
their different co-efficients of refraction. Notwithstanding that the
solutions taken are heavier than water, after some time complete inter-
mixture will ensue. Gay-Lussac convinced himself of this fact by
this particular experiment, which he conducted in the cellars under the
Paris Astronomical Observatory. These cellars are well known as the
locality where numerous interesting researches have been conducted,
because, owing to their depth under ground, they have a uniform tem-
perature during the wrhole year ; the temperature does not change
during the day, and this was indispensable for the experiments on the
diffusion of solutions, in order that no doubt in their results should
arise from a daily change of temperature (the experiment lasted several
months), which would set up currents in the liquids and intermix their
strata. Notwithstanding the uniformity of the temperature, the sub-
stance in solution in time ascended into the water and distributed itself
uniformly through it, proving that there exists between water and a
substance dissolved in it a particular kind of attraction or striving for
mutual interpenetration in opposition to the force of gravity. Further,
this effort, or rate of diffusion, is different for salt or sugar or for

62 PRINCIPLES OF CHEMISTRY

various other substances. Consequently, in solution there acts a
peculiar force, as in actual chemical combinations, and solution is de-
termined by a peculiar kind of movement (by the chemical energy of a
substance) which is proper to the substance dissolved and to the sol-
vent.

Graham made a series of experiments similar to those above
described, and he showed that the rate of diffusion*1 in water is very
variable — that is, a uniform distribution (under perfect rest, and with
such an arrangement of the strata of the solutions that uniformity
takes place in opposition to gravity) of a substance in the water dis-
solving it is attained in different periods of time with different solutions.
Graham compared diffusive capacity with volatility. There are sub-
stances which diffuse easily, and there are others which diffuse with
difficulty, just as there are more or less volatile substances. Seven
hundred cubic centimetres of water was poured into a jar, and by means
of a syphon (or a pipette) 100 cub. centimetres of a solution containing 10
grams of a substance was cautiously poured in so as to occupy the lower
portion of the jar. After the lapse of several days, successive layers of
50 cubic centimetres were taken from the top downwards, and the quan-
tity of substance dissolved in the different layers determined. Thus,
common table salt, after fourteen days, gave the following amounts (in
milligrams) in the respective layers, beginning from the top : 104. 120,
126, 198, 267, 340, 429, 535, 654, 766, 881, 991, 1,090, 1,187, and 2,266
in the remainder ; whilst albumin in the same time gave, in the first
seven layers, a very small amount, and beginning from the eighth layer,
10, 15, 47, 113, 343, 855, 1,892, and in. the remainder 6,725 milli-
grams. Thus, the diffusive power of a solution depends on time and
on the nature of the substance dissolved, which fact may serve, not only
for the explanation of the process of solution, but also in distinguishing
one substance from another. Graham showed that substances which
rapidly diffuse through liquids are able to rapidly pass through mem-
branes and crystallise, whilst substances which diffuse slowly and do not
crystallise are colloids, that is, resemble glue, and penetrate through

17 The researches of Graham, Fick, Nernst, and others showed that the quantity of a
dissolved substance which is transmitted (rises) from one stratum of liquid to another in
a vertical cylindrical vessel is not only proportional to the time and to the sectional area
of the cylinder, but also to the amount and nature of the substance dissolved in a stratum
of liquid, so that each substance has its corresponding co-efficient of diffusion. The cause
of the diffusion of solutions must be considered as essentially the same as the cause of
the diffusion of gases — that is, as dependent on movements which are proper to their
molecules ; but here most probably those purely chemical, although feebly-developed,
forces, which incline the substances dissolved to the formation of definite compounds,
also play their part.

ON WATKR AM» ITS COMPOUNDS M

;i nnMiibrane slowly, and form jellies ; that is, occur in insoluble
forms.18

18 The rate of diffusion — like the rateof transmission — through membranes, or tlitih/*/*
(which plays an important part in the vital processes of organisms and also in technical
work). present*, according to the researches of Graham, a sharply-defined change in
passing from such crystallisable substances as the majority of salts and acids to sub-
stances which are capable of giving jellies (gum, gelatin, il'c.). The former diffuse into
solutions and pass through membranes much more rapidly than the latter, and Graham
therefore distinguishes between cri/xtdlloid.i, which diffuse rapidly, and colloids, which
diffuse slowly. On breaking solid colloids into pieces, a total absence of cleavage is
remarked. The fracture of such substances is like that of glue or glass. It is termed a
• conchoidal ' fracture. Almost all the substances of which animal and vegetable bodies
consist are colloids, and this is, at all events, partly the reason why animals and plants
have such varied forms, which have no resemblance to the crystalline forms of the
majority of mineral substances. The colloid solid substances in organisms — that is, in
animals and plants — are usually soaked with water, and take most peculiar forms, of net-
works, of grannies, of hairs, of mucous, shapeless masses, Arc., which are quite different
from the forms taken by crystalline substances. When colloids separate out from solu-
tions, or from a molten state, they present a form which is similar to that of the liquid
from which they were formed. Glass maybe taken as the best example of this. Colloids
are distinguishable from crystalloids, not only by the absence of crystalline form, but by
many other properties which admit of clearly distinguishing both these classes of solids,
as was shown by the above-mentioned English scientific man, Graham. Nearly all
colloids are capable of passing, under certain circumstances, from a soluble into an
insoluble state. The best example is shown by white of eggs (albumin) in the raw and
soluble form, and in the hard-boiled
and insoluble form. The majority
of colloids, on passing into an in-
soluble form in the presence of
water, give substances having a
gelatinous appearance, which is
familiar to every one in starch,
solidified glue, jelly, itc. Thus

gelatin, or common carpenter's ^^^^f

glue, when soaked in water, swells
up into an insoluble jelly. If this
jelly be heated, it melts, and is then
soluble in water, but on cooling it
again forms a jelly which is in-
soluble in water. One of the pro-
perties which distinguish, colloids Fl(i- 15.- Dialyser. Apparatus for tie separation of sub-
, „ . , . ,, , ,, , stances winch jwss through a membrane from those

from crystalloids is that the former which do nor. Description in text.
pass very slowly through a mem-
brane, whilst the latter penetrate very rapidly. This maybe shown by taking a cylinder,
open at both ends, and by covering its lower end with a bladder or with vegetable parch-
ment (unsized paper immersed for two or three minutes in a mixture of sulphuric acid and
half its volume of water, and then washed), or any other membranous substance (all such
substances are themselves colloids in an insoluble form). The membrane must be firmly
tied to the cylinder, so as not to leave any opening. Such an apparatus is called a
fliuli/ser (fig. 15), and the process of separation of crystalloids from colloids by means of
such a membrane is termed dialysis. An aqueous solution of a crystalloid or colloid,
or a mixture of both, is poured into the dialyser, which is then placed in a vessel con-
taining water, so that the bottom of the membrane is covered with water. Then, after a
certain period of time, the crystalloid passes through the membrane, whilst the colloid,
if it does pass through at all, does so at an incomparably slower rate. The crystalloid

64 I'KIXCII'LES OF CMIK3IJSTKY

If it be desired to increase the rate of solution, recourse must
be had to stirring, shaking, or some such mechanical movement,
obliging the solution formed round the given substance to rise up-
wards if the solution be heavier than water. But if once a uniform
solution is formed, it will remain uniform if the temperature be
uniform, no matter how heavy the dissolved substance is, or how long
the solution be left at rest, which fact again shows the presence of a
force holding together the particles of the body dissolved and of the
solvent.19

naturally passes through into the water until the solution attains the same strength on
both sides of the membrane. By replacing the outside water with fresh water, a fresh
quantity of the crystalloid may be separated from the dialyser. While a crystalloid is
passing through the membrane, a colloid remains almost entirely in the dialyser, and
therefore a mixed solution of these two kinds of substances may be separated from each
other by a dialyser. The study of the properties of colloids, and of the phenomena of
their passage through membranes, should elucidate much respecting the phenomena
which are accomplished in organisms.

19 The formation of solutions may be considered in two aspects, from a physical and from
a chemical point of view, and it is more evident in solutions than in any other department
of chemistry that these provinces of natural science are allied together in a most intimate
manner. On one hand solutions form a particular aspect of a physico-mechanical inter-
penetration of homogeneous substances, and a juxtaposition of the molecules of the sub-
stance dissolved and of the solvent, similar to the juxtaposition which is exhibited in
homogeneous substances. From this point of view this diffusion of solutions is exactly
similar to the diffusion of gases, with only this difference, that the nature and store of
energy is different in gases from what it is in liquids, and th»,t in liquids there is consider-
able friction whilst in gases there is comparatively little. The penetration of a dissolved
substance into water is likened to evaporation, and solution to the formation of vapour.
This resemblance was clearly expressed even by Graham. In recent years the Dutch
chemist, Van't Hoff , has developed this view of solutions in great detail, having shown (in
a memoir in the Transactions of the Swedish Academy of Science, Part 21, No. 17,
' Lois de 1'equilibre chimique dans Petat dilue, gazeux au dissous,' 1886), that for dilute
solutions the osmotic pressure follows the same laws (of Boyle, Mariotte, Gay-Lussac.
and Avogadro-Gerhardt) as for gases. The osmotic pressure of a substance dissolved in
water is determined by means of membranes which allow the passage of water, but not
of a substance dissolved in it, through them. This property is found in animal proto-
plasmic membranes and in porous substances covered with an amorphous precipitate
such as is obtained by the action of copper sulphate on potassium ferrocyanide (Pffeifer
Traube). If, for instance, a one p.c. solution of sugar be placed in such a vessel,
which is then closed and placed in water, then the water passes through the walls
of the vessel and increases the pressure by 50 mm. of the barometric column. If the
pressure be artificially increased inside the vessel, then the water will be expelled
through the walls. The osmotic pressure of dilute sohitions determined in this manner
(from observations made by Pffeifer and De Vries) was shown to follow the same laws
as those of the pressure of gases ; for instance, by doubling or increasing the quantity of
a salt (in a given volume) n times, the pressure is doubled or increases n times. One of
the extreme consequences of the resemblance of osmotic pressure to gaseous pressure
is that the concentration of a uniform solution varies in parts which are heated or cooled.
Soret (1881) indeed observed that a solution of copper sulphate containing 17 parts of
the salt at 20° only contained 14 parts after heating the upper portion of the tube to
80° for a long period of time. This aspect of solution, which is now being very carefully
and fully worked out, may be called the physical side. Its other aspect is purely
chemical, for solution does not take place between any two substances, but requires a

<>N YYATKK AND ITS COMPOUNDS 65

In the consideration of the process of solution, besides the con-
ception of diffusion, another fundamental conception is necessary,
namely, that of the saturation of solutions.

^pecial and particular attraction or affinity between them. A vapour or gas permeates
into any other vapour or gas, but a salt which dissolves in water may not be in the least
soluble in alcohol, and is quite insoluble in mercury. In considering solution as a mani-
festation of chemical forces (and of chemical energy), it must be acknowledged that they
an- here developed to so feeble an extent that the definite compounds (that is, those
Formed according to the law of multiple proportions) which are formed between water
and a soluble substance dissociate at even the ordinary temperature, forming a homo-
geneous system— that is, one where both the compound and the products into which it
decomposes (water and the aqueous compound) occur in a liquid state. The chief diffi-
culty in the comprehension of solutions depends on the fact that the mechanical theory
of the structure of liquids has not yet been so fully developed as the theory of gases, and
solutions are liquids. The conception of solutions as liquid dissociated definite chemical
compounds is based on the following considerations : (1) that there exist certain undoubt-
edly definite chemical crystalline compounds (such as H2SO4, H2O ; or NaCl, 10H2O ; or
CaClo, 6HoO ; ivrc.) which melt on a certain rise of temperature, and then form real solu-
tions ; (2) that metallic alloys in a molten condition are real solutions, but on cooling they
often give entirely distinct and definite crystalline compounds, which are recognised by
the properties of alloys; (3) that between the solvent and the substance dissolved there
are formed, in a number of cases, many undoubtedly definite compounds, such as com-
pounds with water of crystallisation ; (4) that the physical properties of solutions, and
especially their specific gravities (a property which is very accurately observable), vary
with a change in composition, and in such a manner as the formation of one or several
definite but dissociating compounds would require. Thus, for example, on adding
water to fuming sulphuric acid its density is observed to decrease until it attains the
definite composition H2SO4, or SO3 + H2O, when the specific gravity increases, although
on further diluting with water it again falls. Further (Mendeleeff, The Investigation of
Aqueous Solution* from their Specific Gravities, 1887), the increase in specific gravity
(ds), with the augmentation (dp) of the percentage amount of a substance dissolved,
varies in all well-known solutions with the percentage amount of the substance dissolved,

so that a rectilinear dependence is obtained (i.e., ( S = A + B») between the limits of

dp

definite compounds which must be acknowledged to exist in solutions ; this would be
expected to be the case from the dissociation hypotheois. So, for instance, from H2SO4
to H2SO4 + H2O (both these substances exist as definite compounds in a free state), the

fraction ( S = 0'0729-0'000749p (where p is the percentage amount of H2SO4). For

alcohol C2H6O, whose aqueous solutions have been more accurately investigated than all
others, three definite compounds must be acknowledged in its solutions, C2HCO -f 12H2O,
C2H(,O + 3H2O, and 3C2H6O + H2O.

The two aspects of solution above mentioned, and the hypotheses which have as yet
been applied to the examination of solutions, although they have partially different
starting points, yet will doubtless in time lead to a general theory of solutions, because
the same common laws govern both physical and chemical phenomena, inasmuch as the
properties and movements of molecules, which determine physical properties, are depend-
ent on the movements and properties of atoms, which determine chemical mutual actions.
For details of the questions dealing with the theories of solution recourse must now be
had to special memoirs and to works on theoretical (physical) chemistry; for this subject
forms one of special interest at the present epoch of the development of our science.
In working out chiefly the chemical side of solutions I consider it to be necessary to
reconcile the two aspects of the question; this seems to me. to be all the more possible,
as the physical side is limited t,o dilute solutions only, whilst the chemical side deals
mainly with strong solutions.

VOL. -I. P

66 PRINCIPLES OF CHEMISTRY

Just as damp air may be added to any quantity of dry air it be
desired, so also a solvent liquid may be taken in an indefinitely large
quantity and yet a uniform solution will be obtained. But more than
a definite quantity of aqueous vapour cannot be introduced into a
certain volume of air at a certain temperature. The excess above the
point of saturation will remain in the liquid form.-0 The relation
between water and substances dissolved in it is similar. More than a
definite quantity of a substance cannot, at a certain temperature, dis-
solve in a given quantity of water ; the excess does not unite with the
water. Just as air or a gas becomes saturated with vapour, so water
becomes saturated with a substance dissolved in it. If an excess of a

20 A juxtaposition of (chemically or physically) reacting substances taken in various
states — for instance, some solid, others liquid or gaseous — is termed ft heterogeneous system.
Up to now it is only systems of this kind which can be subjected to d- 'tailed examination
in the sense of the mechanical theory of heat. Solutions present liquid homogeneous
systems, which as yet are subjected to investigation with difficulty.

In the case of limited solution of liquids in liquids, the difference hi-firci'it tJn
and the substance dissolved is clearly seen. The former (that is. the solvent) may be
added in an unlimited quantity, and yet the solution obtained will always be uniform,
whilst of the substance dissolved there can only be taken a definite saturating propor-
tion. We will take water and common (sulphuric) ether. On shaking the ether with the
water it will be remarked that a portion of it dissolves in the water, forming a solution.
If the ether be taken in such a quantity that it saturates the water and a portion of it
remains undissolved, then this remaining portion will act as a solvent, and water will
diffuse through it and also form a saturated solution of water in the ether taken. Thus
two saturated solutions will be obtained. One solution will contain ether dissolved in
water, and the other solution will contain water dissolved in ether. These two solutions
will arrange themselves in two layers, according to their density; the ethereal solution
of water will be on the top, as the lightest, and the aqueous solution of ether at the
bottom, as the heaviest. If the upper ethereal solution be poured off from the aqueous
solution, any quantity of ether may be added to it; this shows that the dissolving sub-
stance is ether. If water be added to it, it is no longer dissolved in it : this shows that
water saturates the ether — here water is the substance dissolved. If we act in the same
manner with the lower layer, we shall find that water is the solvent and ether the sub-
stance dissolved. By taking different amounts of ether and water, the degree of
solubility of ether in water, and of water in ether, may be easily determined. Thus, for
example, in the above case it is found that water approximately dissolves ^ of its
volume of ether, and ether dissolves a very small quantity of water. Let. us imagine that the
liquid poured in dissolves a considerable amount of water, and thai water dissolves a
considerable amount of the liquid. For instance, let us imagine that the saturation of
100 parts of water require 80 parts of the liquid, and that 100 parts of the liquid would
require 125 parts of water for its saturation. What would then take place if the liquid
be poured iu water ? Two layers could not be formed, because the saturated solutions
would resemble each other, and therefore they would intermix in all proportions.
Indeed, in the saturated aqueous solution there, would be 0'8 parts of the liquid taken to
1 part of water, and in the solution of water in the liquid taken there would be on
saturation 1 part of water to 0'8 parts of the liquid. There would be no line of demarca-
tion between the layers of the liquids, or, in other words, they would intermix in all
proportions. This is, consequently, a case of a phenomena where two liquids present
considerable co-efficients of solubility in each other, but where it is impossible to say what
these co-efficients are, because it is impossible to obtain a saturated solution.

ON WAT Hi; AND ITS COMPOUNDS

67

substance !>«' added to water which is already saturated with it, it will

remain in its original state, ;:nd will not spread through the water. The

quantity of a substance

(either l>y volume with

gases, or by weight with

solids and liquids) which is

capable of saturating 100

parts of water is called the

co-pftifii-Ht <>f xoltihilitij or

the sot nullify. Tn 100 grams

of water at 15°, there can

be dissolved not more than

35 "86 grams of common

salt. Consequently, its

solubility at 15° is equal

to 35-S6.21 It is most

-1 The solubility, or co-efficient
of solubility, of a substance is de-
termined by various methods.
Either a solution is expressly pre-
pared with a clear excess of the
soluble substance and saturated
at a given temperature, and the
quantity of water and of the sub-
stance dissolved in it determined
by evaporation, desiccation, or
other means ; or else, as is done
with gases, known quantities of
wat«-r and of the soluble sub-
stance are taken, and the amount
remaining undissolved is deter-
mined.

The solubility of a gas in water
is determined by means of an ap-
paratus called an absorptio-
niffcr (fig. 16). It consists of an
iron stand/, on which an india-rub-
ber ring rests. A wide glass tube
is plar-ed on this ring, and is pres-
sed down on it by the ring // and
fhe screws ii. The tube is thus
firmly fixed on the stand. A cock
r, communicating with a funnel r,
passes into the lower part of the
stand. Mercury can be poured
into the wide tube through this
funnel, which is therefore made
of steel, as copper would be
affected by the mercury. The
upper ring h is furnished with a

Ktui-rti's alisorjitionieter. Apparatus
niiiiiiiL' tlic solubility of gases in liquids.

leter-

P 2

68 PRINCIPLES OF CHEMISTRY

important to turn attention to the existence of the solid imsohi1>J<'
substances of nature, because on them depends the shape of the

cover 2^, which can be firmly pressed down on to the wide tube, and hermetically closes it
by means of an india-rubber ring. The tube r r can be raised at will, and so by pouring mer-
cury into the funnel the height of the column of mercury, which produces pressure inside
the apparatus, can be increased. The pressure can also be diminished at will, by letting
mercury out through the cock r, A graduated tube e, containing the gas and liquid to be
experimented on, is placed inside the wide tube. This tube is graduated in millimetres
for determining the pressure, and it is calibrated for volumes, so that the number of
volumes occupied by the gas and liquid dissolving it can be easily calculated. This tube
can also be easily removed from the apparatus. To the right of the figure, the lower
portion of this tube when removed from the apparatus is shown. It will be observed
that its lower end is furnished with a male screw 6, fitting in a nut a. The lower
surface of the nut a is covered with india-rubber, so that on screwing up the tube its
lower end presses upon the india-rubber, and thus hermetically closes the whole tube, for
its upper end is fused up. The nut a is furnished with arms c c, and in the stand f
there are corresponding spaces, so that when the screwed-up internal tube is fixed into
stand/, the arms c c fix into these spaces cut in/. This enables the internal tube to In-
fixed on to the stand/. When the internal tube is fixed in the stand, the wide tube is put
into its right position, and mercury and water are poured into the space between the two
tubes, and communication is opened between the inside of the tube e and the mercury
between the interior and exterior tubes. This is done by either revolving the interior
tube e, or by a key turning the nut about the bottom part of/. The tube e is filled with
gas and water as follows : the tube is removed from the apparatus, filled with mercury,
and the gas to be experimented on is passed into it. The volume of the gas is measured,
the temperature and pressure determined, and the volume it would occupy at 0° and
760 mm. calculated. A known volume of water is then introduced into the tube. The
water must be previously boiled, so as to be quite freed from air in solution. The tube is
then closed by screwing it down on to the india-rubber on the nut. It is then fixed on to
the stand/, mercury and water are poured into the intervening space between it and the
exterior tube, which is then screwed up and closed by the cover j?, and the whole
apparatus is left at rest for some time, so that the tube e, and the gas in it, may attain the
same temperature as that of the surrounding water, which is marked by a thermometer
Jc tied to the tube e. The interior tube is then again closed by revolving it in the nut,
the cover^? again shut, and the whole apparatus is shaken in order that the gas in the
tube e may entirely saturate the water. After several shakings, the tube e is again
opened by revolving it in the nut, and the apparatus is left at rest for a certain time ; it is
then closed and again shaken, and so on until the volume of gas does not diminish after
a fresh shaking — that is, until saturation ensues. Observations are then made of the
temperature, the height of the mercury in the interior tube, and the level of the water in
it, and also of the level of the mercury and water in the exterior tube. All these data
are necessary in order to calculate the pressure under which the solution of the gas takes
place, and what volume of gas remains undissolved, and also the quantity of water which
serves as the solvent. By varying the temperature of the surrounding water, the amount
of gas dissolved at various temperatures may be determined. Bunsen, Carius, and
many others determined the solution of various gases in water, alcohol, and certain
other liquids, by means of this apparatus. If in a determination of this kind it is found
that n cubic centimetres of water at a pressure h dissolve in cubic centimetres of a
given gas, measured at 0° and 760 mm., when the temperature under which solution
took place was t° and pressure h mm., then it follows that at the temperature / flic,

co-efficient of solubility of the gas in 1 volume of the liquid will be equal to m x ' '

This formula is very clearly understood from the fact that the co-efficient of solubility
of gases is that quantity measured at 0° and 760 mm., which is absorbed at a pressure

ON WATER AND ITS COMPOUNDS 69

substance of the earth's surface, and of plants and animals. There
is so much water on the earth's surface, that were the surface of sub-
stances formed of soluble matters it would constantly change, and
however substantial their forms might be, mountains, river banks and
s«-u shores, plants and animals, or the habitations and coverings of men,
could not exist for any length of time.22

of 7<>0 mm. by one volume of a liquid. If n cubic centimetres of water absorb m cubic

•centimetres of a gas, then one cubic centimetre absorbs — . If — c.c. of a gas are ab-

n n

sorbecl under a pressure of It mm., then, according to the law of the variation of
solubility of a ^as with the pressure, there would be dissolved, under a pressure of

760 mm., a quantity varying in the same ratio to — • as 760 : h. In determining the

residual volume of gas its moisture (note 1) must be taken into consideration.

Below are given the number of grams of several substances saturating 100 grams of
water — that is, their co-efficients of solubility by weight at three different temperatures : —

At 0° At 20° At 100°

, Oxygen, O2
Carbonic anhydride, CO2

"^fe ^ _

I Ammonia, NH3 I 90-0 51-8 7'3

, Phenol, CfiH«O ! 4'9 5'2 oo

Liquids Ainyl ak-oliol, C-.H^O . . . . 4'4 2'9

^ Sulphuric acid, H,>SO4 .... oo OO OO

( Gypsum, CaSO4 , 2HoO . j ^ i ^

] Alum, AlKSoOg , 12H..O ...... | 8'3 15'4 857'5

Solids - Anhydrous sodium sulphate, NaoSO4

Common Salt, NaCl
Nitre, KN03

4-5 20

85-7 86-0

18-8 81-7

43

39'7

Sometimes a substance is so slightly soluble that it may be considered as insoluble.
Many such substances are met with both in solids and liquids, and such a gas as oxygen,
although it does dissolve, does so in so small a proportion by weight that it might be
considered as zero did not the solubility of even so little oxygen play an important part
in nature (as in the respiration of fishes) and were not an infinitesimal quantity of a gas
by weight so easily measured by volume. The sign QO, which stands on a line with sul-
phuric acid in the above table, indicates that it intermixes with water in all proportions.
There are many such cases among liquids, and everybody knows, for instance, that spirit
{absolute alcohol) can be mixed in any proportion with water. Common corn spirit
(vodky) is a mixture of about fifty parts by weight of pure spirit to 100 parts by weight
of water.

22 Just as the existence must be admitted of substances which are completely un-
decomposable (chemically) at the ordinary temperature — for there are substances which
are entirely non-volatile at such a temperature (as wood and gold), although capable of
decomposing (wood) or volatilising (gold) at a higher temperature— so also the existence
must be admitted of substances which are totally insoluble in water without some degree
of change in their state. Although mercury is partially volatile at the ordinary tem-
perature, there is no reason to think that it and other metals are soluble in water, alcohol,
or other similar liquids. However, mercury forms solutions, as it dissolves other metals.
On the other hand, there are many substances found in nature which are so very
slightly soluble in water, that in ordinary practice they may be considered as insoluble
<for example, barium sulphate). For the comprehension of that general plan according to
which a change of state of substances (combined or dissolved, solid, liquid, or gaseous)

70 I'KIXCIPLKS OF CHEMISTRY

Substances which are easily soluble in water bear a certain resrin
blance to it. Thus sugar and salt in many of their superficial features
remind one of ice. , Metals, which are not soluble in water, have 110
points in common with it, whilst on the other hand they dissolve each
other in a molten state, forming alloys, just as oily substances dissolve
each other ; for example, tallow is soluble in petroleum and in olive oil,
although they are all insoluble in water. From this it is evident that
the analogy of substances forming a solution plays an important part,
and as aqueous and all other solutions are liquids, there is good reason to
believe that in the process of solution solid and gaseous substances
change in a physical sense, passing into a liquid state. These con-
siderations elucidate many points of solution — as, for instance, the vari-
ation of the co-efficient of solubility with the temperature and the evo-
lution or absorption of heat in the formation of solutions.

The solubility — that is, the quantity of a substance necessary for
saturation — varies with the temperature, and, further, with an increase
in temperature the solubility of solid substances generally increases, and
that of gases decreases ; this might be expected, as solid substances by
heating, and gases by cooling, approach to a liquid or dissolved state.23
A graphic method is often employed to express the variation of solu-
bility with temperature. On the axes of abscissae or on a horizontal
line, temperatures are marked out and perpendiculars are raised corre-
sponding with each temperature, whose length is determined by the
solubility of the salt at that temperature — expressing, for instance, one
part by weight of a salt in 100 parts of water by one unit of length,
such as a millimetre. By joining the summits of the perpendiculars,
a curve is obtained which expresses the degree of solubility at different
temperatures. For solids, the curve is generally an ascending one — i.e.*
recedes from the horizontal line with the rise in temperature. These
curves clearly show by their inclination the degree of rapidity of increase
in solubility with the temperature. Having determined several points

takes place, it is very important to make a distinction at this boundary line (on approach-
ing zero of decomposition, volatility, or solubility) between an insignificant amount and
zero, but the present methods of research and the data at our disposal at the present
time do not yet touch such questions. It must be remarked, besides, that water in a
number of cases does not dissolve a substance as such, but acts on it chemically and forms
a soluble substance. Thus glass and many rocks, especially if taken as powder, are
chemically changed by water, but are not directly soluble in it.

23 Beilby (1883) experimented on paraffin, and found that one cubic decimetre of solid
paraffin at 21° weighed 874 grams, and when liquid, at its melting-point 88°, 788 grams, at
49°, 775 grams, and at 60°, 767 grams, from which the weight of a litre of liquefied paraffin
would be 795-4 grams at 21° if it could remain liquid at that temperature. By dissolving
solid paraffin in lubricating oil at 21° Beilby found that 795'6 grams occupy one cubic
decimetre, from which he concluded that the solution contained liquefied paraffin.

o.\ WATEB AND ITS COMPOUNDS 71

of ;i curve that i>, having made a determination of the solubility for
several temperature tin- solubility at intermediary temperatures may
be determined from the sinuosity and form of the curve so formed ; in
this way the empirical law of solubility may be followed.'2"1 The results of
research have shown that the solubility of certain salts — as, for example,
(in 11 moil table salt — varies comparatively little with the temperature ;
whilst for other substances the solubility increases by equal amounts for
equal increments of temperature. So, for example, for the saturation of

-' (lay-Lu-^ac \vas the first to have recourse to such a graphic method of expressing
solubility, and lie considered, in accordance with the general opinion, that by joining up
the summits of the ordinates in one harmonious curve it is possible to express the entire
change of solubility with the temperature. Now, there are many reasons for doubting
the accuracy of such an admission, for there undoubtedly are critical points in curves of
solubility (for example, of sodium sulphate, as shown further on), and it may be that
definite compounds of dissolved substances with water, in decomposing within known
limits of temperature, give critical points more often than would be imagined; it may
even be, indeed, that instead of a continuous curve, solubility should be expressed — if
not always, then not unfrequently — by straight or broken lines. According to Ditte, the
solubility of sodium nitrate, NaXO,-, is expressed by the following figures per 100 parts of
water : —

0° 1 10° 15° 21° 29° 36° 51° 68°

()C>-7 71-0 7<i'o 80-6 85'7 92".> 91)'4 13'6 12;V1

According to my opinion (iHHlj, these data should be expressed with exactitude by a
straight line. (\7',~> -f O'STf, which entirely agrees with the results of experiment. Accord-
ing to this the figure expressing the solubility of the salt at 0° exactly coincides with
the composition of a definite chemical compound — NaXO5,7H2O. The experiments
made by Ditte showed that all saturated solutions between 0° and — 15'7 have such a
composition, and that at the latter temperature the solution completely solidifies into one
homogeneous whole. Ditte shows, in the first place, that the solubility of sodium nitrate
is expressed by a broken straight line, and, in the second place, confirms the idea,
which I had already traced, that in solutions we have definite chemical compounds in a
state of dissociation. In recent times (IHHH) Etard discovered a similar phenomenon in
many of the sulphates. Brandes, in 1830, shows a diminution in solubility below 100°
for manganese sulphate. The percentage by weight (i.e., per 100 parts of the solution, and
not of wateri of saturation for ferrous sulphate, FeSO4, from — 2° to + 65° = 13'5 + 0'3784f —
that is, the solubility of the salt increases. The solubility remains constant from 65° to
98° (according to Brandes the solubility then increases ; this divergence of opinion
requires proof), and from 98° to 150° it falls as = 104'35- 0'6685£. Hence, at about
+ 156° the solubility should =0, and this has been confirmed by experiment. I observe,
on my side, that Etard's formula gives 38'1 p.c. of salt at (55° and 38"8p.c. at 92°, and this
maximum amount of salt in the solution very nearly corresponds with the composition
FeSO4,14H2O, which requires 37'6 p.c. Thus, in this case, as in that of sodium nitrate,
the formation of a definite solution may be presupposed. From what has been said, it is
evident that the data concerning solubility require a new method of investigation, which,
in the first place, should have in view the entire scale of solubility— from the formation
of completely solidified solutions (cryohydrates, which we shall speak of presently) to the
separation of salts from their solutions, if this is accomplished at a higher temperature
(for manganese and cadmium sulphates there is an entire separation, according to Etard),
or to the formation of a constant solubility (forpotassium sulphate the solubility, accord-
ing to Etard, remains constant from 163° to 220° and equals 24'9 p.c.) ; and, in the second
place, should endeavour to apply the conception of definite compounds existing in solu-
tions to constant and critical solutions, corresponding with a maximum of solubility or
of its limits. From these aspects solution should present a new and particular inter.

7k2 PRINCIPLES OF CHEMISTRY

100 parts of water by potassium chloride there is required at 0°, 29*2
parts, at 20°, 34*7, at 40°, 40'2, at 60°, 45-7 ; and so on, for every 10°
the solubility increases by 2 -75 parts by weight of the salt. Therefore
the solubility of potassium chloride in water may be expressed by a
direct equation : a=29*2 + 0*2752, where a represents the solubility at t".
For other salts, more complicated equations are required. For exam pie,
for nitre: a=13*3 + 0*5742 + 0*0171722 + O0000036*3, which shows
that when 2=0° a=13*3, when 2 = 10° a=20'8, and when 2 = 100°
a=246*0.

Curves of solubility give the means of judging with accuracy the
amount of a salt separated by the cooling to a known extent of a
solution saturated at a given temperature. For instance, if 200 parts
of a solution of potassium chloride in water saturated at a temperature
of 60° be taken, and it be asked how much of the salt will be separated
by cooling the solution to 0°, if its solubility at 60° = 45'7 and at
0°=29*2 ? The answer is obtained in the following manner : At 60° a
saturated solution contains 45*7 parts of potassium chloride per 100
parts by weight of water, consequently 145*7 parts by weight of the
solution contains 45*7 parts, or, by proportion, 200 parts by weight of
the solution contains 62*7 parts of the salt. The amount of salt
remaining in solution at 0° is calculated as follows : In 200 grams
taken there will be 137*3 grams of water ; consequently, this amount of
water is capable of holding only 40*1 grams of the salt, and therefore
in lowering the temperature from 60° to 0° there should separate from
the solution 62*7 — 40*1 = 22*6 grams of the dissolved salt.

The difference in the solubility of salts, <fcc., with a rise or fall of
temperature is often taken advantage of, especially in technical
work, for the separation of salts in intermixture from each other.
Thus a mixture of potassium and sodium chlorides (this mixture is met
with in nature at Stassfiirt) is separated from a saturated solution by
subjecting it alternately to boiling (evaporation) and cooling. The
sodium chloride separates out in proportion to the amount of water
expelled from the solution by boiling, and is removed, whilst the
potassium chloride separates out on cooling, as the solubility of this
salt rapidly decreases with a lowering in temperature. Nitre, sugar, and
many other soluble substances are purified (refined) in a similar
manner.

Although in the majority of cases the solubility of solids increases
with the temperature, yet just as there are substances whose volume
diminishes with a rise in temperature (for example, water from 0° to
4°), so there are not a few solid substances whose solubilities fall on
heating. Glauber's salt, or sodium sulphate, historically forms a particu-

ON WATER AND ITS COMPOUNDS 78

larly instructive example of the case in question. If this salt be taken
in an ignited state (deprived of its water of crystallisation), then its
solubility in 100 parts of water varies with the temperature in the
following manner : at 0°, 5 parts of the salt form a saturated solution ;
at 20°, 20 parts of the salt, at 33° more than 50 parts. As will be
seen, the solubility increases with the temperature, as is the case
with nearly all salts ; but starting from 33° it suddenly diminishes,
and at a temperature of 40°, there dissolves less than 50 parts of
the salt, at 60° only 45 parts of the salt, and at 100° about 43
parts of the salt in, 100 parts of water. This phenomenon may be
traced to the following facts : Firstly, that this salt forms various
compounds with water, as will be afterwards explained ; secondly,
that at 33° the compound Na2SO4 + 10H.20 formed from the solu-
tion at lower temperatures, melts ; and thirdly, that on evaporation
at a temperature above 33° there separates out an anhydrous salt,
Na2S04. It will be seen from this example how complicated such a
seemingly simple phenomenon as solution really is ; and all data con-
cerning solutions lead to the same conclusion. This complexity becomes
evident in investigating the heat of solution. If solution consisted of
a physical change only, then in the solution of gases there would be
evolved — and in the solution of solids, there would be absorbed — so
much heat as answers to the change of state ; but in reality a large
amount of heat is always evolved in solution, depending on the fact
that in the process of solution there is accomplished an act of chemical
combination, accompanied by an evolution of heat. Seventeen grams of
ammonia (this weight corresponds with its formula NH3), in passing
from a gaseous into a liquid state, evolve 4,400 units of heat (latent
heat) ; that is, the quantity of heat necessary to raise the temperature
of 4,400 grams of water 1°. The same quantity of ammonia, in dissolv-
ing in an excess of water, evolves twice as much heat — namely 8,800
units — showing that the combination with water is accompanied by the
evolution of 4,400 units of heat. Further, the chief part of this -heat
is separated in dissolving in small quantities of water, so that 17 grams
of ammonia, in dissolving in 18 grams of water (this weight corre-
sponds with its composition H2O), evolve 7,535 units of heat, and there-
fore the formation of the solution NH3 + H2O evolves 3,135 units of
heat beyond that due to the change of state. As in the solution of
gases, the heat of liquefaction (of physical change of state) and of chemi-
cal combination with water are both positive ( + ), therefore in the
solution of gases in water a heat effect is alwa}*s observed. This pheno-
menon is different in the solution of solid substances, because their
passage from a solid to a liquid state is accompanied by an absorption

74 PRINCIPLE.-

of heat (negative, — heat), whilst their chemical combination with water
is accompanied by an evolution of heat ( 4- heat) ; consequently, their
sum may either be a cooling effect, when the positive (chemical) portion
of heat is less than the negative (physical), or it may be, on the
contrary, a heating effect. This is actually the case. 124 grams of
sodium thiosulphate (employed in photography) Na,SsO3,5H20 in
melting (at 48°) absorbs 9,700 units of heat, but in dissolving in a large
quantity of water at the ordinary temperature it absorbs 5,700 units of
heat, which shows the evolution of heat (about + 4,000 units), not-
withstanding the cooling effect observed in the process of solution, in
the act of the chemical combination of the salt with water.-' But in

25 The latent heat of fusion is determined at the temperature of fusion, whilst solution
takes place at the ordinary temperature, and one must think that at this temperature
the latent heat would be different, just as the latent heat of evaporation varies with the
temperature (see note 11, p. 52). Besides which, in solution there occurs a disunion (dis-
integration) of the particles of both the solvent and the substance dissolved, which in its-
mechanical aspect resembles evaporation, and which therefore must consume much
heat. The heat emitted during the solution of a solid must be therefore considered
(Personne) as composed of three factors — (1) positive, the effect of combination; (2).
negative, the effect of transference into a liquid state ; and (3) negative, the effect of dis-
integration. In the solution of a liquid by a liquid the second factor is removed ; and
therefore if the heat evolved in combination is greater than that absorbed in disintegra-
tion a heating effect is observed, and in the reverse case a cooling effect ; and, indeed,
sulphuric acid, alcohol, and many liquids evolve heat in dissolving in each other. But the
solution of chloroform in carbon bisulphide (Bussy and Binget), or of phenol (or aniline)
in water (Alexeeff), produces cold. In the solution of a small quantity of water in acetic
acid (Abasheff), or hydrocyanic acid (Bussy and Binget), or amyl alcohol (Alexeeff), cold
is produced, whilst in the solution of these substances in an excess of water heat is
evolved.

The fullest information concerning the solution of liquids in liquids has been
gathered by W. T. Alexe'eff (1883-1885), still these data are far from being sufficient to
resolve the mass of problems respecting this subject. He showed that two liquids which
dissolve in each other, intermix together in all proportions at a certain temperature.
Thus the solubility of phenol, C6H6O, in water, and the converse, is limited up to
70°, whilst above this temperature they intermix in all proportions. This is seen
from the following figures, where p is the percentage amount of phenol and t the
temperature at which the solution becomes cloudy — that is, that at which it is satu-
rated :—

j? = 7'12 10"20 15-31 26-15 28'55 36'70 48'K<> (51-15 71'97
t = l° 45° 60° 67D 67° 67° (55° 53° 20°

It is exactly the same in the solution of benzene, aniline, and other substances in
molten sulphur. Alexeeff discovered a similar complete intermixture for solutions of
secondary butyl alcohol in water at about 107° ; at lower temperatures the solubility is
not only limited, but between 50° and 70° it is at its minimum, both for solutions of the
alcohpl in water and for water in the alcohol ; and at a temperature of 5° both solutions
exhibit a fresh change in their scale of solubility, so that a solution of the alcohol in
water which is saturated between 5° and 40° will become cloudy when heated to 60°.
In the solution of liquids in liquids, Alexeeff observed a lowering in temperature (an
absorption of heat) and an absence of change in specific heat (calculated for the mixture)
much more frequently than had been done by previous observers. As regards his affir-

"N WATEB AND ITS COMPOUNDS 75

most cases solid substances in dissolving in water evolve heat, notwith-
standing the passage into a liquid state, which indicates so considerable
an evolution of ( + ) heat in the act of combination with water that it
exceeds the absorption of ( — ) heat dependent on the passage into a
liquid state. Thus, for instance, calcium chloride, CaCl,, magnesium
sulphate, ^IgSO,, and many other salts in dissolving evolve heat ; for
example, 60 grams of magnesium sulphate evolves about 10,000 units
of heat. Therefore, in the solution of solid bodies there is produced
either a cooling 2G or a heating 27 effect, according to the difference of
the reacting affinities. When they are considerable — that is, when
water is with difficulty separated from the resultant solution, and only
with a rise of temperature (such substances absorb water) — then
much heat is evolved in the process of solution, just as in many
reactions of direct combination, and therefore a considerable heating of
the solution is observed. "Of such a kind, for instance, is the solution

matioii (in the sense of a mechanical and not a chemical representation of solutions) that
substances in solutions preserve their physical states (as gases, liquids, or solids), it is
very doubtful, for it would necessitate admitting the presence of ice in water or its
vapour. His theory starts from an unsupported hypothesis — which is, however, held by
many — that the sizes (weights) of the molecules of one and the same substance are very
different in different physical states. At present the weight of gaseous molecules is
determined from the freezing of solutions (see later), and therefore it must either be
admitted that solutions contain gaseous molecules or else that the weight of liquid
molecules is the same as that of gaseous molecules, which is far simpler and more
probable.

From what has been said above, it will be clear that even in so very simple a case as
solution, it is impossible to calculate the heat emitted by chemical action alone, and that
the chemical process cannot be separated from the physical and mechanical.

16 The cooling effect produced in the solution of solids (and also in the expansion of
gases and in evaporation) is applied to the production of low temperatures. Ammo-
nium nitrate is very often used for this purpose ; in dissolving in water it absorbs 77
units of heat per each part by weight. On evaporating the solution thus formed, the
solid salt is re-obtained. The application of the various freezing mixtures is based on
the same principle. Snow or broken ice frequently enters into the composition of these
mi. rin res, advantage being taken of its latent heat of fusion in order to obtain the
lowest possible temperature (without altering the pressure or employing heat, as in other
methods of obtaining a low temperature). For laboratory work recourse is most often
had to a mixture of three parts of snow and one part of common salt, which causes the
temperature to fall from 0° to - 21° C. Potassium thiocyanate, KCNS, mixed with water
(f by weight of the salt) gives a still lower temperature. By mixing ten parts of crystal-
line calcium chloride, CaCl2,6H2O, with seven parts of water, the temperature may even
fall from 0° to - 55°.

27 The heat which is evolved in solution, or even iu the dilution of solutions, is also
sometimes made use of in practice. Thus caustic soda (NaHO), in dissolving or on the
addition of water to a strong solution of it, evolves so much heat that it can replace fuel.
In a steam boiler, which has been previously heated to the boiling point, another boiler
is placed containing caustic soda, and the exhaust steam is made to pass through the
latter ; the formation of steam then goes on for a somewhat long period of time without
any other heating. Norton makes use of this for smokeless street boilers.

of Milphuric arid (nil of vitro! II 2S( ) ,), and of can-tic soda (Xall< >),
Are., in water.-"

Solution exhibits a reverse reaction : that is to >av. if the water be
expelled from a solution, the sulistanee originally taken is re-ol)t;iiiied.
Unt it must l»e borne in mind that the expulsion of the water taken for
-olution is not accomplished with equal facility throughout, because
watei1 lias different decrees of chemical atHnit v for the substance di>-
-.olved. Thus, if a solution of sulphuric acid, which mixes with water
in all proportions, lie heated, it will be found that very different
decrees of heat are required to expel the water. When it is in a lari^e

• i-,,,.; :, ,n. ,,-,,!-,, t',i.- -r. '.d.-t ri-c ..f temperature, corre^pi'iiiN witli the f.u-niMt i<m <•[ :\
?nh\dr,tte II SO ,.-jH < ) ~:',-\ p.c. H S( ) , . wliii-li \ erv lil<ely repeals it -elf in a similar
f, ,,.,,, I,, other -.nhiticii-. alllHiu.L'h all t he phenomena i of cunt raeti.ni, ••vnlut imi of heat, ami
,-;„. ,,i temperature- are \rry c..mplex ami are depemlenl 1.11 many <-i rcii mst a nces. One
.....Mil, I tliinl;. h.-u. ••..-!•. ju. i-iii-.' from the alm\e example-, that all other influcnc<-s are
(,.(.l,ler in their .1, -tii. ii than ehemiral attraction, especially when it is so er,nsi(leral>le as
I,, t'Ai-«-ii -ulphunr aci.l ainl v.ater.

ON WATKK AND ITS r< >M I'< H'NDS 77

excess, water already begins to come off at a temperature slightly
above 100°, but if it be in but a small proportion there is such a
relation between it and the sulphuric acid that at 120°, 150°, 200°, and
even at 300°, water is still held by the sulphuric acid. The bond
between the remaining quantity of water and the sulphuric acid is
evidently stronger than the bond between the sulphuric acid and the
excess of water. The force acting in solutions is consequently of
different intensity, starting from so feeble an attraction that the proper-
ties of water — as, for instance, its power of evaporation — are but very
little changed, and ending with cases of strong attraction between the
water and the substance dissolved in or chemically combined with it. In
consideration of the very important signification of the phenomena, and
of the cases of the breaking up of solutions with separation of water
or of the substance dissolved from them, we shall further discuss them
separately, after having acquainted ourselves with certain peculiarities
of the solution of gases and of solid bodies.

The solubility of gases, which is usually measured by the volume
cf gas29 (at 0° and 760 mm. pressure) per 100 volumes of water, varies
not only with the nature of the gas (and also of the solvent), and
with the temperature, but also with the pressure, because gases them-
selves change their volumes considerably with the pressure. As might
be expected, (1) gases which are easily liquefied (by pressure and cold) are
more soluble than those which are liquefied with difficulty. Thus, in
100 volumes of water there dissolve at 0° and 760 mm. only two volumes
of hydrogen, three volumes of carbonic oxide, four volumes of oxygen,
&c., for these are gases which are liquefied with difficulty ; whilst

-p If a volume of gas v be measured under a pressure of // mm. of mercury (at 0°)
and at a temperature t° Centigrade, then, according to the laws of Boyle, Mariotte, and
of Gay-Lussac combined, its volume at 0° and 760 mm. will equal the product of v into
760 divided by the product of h into l + at°, where a is the co-efficient of expansion of
gases, which is equal to 0'00367. The weight of the gas will be equal to its volume at
0° and 760 mm. multiplied by its density referred to air and by the weight of one volume
of air at 0° and 760 mm. The weight of one litre of air under these conditions being =
1-293 grams. If the density of the gas be given in relation to hydrogen this must be
divided by 14'4 to bring it in relation to air. If the gas be measured when saturated
with aqueous vapour, then it must be reduced to the volume and weight of the gas when
dry, according to the rules given in Note 1. If the pressure be determined by a
column of mercury having a temperature /, then by dividing the height of the column by
1 + 0'00018£ the corresponding height at 0° is obtained. If the gas be enclosed in a
tube in which a liquid stands above the level of the mercury, the height of the column
of the liquid being = H and its density = D, then the gas will be under a pressure which

TTITv

is equal to the barometric pressure less , where 13'59 is the density of mercury. By

13' 59

these methods the quantity of a gas is determined, and its observed volume reduced to
normal conditions or to parts by weight. The physical data concerning vapours and
gases must be continually kept in sight in dealing with and measuring gases. The student
must become perfectly familiar with the calculations relating to gases.

78 PRINCIPLES OF CHEMISTRY

there dissolve 180 volumes of carbonic anhydride, 130 of nitrous oxide,
and 437 of sulphurous anhydride, for these are gases which are rather
easily liquefied. (2) The solubility of a gas is diminished by heating,
which is easy to understand from what has been said previously — that
the elasticity of a gas becomes greater as it is further removed from a
liquid state. Thus 100 volumes of water at 0° dissolve 2-5 volumes of
air, and at 20° only 1*7 volumes. For this reason cold water, when
brought into a warm room, parts with a portion of the gas dissolved in
it.30 (3) The quantity of the gas dissolved varies directly with the pres-
sure. This rule is called the late of Henry and Dalton, and is applicable
to those gases which are little soluble in water. Therefore a gas is
separated from its solution in water in a vacuum, and water saturated
with a gas under great pressure parts with it if the pressure be dimi-
nished. Thus many mineral springs are saturated underground with
carbonic anhydride under the great pressure of the column of water
above them. On coming to the surface, the water of these springs
boils and foams in giving up the excess of dissolved gas. Sparkling
wines and aerated waters are saturated under pressure with the same
gas. They hold the gas so long as they are in a well-corked vessel.
When the cork is removed and the liquid comes in contact with air at
a less pressure, part of the gas, unable to remain in solution at a lesser
pressure, is separated as foam with the hissing sound familiar to all.
It must be remarked that the law of Henry and Dal ton belongs to the
class of approximate laws, like the laws of gases (Gay-Lussac's and
Mariotte's) and many others — that is, it expresses only a portion of a
complex phenomenon, the limit towards which the phenomenon aims.
The matter is rendered complicated from the influence of the degree of
solubility and of affinity of the dissolved gas for water. Gases which
are little soluble — for instance, hydrogen, oxygen, and nitrogen — follow
the law of Henry and Dalton the most closely. Carbonic anhydride
exhibits a decided deviation from the law, as is seen from the determi-
nations of Wroblewski (1882). He showed that at 0° a cubic centi-
metre of water absorbs 1 -8 cubic centimetres of the gas under a pressure
of one atmosphere ; under 10 atmospheres, 16 cubic centimetres (and
not 18, as it should be according to the law) ; under 20 atmospheres,

•~° According to Bunsen, 100 volumes of water under a pressure of one atmosphere
absorb the following volumes of gas (measured at 0° and 7(50 mm.) : —

123 456 7 89 10

0° 4-11 2-03 1-93 179-7 3'3 ISO'S 437'1 688-6 5'4 104960
10° 3-25 1-61 1-93 118-5 2'6 92'0 358-G 513-8 4'4 81280

20° 2-84 1-40 1-93 90'1 2"3 67'0 290'5 362'2 3'5 65400

I, oxygen ; 2, nitrogen : 3, hydrogen ; 4, carbonic anhydride ; 5, carbonic oxide; 6, nitrous oxide;
7, hydrogen sulphide ; 8, sulphurous anhydride ; 9. marsh gas ; 10, ammonia.

ON WATKi; AM) ITS Co.MI'orNliS

79

26'6 cubic centimetres (instead of 36) ; and under 30 atmospheres, 33'7
cubic centimetres.31 However, as the researches of Sechenoff show,
the absorption of carbonic anhydride within certain limits of change
of pressure, and at the ordinary temperature, by water — and even by
solutions of salts which are not chemically changed by it, or do not
form compounds with it — very closely follows the law of Henry and
Dalton, so that the chemical bond between this gas and water is so
feeble that the breaking up of the solution with separation of the gas
is accomplished by a decrease of pressure alone.32 The case is different
if a considerable affinity exists between the dissolved gas and water.
Then it might even be expected that the gas would not be entirely
separated from water in a vacuum, as should be the case with gases
according to the law of Henry and Dalton. Such gases — and, in
general, all which are very soluble — exhibit a distinct deviation from
the law of Henry and Dalton. As examples, ammonia and hydro-
chloric acid gas may be taken. The former is separated by boiling and
decrease of pressure, while the latter is not, but they both deviate dis-
tinctly from the law.

1 'iv-sure in mm.
of mercury

Ammonia dissolved
in 10(1 grams of
water at 0°

Hydrochloric acid
gas dissolved in 100
trnmis of water at 0°

( I rams

Grama

100

28*0

65-7

500

M~> 78-2

1,000

112-6 85-6

1,500

165-6

It will be remarked, for instance, from this table that whilst the pres-

31 These figures show that the co-efficient of solubility decreases with an increase of
pressure, notwithstanding that the carbonic anhydride approaches a liquid state. And,
indeed, liquefied carbonic anhydride does not intermix with water, and does not exhibit a
rapid increase in solubility at its temperature of liquefaction. This indicates, in the first
place, that solution does not consist in liquefaction, and in the second place that the solu-
liility of a substance is determined by a peculiar attraction of water for the substance
dissolving. Wroblewski even considers it possible to admit that a dissolved gas retains
its properties as a gas. This he deduces from his experiments, which showed that the
rate of diffusion of gases in a solvent is, for gases of different densities, inversely propor-
tional to the square roots of their densities, just as the velocities of movement of gaseous
molecules (see Note 34 on p. 80). Wroblewski showed the affinity of water, H2O, for carbonib
anhydride, COo, from the fact that on expanding moist compressed carbonic anhydride
(compressed atO° under a pressure of 10 atmospheres) he obtained (a fall in temperature
takes place from the expansion) a very unstable definite crystalline compound, COo + 8H2O.

32 As, according to the researches of Roscoe and his collaborators, ammonia exhibits
a considerable deviation at low temperatures from the law of Henry and Dalton, whilst
at 100° the deviation is small, it would appear that the dissociating influence of tem-
perature tells on all gaseous solutions ; that is, at high temperatures, the solutions of
all gases will follow the law, and at lower temperatures there will in all cases be a
deviation from it.

80 PRINCIPLES OF CHEMISTRY

sure increased 10 times, the solubility of ammonia only increased 4!,
times.

A number of examples of such cases of the absorption of gases
by liquids might be cited which do not in any \\;iv, even approximately,
agree with the laws of solubility. Thus, for instance, carbonic anhy-
dride is absorbed by a solution of caustic potash in water, and if there
be sufficient caustic potash it is not separated from the solution by a
decrease of pressure. This is a case of more intimate chemical com-
bination. A less completely studied, but similar and clearly chemical,
correlation appears in certain cases of the solution of gases by water,
and we shall afterwards take an example of this in the solution of
hydrogen iodide ; but first we will stop to consider a remarkable appli-
cation of the law of Henry and Dalton33 in the case of the solution of
a mixture of two gases, and this we must do all the more because the
phenomena which then take place cannot be foreseen without a clear
theoretical representation of the nature of gases.34

53 The ratio between the pressure and the amount of gas dissolved was discovered by
Henry in 1805, and Dalton in 1807 pointed out the adaptability of this law to cases of
gaseous mixtures, introducing the conception of partial pressures which is absolutely
necessary for a right comprehension of Dalton's law. The conception of partial pressures
essentially enters into that of the diffusion of vapours in gases (footnote 1) ; for the
pressure of damp air is equal to the sum of the pressures of dry air and of the aqueous
vapour in it, and it is admitted as a sequence to Dalton's law that evaporation in dry
air takes place as in a vacuum. It is, however, necessary to remark that the volume of
a mixture of two gases (or vapours) is only approximately equal to the sum of the volumes
of its constituents (the same, naturally, also refers to their pressures) — that is to say, in
mixing gases a change of volume occurs, which, although small, is quite apparent when
carefully measured. For instance, in 1888 Brown showed that on mixing various volumes
of sulphurous anhydride (SO2) with carbonic anhydride (at equal pressures of 7<>0 mm.
and equal temperatures) a decrease of pressure of 3'9 millimetres of mercury was
observed The possibility of a chemical action in similar mixtures is evident from the
fact that equal volumes of sulphurous and carbonic anhydrides at — 19° form, according
to Pictet's researches in 1888, a liquid having the signs of a chemical compound, or a
solution similar to that given when sulphurous anhydride and water combine into an
unstable chemical whole.

51 The origin of the now generally-accepted kinetic theory of gases, according to
which they are animated by a rapid progressive movement, is very ancient (Bernoulli and
others in the last century had already developed a similar representation), but it was
only generally accepted after the mechanical theory of heat had been established, and
after the work of Krb'nig (1855), and especially after its mathematical side had been
worked out by Clausius and Maxwell. The pressure, elasticity, diffusion, and internal
friction of gases, the laws of Boyle, Mariotte, and of Gay-Lussac and Avogadro-Gerhardt
are not only explained (deduced) by the kinetic theory of gases, but also expressed with
perfect exactitude ; thus, for example, the magnitude of the internal friction of different
gases was foretold with exactitude by Maxwell, by applying the theory of probabilities to
the concussion of gaseous particles. The kinetic theory of ga^es must therefore be con-
sidered as one of the most brilliant acquisitions of the latter half of the present century.
The velocity of the progressive movement of the gaseous particles of a gas, one cubic
centimetre of which weighs d grams, is found, according to the theory, to be equal to
the square root of the product of SpDg divided by d, where p is the pre>suiv under which

oN WATER AND ITS COMPOUNDS 81

Tin l<nr of partial pressures is as follows : — The solubility of gases
in intermixture with each other does not depend on the influence of
the total pressure acting on the mixture, but on the influence of that
portion of the total pressure which is due to the volume of each given gas
in the mixture. Thus, for instance, if oxygen and carbonic anhydride
were mixed in equal volumes and exerted a pressure of 760 millimetres,

(1 is determined expressed in centimetres of the mercury column, D the weight of a cubic
centimetre of mercury in grams (-0 = 13*59,^ = 76, consequently the normal pressure =
l.o:i:; grams on a sq. c. m.), and g the acceleration of gravity in centimetres (^ = 980'5,
at the sea level and long. 45°, = 981'92 at St. Petersburg ; in general it varies with the
longitude and altitude of the locality). Therefore, at 0° the velocity of hydrogen is 1,843,
and of oxygen 461, metres per second. This is the average velocity, and (according to-
Maxwell and others) it is probable that the velocities of individual particles are different,
that is, they occur in, as it were, different conditions of temperature, which is very im-
portant to take into consideration in the investigation of many phenomena proper to-
matter. It is evident from the above determination of the velocity of gases, that
different gases at the same temperature and pressure have average velocities, which are
inversely proportional to the square roots of their densities ; this is also shown by direct
experiment on the flow of gases through a fine orifice, or through a porous wall. This
<l/Nfii>iitt<ii- n -J ncit if of flow for different gases is frequently taken advantage of in
chemical researches (see Chap. II. and also Chap. VII. on the law of Avogadro-Gerhardt)
in order to separate two gases having different densities and velocities. The difference
of the velocity of flow of gases also determines the phenomenon cited in the following
footnote for demonstrating the existence of an internal movement in gases.

If for a certain mass of a gas which fully and exactly follows the laws of Mariotte
and Gay-Lussac the temperature t and the pressure p be simultaneously changed, then
the entire change would be expressed by the equation pr = C (1 + at), or, what is the
same, pv = RT, where T-t + 273 and C and R are constants which vary not only with the
units of measurement but with the nature of the gas and its mass. But as there are
discrepancies from both the fundamental laws of gases (which will be spoken of in the
following chapter), and as, on the one hand, a certain attraction between the gaseous
molecules must be admitted, and on the other hand it must be acknowledged that the
gaseous molecules themselves occupy a portion of a space, therefore for ordinary gases,,
within any considerable variation of pressure and temperature, recourse should be had
to Van der Waal's formula —

(p + ^r) (v—p) = R (I— at)

where a is the true co-efficient of expansion of gases. As the actual co-efficient of ex-
pansion of air at the atmospheric pressure and between temperatures of 0° and 100° =
0'00367, when determined from the change of pressure (according to Kegnault's data)
and when determined from the change of volume = 0'00368 (according to Mendeleeff and
Kayander), and for other gases there is a discrepancy, although not a large one (see the
following chapter), which is considerable at high pressures and for great densities, there-
fore that co-efficient of expansion should be taken which all gases have at low pressures.
This quantity is approximately 0'00367.

The formula of Van der Waal has an especially important significance in the case
of the passage of a gas into a liquid state, because the fundamental properties of both
pi M-. and liquids are equally well expressed, although only in their general features,
by it.

The further development of the questions referring to the subjects here touched on,
which are of especial interest for the theories of solutions, must be looked for in special
memoirs and works on theoretical and physical chemistry. A small part of this subject
will be partially considered in the footnotes of the following chapter.
VOL. I.

82 PRINCIPLES OF CHEMISTRY

then water would dissolve so much of each of these gases as would be
dissolved if each separately exerted a pressure of half an atmosphere,
and in this case, at 0° one cubic centimetre of water would dissolve
0-02 cubic centimetre of oxygen and 0*90 cubic centimetre of carbonic
anhydride. If the pressure of a gaseous mixture equals It, and in u
volumes of the mixture there be a volumes of a given gas, then its
solution will proceed as though this gas were dissolved under a pres-
sure - . That portion of the pressure under influence of which the

solution proceeds is termed the ' partial ' pressure.

In order to represent to oneself the cause of the law of partial
pressures, an explanation must be given of the fundamental properties
of gases. Gases are elastic and disperse in all directions. All that is
known of gases obliges one to think that these fundamental properties
of gases are due to a rapid progressive movement, in all directions,
which is proper to their smallest particles (molecules).35 These mole-
cules in impinging against an obstacle produce a pressure. The greater
the number of molecules impinging against an obstacle in a given time,
the greater the pressure. The pressure of a separate gas or of a gaseous
mixture depends on the sum of the pressures of all the molecules, on
the number of blows in a unit of time on a unit of surface, and on the
mass and velocity (or the vis viva) of the impinging molecules. To the
obstacle all molecules (although different in nature) are alike ; it is
submitted to a pressure due to the sum of their vis viva. But, in a
chemical action such as the solution of gases, on the contrary, the

50 Although the actual movement of gaseous molecules, which is acknowledged by the
kinetic theory of gases, cannot be seen, yet its existence may be rendered evident by
taking advantage of the difference in the velocities which undoubtedly belongs to
different gases which are of different densities under equal pressures. The molecules of a
light gas must move more rapidly than the molecules of a heavier gas in order to produce
the same pressure. Let us take, therefore, two gases — hydrogen and air ; the former is
14'4 times lighter than the latter, and hence the molecules of hydrogen must move almost
four times more quickly than air (more exactly 3'8, according to the formula given in the
preceding footnote). Consequently, if air occurs inside a porous cylinder and hydrogen
outside, then in a given time the volume of hydrogen which succeeds in entering the
cylinder will be greater than the volume of air leaving the cylinder, and therefore the
pressure inside the cylinder will rise until the gaseous mixture (of air and hydrogen)
attains an equal density both inside and outside the cylinder. If now the experiment
be reversed and air surround the cylinder, and hydrogen be inside the cylinder, then more
gas will leave the cylinder than enters it, and hence the pressure inside the cylinder
will be diminished. In these considerations we have replaced the idea of the number
of molecules by the idea of volumes. We shall learn afterwards that equal volumes
of different gases contain an equal number of molecules (the law of Avogadro-Ger-
hardt), and therefore instead of speaking of the number of molecules we can speak of
the number of volumes. If the cylinder be partially immersed in water the rise and fall
of the pressure can be observed, and consequently the experiment can be rendered self-
evident.

ON WATKII AND ITS COMPOUNDS 83

nature of the impinging molecules plays the most important part. In
impinging against a liquid, a portion of the gas enters into the liquid
itself, and is held by it so long as other gaseous molecules impinge
against the liquid — exert a pressure on it. As regards the solubility of
a given gas, for the number of blows it makes on the surface of a liquid,
it is immaterial whether other molecules of gases impinge side by side
with it or not. Therefore, the solubility of a given gas will be propor-
tional, not to the total pressure of a gaseous mixture, but to that por-
tion of it which is due to the given gas separately. Further, the satura-
tion of a liquid by a gas depends on the fact that the molecules of
gases that have entered into a liquid do not remain at rest in it,
although they enter in a harmonious kind of movement with the mole-
cules of the liquid, and therefore they throw themselves off from the
surface of the liquid (just like its vapour if the liquid be volatile). If
in a unit of time an equal number of molecules penetrate into (leap
into) a liquid and leave (or leap out of) a liquid, it is saturated. It
is a case of mobile equilibrium, and not of rest. Therefore, if the
pressure be diminished, the number of molecules departing from the
liquid will exceed the number of molecules entering into the liquid,
and a fresh state of mobile equilibrium only takes place under a fresh
equality of the number of molecules departing from and entering into
the liquid. Thus are explained the main features of the solution, and
furthermore of that special (chemical) attraction (penetration and har-
monious movement) of a gas for a liquid, which determines both the
measure of solubility and the degree of stability of the solutions pro-
duced.

The consequences of the law of partial pressures are exceedingly
numerous and important. All liquids in nature are in contact with the
atmosphere. The atmosphere, as we shall afterwards see more fully,
consists of an intermixture of gases, chiefly four in number — oxygen,
nitrogen, carbonic anhydride, and aqueous vapour. 100 volumes of
air contain, approximately, 78 volumes of nitrogen, and about 21
volumes of oxygen ; the quantity of carbonic anhydride, by volume,
does not exceed 0'05. Under ordinary circumstances, the quantity of
aqueous vapour is much greater, but it varies with the moisture of the
atmosphere. Consequently, the solution of nitrogen in a liquid in
contact with the atmosphere will proceed under a partial pressure equal
to j7(*0 x 760 mm. if the atmospheric pressure equal 760 mm. ; con-
sequently, under a pressure of 600 mm. of mercury, the solution of
oxygen will proceed under a partial pressure of about 160 mm., and
the solution of carbonic anhydride only under the very small pressure
of 0'4 mm. Therefore, although the amount of nitrogen in air is

G2

84 PRINCIPLES OF CHEMISTRY

large, yet, as the solubility of oxygen in water is twice that of the
nitrogen in water, the proportion of oxygen dissolved in water will be
greater than its proportion in air. It is easy to calculate what quantity
of each of the gases will be contained in water, and we will take the
most simple case, and calculate what quantity of oxygen, nitrogen, and
carbonic anhydride will be dissolved from air having the above com-
position at 0° and 760 mm. pressure. Under a pressure of 760 mm. 1
cubic centimetre of water dissolves 0*0203 cubic centimetre of nitrogen,
or under the partial pressure of 600 mm. it will dissolve 0*0203 x Jg#,
or 0*0160 cubic centimetre ; of oxygen 0*041 1 x 1 •: ", or 0*0086 cubic cen-

0*4

timetre ; of carbonic anhydride 1*8 x~ - or 0*00095 cubic centimetre;

760

consequently, 100 cubic centimetres of water will contain at 0° altogether
2*55 cubic centimetres of atmospheric gases, and 100 volumes of air
dissolved in water will contain about 62 p.c. of nitrogen, 34 p.c. of
oxygen, and 4 p.c. of carbonic anhydride. The water of rivers, wells,
<tc., usually contains more carbonic anhydride. This proceeds from
the oxidation of organic substances falling in the water. The amount
of oxygen, however, dissolved in water appears to be actually about ^
the dissolved gases, whilst air contains only 1 of it by volume. .

According to the law of partial pressures, whatever gas be dissolved in
water will be expelled from the solution in an atmosphere of another gas.
This depends on the fact that gases dissolved in water escape from it
in a vacuum, because the pressure is nil. An atmosphere of another
gas acts like a vacuum on a gas dissolved in water. Separation then
proceeds, because the molecules of the dissolved gas no longer impinge
upon the liquid, are not dissolved in it, and those previously held in solu-
tion depart from the liquid in virtue of their elasticity.3'"' For the same

3(5 Here there may be, properly speaking, two cases : either the atmosphere surround-
ing the solution may be limited, or it may be proportionally so vast as to be unlimited,
like the earth's atmosphere. If a gaseous solution be brought into an atmosphere of
another gas which is limited — for instance, as in a closed vessel — then a portion of the
gas held in solution will be expelled, and thus pass over into the atmosphere surrounding
the solution, and will evince its partial pressure. Let us imagine that water saturated
with carbonic anhydride at 0° and under the ordinary pressure be brought into an
atmosphere of a gas which is not absorbed by water; for instance, that 10 c.c.
of an aqueous solution of carbonic anhydride be introduced into a vessel holding
10 c.c of such a gas. The solution will contain 18 c.c of carbonic anhydride. The
expulsion of this gas goes on until a state of equilibrium is arrived at. The liquid
will then contain a certain amount of carbonic anhydride, which is retained under
the partial pressure of that gas which has been expelled. Now, how much gas will
remain in the liquid and how much will pass over into the surrounding atmosphere ?
In order to solve this problem, let us suppose that x cubic centimetres of carbonic
anhydride are retained in the solution. It is evident that the amount of carbonic anhy-
dride which passed over into the surrounding atmosphere will be 18 — a", and the total
volume of gas will be 10 + 18 — a: or 28 — # cubic centimetres. The partial pressure under

ON WATER AND ITS O >.M P< )TNDS 85

reason a gas may be entirely expelled from a gaseous solution by
boiling — at least, in many cases when it does not form particularly stable
compounds with water. In fact the surface of the boiling liquid will
be occupied by aqueous vapour, and therefore all the pressure acting
on the gas will belong to the aqueous vapour. Consequently, the partial
pressure of the dissolved gas will be very inconsiderable. For this, and
for no other reason, a yas separates from a solution on boiling the liquid
holding it. At the boiling point of water the solubility of gases in
water is still sufficiently great for a considerable quantity of a gas to
remain in solution. The gas dissolved in the liquid is carried away,
together with the aqueous vapour ; if boiling be continued for a long
time, then in the end all the gas will be separated.37

which the carbonic anhydride is then dissolved will be (supposing that the common

JQ ~

pressure remains constant the whole time) equal to OQ_~> consequently there is not in
solution 18 c.c of carbonic anhydride (as would be the case were the partial pressure
•equal to the atmospheric pressure), but only 18 2Q_ , which is equal to x, and conse-

•I Q ~

quently we obtain the equation 18 ou_ =#> hence # = 8'69. Again, where the atmo-
sphere into which the gaseous solution is introduced is not only that of another gas but also
unlimited, then the gas dissolved will, on passing over from the solution, diffuse itself
through this atmosphere, and from its limitedness produce an infinitely small pressure
in the unlimited atmosphere. Consequently, no gas can be retained in solution under
this infinitely small pressure, and it will be entirely expelled from the solution. For
this reason water saturated with a gas which is not contained in air, will be entirely de-
prived of the dissolved gas if left exposed to air. Water also passes off from a solution
into the atmosphere, and it is evident that there might be such a case as a constant
proportion between the quantity of water vaporised and the quantity of a gas expelled
from a solution, so that not the gas alone, but the entire gaseous solution, would pass off.
A similar case is exhibited in solutions which are not decomposed by heat (such as those
of hydrogen chloride and iodide), as will afterwards be considered.

37 However, in those cases when the variation of the co-efficient of solubility with the
temperature is not sufficiently great, and when a known quantity of aqueous vapour
and of the gas passes off from a solution at the boiling point, an atmosphere may be
obtained having the same composition as the liquid itself. In this case the amount of
gas passing over into such an atmosphere will not be greater than that held by the
liquid, and therefore such a gaseous solution will distil over without change, and without
altering its composition during the whole period of boiling or distillation. The solution
will then represent, like a solution of hydriodic acid in water, a liquid which is not
changed by distillation, while the pressure under which this distillation takes place re-
mains constant. Thus in all its aspects solution presents gradations from the most feeble
affinities to examples of intimate chemical combination. The amount of heat evolved in
the solution of equal volumes of different gases is in distinct relation with these variations
of stability and solubility of different gases. 22 '3 litres of the following gases (at 700
mm. pressure) evolve the following number of (gram) units of heat in dissolving in a
large mass of water ; carbonic anhydride 5,600, sulphurous anhydride 7,700, ammonia
8,800, hydrochloric acid 17,400, and hydriodic acid 19,400. The two last-named gases,
which are not expelled from their solution by boiling, evolve approximately twice as
much heat as such gases as ammonia, which are separated from their solutions by boiling,
whilst gases which are only slightly soluble evolve less heat than the latter gases.

86 PRINCIPLES OF CHEMISTRY

It is_ evident that the conception of the partial pressures of gases
should not only be applied to the formation of solutions, but also to all
cases of chemical action of gases. Especially numerous are its appli-
cations to the physiology of respiration, for in these cases it is only the
oxygen of the atmosphere that acts.38

The solution of solids, whilst depending only in a small mea-
sure on the pressure under which solution takes place (because solids
and liquids are almost incompressible), is very clearly dependent on
the temperature. In the great majority of cases the solubility of
solids in water increases with the temperature ; and further, the
rapidity of solution increases also. The latter is determined by the
rapidity of diffusion of the solution formed into the remainder of the
water. The solution of a solid in water, although it is as with gases,
a physical passage into a liquid state, is determined, however, by its
chemical affinity for water ; which is particularly clear from the fact
that in solution there occurs a diminution in volume, a change in the
boiling point of water, a change in the tension of its vapour, in. the
freezing point, and in many similar properties. Were solution a physical,
and not a chemical, phenomenon, it would naturally be accompanied
by an increase and not by a diminution of volume, because generally in
melting a solid increases in volume (its density diminishes). Con-
traction is the usual phenomenon accompanying solution, and takes
place even in the addition of solutions to water,39 and in the solution

58 Among the numerous researches concerning this subject, certain results obtained
by Paul Bert are cited in Chapter III., and here we will point out that Prof. Sechenoff,
in his researches on the absorption of gases by liquids, very fully investigated the
phenomena of the solution of carbonic anhydride in solutions of various salts, and
arrived at many important results, which showed that, on the one hand, in the solution
of carbonic anhydride in solutions of salts on which it is capable of acting chemically (for
example, sodium carbonate, borax, ordinary sodium phosphate), there is not only an
increase of solubility, but also a distinct deviation from the law of Henry and Dalton ;
and, on the other hand, that solutions of salts which are not acted on by carbonic anhy-
dride (for example, the chlorides, nitrates, and sulphates) absorb less of it, by reason of
the competition of the already dissolved salt, and follow the law of Henry and Dalton,.
but all the same show undoubted signs of a chemical action between the salt, water, and
carbonic anhydride. Sulphuric acid (whose co-efficient of absorption is 92 vols. per 100),
when diluted with water, absorbs less and less carbonic anhydride, until the hydrate
H2SO4,H2O (co-eff. of absorption then equals 66 vols.) is formed ; then on further
addition of water the solubility again rises until a solution of 100 p.c. of water ia
obtained.

39 Kremers made this observation in the following simple form : — He took a narrow-
necked flask, with a mark on the narrow part (like that on a litre flask which is used for
accurately measuring liquids), poured water into it, and then inserted a funnel, having a
fine tube which reached to the bottom of the flask. Through this funnel he carefully
poured a solution of any salt, and (having removed the funnel) allowed the liquid to
attain a definite temperature (in a water bath) ; he then filled the flask up to the mark
with water. In this manner two layers of liquid were obtained, the heavy saline solution

ON WATER AND ITS COMPOUNDS 87

of liquids in water,40 just as happens in the combination of substances
when evidently new substances are produced.41 The contraction which
takes place in solution is, however, very small, a fact which depends on
the small compressibility of solids and liquids, and on the insignificance
of the compressing force acting in solution.42 The change of volume
which takes place in the solution of solids and liquids, or the altera-
tion in specific gravity 43 corresponding with it, depends on peculiari-
ties of the dissolving substances, and of water, and, in the majority
of cases, is not proportional to the quantity of the substance dis-

below and water above. The flask was then shaken in order to accelerate diffusion, and
it was observed that the volume became less if the temperature remained constant.
This can be proved by calculation, if the specific gravity of the solutions and water be
known. Thus at 15° one c.c. of a 20 p.c. solution of common salt weighs 1'1500 grams,
hence 100 grams occupy a volume of 86'96 c.c. As the sp.gr. of water at 15° = 0'99916,
therefore 100 grains of water occupy a volume of 100'OB c.c. The sum of the volumes is
187'04 c.c. On mixing, 200 grams of a 10 p.c. solution are obtained. Its specific gravity is
1-0725 (at 15° and referred to water at its maximum density), hence the 200 grams will
occupy a volume of 186'48 c.c. The contraction is consequently equal to 0'56 c.c.

40 The contractions produced in the case of the solution of sulphuric acid in water
are shown in the diagram Fig. 17 (page 7.6). Their maximum is 10' 1 c.c. per 100 c.c. of
the solution formed. A maximum contraction of 4'15 at 0°, 3'78 at 15°, and 3'50 at 30°,
takes place in the solution of 46 parts by weight of anhydrous alcohol in 54 parts of
water. This signifies that if, at 0°, 46 parts by weight of alcohol be taken per 54 parts by
weight of water, then the sum of their separate volumes will be 104'15, and after mixing
their total volume will be 100.

41 This subject will be considered later in this work, and we shall then see that the
contraction produced in reactions of combination (of solids or liquids) is very variable
in its amount, and that there are, although very rare, reactions of combination in which
contraction does not take place, or when an increase of volume is produced.

4- The compressibility of solutions of common salt is less, according to Grassi, than
that of water. At 18° the compression of water per million volumes =48 vols. for a
pressure of one atmosphere ; for a 15 p.c. solution of common salt it is 82, and for a
24 p.c. solution 26 vols. Similar determinations were made by Brown (1887) for saturated
solutions of sal ammoniac (38 vols.), alum (46 vols.), common salt (27 vols.), and sodium
sulphate at + 1°, when the compressibility of water =47 per million volumes. This inves-
tigator also showed that substances which dissolve with an evolution of heat and with an
increase in volume (as, for instance, sal-ammoniac) are partially separated from their
saturated solutions by an increase of pressure (this experiment was especially convincing
in the case of sal-ammoniac), whilst the solubility of substances which dissolve with an
absorption of heat or diminution in volume increases, although very slightly, with an
increase of pressure. Sorby observed the same phenomenon with common salt (1863).

43 The most trustworthy data relating to the variation of the specific gravity of
solutions with a change of their composition and temperature, are collected and discussed
in my work cited in footnote 19. The practical (for the amount of a substance in
solution is determined by the aid of the specific gravities of solutions, both in works and
in laboratory practice) and the theoretical (for specific gravity can be more accurately
observed than other properties, and because a variation in specific gravity governs the
variation of many other properties) interest of this subject, besides the strict rules and laws
to which it is liable, make one wish that this province of data concerning solutions
may soon be enriched by further observations of as accurate a nature as possible. Their
collection does not present any great difficulty, although requiring much time and
attention.

<SS I'RIXCIPLKS OF rilKMJSTRY

solved,14 showing the existence of a chemical action between the solvent
and the substance dissolved which is of the same nature as in all other
forms of chemical relation.1'

Although an alteration of the external pressure does not usually
decompose solutions of solids, nevertheless the feeble development of

•*' Inasmuch us the decree of change exhibited in many properties on the formation of
solutions, is not large, so. owing to the insuflii-ient ac-curacy of observations, a proportion-
ality between this change and a change of composition may, in a first rough approximation
and especially \vithin narrow limits of change of composition, easily be imagined in cases
where it does not even exist. The conclusion of Michel and Kraft is particularly instruc-
tive in this respect: in lsf>4. on the basis of their incomplete researches, they supposed
the increment of the specific gravity of solutions to be proportional to the increment of
a salt in a given volume of a solution, which is only true for determinations of specific
gravity which are exact to the second decimal place — an accuracy insufficient even for
technical determinations. Accurate measurements do not confirm a proportionality
either in this case or in many others where a ratio has been generally accepted ; as, for
example, for the rotatory power (with respect to the plane of polarisation i of solutions, and
for their capillarity, Arc. Nevertheless, such a method is not only still made use of, but
even has its advantages when applied to solutions within a limited scope — as, for instance,
very weak solutions, and for a first acquaintance with the phenomena accompanying
solution, and also as a means for facilitating the application of mathematical analysis to
the investigation of the phenomenon of solution. Judging by the results obtained in my
researches on the specific' gravity of solutions, I think that in many cases it would be
nearer the truth to take the change of properties as proportional, not to the amount of a
substance dissolved, but, to the product of this quantity and the amount of water in
which it is dissolved; all the more so as many chemical re'ations vary in proportion to
the reacting masses, and a similar ratio has been established for many phenomena of
attraction studied by mechanics. This product is easily arrived at when the quantity of
water in the solutions to be compared is constant, as is shown in investigating the fall of
temperature in the formation of ice (nee footnote 41), p. IK)'.

'•' All the different forms of chemical reaction may be said to take place in the process
of solution, il \ CinnbiiKiiiona between the solvent and the substance dissolved, which
are more or less stable (more or less dissociated). This form of reaction is the most
probable, and is that most often observed. ('2 1 Reactions of substitution or of double,
ih-coiHjioxitiun between the molecules. Thus it may be supposed that in the solution of
sal-ammoniac, XII, Cl. the action of water produces ammonia, NH.,HO, and hydrochloric
acid. HC1. which are dissolved in the water and simultaneously attract each other. As
these solutions and many others do indeed exhibit signs which are sometimes indispu-
table of similar double decompositions (thus solutions of sal-ammoniac yield a certain
amount of ammoniai. it is probable that this form of reaction is more often met with
than is generally thought. (Mi Reactions of ixuHK'nmit or rcylnceiui'iit are also probably
met with in solution, all the more as here molecules of dim-rent kinds come into intimate
contact, and it is very likely that the configuration of the atoms in the molecules under
these influences is somewhat different from what it was in its original and isolated
state. One is led to this supposition especially from observations made on solutions of
substances which rotate the plane of polarisation land observations of this kind are very
sensitive with respect to the atomic structure of molecules), because they show, for
example (according to Schneider, iH.slj, that strong solutions of malic acid rotate the
plane of polarisation to the right, whilst its ammonium salts in all degrees of concentra-
tion rotate the plane of polarisation to the left. (4 1 Reactions of <li-<-<>nij>u.ii(H>n under
the influences of solution are not only rational of themselves, but. have in recent years
been recognised by Arrhenins, Ostwald. and others, particularly on the basis of electro-
lytical determinations. If a portion of the molecules of a solution occur in a condition of
decomposition, the other portion mav occur in a yet more complex state of combination,

ON WATKi; AND ITS COMPOUNDS 89

the chemical atlinitics acting in solutions of solids becomes evident
from those multifarious methods by \vhich their solutions are drcum
jiowd, whether they be saturated or not. On heating (absorption of
heat), on cooling, and by internal forces alone, aqueous solutions in
many cases separate into their components or their definite com-
pounds. The water contained in solutions is removed from them
as vapour, or, by freezing, in the form of ice,46 but the tension of the
rn/iour of water 47 held in solution is less than that of water in a free

just as the velocity of the movement of different gaseous molecules may be far from
being the same (see Note 34, p. 80).

It is, therefore, very probable that the reactions taking place in solution vary both
quantitatively and qualitatively with the mass of water in the solution, and the great
difficulty in arriving at a lasting decision on the question as to the nature of the chemical
relations which take place in the process of solution will be understood, and if besides
this the existence of a physical process, like the sliding between and interpenetration of
two homogeneous liquids, be also recognised in solution, then the complexity of the
problem as to the actual nature of solutions, which is now to the fore, appears in its
true light. However, the efforts which are now being applied to the solution of this
problem are so numerous and of such varied aspect that they will offer the coming
investigators a vast mass of material towards the construction of a complete theory of
solution.

For my part, I think that the study of the physical properties of solutions (and
especially of weak ones) which now reigns, cannot give any fundamental and complete
solution of the problem whatever (although it should add much to both the provinces of
physics and chemistry), but that, parallel with it, should be undertaken the study of the
influence of temperature, and especially of low temperatures, the application to solu-
tions of the mechanical theory of heat, and the comparative study of the chemical pro-
perties of solutions. The beginning of all this is already established, but it is impossible
to consider in so short an exposition of chemistry the further efforts of this kind which
have been made up to the present date.

46 If solutions are regarded as being in a state of dissociation (see footnote 19, p. 64) it
would be expected that they would contain free molecules of water, which form one of the
products of the decomposition of those definite compounds whose formation is the cause
of solution. In separating as ice or vapour, water makes, with a solution, a heteroge-
neous system (made up of substances in different physical states) similar, for instance,
to the formation of a precipitate or volatile substance in reactions of double decom-
position.

47 If the substance dissolved is non-volatile (like salt or sugar), or only slightly volatile,
then the whole of the tension of the vapour given off belongs to the water, but if a
solution of a volatile substance — for instance, a gas or a volatile liquid — evaporates, then
only a proportion of the pressure belongs to the water, and the whole pressure observed
consists of the sum of the pressures of the vapours of the water and of the substance
dissolved. The majority of researches bear on the first case, which will be spoken of
presently, and the observations of D. P. Konovoloff (1881) refer to the second case. He
showed that in the case of two volatile liquids, mutually soluble in each other, forming
two layers of saturated solutions (for example, ether and water, note 20, p. 66), both solu-
tions have an equal vapour tension (in the case in point the tension of both is equal to
481 mm. of mercury at 19'8°). Further, he found that for solutions which are formed
in all proportions, the tension is either greater (solutions of alcohol and water) or lesa
(solutions of formic acid) than that which answers to the rectilinear change (proportional
to the composition) from the tension of water to the tension of the substance dis-
solved ; thus the tension, for example, of a 70 p.c. solution of formic acid is less, at all

state, and M- /• //,/" r<ttnr> <>f //<> t'ormntini, of in from solutions is lower
tli:tn O . I-'urther. both the diminution of vapour tension and the
lowering of tin- freexing point proceed, at lea-t in dilute solutions,
alnm-i in proportion to the amount of a substance dis.-olved. ls Thus,
if ]ier 1 I.H » Drains of water there lie in solution 1 . ">. 1 (J grains of common
salt (Na('l), then at 100 the vapour tension of the solutions decreases
li\" 1. L' 1 , -1.) mm. ot the baromet ne eohimn. a^atn.-t 7'iO mm., or the
vapour tension of water, whilst the free/in^ point.- are — U'">> . — '_M'l .
and — t'rl'J respectively. The above figures '•' are almost proportional

temperature-,, than the ten-ion of water and of formic acid itself. Tim-, in thi- case the
ten-ion of a solution i- never equal to the sum of the tension of the di--ol\ in-- liquid-, us
Kt •jnaiili already showed when he dist in^ui-hed thi- ca-e from that in which a mixture
of liquids, which arc insoluble in cadi other, evaporates. I-'IMHI this it is evident that a.
mutual action occurs in solution, which diiniui>lie- the \apoiir ten-ion- pi-oper to the
indi\idual substances, as would lie expected mi the suppo-itioii of the fonnatiou of com-
pounds ,,(' the di>-ol\iiiL: >ul)-tance- in -olutioii-. l)ecau-e the ela-ticity then alway-

'" Thi- amount i- u-ually exprt-ssed liy the wei-ht of the -ul.-tance dissolved per l*»u
pai't- liy weight of water. 1 'rohalily it would l»e lietter to expre-s it \>\ the (|uantity of
the -nli-taiice in a definite volume of the-olution — for instance, in a litre. 1 -peak in
detail of t lie different method- of expres-in^- the competition of solutions in the work
mentioned in note lit. p. f, J.

•' The vai'iatiou of the \apour tension of solutions ha- lieen inve-ti^ated liy many.
Thebe>t kniiwn researches aiv those of Wiillner . ls:,s-isf,e and of Tamilian ilHHTi. Tin-
re>earclie- on the temperature of the formation of ice from various solutions ai'i' ulso
very numerous; Bla^den i_17«wi, Kiidorfl i lsf.1 .. and \^<- L'oppet 1 1*71 1 e-tal>lislied the
1 H"j inn i ]r_r. lint thi- kind of m\'e-ti^'at ion takes its chief inteie>t fi'oin the woi'k ot
Itaoult. lie_run in l^-i> on aijiieous -olution-. and afterward- continued tor -olutions in
\ariou- ot hei- eii~ii\-lro/.en licpiid- - for in-tance. lien/eiie. (', 1 1. 'inch- at I'lU'i I. acetic
acid. ('.,}!,(.)., i It'rT'i . and other-. An especially important intei'ot is attached to these
investigations of llaoult on the lowering of the free/in^ point, l.ecau-e he took >olut ion-
of nmiiv well-kn..wn carlxi

mpolllHls ana discovered a -mi]
molecular weight of the Mili^tances and the temperature of cr\ -talli-at ion of the
solvent, which enaliled this kind of research to he applied to the m\ e-t i-at ion of the
nature of stil>Mtance>. \Ve >hall meet with the ajiplication of Kaoult's reMilts Inter on,
and at pic-enl will only cite the deduction arrived at from the-e re-lilt -. The solution
o! one - liundreilt h part of t hat molecular i: ram u ei- lit \\ Inch corre-pond- \\ ith the formula
of a -ul'-taiice di—olved I for example, Na('l :.s-:,. ( ' .1 1, ( ) ir,. ,\ ,-. in ]oiipart> of a
solvent lower- the free/in^ ]ioint of it - -olution in u a t er if 1 s.'i . iii lien /cue ()' I'.i . and in
acetic acid H-o'.i .or twice an much as with water. And a- in weak >olut ion- t he fall of free/in-
point i- proportional to the amount of the -uli-taiice di--ol\ed. it follow- that the fall of
free/in^' point for all other -olution- may lie calculated from tin- rule. So. for in-tance,

the weight which corre-pond- with the formula of acetone, ( '-H, <>. i- ,"•« ; a -oluti 'on-

t a mi i ix •!' \:i. i-i"J-J. .11 id 1 •!".',:> ^ram- of acet • pel- loo ^ram- o] water lorin- ice ' according

to the determination- of heckmaiim at tt'TTd . r:t:;ii . and :) -_!o . ;(iid the.-e ti^ure- -how
that uiih a Milulion containing o-;,> -ram- of acetone per loo ,,| water the fall of the
temperature ,,| the formation ,,f ice will lie d'ls.") , If 1 -o . and d'17'.H. It mu-t lie
remarked that the la\s of projiort iomtlity I.etween the fall of temperature of tlie forma-
tion of ice, and the composition of a solution.i> in general oul\ approximate, and i- only

applicable to ueak -ollll ion-.

We will here remark that the theoretical intere-t o! tlii- Mibjecl was -t ren^t hened
on the disco\er\ of tin connection existili" between the tall o! teli-i ui. the fall of the

ox WATKK AND ITS COMPOUNDS 91

to the amounts of salt in solution (1, 5, and 10 per 100 of water).
Furthermore, it has been shown by experiment that the ratio of the
diminution of vapour tension to the vapour tension of water at different
temperatures in a given solution is an almost constant quantity/"0 and

temperature of the formation of ice, of osmotic pressure (Van't Hoff, note 19), and of the
electrical conductivity <>f solutions, and we will therefore supplement what we have
ul ready said on the subject by some short remarks on the method of investigating the
phenomenon, and on its theretical results.

In order to determine the temperature of the formation of ice (or of crystallisation
of other solvents), a known solution is prepared and poured into a cylindrical vessel
surrounded by a second similar vessel, leaving a layer of air between the two, which,
being a bad conductor, prevents any rapid change of temperature. The bulb of a sensi-
tive and corrected thermometer is immersed in the solution, and also a bent platinum
wire for stirring the solution ; the whole is then cooled (by immersing the apparatus in a
freezing mixture), and the temperature at which ice begins to separate observed. If the
temperature at first falls slightly lower, nevertheless, it becomes constant when ice
begins to form. By then allowing the liquid to get just warm, and then again observing
the temperature of the formation of ice, an exact determination may be arrived at. If
there be a large mass of solution, the formation of the first crystals may be accelerated
by dropping a small lump of ice into the solution already partially over-cooled. This
only imperceptibly changes the composition of the solution. The observation should be
made at the point of formation of only a very small amount of crystals, as otherwise the
composition of the solution will become altered from their separation. Every precaution
must be taken to prevent the access of moisture to the interior of the apparatus, which
might also alter the composition of the solution or properties of the solvent (for instance,
when using acetic acid).

The very great theoretical interest of these observations on the fall of the tempera-
ture of the formation of ice, which are essentially very simple, dates from the time when
Van't Hoff (note 19) showed that their consequences are in complete accord with those
derived from observations on osmotic pressure. These latter showed that a molecular
(expressed by formulae) quantity of a substance evinces an osmotic pressure in a solu-
tion, which is equal to the atmospheric pressure (when i = 1), or which is greater than it
by i times. The magnitude i, determined from osmotic observations on aqueous solutions,
is also obtained from observations on the fall of the temperature of the formation of ice,
if the fall corresponding with a solution containing 1 gram of a substance per 100 parts
water be multiplied by the molecular weight (according to the formula of the substance,
and expressing the weight of a molecule) of the substance dissolved, and divided by
18'5. Thus from the above data for acetone, it is seen that with a solution containing
1 gram, the fall of temperature of the formation of ice equals 0'818°, and after multiply-
ing by the molecular weight (58), and dividing by 18'5, we have i=l. With sugar and
many other substances (among salts, magnesium sulphate, for instance), with carbonic
anhydride, ttc., both methods give a figure which is nearly unity. For potassium and
sodium chlorides, potassium iodide, nitre, and others, i is greater than 1 but less than
2 ; for sulphuric and hydrochoric acids, sodium and calcium nitrates, and others, i is
nearly 2 ; for solutions of barium and magnesium chlorides, potassium carbonate and
dicliromate, i, according to both methods, is greater than 2 but less than 3. The further
investigation of this subject should show whether these conclusions are entirely general,
and would probably explain better than they do now those remarkable correlations
which are arrived at with the present data.

M This fact, which was established by Gay-Lussac, Prinsep, and v. Babo, is confirmed
by the latest observations, and enables us to express not only the fall of tension (p — p')

its.!!, but its ratio to the tension of water (2—£.\. It is to be remarked that in the

V p I

absence of any chemical action, the fall of tension is either very small, or does not

that for every (dilute) solution the rat io bet wee ti the diminution of vapour

tension and of the free/in^- point is also a sufficiently constant quant it v.'1
1 he diminution of the vapour tension of solutions explains the rise
in boiling point through the solution of soli-,i inui- volatile bodies in
water. The temperature of a vapour is the same as that of the solu-
tion from which it is generated, and therefore it follows that the
aqueous vapour ^i\en oil' from a solution will be superheated. A
saturated solution of common salt boils at 1 US 1 .a solution of :'>:$.">
parts of nitre in 100 parts of water at 11-V[I . and a solution of '.\'2~>
parts of potassium chloride in 1(H) parts of water al 17'.1 . if the tempera-
ture of ebullition be determined bv immersing the thermometer bulb in
the liquid itself. This is another proof of the bond which exists between
water and the substance dissolved. And this bond is seen still more
clearly in those cases (for example, in the solution of nitric or formic
acid in \\ater) where the solution boils at a higher temperature than
either water or the volatile substance dissolved in it. For this reason
the solutions of certain u'a>es for instance, hydriodie or hydrochloric
acid boil above 100'.

The separation of ice from solutions •'- explains both the phenome-
non, well known to seamen, that the ice formed from salt water gives
fresh water, and also the fact that by free/ing, just as by evaporation,
a solution is obtained which is richer in salts than before. This is
taken advantage of in cold countries for obtaining a liquor from sea-
water, which is then evaporated for the extraction of .salt.

< >n the removal of part of the water from a solution (hv evaporation
or the separation of ice), there should be obtained a saturated solution,
and then the substance dissolved should separate out. Solutions satu-
rated at a certain temperature should also separate out a corresponding
part of the substance dissolved if thev be reduced, by cooling, '3 to a

. . • at all note :;:'. . and is not proportional to the quantity of the -ul, -lance added. As

lie. the ten-ion - then equal, accordiut,' to the law of Dalton. to the sum of the

ten-ion- oi the -uh-tanees taken. Therefore, liquids which are ins,, ]uhle in each other

i for example, water ,md chloride of earhom pre-eiit a tension eipial to the -urn of their

dual ten-ions, and the'-efore -udi a mixture hoils at a louei temperature than the

': It. in our example, the fall of tension !„• di\ ided 1-y the tei - on of water, a figure is
ol.taim i .\liielii- nearh llir, t iinc- le— than t he magnitude of 1 . • i temperat lire of

M] the application ot the mechanical theory of heat, and is repeated l,\ man\ imesti^'ated

it t

<>.\ VTATKK AND ITS COMPOUNDS 93

temperature at which the water can no longer hold the former quantity
of the substance in solution. If this separation, by cooling a saturated
solution or by evaporation, take place slowly, cryxtal* of the substance
dissolved an- in many cases formed ; and this is the method by which
crystals of soluble salts are usually obtained. Certain solids very
easily separate out from their solutions in perfectly-formed crystals,
which may attain very large dimensions. Such are nickel sulphate,
alum, sodium carbonate, chrome-alum, copper sulphate, potassium ferri-
cvanidc, and a whole series of other salts. The most remarkable circum-
stance in this is that many solids in separating out from an aqueous
solution retain a portion of water, forming crystallised solid substances
which contain water. A portion of the water previously in the solution
remains in the separated crystals. The water which is thus retained
is called the water of crystallisation. Alum, copper sulphate, Glauber's
salt, and magnesium sulphate contain such water, but neither sal-
ammoniac, nor table salt, nor nitre, nor potassium chlorate, nor silver
nitrate, nor sugar, contains any water of crystallisation. One and the
same substance may separate out from a solution with or without water
of crystallisation, according to the temperature at which the crystals are
formed. Thus common salt in crystallising from its solution in water
at the ordinary or a higher temperature does not contain water of
crystallisation. But if its separation from the solution takes place at
a low temperature, namely below —5°, then the crystals contain 38
parts of water in 100 parts. Crystals of the same substance which
separate out at different temperatures may contain different amounts
of water of crystallisation. This proves to us that a solid dissolved in
water may form various compounds with it, differing in their properties
and composition, and capable of appearing in a solid separate form like
many ordinary definite compounds. This is indicated by the numerous
properties and phenomena connected with solutions, and gives reason
for thinking that there exist in solutions themselves such compounds of

note 24), so these substances do not separate from their saturated solutions on cooling
but on heating. Thus a solution of manganese sulphate, saturated at 70°, becomes cloudy
on further heating. The point at which a substance separates from its solution with a
change of temperature gives an easy means of determining the co-efficient of solubility,
and this was taken advantage of by Prof . Alexeeff for determining the solubility of many
substances. The phenomenon and method of observation is here essentially the same
as in the determination of the temperature of formation of ice. If a solution of a sub-
stain •<• which separates out on heating be taken (for example, the sulphate of calcium
or manj_rane>ei. then at a certain fall of temperature ice will separate out from it, and at
a certain rise of temperature the salt will separate out. From this example, and from
general considerations, it is clear that the separation of a substance dissolved from a
solution should present a certain analogy to the separation of ice from a solution. In
both cases, a heterogeneous system of a solid and a liquid is formed from a homogeneous
(liquid) system.

the substance di>solved, and the sohent or compounds similar to them,
only in a liquid partly decomposed form. Kven the <•<>?<>///• nf mlnt'tun*
may often conlirm this opinion. Copper sulphate forms crystals having
a blue colour and containing- water of crystallisation. If the water of
crystallisation be removed by heating the crystals to redness, a colour-
less anhydrous substance is obtained (a white powder). 1'Yom this it
may be seen that the blue colour belongs to the compound of the copper
salt with water. Solutions of copper sulphate are all blue, and con-
sequently "hey contain a compound similar to the compound formed by
the salt with its water of crystallisation. Crystals of cobalt chloride
when dissolved in an anhydrous liquid like alcohol, for instance <_nve
a blue solution, but when they are dissolved in water a red solution is
obtained, ('rystals from the aqueous solution, according to Professor
Potilit/in. contain six times as much "water (CoCl.,,f>H.,< )) for a ijiven
\\ eiu'lit of the salt, as t hose violet crystals (CoCb. II .,< > ) which are formed
by the evaporation of an alcoholic solution.

That solutions contain particular compounds with water is further
shown bv the phenomena of supersaturated solutions, of so-called crvo-
hvdrates. of solutions of certain acids having constant boiling ]>oints.
and the properties of compounds containing water of crystallisation
whose data it i- indispensable to keep in view in tne consideration of

-i i] Ut li i! IS.

The phenomenon of supersaturated solutions consists in the follow-
ing : •< Mi the refrigeration of a saturated solution of certain salts,-'1
if the liuuid be brought tinder certain conditions, the excess of the solid
ina\' -omctiiiies remain in solution and not separate out. A "Teat
number of substances, and especially sodium sulphate, Na._,S(),, or
( Haulier's sab. ea-ilyform supersaturated solution-. If boiling water
be saturated \\ith tin- salt, and the solution be poured ot]' from any
reniainiiiLf undis-olved salt, and. the boiling beinif still continued, the
Vessel holding the solution be \\-ell closed by cot toll wool, or by fusing up
the vcs>cl. or by covering t he sol ut ion with a layer of oil. i hen it will he
found that this saturated solution does not separate out anv (dauber's
^alt whatever on cooling do\\ ii to the ordinary or even to a much
lower temperature : a It lioii^h \\iihout the abo\e precautions a salt
M -pa rate- out on eoolinir. in the form of crystals \\ hidi contain water of

• 'I , ., , ,!< v.hirli c}..irate ..Hi witli \siiti-r ••!' cry-tiilli-iitii.il t'l.nn sii|M-rsiitunitf<l

n-r.,11- p- -i-.ir.-li. •- hasc |.r(.\ci| that -ii| n-r-at urat.'il snlut ii.ns «!M imt
• i ,,| • , i- ,- -.-in ial |in.|.i-rt , -. 'i lie \ ariation M!'

•, M i',,cin.il "ii "i i, ... A ,-.. take |iliici. an Mi-.lin- ti. tlic i.rilimiry

<'N WATKR AND ITS COMPOUNDS 95

<•! •ystallisation to the amount of Na2S04,10H2O — that is, 180 parts of
water for 142 parts of anhydrous salt. The supersaturated solution
may be moved about or shaken inside the vessel holding it, and no
i-ry>tallisatioii will take place; the salt remains in the solution in as
laruv an amount as at. a higher temperature. If the vessel holding
the supersaturated solution be opened and crystals of Glauber's salt be
thrown in, crystallisation suddenly takes place.53 A considerable rise
in temperature is noticed during this rapid separation of crystals, which
is explained by the salt, previously in a liquid state, passing into a solid
state, by which, as is known, latent heat is evolved. This somewhat
resembles the fact that water may be cooled below 0° (even to — 10°) if
it be left at rest, under certain circumstances, and evolves heat in
suddenly crystallising. Although from this point of view there is a
resemblance, yet in reality the phenomenon of supersaturated solutions
is much more complicated. Thus, on cooling, a saturated solution of
Glauber's salt deposits crystals containing Na2SO4,7H2O,56 or 126 parts

65 Inasmuch as air, as has been shown by direct experiment, contains, although in
very small quantities, minute crystals of salts, and among them of sodium sulphate, air
can bring about the crystallisation of a saturated solution of sodium sulphate in an open
vessel, but it has no effect on saturated solutions of certain other saTts ; for example, lead
acetate. According to the observations of De Boisbaudran, Gernez, and others, isomor-
phous salts (analogous in composition) are capable of evoking crystallisation. Thus, a
supersaturated solution of nickel sulphate crystallises by contact with crystals of sul-
phates of other metals analogous to it, such as those of magnesium, cobalt, copper, and
manganese. The crystallisation of a supersaturated solution, brought about by the con-
tact of a minute crystal, starts from it in rays with a definite velocity, and it is evident
that the crystals as they form propagate the crystallisation in definite directions. This
phenomenon recalls the evolution of organisms from germs. An attraction of similar
molecules ensues, and they dispose themselves in definite similar forms.

56 In these days a view is very generally accepted, which regards supersaturated
solutions as homogeneous systems, which pass into heterogeneous systems (composed of
a liquid and a solid substance), in all respects exactly resembling the passage of water
cooled below its freezing point into ice and water, or the passage of crystals of rhombic
sulphur into monoclinic crystals, and of the monoclinic crystals into rhombic. Although
many phenomena of supersaturation are thus clearly understood, yet the spontaneous for-
mation of the unstable hepta-hydrated salt (with 7H2O), in the place of the more stable
deca-hydrated salt (with mol. 10H2O), indicates a property of a saturated solution of sodium
sulphate which obliges one to admit that it has a different structure form an ordinary
solution. Stcherbacheff affirms, on the basis of his researches, that a solution of the
deca-hydrated salt gives, on evaporation, without the aid of heat, the deca-hydrated salt,
whilst after heating above 33° it forms a supersaturated solution and the hepta-hydrated
salt, which gives reason for thinking that the state of salts in supersaturated solutions
is different from that in ordinary solutions. But in order that this view should be
accepted, some signs must be discovered distinguishing solutions (which are, according to
this view, isomeric) containing the hepta-hydrated salt from those containing the deca-
liydrated salt, and all efforts made in this direction (the study of the properties of the
solutions) have given negative results. Further, according to this view, one would expect
that all supersaturated solutions would contain particular forms of crystallohydrates,
ami, although this is possible, yet up to now nothing of the kind has been observed,

96 PRINCIPLES OF CHEMISTRY

of water per 142 parts of anhydrous salt, and not 180 parts of water, as
in the above-mentioned salt. Further, the crystals containing TH2O
are distinguished for their instability ; if they stand in contact not only
with crystals of Na2SO4,10H2O, but with many other substances, they
immediately become opaque, forming a mixture of anhydrous and deca-
hydrated salts. It is evident that between water and a soluble sub-
stance there may be established different kinds of greater or less stable
equilibrium, of which solutions form one aspect/'7

and one must think that the connection with the fusibility of the deca-hydrated salt
(and of all salts which easily give supersaturated solutions and are capable of forming
several crystallohydrates), and with that decomposition (formation of the anhydrous
salt) which the deca-hydrated salt suffers on melting — plays its part here. As some
crystallohydrates of salts (alums, sugar of lead, calcium chloride) melt without
decomposing, whilst others (like Na2SO4,H.2O) are decomposed, then it may be that the
latter are only in a state of equilibrium at a higher temperature than their melting point.
Did experiment show that the hepta-hydrated salt began to crystallise below 33°, and
that then only the crystals grow, then all the data concerning supersaturated solutions of
sodium sulphate could be explained exclusively in the sense of a super-cooling effect.
At present, however, these questions, notwithstanding the mass of research to which
they have been subjected, cannot be considered as fully resolved. It may here be
observed that in melting crystals of the deca-hydrated salt, there is formed, besides
the solid anhydrous salt, a saturated solution giving the hepta-hydrated salt, so that this
passage from the deca- to the hepta-hydrated salt, and the reverse, takes place with the
formation of the anhydrous (or it may be, mono-hydra ted) salt.

The researches of Pickering (1887) on the amount of heat which is evolved in the
solution of hydrous and anhydrous salts at different temperatures, give reason to think
that at a certain temperature no heat will be evolved in the combination with water; that
is, that probably such a combination will not take place. Thus 106 grams (the molecular
weight in grams) of anhydrous sodium carbonate, NaoCOj, in dissolving in 7,200 grams
( = 400 H2O) of water, evolve 4,300 calories at 4°, 5,300 at 16°, and 5,850 calories at 25° (in
other cases the heat evolved in solution also increases with a rise of temperature). If,
however, the crystallo- hydrate, NaoCO^ , 10H.2O,be taken, then (for the same quantity of
anhydrous salt) an absorption of heat is observed; at 4° -16,250, at 16° — 16,150, and at
25° — 16,300 calories. As in this case a portion of the heat absorbed is due to the fact that
the water of crystallisation taken in a solid state appears in a liquid state, Pickering sub-
tracts the latent heat of liquefaction of ice, and obtains in the given case at 4° -1,700, at
16° — 600, and at 28° -0 calories. From this, the heat of the formation of the crystallo-
hydrate, or the heat evolved by the combination of Na2CO3 with 10H2O, may be
calculated (by subtracting the former quantities from the first). At 4° it is equal to
+ 6,000, at 16° + 5,900, at 25° + 5,850 calories; that is, it distinctly decreases, although
but slightly, with the rise of temperature. It may be that for Na2SO4 at 33° the heats
of the formation of + lOHoO and 7H2O differ but very slightly.

57 Emulsions, like milk, are composed of a solution of glutinous or like substances,
or of oily liquids suspended in a liquid in the form of drops, which arc clearly visible
under a microscope, and form an example of a mechanical formation which resembles
solutions. But the difference from solutions is here evident. There are, however,
solutions which approach very near to emulsions in the facility with which the substance
dissolved separates from them. It has long been known, for example, that a particular
kind of Prussian blue, KFe2(CN)6, dissolves in pure water, but, on the addition of the
smallest quantity of either of a number of salts, it curdles and becomes quite insoluble.
If copper sulphide (CuS), cadmium sulphide (CdS), arsenic sulphide (As2S-), and many
other metallic sulphides, be obtained by a method of double decomposition (by precipi-

ON WATKK AND ITS COMPOUND- 97

Solutions of salts on refrigeration below 0° deposit ice or crys-
tals (\vhich then usually contain water of crystallisation) of the salt
dissolved, and on arriving by this means at a certain degree of con-
centration they solidify in their entire mass. These solidified masses
are termed r>7/o// //'//•<//' '*. My researches on solutions of common salt
(1868) showed that its solution solidifies when it reaches a composition
NaCl + 10H2O (180 parts of water per 58'5 parts of salt), which takes
place at about — 23°. The solidified solution melts at the same temper-
ature, and both the portion melted and the remainder preserve the
above composition. Guthrie (1874-1876) obtained the cryohydrates of
many salts, and he showed that certain of them are formed at com-
paratively low temperatures, whilst others (for instance, corrosive
sublimate, alums, potassium chlorate, and various colloids) are formed
on a slight cooling, to — 2° or even before, and that these contain a
very large amount of water. One can easily imagine that these two
series of cryohydrates differ considerably from each other, but the in-
sufficiency of the existing data58 does not permit of a true judgment
being formed. Nevertheless, in the case of common salt, the cryo-

tating salts of these metals by hydrogen sulphide), and be then carefully washed (by
allowing the precipitate to settle, pouring off the liquid, and again adding sulphuretted
hydrogen water), then, as was shown by Schulze, Spring, Prost, and others, the pre-
viously insoluble sulphides pass into transparent (for mercury, lead, and silver, reddish
brown ; for copper and iron, greenish brown ; for cadmium and indium, yellow ; and for
zinc, colourless) solutions, which may be preserved (the weaker they are the longer they
keep) and even boiled, but which, nevertheless, in time become curdled — that is, settle
in an insoluble form, and then sometimes become crystalline and quite incapable of
re-dissolving. Graham and others observed the power shown by colloids (see note 18) of
forming similar hydrusols or solutions of gelatinous colloids, and, in describing alumina,
and silica, we shall have occasion to speak of such solutions once more.

In the existing state of our knowledge concerning solution, such solutions may be
looked on as a transition between emulsion and ordinary solutions, but no fundamental
judgment can be formed about them until a study has been made of their relations to
ordinary solutions (the solutions of even soluble colloids freeze immediately on cooling
below 0°, and, according to Guthrie, do not form cryohydrates), and to supersaturated
solutions, with which they have certain points in common.

58 Offer (1880) concludes, from his researches on cryohydrates, that they are simple
mixtures of ice and salts, having a constant melting point, just as there are alloys having a
constant point of fusion, arid solutions of liquids with a constant boiling point (see note 60).
This does not, however, explain in what form a salt is contained, for instance, in the
cryohydrate, NaCl + 10H2O. At temperatures above — 10° common salt separates out in
anhydrous crystals, and at temperatures near —10°, in combination with water of
crystallisation, NaCl + 2H2O, and, therefore, it is very improbable that at still lower
temperatures it would separate without water. If the possibility of the solidified cryo-
hydrate containing XaCl + 2H2O and ice be admitted, then it is not clear why one of
these substances does not melt before the other. If alcohol does not extract water from
the solid mass, leaving the salt behind, this does not prove the presence of ice, because
alcohol also takes up water from the crystals of many hydrated substances (for instance,
from NaCl + 2H2O) at about their melting-points. Besides which, a simple observation
on the cryohydrate, NaCl + lOH.^O, shows that with the most careful cooling it does not

VOL. I. H

98 PRINCIPLES OF CHKMJSTKY

hydrate with 10 molecules of water, and in the case of sodium nitrate,
the cryohydrate ~'9 with 7 molecules of water (i.e., 126 parts of water
per 85 of salt) should be accepted as established substances, capable of
passing from a solid to a liquid stare and conversely ; and therefore it
may be thought that in cryohydrates we have solutions which are not
only undecomposable by cold, but also have a definite composition which
would present a fresh case of definite equilibrium between the solvent
and the substance dissolved.

The formation of definite but unstable compounds in the process of
solution becomes evident from the phenomena of a marked decrease of
vapour tension, or from the rise of the temperature of ebullition which
occurs in the solution of certain volatile liquids and gases in water. As
an example, we will take hydriodic acid, HI, a gas which liquefies on
a very considerable reduction of temperature, giving a liquid which
boils at - 20°. A solution of it containing 57 p.c. of hydriodic acid is
distinguished by its great stability. If it be evaporated by heating,
the hydriodic acid volatilises together with the water in the same
proportions as they occur in the solution, so that the gas passes off
together with the aqueous vapour, and therefore such a solution may be
distilled unchanged, for the distillate will contain the same proportion
of hydriodic acid and water as was originally taken. The solution
boils at a higher temperature than water. The physical properties of
the gas and water in this case already disappear ; there is formed a
stable compound between water and the gas, a new substance which
has its definite boiling point. To put it more correctly, this is not the
temperature of ebullition, but the temperature at which the compound
formed decomposes, forming the vapours of the products of dissociation,
which, on cooling, re-combine. The above-described aqueous solution
boils at 127°. Should a less amount of hydriodic acid be dissolved
in water than the above, then, on heating such a solution, water only
will at first be 'distilled over, until the solution attains the above-
mentioned composition ; it will then distil over unaltered. If more
hydriodic acid be passed into such a solution a fresh quantity of the
gas will dissolve, which, however, may be very easily removed. It
must not, however, be thought that those forces which determine the

on the addition of ice deposit ice, which would occur if ice in intermixture with the- salt
were formed on solidification.

I may add with regard to cryohydrates that, in investigating aqueous solutions of
alcohol (note 19), I concluded, on the basis of the specific gravity, that a compound,
C2H6O + 12H2O, existed, and a solution -of this composition completely solidifies on cool-
ing to —20°, forming well-formed crystals, which melt at about —18°, as was shown by
observations made by W. E. Tischenko and myself. This definite compound reminds
one of cryohydrates in many respects.

59 See note 24.

M.\ WATKK AM) ITS COMPOUNDS !M)

formation of ordinary gaseous solutions play no part whatever in the
formation of a solution having a definite boiling point ; that they do
act is shown from the fact that such constant gaseous solutions vary in
their composition under different pressures/'0 Therefore, it is not at

;o For this reason ('the want of entire constancy of the composition of constant boiling
solutions with a ch-inge of pressure) nrmy deny the existence of definite hydrates formed
by volatile snl»st inces — for instance, by hydrochloric acid and water. They generally
argue as follows: If there did exist a constancy of composition, then it would net be
altered by a change of pressure. But the distillation of constant boiling hydrates is un-
doubtedly accompanied (judging by the vapour densities determined by Binean). like the
distillation of sal-ammoniac, sulphuric acid. Arc., by an entire decomposition of the
previous compound — that is, these substances do not exist in a state of vapour, but
their products of decomposition (hydrochloric acid and water) are gases at the tempera-
ture of volatilisation, whi:-h dissolve in the volatilised and condensed liquids ; but the
solubility of gases in liquids depends on the pressure, and, therefore, the composition of
constant boiling solutions may, and even ought to, vary with a change of pressure, and.
further, the smaller the pressure and the lower the temperature of volatilisation, the
more likely is a true compound to be obtained. According to the researches of Koscoe
and Dittmar (1859), the constant boiling solution of hydrochloric acid proved to contain
18 p.c. of hydrochloric acid at a pressure of 3 atmospheres, 20 p.c. at 1 atmosphere,
and 28 p.c. at ^ of an atmosphere. On passing air through the solution until its
composition became constant (i.e., forcing the excess of aqueous vapour or of hydro-
chloric acid to pass away with the air), then acid was obtained containing about
20 p.c. at 100°, about 23 p.c. at 50°, and about 25 p.c. at 0°. From this it is seen
that by decreasing the pressure and lowering the temperature of evaporation one
arrives at the same limit, where the composition should be taken as HC1 + 6H2O, which
requires 25'26 p.c. of hydrochloric acid. Fuming hydrochloric acid contains more than
this.

The most important fact in evidence of the existence of definite compounds in acids
boiling at a constant temperature is the fall of tension. The gas loses its tension, does not
follow the law of Henry and Dalton with a diminution of pressure ; its solution oaly parts
with water ; the vapour tension of a volatile liquid in solution is less than its own or that
of the water combined with it. This loss of tension is a loss of movement brought about
by the action of the attraction existing between the water and the substance dissolved. In
the case already considered, as in the case of formic acid in the researches of D. P.
Konovaloff (note 47), the constant boiling solution corresponds with a minimum tension —
that is, with a boiling point higher than that of either of the component elements. But
there is another case of constant boiling solutions similar to the case of the solution of
propyl alcohol, C.'-H^O, when a solution, undecomposed by distillation, boils at a lower
point than that of the more volatile liquid. However, in this case also, if there be
solution, the possibility cannot be denied of the formation of a definite compound in the
form C-,HsO-fH2O, and the tension of the solution is not equal to the sum of tensions
of the components. There are possible cases of constant boiling mixtures even when there
is no solution nor any loss of tension, and consequently no chemical action, because the
amount of liquids that, are volatilised is determined by the product of the vapour den
into their vapour tensions (Wanklyn), in consequence of which liquids whose boiling
point is above 100° — for instance, turpentine and ethereal oils in general — when distilled
with aqueous vapour, pass over at a temperature below 100°. Consequently, it is not in
the constancy of composition and boiling point (temperature of decomposition) that the
signs of a clear chemical action should be seen in the above-described solutions of acids,
but in the great loss of tension, which completely resemble* the loss of tension ob-
>erved. for instance, in the perfectly-definite combinations of substances with water of
crystallisation (see later, note i'i.">). Sulphuric acid. H..SO,. as we shall learn later, is a!-o
decomposed by distillation, like HC1 + 6H.2O, and exhibits, moreover, all the signs of a

II L'

100 FIUNril'LES OF CHEMISTRY

every, but only at the ordinary, atmospheric pressure that a constant
boiling solution of hydriodic acid will contain 57 p.c. of the gas. At
another pressure the proportion of water and hydriodic acid will be
different. It varies, however, judging from observations made by Roscoe,
very little for considerable variations of pressure. This variation in
composition directly indicates that pressure exerts an influence on the
formation of unstable chemical compounds which are easily dissociated
(with formation of a gas), just as it influences the solution of gases,
only the latter is influenced to a more considerable degree than the
former/'1 Hydrochloric, nitric, and other acids form solutions 1iarin</
definite boiling points, like that of hydriodic acid. They show further
the common property, if containing but a small proportion of water, that
they fume in air. Strong solutions of nitric, hydrochloric, hydriodic,
and other gases are even termed ' fuming acids.' The fuming liquids
contain a definite compound, whose temperature of ebullition (decom-
position) is higher than 100°, and contain also an excess of the volatile
substance dissolved, which (the substance) exhibits a capacity to com-
bine with water and form a hydrate, whose vapour tension is less than
that of aqueous vapour. On evaporating in air, this dissolved substance
meets the atmospheric moisture and forms a visible vapour (fumes) with
it, which consists of the above-mentioned compound. The attraction
or affinity which binds, for instance, hydriodic acid with water is
evinced not only in the evolution of heat and the diminution of vapour
tension (rise of boiling point), but also in many purely chemical rela-
tions. Thus hydriodic acid is produced from iodine and hydrogen
sulphide in the presence of water, but unless water is present this re-
action does not take place/'2

definite chemical compound. The study of the variation of the specific gravities of
solutions as dependent on their composition (see note 19) shows that phenomena of a
similar kind, although of different dimensions, take place in the formation of both H2SO4
from H2O and SO3, and of HC1 + 6H.2O (or of aqueous solutions analogous to it) from HC1
and H20.

61 The essence of the matter may be thus represented. A substance A, either gaseous
or easily volatile, forms with a certain quantity of water, ?zHoO, a definite complex com-
pound AnH^O, which is stable up to a temperature t3 higher than 1003. At this tempera-
ture it is decomposed into two substances, A + H2O. Both boil below t° at the ordinary
pressure, and therefore at t° they distil over and re-combine in the receiver. But if a
part of the substance AnfL^O is decomposed or volatilised, there still remains a portion of
undecomposed liquid in the vessel, which can partially dissolve one of the products of
decomposition, and that in quantity varying with the pressure and temperature, and
therefore the solution at a constant boiling point will have a slightly-different composition
at different pressures.

62 For solutions of hydrochloric acid in water there are still greater differences in
reactions. For instance, strong solutions decompose antimony sulphide (forming hydro-
gen sulphide, H2S), and precipitate common salt from its solutions whilst weak solutions-
do not act thus.

<>N WATKK AND ITS COMPOUNDS 101

.Many compounds containing water of crystallisation are solid sub-
stances (when melted they are already solutions — i.e., liquids) ; further-
more, they are capable of being formed from solutions, as is ice or
aqueous vapour. I propose calling them • •/•//*/'/'/"-// //'//v/A-x. Inasmuch
as the direct presence of ice or aqueous vapour cannot be admitted in
solutions (for these are liquids), although the presence of water may
be, so also there is no basis for acknowledging the presence in solu-
tions of substances in an already -existing state of combination with
water of crystallisation, although they are obtained from solutions as
siu-h.'::{ It is evident that such substances present one of the many
forms of equilibrium between water and a substance dissolved in it.
This form, however, reminds one, in all respects, of solutions — that is,
aqueous compounds which are more or less easily decomposed, with
separation of water and the formation of a less aqueous or an anhydrous
compound. In fact, there are not a few crystals containing water
which lose a part of their water at the ordinary temperature. Of such
a kind, for instance, are the crystals of soda, or sodium carbonate,
which, when separated from an aqueous solution at the ordinary
Temperature, are quite transparent; but when left exposed to air,
lose a portion of their water, becoming opaque, and, in the process,
lose their crystalline appearance, although preserving their original
form. This process of the separation of water at the ordinary tempera-
ture is termed the efflorescence of crystals. Efflorescence takes place
more rapidly under the receiver of an air pump, and especially at a
gentle heat. This breaking up of a crystal is dissociation at the
ordinary temperature. Solutions are decomposed in exactly the same
manner.64 The tension of the aqueous vapour, which is given off from

63 Supersaturated solutions give an excellent proof in this respect. Thus a solution
of copper sulphate generally crystallises in penta-hydrated crystals, CuSC>4 + 5H2O, and
ii-- -uturated solution gives such crystals if it be brought into contact with the minutest
possible crystal of the same kind. But, according to the observations of Lecoq de Bois-
baudran, if a crystal of ferrous sulphate (an isomorphous salt, see note 55), FeSO4 + 7H2O,
be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt,
( 'uSO.j+7H2O, are obtained. It is evident that neither the penta- nor the hepta-hydrated
salt is contained as such in the solution. The solution presents its own particular liquid
form of equilibrium.

64 Efflorescence, like every evaporation, proceeds from the surface. Inside crystals
which have effloresced there is usually found a non-effloresced mass, so that the majority
of effloresced crystals of washing soda show, in their fracture, a transparent nucleus
coated by an effloresced, opaque, powdery mass. It is a remarkable circumstance in this
respect that efflorescence proceeds in a completely regular and uniform manner, so that
the angles and planes of similar crystallographic character effloresce simultaneously,
and i)i this respect the crystalline form determines those part s of crystals where efflo-
rescence starts, and the order in which it continues. In solutions evaporation also
proceeds from the surface, and the first crystals which appear on its reaching the
required degree of saturation are also formed at the surface. After falling to the
bottom the crystals naturally continue to grow (see Chap. X.).

102 PRINCIPLES OF CHK-MJSTKY

crystallo-hydrates is naturally, as with solutions, less than the vapour
tension of water itself '"' at the same temperature, and therefore many
anhydrous salts which are capable of combining with water absorb
aqueous vapour from moist air ; that is, they act like a cold body on
which water is deposited from steam. It is on this that the desiccation
of gases is based, and it must farther be remarked in this respect that
certain substances — for instance, potassium carbonate (Iv3CO3) and
calcium chloride (CaCL>) — not only absorb the water necessary for the
formation of a solid crystalline compound, but also give solutions, or
deliquesce, as it is termed, in moist air. Many crystals do not effloresce
in the least at the ordinary temperature ; for example, copper sulphate,
which may be preserved for an indefinite length of time without efflo-
rescing, but when placed under the receiver of an air pump, if efflores-
cence be once started, it goes on at the ordinary temperature. The
temperature at which the entire separation of water from crystals takes
place varies considerably, not only for different substances but also for
different portions of the contained water. Very often the temperature
at which dissociation begins is very much higher than the boiling point
of water. So, for example, copper sulphate, which contains 36 p.c. of
water, gives up 2 8 '8 p.c. at 100°, and the remaining quantity, namely
7*2 p.c., only at 240°. Alum, out of the 45'5 p.c. of water which it con-
tains, gives up 18-9 p.c. at 100°, 17'7 p.c. at 120°, 7-7 p.c. at 180°, and
1 p.c. at 280°; it only loses the last quantity (1 p.c.) at their temperature
of decomposition. These examples clearly show that the annexation of
water of crystallisation is accompanied by a rather profound, although,
in comparison with instances which we shall consider later, still incon-

65 According to Lescoeur (1883), at 100° a thick solution of barium hydroxide, BaH2O2r
on first depositing crystals (with + H.>O) has a tension of about 630 mm. (instead of 7(50 mm.,
the tension of water), which decreases (because the solution evaporates! to 45 mm., when
all the water is expelled from the crystals, BaH2O.2 + HoO, which are formed, but they
also lose water (dissociate, effloresce at 100°), leaving the hydroxide, BaH^O^, which is per-
fectly undecomposable at 100° — that is, does not part with water. At 73° (the tension of
water is then 265 mm.) a solution, containing 33H.^>O, on crystallising has a tension of
280 mm. ; the crystals BaH2O + 8H2O, which separate out, have a tension of 1(10 mm. ; on
losing water they give BaH2O2 -»-HoO. This substance does not decompose at 7:! . and
therefore its tension =0. Miiller-Erzbach (1884) determines the tension (with reference
to liquid water) by placing similar long tubes with water and tin- substances experi-
mented with in a desiccator, the rate of loss of water giving the relative tension. Thus,
at the ordinary temperature, crystals of sodium phosphate, Na.,HPO j -r 12H.->O, present
a tension of 0*7 compared with water, until they lose 5H2O, then 0'4 until they lose ">HoO
more, and on losing the last equivalent of water the tension falls to 0'04 compared with
water. It is clear that the different molecules of water are held by an unequal force.
Out of the five molecules of water in copper sulphate the two first are comparatively
easily separated, even at the ordinary temperature (but only after several days in a
desiccator, according to Latchinoff) ; the next two are more difficultly separated, and the
last equivalent is held firmly, even at 100°.

ON AVATKK AND ITS COMPOUNDS 103

, <-liaii!_;<' ft its properties. In certain cases the water of crys-
tallisation is only given oft' when the solid form of the substance is
destroyed : when the crystals melt on heating. The crystals are then
said to ma/t in their water of crystallisation. Further, after the separa-
tion of the water, a solid substance remains behind, so that by further
heating it acquires a solid form. This is seen most clearly in crystals
of sugar of lead or lead acetate, which melt in their water of crystalli-
sation at a temperature of 56*25°, and in so doing begin to lose water.
On reaching a temperature of 100° the sugar of lead solidifies, having
lost all its water ; and then at a temperature of 280° the anhydrous and
solidified salt again melts. Sodium acetate (C2H3Na02,3H.,O) melts
at .">8° (but resolidifies only on contact with a crystal, otherwise it may
remain liquid even at 0° ; as the temperature does not change during
solidification, the melted salt can be used for obtaining a constant
temperature of 58°). According to Jeannel, the latent heat of fusion is
about 28 calories, and, according to Pickering, the heat of solution is 35
calories. When melted, this salt boils at 123° — that is, the tension of
the aqueous vapour given off then equals the atmospheric pressure.

It is most important to recognise in respect to the water of crys-
tallisation that its ratio to the quantity of the substance with which it
is combined is always a constant quantity. However often we may
prepare copper sulphate, we shall always find 36*14 p.c. of water in its
crystals, and these crystals always lose four-fifths of their water at
100°, and one-fifth of the whole amount of the water contained remains
in the crystals at 100°, and is only expelled from them at a temperature
of about 240°. The determination of the amount of water of crystal-
lisation is easily made if a weighed quantity of crystals is dried in an
air or other bath. What has been said about crystals of copper sulphate
refers also to crystals of every other substance which contain water of
crystallisation. It is impossible to here increase either the relative
proportion of the salt or of the water, without changing the homo-
geneity of the substance. If once a portion of the water be lost — for
instance, if once efflorescence takes place— a mixture is obtained, and
not a homogeneous substance, namely a mixture of a substance deprived
of water with a substance which has not yet lost water — i.e., decom-
position has already commenced. This constant ratio is an example of
the fact that in chemical compounds the quantity of the component
parts is quite definite ; that is, it is an example of the so-called definite
(•lumical compounds. They may be distinguished from solutions, and
from all other so-called indefinite chemical compounds, in that at least
one, and sometimes both, of the component parts may be added in a
large quantity to an indefinite chemical compound without destroying

104 PRINCIPLES OF CIIK.MJSTRY

its homogeneity, as in solutions, whilst it is impossible to add any one
of the component parts to a definite chemical compound without de-
stroying the homogeneity of the entire mass. Definite chemical com-
pounds only decompose at a certain rise in temperature ; on a lowering
in temperature they do not, at least with very few exceptions, yield
their components like solutions which form ice or compounds with water
of crystallisation. This obliges one to consider that solutions contain
water as water, Mj although it may sometimes be in a very small quan-
tity. Therefore solutions which are capable of entirely solidifying (for
instance, cryohydrates 'and crystallo-hydrates — i.e., compounds with
water of crystallisation which are capable of melting — or the compound
of 84^ parts of sulphuric acid, H2SO4, with hH parts of water, H20,
or H2SO4,H2O, or H4SO^) appear as true definite chemical compounds,
If, then, we imagine such a definite compound in a liquid state, and
admit that it partially decomposes in this state, separating water —
not as ice or vapour (for then the system would be heterogeneous.
including substances in different physical states), but in a liquid form,
when the system will be homogeneous — -then we shall form an idea of
a solution as an unstable, decomposing fluid equilibrium between water
and the substance dissolved. Just as the component elements may be
added to a gaseous mixture without destroying its homogeneity, so both
the solvent may be added to a solution (the solution will then be
obtained diluted, and no longer presenting a definite composition), arid
also the substance dissolved may be added (with a solid and a saturated
solution a supersaturated solution will be obtained), which may, how-
ever, owing to the force of the cohesion of its parts, separate out from
the solution in a crystallised form. In adding the solvent, or the
substance dissolved, without destruction of the homogeneity of the
whole, we altered their relative quantity (the proportion of the acting
masses), by which there will be an alteration, both in the quantity of the
water, forming one of the products of dissociation, and also of the relative
quantity of one or many of the definite compounds between the water
and the substance dissolved. Owing to this change, there occurs an
alteration in the properties of a solution (contraction, change of vapour
tension, &c.) ; not in the sense of a purely mechanical change in the
proportion of the components (as in the intermixture of non reacting

66 Such a phenomenon frequently presents itself in purely chemical action. Km-
instance, let a liquid substance A give, with another liquid substance J3, under the condi-
tions of an experiment, a mere minute quantity of a solid or gaseous substance C. This
small quantity will separate out (pass away from the sphere of action, as Berthollet
expressed it), and the remaining masses of A and I? will again give C ; consequently,
under these conditions, action will go on to the end. Such, it seems to me, is the action
in solutions when they yield ice or vapour indicating the presence of water.

OX WATKK AND ITS COMPOUNDS 105

gases), 1'iit in the sense of an alteration in the quantity of those definite
liquid chemical compounds which are determined by the chemical attrac-
tion between water and the substance dissolved in it, and by their
capacity for forming with it 'liri-rxe compounds,*'" which is seen in the
capacity of one substance to form with water many various crystal I »-
},,/,} rofrx, or compounds with water of crystallisation, showing diverse
and independent properties. From these considerations, solution
ma ;/ l»> regarded as fluid, unstable, definite chemical compounds in a
state of dissociation.™

67 Certain substances are capable of forming only one compound, others several, and
these of the most varied degrees of stability. The compounds of water are instances of
tins kind. In solutions of sulphuric- ;ic-icls (nee note 19), for example, the existence must
l)f acknowledged of several different definite compounds. Many of these have not yet
In 'en obtained in a free state, and it may be that they cannot be obtained in any other
but a liquid form— that is, dissolved; just as there are many undoubted definite com-
pounds which only exist in one physical state. Among the hydrates such instances
occur. The compound CO2 + 8HUO (see note 31), according to Wroblewski, only occurs in
a solid form. Hydrates like H.,S + 1'2H2O (De Forcrand and Villard), HBr + H2O (Rooze-
boom), can only be accepted on the basis of a decrease of tension, but present themselves
as very transient substances, incapable of existing in a stable free state. Even sulphuric
acid, H.iSO4, itself, which undoubtedly is a definite compound, fumes in a liquid form,
evolving the anhydride, SO5 — that is, exhibits a very unstable equilibrium. The crystallo-
hydrates of chlorine, C13 + 8H2O, of hydrogen sulphide, H2S + 12H<>O (it is formed at 0°,
and is completely decomposed at +1°, as then 1 vol. of water only dissolves 4 vols. of
hydrogen sulphide, while at 0'1° it dissolves about 100 vols.), and of many other gases,
are instances of hydrates which are very unstable.

68 Of such a kind are also other indefinite chemical compounds; for example,
metallic alloys. These are solid substances or solidified solutions of metals. They also
contain definite compounds, and may contain an excess of one of the metals. According
to the experiments of Laurie (1888), the alloys of zinc with copper in respect to the electro-
motive force in galvanic batteries behave just like zinc if the proportion of copper in the
alloy does not exceed a certain percentage — that is, until a definite compound is attained
— for then there are yet particles of free zinc ; but if a copper surface be taken, and it be
covered by only one-thousandth part of its area of zinc, then only the zinc will act in a
galvanic battery.

69 According to the above supposition, the condition of solutions in the sense of the
kinetic hypothesis of matter (that is, on the supposition of an internal movement of
molecules and atoms) may be represented in the following form: — In a homogeneous
liquid — for instance, water — the molecules occur in a certain state of, although mobile,
>till stable, equilibrium. When a substance A dissolves in water, its molecules form with
-cveral molecules of water, systems AnHoO, which are so unstable that when surrounded
by molecules of water they decompose and re-form, so that A passes from one mass of
molecules of water to another, and the molecules of water which were at this moment in
harmonious movement with A in the form of the system AnH.^O, in the next instant
may have already succeeded in getting free. The addition of water or of molecules of A
may either only alter the number of free molecules, which in their turn enter into systems
A n\ LO, or they may introduce conditions for the possibility of building up new systems
. 1 ,, H..O, where m is either greater or less than n. If in the solution the relation of the
molecules be the same as in the system AmH<>O, then the addition of fresh molecules of
w;iter or of A would be followed by the formation of new molecules ^4»H2O. The relative
quantity, stability, and composition of these systems or definite compounds will vary in
one or another solution. Such a view of solutions came to me from a most intimate
study of the variation of their specific gravities, to which my book, cited in note 19, is

106 PRINCIPLES OF CHEMISTRY

In regarding solutions from this point of view they come under the
head of those definite compounds which chemistry mainly treats of.70
For this reason we will direct our particular attention to one side of
the subject under consideration, which touches on the essential property

devoted. Definite compounds, Ati^R.^O and Jj^HoO. existing in a tree — for instance,
solid — form, may in certain east's be held in solutions in a dissociated state (although but
partially) ; they are similar in their structure to those definite substances which are.
formed in solutions, but nothing obliges one to think that it is such systems as, for
instance, Na2SO4 + 10H3O, or Na3SO4 + 7H2O, or Xa.>S().4, that are contained in solu-
tions. The comparatively more stable systems J.^jH.,0 which exist in a tree state and
change their physical state must present, although within certain limits of temperature,
an entirely harmonious kind of movement of A with /^H.jO ; the property also and state
of systems AnH^Q and AmU^O, occurring in solutions, is that they are in a liquid
form, although partially dissociated. Substances A}, which give solutions, are distin-
guished by the fact that they can form such unstable systems .l//Ho(), but besides them
they can give other much more stable systems J/^H.,0. Thus ethylene, C'oll,. in dis-
solving in water, probably forms a system C3H4nHoO, which easily splits up into (' .,Il[
and HoO, but it also gives the system of alcohol, CoH.^HoO or C,.HGO, which is compara-
tively stable. Thus oxygen can dissolve in water, and it can combine with it, forming
peroxide of hydrogen. Turpentine, C10H1(3, does not dissolve in water, but it combines
with it in a comparatively stable hydrate. In other words, the chemical structure of
. hydrates, or of the definite compounds which are contained in solutions, is distinguished
not only by its original peculiarities but also by a diversity of stability. A similar struc-
ture to hydrates must be acknowledged in crystallo-hydrates. On melting t hey give actual
(real) solutions. As substances which give crystallo-hydrates, like salts, are capable of
forming a number of diverse hydrates, and as the greater the number of molecules of
water (n) they (J.«H2O) contain the lower is the temperature of their formation, and as
the more easily they decompose the more water they hold, therefore, in the first place,
the isolation of hydrates holding much water existing in aqueous solutions may be
soonest looked for at low temperatures (although, perhaps, in certain cases they cannot
exist in the solid state) ; and secondly, the stability also of such higher hydrates will be
at a minimum under the ordinary circumstances of the occurrence of liquid water.
Hence a further more detailed investigation of cryohydrates (note 58 j may help to the
elucidation of the nature of solutions. But it may be foreseen that certain cryohydrates
will, like metallic alloys, present solidified mixtures of ice with the salts themselves and
their more stable hydrates, and others will be definite compounds.

70 The above representation of solutions, &c., considering them as a particular state
of definite compounds, excludes the independent existence of indefinite compound! ;
by this means that unity of chemical conception is obtained which cannot be arrived
at by admitting the physico-mechanical conception of indefinite compounds. The
gradual transition from typical solutions (as of gases in water, and of weak saline
solutions) to sulphuric acid, and from it and its definite, but yet unstable and liquid,
compounds, to clearly definite compounds, such as salts and their crystallo-hydi -ates,
is so imperceptible, that by denying that solutions pertain to the number of definite
but dissociating compounds, we risk denying the definiteness of the atomic com-
position of such substances as sulphuric acid or of molten crystallo-hydrates. I
repeat, however, that for the present the theory of solutions cannot be considered as
firmly established. The above opinion about them is nothing more than a hypothesis
which endeavours to satisfy those comparatively limited data which we ha\e for the
present about solutions, and of those cases of their transition into definite compounds.
By submitting solutions to the Daltonic conception of atomism, 1 hope that we may not
only attain to a general harmonious chemical doctrine, but also that new motives for
investigation and research will appear in the problem of solutions, which must either
confirm the proposed theory or replace it by another fuller and truer one.

<>N WATF.i; AND ITS CoMI'orNhS 107

of definite compounds as a class to whose number solutions should (or
at least, may) be referred.

\Vr >a\\ above that copper sulphate loses four- fifths of its water at
100° and the remainder at 240°. This means that there are two definite
compounds of water with the anhydrous salt. Washing soda or car-
bonate of sodium, Na2CO3, separates out as crystals, Na2CO3,lCH2O,
containing G'2 '9 p.c. of water by weight, from its solutions at the
ordinary temperature. When a solution of the same salt deposits crystals
at a low temperature, about — 20°, then these crystals contain 71*8 parts
of water per 2S-2 parts of anhydrous salt. Further, the crystals are
obtained together with ice, and are left behind when it melts. If
ordinary soda, with 62-9 p.c. of water, be cautiously melted in its own
water of crystallisation, there remains a salt, in a solid state, containing
only 14-5 p.c. of water, and a liquid is obtained which contains the solu-
tion of a salt which separates out crystals at 34°, which contain 46 p.c.
of water and do not effloresce in air. Lastly, if a supersaturated solu-
tion of soda be prepared, then at temperatures below 8° it deposits
crystals containing 54'3 p.c. of water. Thus there are known ag many
as five compounds of anhydrous soda with water ; and they are dis-
similar in their properties and crystalline form, and even in their
solubility. "We will mention that the greatest amount of water in the
crystals corresponds with a temperature of 20°, and the smallest to the
highest temperature. There is apparently no relation between the
above quantities of water and the salts, but this is only because in each
case the amount of water and anhydrous salt was given in percentages,
but if it be calculated for one and the same quantity of anhydrous salt,
or of water, a great regularity will be observed in the amounts of the
component parts in all these compounds. It appears that for 106 parts
of anhydrous salt in the crystals separated out at — 20° there are 270
parts of water ; in the crystals obtained at 15° there are 180 parts of
water ; in the crystals obtained from a supersaturated solution 126 parts,
in the crystals which separate out at 34°, 90 parts, and the crystals with
the smallest amount of water, 18 parts. On comparing these quantities
of water it may be easily seen that they are in simple proportion to each
other, for they are all divisible by 18, and are in the ratio 15 : 10 : 7 : 5 : 1.
Naturally, direct experiment, however carefully it be conducted, is
hampered with errors, but taking these inevitable errors into con-
sideration, it will be seen that for a given quantity of an anhydrous
substance there occur, in several of its compounds with water,
quantities of water which are in very simple multiple proportion. This
is observed in, and is common to, all definite chemical compounds.
This rule is called the law of multiple proportions. It was discovered

108 PRINCIPLES OF CIIK.Al

by Dal ton, and will be evolved in detail in tiie farther exposition in
this work. For the present we will only state that the law of definite
composition enables the composition of substances to be expressed by
formulae, and the law of multiple proportions permits the application
of co-efficients in a weight of whole numbers, in formulae. Thus the
formula, Na2CO3, 10H.2O, directly shows that in this crystallo-hydrate
there are 180 parts of water to 106 parts by weight of the anhydrous
salt, because the formula of soda, Xa.,C03, directly answers to a weight
of 106, and the formula of water to 18 parts, by weight, which are hnv
taken 10 times.

Tn the above examples of the combinations of water, we saw the
gradually-increasing intensity of the bond between water and a
substance with which it forms a homogeneous compound. There is a
series of such compounds with water, in which the water is held with
very great force, and is only given up at a very high temperature, and
sometimes cannot be separated by any degree of heat without the entire
decomposition of the substance. In these compounds there is generally
. no outward sign whatever of their containing water. A perfectly new
substance is formed from an anhydrous substance and water, in which
sometimes the properties of neither one nor the other substance are
observable. In the majority of cases, a considerable amount of heat is
evolved in the formation of such compounds with water. Sometimes
the heat evolved is so intense that a red heat is produced and light
is emitted. It is hardly to be wondered at, after this, that stable
compounds are formed by such a combination. Their decomposition
requires great heat ; a large amount of work is necessary to separate
them into their component parts. All such compounds are definite,
and, generally, completely and clearly definite. The number of such
definite compounds with water or hydrates, in the narrow sense of the
word, is generally inconsiderable for each anhydrous substance ; in the
greater number of cases, there is formed only one such combination of a
substance with water, one hydrate, having so great a stability. The
water contained in these compounds is often called water of constitution
— i.e., water which enters into the structure or composition of the given
substance. By this it is desired to express, that in other cases tin-
molecules of water are as it were separate from the molecules of that
substance with which it is combined. It is supposed that in the forma-
tion of hydrates this water, even in the smallest particles, forms one
complete whole with the anhydrous substance. Many examples of
the formation of such hydrates might be cited. The most familiar
example in practice is the hydrate of lime, or so-called * slaked ' lime.
Lime is prepared by burning limestone, by which the carbonic anhydride

(>N WATEB AND ITS COMPOUNDS 109

is expelled fnmi it, and there remains a \\liitc stony mass, whi«-h U
dense, compact, and rather tenacious. Lime is usually sold in t In-
form, and hears the name of 'quick' or 'unslaked' lime. If water be
poured over such lime, a great rise in temperature is remarked either
directly, or after a certain time. The whole mass becomes hot, part of
the water is evaporated, the stony mass in absorbing water crumbles into
ponder, and if the water be taken in sufficient quantity and the lime
be pure and well burnt, not a particle of the original stony mass is left —
it all crumbles into powder. If the water be in excess, then naturally
a portion of it remains and forms a solution. This process is called
' slaking ' lime. Slaked lime is used in practice in intermixture with
sand as mortar. Slaked lime is a definite hydrate of lime. If it is
dried at 100° it retains 24-3 p.c. of water. This water can only be
expelled at a temperature above 400°, and then quicklime is re-obtained.
The heat evolved in the combination of lime with water is so intense
that it can set fire to wood, sulphur, gunpowder, &c. Even on mixing
lime with ice the temperature rises to 100°. If lime be melted with a
small quantity of water in the dark, a luminous effect is observed. But,
nevertheless, water may still be separated from this hydrate.71 If
phosphorus be burnt in dry air, a white substance called ' phosphoric
anhydride ' is obtained. It combines with water with such energy, that
the experiment must be conducted with great caution. A red heat is
produced in the formation of the compound, and it is impossible to
separate the water from the resultant hydrate at any temperature.
The hydrate formed by phosphoric anhydride is a substance which is
totally undecomposable into its original component parts by this action
of heat. Almost as energetic a combination occurs when sulphuric
anhydride, SO3, combines with water, forming its hydrate, sulphuric
acid, H2SO,. In both cases definite compounds are produced, but
the latter substance, as a liquid, and capable of decomposition by heat,
giving off the vapour of its volatile anhydride even at the ordinary
temperature, forms an evident link with solutions, and, with an
excess of water, it gives, as a soluble substance, a true solution.
If 80 parts of sulphuric anhydride retain 18 parts of water, this
water cannot be separated from the anhydride, even at a tempera-
ture of 300°. It is only by the addition of phosphoric anhy-
dride, or by a series of chemical transformations, that this water can be
separated from its compound with sulphuric anhydride. Oil of vitriol,

71 In combining with water one part by weight of lime evolves 245 units of heat. A
high temperature is obtained, because the specific heat of the resulting product is small.
Sodium oxide, NaoO, in reacting on water, H2O, and forming caustic soda (sodium
hydroxide), NaHO, evolves 552 units of heat for each part by weight of sodium oxide.

110 PRINCIPLES OF CIIE.M:

or sulphuric acid, is such a compound. If a larger proportion of water
be taken, it will combine with the H2SO, ; for instance, if M parts of
water per 80 parts of sulphuric anhydride be taken, a compound is
formed which crystallises in the cold, and melts at -+- 8°, whilst oil of vitriol
does not solidify at even — 30°. If still more water be taken, the oil of
vitriol will dissolve in the remaining quantity of water. An evolution
of heat takes place, not only on the addition of the water of constitu-
tion, but in a less degree on further additions of water.72 And
therefore there is no distinct boundary, but only a gradual transition,
between those chemical phenomena which are expressed in the forma-
tion of solutions and those which take place in the formation of the
most stable hydrates.73

72 The diagram given in note 28 shows the evolution of heat <m the mixture of
sulphuric acid, or mono-hydrate (HoSO4, i.e. SOs + H-jO), with different quantities of uat ri-
per 100 vols. of the resultant solution. Per 98 grams of sulphuric acid iH..SO(l there are
evolved, on the addition of 18 grams of water, 6,379 units of heat ; with <l<ml>le or three
times the quantity of water 9,418 and 11,187 units of heat, and with an infinitely large
quantity of water 17,860 units of heat, according to the determinations of Thomsen. He
also showed that when HoSO4 is formed from SO3 ( = 80) and H.2O ( = ]KI. 21. ms units of
heat are evolved per 98 parts by weight of the resultant sulphuric acid.

" Thus, for different hydrates the stability with which they hold water is very dis-
similar. Certain hydrates hold water very loosely, and in combining with it evolve
little heat. From other hydrates the water cannot be separated by any degree of heat,
even if they are formed from anhydrides (i.e., anhydrous substances) and water with
little evolution of heat; for instance, acetic anhydride in combining with water evolves an
inconsiderable amount of heat, but the water cannot then be expelled from it. If the
hydrate (acetic acid) formed by this combination be strongly heated it either volatilises
Avithout change, or decomposes into new substances, buj> it does not again yield the original
substances — i.e., the anhydride and water. Here is an instance which gives the reason
for calling the water entering into the composition of the hydrate, water of constitution.
Such, for example, is the water entering into the so-called caustic soda or sodium
hydroxide (see note 71). But there are hydrates which easily part with their water; yet
this water cannot be considered as water of crystallisation, not only because sometimes
such hydrates have no crystalline form, but also because, in perfectly analogous cases,
very stable hydrates are formed, which are capable of particular kinds of chemical
reactions, as we shall learn afterwards. In a word, there is not a distinct boundary
either between the water of hydrates and of crystallisation, or between solution and
hydration.

It must be observed that in separating from an aqueous solution, many substances,
without having a crystalline form, hold water in the same unstable state as in crystals ;
only this water cannot be termed 'water of crystallisation' if the substance which
separates out has no crystalline form. The hydrates of alumina and silica are examples
of such unstable hydrates. If these substances are separated from an aqueous solu-
tion by a chemical process, then they always contain water, and when dried at a
definite temperature, so that the hvgroscopic water may pass off, these substances hold
water in a definite proportion. The formation of a new chemical compound containing
water is here particularly evident, for alumina and silica in an anhydrous stat • have
properties differing from those they show when combined with water, and do not combine
directly with it. The entire series of colloids on separating from water form similar
compounds with it, which have the aspect of solid substances generally, without crystal-
line structure. Besides which, colloids retain water in other different states (srr notes :>7

<)N W.\TKK AND ITS COMPOUNDS 111

\Vt- liave thus considered many aspects and decrees of combination
of various substances with water, or instances of the compounds of
water, when it and other substances form new homogeneous substances,
which in this case will evidently be complex — i.e., made up of different
substances — and although they are homogeneous, yet it must be admitted
that in them there exist those component parts which entered into their
composition, inasmuch as these parts may be re-obtained from them. It
must not be imagined that water really exists in hydrate of lime, any
more than that ice or steam exists in water. When we say that water
occurs in the composition of a certain hydrate, we only wish to point
out that there are chemical transformations in which it is possible to
obtain that hydrate by means of water, and other transformations in
which this water may be separated out from the hydrate. This is all
simply expressed by the words, that water enters into the composition
of this hydrate. If a hydrate be formed by feeble bonds, and be decom-
posed at even the ordinary temperature, then the water appears as one
of the products of dissociation, which in all likelihood is the case in
solutions, and forms the fundamental distinction between them and
other hydrates in which the water is combined with greater stability
and forms a solid substance.

and 18), and most often form gelatinous masses. Water is held in a considerable quan-
tity in solidified glue or boiled albumin. It cannot be expelled from them by pressure ;
hence, in this case there has ensued some kind of combination of the substance with water,
This water, however, is easily separated by drying ; but not the whole of it, a portion
being retained, and this portion belongs, as they say, to the hydrate, although in this
CUM' it is very difficult, if possible, to obtain definite compounds. The absence of any
distinct boundary lines between solutions, crystallo-hydrates, and ordinary hydrates
above referred to, is very clearly seen in such examples.

112 PRINCIPLES OF CHEMISTRY

CHAPTER II

THE COMPOSITION OF WATER, HYDROGEN

THE question now arises, Is not water itself a compound substance ?
Cannot it be formed by the mutual combination of some component
parts ? Cannot it be broken up into its component parts ? There can-
not be the least doubt that if it does split up, and if it is a compound,
then it is a definite one characterised by the stability of the union
between those component parts from which it is formed. From the
fact alone that water passes into all physical states as a homogeneous
whole, without in the least varying in its properties and without split-
ting up into its component parts (neither solutions nor many hydrates
can be distilled— they are split up), we must already conclude, from this
fact alone, that if water is a compound then it is a stable and definite
chemical compound. Like many other great discoveries in the province
of chemistry, it is to the end of the last century that we are indebted
for the important discovery that water is not a simple substance, that
it is composed of two substances like a number of other compound sub-
stances. This was proved by two of the methods by which the com-
pound nature of bodies may be determined as self-evident ; by analysis
and by synthesis — -that is, by a method of the decomposition of water
into, and of the formation of water from, its component parts. In 1781
Cavendish first obtained water by burning hydrogen in oxygen, both of
which gases were already known to him. He concluded from this that
water was composed of two substances. But he did not make more
accurate experiments, which would have shown the relative quantities
of the component parts in water, and which would have determined its
complex nature with certainty. Although his experiments were the
first, and although the conclusion he drew from them was true, yet such
novel ideas as the complex nature of water are not easily recognised so
long as there is no series of researches which entirely and indubitably
proves the truth of such a conclusion. The fundamental experiments
which proved the complexity of water by the method of synthesis, and
of its formation from other substances, were made in 1789 by Monge,

THE ro.MI'OSlTlnN OF WATEU, HYDROGEN' 118

Lavoisier, Fourcroy, and Vauquelin. They obtained four ounces of
water by burning hydrogen, and found that water consists of 15 parts
of hydrogen and 85 parts of oxygen. It was also proved that the
weight of water formed was equal to the sum of the weights of the
component parts entering into its composition ; consequently, water con-
tains all the matter entering into oxygen and hydrogen. The com-
plexity of water was proved in this manner by a method of synthesis.
But we will turn to its analysis — i.e., to its decomposition into its com-
ponent parts. The analysis may be more or less complete. Either
both component parts may be obtained in a separate state, or else
only one is separated and the other is converted into a new compound
in which its amount may be determined by weighing. This will be a
reaction of substitution, such as is often taken advantage of for
analysis. The first analysis of water was thus conducted in 1784 by
Lavoisier and Meusnier. The apparatus they arranged consisted of a
glass retort containing water, naturally purified, and whose weight had
been previously determined. The neck of the retort was inserted into
a porcelain tube, placed inside an oven, and heated to a red heat by
charcoal. Iron filings, which decompose water at a red heat, were
placed inside this tube. The end of the tube was connected with a
worm, for condensing any water which might pass through the tube
unclecomposed. This condensed water was collected in a separate flask.
The gas formed by the decomposition was collected over a water bath
in a bell jar. The aqueous vapour in passing over the red-hot iron was
decomposed, and a gas was formed from it whose weight could be
determined from its volume, its density being known. Besides the
water which passed through the tube unaltered, a certain quantity of
water disappeared in the experiment, and this quantity, in the experi-
ments of Lavoisier and Meusnier, was equal to the weight of the gas
which was collected in the bell jar plus the increase in weight of the
iron filings. Hence the water was decomposed into a gas, which was
collected in the bell jar, and a substance, which combined with the
iron ; consequently, it is composed of these two component parts. This
was the first analysis of water ever made ; but here only one (and not
both) of the gaseous component parts of water was collected separately.
Both the component parts of water can, however, be simultaneously
obtained in a free state. For this purpose the decomposition is brought
about by a galvanic current or by heat, as we shall learn directly.1

1 The first experiments of the synthesis and decomposition of water did not afford,
however, an entirely convincing proof that water was composed of hydrogen and oxygen
only. Davy, who investigated the decomposition of water by the galvanic current,
thought for a long time that, besides the gases, an acid and alkali were also obtained.

VOL. I. I

114 PRINCIPLES OF CHEMISTRY

Water is a bad conductor of electricity— that is, pure water does
not transmit a feeble current ; but if any salt or acid be dissolved in
it, then its conductivity increases, and on the passage of a current
through acidified water it is decomposed into its component parts.
Some sulphuric acid is generally added to the water. By immersing
platinum plates (electrodes) in this water (platinum is chosen because
it is not acted on by acids, whilst many other metals are chemically
acted on by acids), and connecting them with a galvanic battery, it
will be observed that bubbles of gas appear on these plates. The gas
which separates is called detonating gas? because, on approaching a
light, it very easily explodes.3 What takes place is as follows : — First,
the water, by the action of the current, is decomposed into two gases.
The mixture of these gases forms detonating gas. When detonating
gas is brought into contact with an incandescent substance — for instance,
a lighted taper — the gases re-combine, forming water, the combination
being accompanied by a great evolution of heat, and therefore the
vapour of the water formed expands considerably, which it does very
rapidly, and as a consequence of which an explosion takes place -that
is, sound and increase of pressure, and atmospheric commotion, as in
the explosion of gunpowder.

In order to discover what gases are obtained by the decom-
position of water, the gases which separate at each electrode must
be collected separately. For this purpose a V-shaped tube is taken ;
one of its ends is open, and the other fused up. A platinum wire,
terminating inside the tube in a plate, is fused into the closed end ;

He was only convinced of the fact that water contains nothing but hydrogen and oxygen
by a long series of researches, which showed him that the appearance of an acid and
alkali in the decomposition of water proceeds from the presence of impurities (especially
from the presence of ammonium nitrate) in water. A final understanding of the com-
position of water is obtained from the determination of the quantities of the component
parts which enter into its composition. It will be seen from this how many data are
necessary for proving the composition of water — thai is, of the transformations of
which it is capable. What has been said of water refers to all other compounds ; the
investigation of each one, the entire proof of its composition, can only be obtained by the
juxtaposition of a large mass of data referring to it.

2 This gas is collected in a voltameter.

3 In order to observe this explosion without the slighest danger, it is best to proceed
in the following manner. Some soapy water is prepared, so that it easily forms soap
bubbles, and it is poured into an iron trough. In this water, the end of a gas-conducting
tube is immersed. This tube is connected with any suitable apparatus, in which
detonating gas is evolved. Soap bubbles, full of this gas, are then formed. If the
apparatus in which the gas is produced be then removed (otherwise the explosion might
travel into the interior of this apparatus), and a lighted taper be brought to the soap
bubbles, a very sharp explosion takes place. The bubbles should be small to avoid any
danger ; ten, each about the size of a pea, suffice to give a sharp report, like a pistol
shot.

THK COMPOSITION ol- WATKK. HYDROGEN 115

the closed end is entirely filled with water 4 acidified with sulphuric
acid, and another platinum wire, terminating in a plate, is immersed in
the open end. If a current from a galvanic battery be now passed
through the wires an evolution of gases will be observed, and the gas
which is obtained in the open branch mixes with the air, while that in
the closed branch accumulates above the water. As this gas accumu-
lates it displaces the water, which continues to descend in the closed
and ascend into the open branch of the tubes. When the water, in
this way, reaches the top of th'e open end, the passage of the current is
stopped, and the gas which was evolved from one of the electrodes only
is obtained in the apparatus. By this means it is easy to prove that a
particular gas appears at each electrode. If the closed end be con-
nected with the negative pole — i.e., with that joined to the zinc — then
the gas collected in the apparatus is capable of burning. This may be
demonstrated by the following experiment : — The bent tube is taken
off the stand, and its open end stopped up with the thumb and inclined
in such a manner that the gas passes from the closed to the open end.
It will then be found, on applying a lighted lamp or taper, that the
gas burns. This combustible gas is hydrogen. If the same experiment
be carried on with a current passing in the opposite direction — that is,
if the closed end be joined up with the positive pole (i.e., with the
carbon, copper, or platinum), then the gas which is evolved from it does
not burn of itself, but it supports combustion very vigorously, so that
in it a smouldering taper immediately bursts into flame. This gas,
which is collected on the anode or positive pole, is oxygen, which is
obtained, as we saw before (in the Introduction), from mercury oxide
and is contained in air.

Thus in the decomposition of water oxygen appears at the positive
pole and hydrogen at the negative pole, so that detonating gas will be
a mixture of them both. Hydrogen burns in air from the fact that in
doing so it re-forms water, with the oxygen of the air. Detonating
gas explodes from the fact that the hydrogen burns in the oxygen
mixed with it. It is very easy to measure the relative quantities of one
Miid the other gas which are evolved in the decomposition of water.
For this purpose a funnel is taken, whose orifice is closed by a cork
through which two platinum wires pass. These wires are connected
with a battery. Acidified water is poured into the funnel, and a glass
cylinder full of water is placed over the end of each wire (fig. 18).
On passing a current, hydrogen and oxygen collect in these cylinders,

4 In order to fill the tube with water, it is turned up, so that the closed end points
(1 >\v 11 wards and the open end upwards, and water acidified with sulphuric acid is poured
into it.

i 2

116

PRINCIPLES OF CHEMISTKV

and it will easily be seen that two volumes of hydrogen are evolved for
every one volume of oxygen. This signifies that, in decomposing, water
gives two volumes of hydrogen and one volume
of oxygen.

Water is also decomposed into its com-
ponent parts by the action of heat. At the
melting point of silver (960°), and in its pre-
sence, water is decomposed and the oxygen
absorbed by the molten silver, which dissolves
it so long as it is liquid. But directly the
silver solidifies the oxygen is expelled from it.
However, this experiment is not entirely con-
vincing ; it might be thought that in this case
the decomposition of the water did not proceed
from the action of heat, but from the action
of the silver on water — that silver decom-

p°ses water' takins UP the

t s m-

determining the relation be- possible to directly show the decomposition

tween the volumes of hydrogen * . * L

and oxygen. of water by the action of heat, because the

component parts of water, if they remain

together, re-combine with a fall of temperature, and give water back
again. For instance, if steam be passed through a red-hot tube,
whose internal temperature attains 1,000°, then a portion5 of the water
decomposes into its component parts, forming detonating gas. But on
passing into the cooler portions of the apparatus this detonating gas
again reunites and forms water. The hydrogen and oxygen obtained
combine together at a lower temperature.6 Apparently the problem —

5 As water is formed by the combination of oxygen and hydrogen, the reaction evolving
much heat, and as it can also be decomposed, therefore this reaction is a reversible
one (see Introduction), and consequently at a high temperature the decomposition of
water cannot be complete — it is limited by the opposite reaction. Strictly speaking, it is
not known how much water is decomposed at a given temperature, although many efforts
(Bunsen, and others) have been made in various directions to solve this question. Not
knowing the coefficient of expansion, and the specific heat of gases at such high tem-
peratures, renders all calculations (from observations of the pressure on explosion)
doubtful.

6 Grove, about 1840, observed that a platinum wire fused in the flame of detonating
gas — that is, having acquired the temperature of the formation of water — and having
formed a molten drop at its end which fell into water, evolved detonating gas — that
is, decomposed water. It therefore follows that water already decomposes at the tem-
perature of its formation. At that time, this formed a scientific paradox ; this we shall
unravel only with the development of the conceptions of dissociation, introduced into
science by Henri Sainte-Claire Deville, about 1850. These conceptions form an im-
portant epoch in science, and their development is one of the problems of contemporary
chemistry. The essence of the matter is that, at high temperatures, water exists but also
decomposes, just as a volatile liquid, at a certain temperature, exists both as a liquid and

THE COMPOSITION OF WATKK, II VJ>1;< >< i F.N

117

to show the decomposability of water at high temperatures— is un-
attainable. It was considered as such before Henri Sainte- Claire
Deville (in the fifties) introduced the conception of dissociation into
chemistry, as of a change of chemical state resembling evaporation, if
decomposition be likened to boiling, and before he had demonstrated
the decomposability of water by the action of heat in an experiment
which will presently be described. In order to demonstrate clearly the
dissociation of water, or its decomposability by heat, at a temperature
approaching that at which it is formed (as a volatile liquid, at a given
temperature, can be either in a liquid or vaporous condition) it was
necessary to separate the hydrogen from the oxygen at a high tempe-
rature, without allowing the mixture to cool. Deville took advantage
of the difference between the densities of hydrogen and oxygen.

A wide porcelain tube p (fig. 19) is placed in a furnace giving a

FK;. 19. Decomposition of water by the action of heat, and the separation of the hydrogen formed by
its permeating through a porous tube.

strong heat (it should be heated with small pieces of good coke). In
this tube there is inserted a second tube T, of less diameter, and made
of unglazed earthenware and therefore porous. The ends of the tube
are luted to the wide tube, and two tubes, c and c', are inserted into
the ends, as shown in the drawing. With this arrangement it is
possible for a gas to pass into the annular space between the walls
of the two tubes, from whence it can be collected. Steam from

as a vapour. Similarly as a volatile liquid saturates a space, attaining its maximum
tension, so also the products of dissociation have their maximum tension, and once that is
attained decomposition ceases, just as evaporation ceases. Under like conditions, if
the vapour be allowed to escape (and therefore its partial pressure be diminished), evapora-
tion recommences, so also if the products of decomposition be removed, decomposition
again continues. These simple conceptions of dissociation introduce infinitely varied
consequences into the mechanism of chemical reactions, and therefore we shall have
occasion to return to them very often.

118 PRINCIPLES OF CHEMISTRY

a retort or flask is passed through the tube D, into the internal porous
tube T. This steam on entering the red hot space is decomposed into
hydrogen and oxygen. The densities of these gases are very different,
hydrogen being sixteen times lighter than oxygen. Light gases, as \ve
saw above, penetrate through porous surfaces very much more rapidly
than denser gases, and therefore the hydrogen passes through the pores
of the tube into the annular space very much more rapidly than the
oxygen. The hydrogen which separates out into the annular space
can only be collected when this space does not contain any oxygen.
If any air remains in this space, then the hydrogen which separates
out will combine with its oxygen and form water. For this reason a
gas incapable of supporting combustion — for instance, nitrogen— is pre-
viously passed in the annular space. Thus the nitrogen is passed
through the tube c, and the hydrogen, separated from the steam, is
collected through the tube c', and will be partly mixed with nitrogen.
A certain portion of the nitrogen will penetrate through the pores of
the unglazed tube into the interior of the tube T. The oxygen will
remain in this tube, and the volume of the remaining oxygen
will be half that of the volume of hydrogen which separates out from
the annular space. Part of the oxygen will also penetrate through
the pores of the tube ; but, as was said before, a much smaller quan-
tity than the hydrogen, and as the density of oxygen is sixteen
times greater than that of hydrogen, the volume of oxygen which
passes through the porous walls will be four times less than the volume
of hydrogen (the quantities of gases passing through porous walls are
inversely proportional to the square roots of their densities). The
oxygen which separates out into the annular space will combine, at a
certain fall of temperature, with the hydrogen ; but as each volume of
oxygen only requires two volumes of hydrogen, whilst at least four
volumes of hydrogen will pass through the porous walls for every
volume of oxygen that passes, therefore, part of the hydrogen will
remain free, and can be collected from the annular space. A corre-
sponding quantity of oxygen remaining from the decomposition of the
water can be collected from the internal tube.

The decomposition of water is produced much more easily by a
method of substitution, taking advantage of the affinity of substances
for the oxygen or the hydrogen of water. If a substance be added to
water, which takes up the oxygen and replaces the hydrogen — then we
shall obtain the latter gas from the water. Thus with sodium, water
gives hydrogen, and with chlorine, which takes up the hydrogen,
oxygen is obtained.

Hydrogen is evolved from water by many metals, which are capable

TIIK COMPOSITION OK WATKK. HYDROGEN 119

of forming oxides (rusts or earths, as Stahl called them) in air— that is,
which are capable of burning or combining with oxygen. The capacity
of metals for combining with oxygen, and therefore for decomposing
water, or for the evolution of hydrogen, is very dissimilar.7 Among
metals, potassium and sodium have the greatest energy in this respect.
The first occurs in potash, the second in soda. They are both lighter than
water, soft, and easily change in air. By bringing one or the other of
them in contact with water at the ordinary temperature,8 a quantity of

7 In order to demonstrate the difference of the .affinity of oxygen for different
elements, it is enough to compare the amounts of heat which are evolved in their combi-
nation with 16 parts by weight of oxygen ; in the case of sodium (when Na2O is formed,
or 46 parts of Na combine with 16 parts of oxygen, according to Beketoff) 100,000 calories
(or units of heat) are evolved, for hydrogen (when water, H2O, is formed) 69,000 calories,
for iron (when the oxide, FeO, is formed) 69,000, and if the oxide FeoO3 is formed,
64,000 calories, for zinc (ZnO is formed) 86,000 calories, for lead (when PbO is formed)
51,000 calories, for copper (when CuO is formed) 38,000 calories, and for mercury (HgO is
formed) 31,000 calories.

These figures cannot correspond directly with the magnitude of the affinities, for the
physical and mechanical side of the matter is very different in the different cases.
Hydrogen is a gas, and, in combining with oxygen, gives a liquid ; consequently it changes
its physical state, and, in doing so, evolves heat. But zinc and copper are solids, and,
in combining with oxygen, give solid oxides. The oxygen, previously a gas, now passes
into a solid or liquid state, and, therefore, also must have given up its store of heat in
forming oxides. As we shall afterwards see, the degree of contraction (and conse-
quently of mechanical work) was different in the different cases, and therefore the
figures expressing the heat of combination cannot directly depend on the affinities, on
the loss of internal energy previously in the elements. Nevertheless, the figures above
cited correspond, in a certain degree, with the order in which the elements stand hi
respect to their affinity for oxygen, as may be seen from the fact that the mercury oxide,
which evolves the least heat (among the above examples), is the least stable, is easily
decomposed, giving up its oxygen ; whilst sodium, the formation of whose oxide is accom-
panied by the greatest evolution of heat, is able to decompose all the other oxides, taking
up their oxygen. In order to generalise the connection between affinity and the evolu-
tion and the absorption of heat, which is evident in its general features, and was firmly
established by the researches of Favre and Silberman (about 1840), and then of Thomsen
(in Denmark) and Berthelot (in France), many investigators, especially the one last
mentioned, established the law of maximum work. This states that only those chemical
reactions take place of their own accord in which the greatest amount of chemical
(latent, potential) energy is transformed into heat. But, in the first place, we are not
able, judging from what has been said above, to distinguish that heat which corresponds
with purely chemical action from the sum total of the heat observed in a reaction (in the
calorimeter) ; in the second place, there are evidently endothermal reactions which
proceed under the same circumstances as exothermal (carbon burns in the vapour of
sulphur with absorption of heat, whilst in oxygen it evolves heat) ; and, in the third
place, there are reversible reactions, which when taking place in one direction evolve
heat, and when taking place in the opposite direction absorb it ; and, therefore, the
principle of maximum work in its elementary form is not supported by science. But the
subject continues to be developed, and will probably lead to a general law, such as
thermal chemistry does not at present possess.

8 If a piece of metallic sodium be thrown into water, it floats on it (owing to its light-
ness), keeps in a state of continual movement (owing to the evolution of hydrogen on
nil sides), and immediately decomposes the water, evolving hydrogen, which can be

hydrogen, ct irre-pt Hiding with tin- amount of the metal taken, mav be
difectlv obtained. < )nc i_;ram of hvdro^vn. occupying' a \olmiic of

ll'lt* litre-, at () and <<>(>nim.. is evolved jicr -"i!1 plains of i>< itassium,
or •_!•"> u'i'am- of sodium. Tin- phenomenon niav lie observed in tlie
following way :a solution of sodium in mercurv or ' sodium {inuilgain,;
as it is "jeiierallv called i- poured into a vessel containing water, and
ownm' TO its weight sinks to the bottom : the -odium held in the
nierciir\" 1 hen acts on the water like pure sodium, liberating hydrogen.

liif 1 1 n MTU i v doe- not act here, a i id the sail it1 amount of it as \vas taken
for dissolving the sodium i- obtained in the residue. The hydrogen i-
evolved little by little in the form of bubbles, which pa-- through
the liquid.

]>evoiid the hydrogen evolved and a -olid substance, which remains
in solution (it may be obtained by evaporating the iv-ultant solution),
no other products are here obtained. Consequently, from the t wo sub-
stances (water and sodium) taken, the same number (if new substances
(hvdrogen and the substance dis-<il\ed in water) have been obtained,
'fi'oin \\hieh we ma\" conclude that the reaction which here takes place
i- a reaction of double decomposition or of substitution. The sub-
stance- taken were, sodium in a. free -late, and water, which consists of
two ^'ases, hydrogen and oxygen. rl'he pnduct- obtained \\cre.
hvdro^en in the free state and a solid, which i- nothing else lr,;t the so-
ealled dtisticsoda (-odium hvdroxide), \\hich is made ti]» of sodium.
oxvgen. and half of the hydrogen contained in the water. Therefore,
the -ubstitution took" jilace between the hydrou'en and the sodium,
namely half of the hvdrogen in the water was replaced bv the sndiuin.
and was evolved iii a free state. < >n this basis it mav be said that
ca.u si ic soda is nothing else but \\ater. in \\'hich half the hvdrogen
is replaced bv metallic sodium. The reaction which Takes place

Illdll IllilV. lld\Vc\ I'l'. |i-:nl Id ill! i-Xjili - "II -llc.lllil tllr -ndillin -lii k In

• , •, i-l. MIM! IM.-III tu act <nlli«- limited inn - ..I' \\ at"T i icdi;itf]\ adjucciit

- • |. NaUO I'LVIUS witll N;l. \a..n.wll it'll art- till lllf \\atrl'. r\<il\ I II-
It n] . 'I .' i dc-i-i i|il|" i-il ii ill nl Water l'\ MiilillMI lllil\ lie lieltrr (lellltill-trateil.

eati-i f.-ty. iii the full.. wiii.i iiiaiini I'. lulu ;i la-- r\ linder lille.l \\ iih mer

elU-y. • ' ed ill .: I II. • l'( '1 1 1'\ I,,, ill. \\atiT I- lil'-l i 1 1 1 1'ud 1 le. -d . ullirll W 1 1 1. UW i I1LT t ( I its

• , , 1 1 ' • t..|i. and 1 h.'ll a |>iere ui" -uiliiiiii V. l'a|ij.ed in paper is illt nxillced with

. ;, . hi.-l • .. nail.-, iii.il e\.,l\. 1, Mir, ,.•..!,. which riili. ••!-. in tlie cylinder, and

. ;, :, ,| :::, , :'. . :.-.',.. i • i, : i.i-i-n c.miplet. d. 'I'll- -afc-l method ..f inakin-

,.-,p, , • ...- ful|u\\ -. The .udiiiin 'iN am d fi'um the n.i|.litli.i iii \\ Inch

,,•••:. rpi i • . . upper -au/c and hdd l.\ luiveps. «.r el.-e held in

!,,!•< i-p- ,,t the end .1 \\hicli ., mall cupper , , . attached. ,md i- then hdd under

'I'll,. , .. | '.dru^'cli ;_'.,e 1.11 ijlliet i\ 1 it Ilia \ he cullected ill a hell

,11- aii'l thdi IL'liti-l.

THE COMPOSITION <>K WATKK. HYDROGEN 121

may be expressed by the equation : H2O 4- Na=NaHO + H ; the mean-
ing of this is clear from what has been already said.'1

Sodium and potassium act on water at the ordinary temperature.
< >ther heavier metals only act on it with a rise of temperature, and
then not so rapidly or vigorously. Thus magnesium and calcium only
liberate hydrogen from water at its boiling point, and zinc and iron only
at a red heat, whilst a whole series of heavy metals, such as copper, lead,
mercury, silver, gold, and platinum, do not in the least decompose
water at any temperature, and do not replace its hydrogen.

From this it is clear that hydrogen may be obtained by the decom-
position of steam by the action of iron (or zinc) with a rise of tempera-
ture. The experiment is conducted in the following manner : pieces
of iron (tilings, nails. Arc.), are laid in a porcelain tube, which is then

9 This reaction is vigorously exothermal. If a sufficient quantity of water be taken
the whole of the sodium hydroxide, NaHO, formed is dissolved, and about 42,500 units of
heat are evolved per 23 grams of sodium taken. As 40 grams of sodium hydroxide
arc produced, and they in dissolving, judging from direct experiment, evolve about 10,000
calories ; therefore, without an excess of water, and without the formation of a solution,
the reaction Xa + H2O = H + NaHO would evolve about 32,500 calories. We shall after-
wards learn that hydrogen contains in its smallest isolable particles H2 and not H,
and therefore it follows that the reaction should be written thus — 2Na + 2H2O = H2 +
'JXaHO, and it then corresponds with an evolution of heat of 4- 05,000 calories. And as
X. X. Beketoff showed that Na^O, or anhydrous oxide of sodium, forms the hydrate, or
sodium hydroxide (caustic soda), 2NaHO, with water, evolving about 35,500 calories, there-
fore the reaction 2N a + H2O = H2 + NaoO corresponds to 29,500 calories. This quantity
of heat is less than that which is evolved in combining with water, in the formation
of caustic soda, and therefore it is not to be wondered at that the hydrate, NaHO, is always
formed and not the anhydrous substance Na^O. That such a conclusion, which agrees
with facts, is inevitable is also seen from the fact that, according to Beketoff, the anhy-
drous sodium oxide, NaoO, acts directly on hydrogen,with separation of sodium Na^O -t- H =
NaHO + Na. This reaction is accompanied by an evolution of heat equal to about
3,000 calories, because Na2O + H2O gives, as we saw, 35,500 calories and Na + H>O evolves
32,500 calories. However, an opposite reaction also takes place — XaHO + Na = NaoO + H
(both with the aid of heat) — consequently, in this case heat is absorbed. In this we see
an example of calorimetric calculations and the small use of the law of maximum work
for the general phenomena of reversible reactions, to which the case just considered
belongs. But it must be remarked that all reversible reactions evolve or absorb but
little heat, and judging from what has been said in Note 6 (and in Note 25 of Chap. I.),
the reason of the discrepancy between the law of maximum work and reality must
before all be looked for in the fact that we have no means of separating the heat which
corresponds with the purely chemical process from the sum total of the heat observed,
and as the structure of a number of substances is altered by heat alone and also by
contact, we can scarcely hope that the time approaches when such a distinction will be
possible. A heated substance, in point of fact, has no longer the original energy of its
atoms — that is, the act of heating not only alters the store of movement of the molecule^
but also of the atoms forming the molecules, in other words, it makes the beginning of or
preparation for chemical change. From this it must be concluded that thernio-chemistry,
or the study of the heat accompanying chemical transformations, cannot be identified
with chemical mechanics. Thermo-chemical data form a part of it, but they alone
cannot give it.

122 PRINCIPLES OF CHEMISTRY

subjected to a strong heat and steam passed through it. The steam,
coming into contact with the iron, gives up its oxygen to it, and thus
the hydrogen is set free and passes out at the other end of the tube
together with undecomposed steam. This method, which is historically
very significant,10 is practically inconvenient, as it requires a rather
high temperature. Further, this reaction, as a reversible one (a red-
hot mass of iron decomposes a current of steam, forming oxide and
hydrogen ; and a mass of oxide of iron, heated to redness in a stream
of hydrogen, forms iron and steam), does not proceed in virtue of the
comparatively small difference between the affinity of oxygen for iron
(or zinc), and for hydrogen, but only because the hydrogen escapes, as
it is formed, in virtue of its elasticity.11 If the oxygen compounds — that
is, the oxides — which are obtained from the iron or zinc, be able to pass
into solution, then the affinity acting in solution is added, and the
reaction may become non-reversible, and proceed with comparatively
much greater facility.12 As the oxides of iron and zinc, by themselves

10 The composition of water, as we saw above, was determined by passing steam over
red-hot iron ; the same method has been used for making hydrogen for filling balloons.
An oxide having the composition FesC^ is formed in the reaction, so that it is expressed
by the equation 3Fe + 4H^O = Fe5O4+8H. It is very important to remark that this re-
action is reversible. By heating the scoria in a current of hydrogen, water and iron
are obtained. From this it follows, from the principle of chemical equilibria, that if
there be taken iron and hydrogen, and also oxygen, but in such a quantity that
it is insufficient for combination with both substances, then it will divide itself
between the two ; part of it will combine with the iron and the other part with the
hydrogen, but a portion of both will remain in an uncombined state. Here again (see
note 9) the reversibility is connected with the small heat effect, and here again both re-
actions (direct and reverse) proceed at a red heat. But if, in the above-described re-
action, the hydrogen escapes as it is evolved, then its partial pressure does not increase
with its formation, and therefore all the iron can be oxidised by the water, which could
not take place were the iron and water heated to the temperature of reaction in a closed
vessel. In this we see the elements of that influence of mass to which we shall have
occasion to return later.

11 Therefore, if iron and water be placed in a closed space, decomposition of the water
will proceed on heating to the temperature at which the reaction 3Fe + 4H...O = Fe3O4 + 8H
commences ; but it ceases, does not go on to the end, because the conditions for a
reverse reaction are attained, and a state of equilibrium will ensue after the decomposi-
tion of a certain quantity of water. Judging from what has been said in Note 9,
something of the same kind takes place if the iron be replaced by sodium, only
then the mass of the water decomposed will be greater, and equilibrium will ensue,
with the formation of the hydrate, NaHO, and not of anhydrous oxide, NaoO — that is,
the water will remain in the form of hydrate only. With copper and lead there will be
no decomposition, either at the ordinary or at a high temperature, because the affinity of
these metals for oxygen is much less than that of hydrogen.

12 In general, if reversible as well as non-reversible reactions can take place between
substances acting on each other, then, judging by our present knowledge, the non-
reversible reactions take place in the majority of cases, which obliges one to acknowledge
the action, in this case, of comparatively strong affinities. The reaction, Zn + H3SO4 —
H2 + ZnSO4, which takes place in solutions at the ordinary temperature, is scarcely re-
versible under these conditions, but at a certain high temperature it becomes reversible,

T1IK COMPOSITION OF AVATKK. HYDROGEN 123

insoluble in water, are capable of combining with (have an affinity for)
acid oxides (as we shall afterwards fully consider), and form saline and
soluble substances, with acids, or hydrates having acid properties, hence
by the action of such hydrates, or of their aqueous solutions,13 iron
and zinc are able to liberate hydrogen with great ease at the ordinary
temperature — that is, they act on solutions of acids just as sodium acts
on water.14 Sulphuric acid, or oil of vitriol, H2S04, is usually chosen

because at this temperature zinc sulphate and sulphuric acid split up, and the action must
take place between the water and zinc. From the preceding proposition results proceed
which are in some cases verified by experiment. If the action of zinc or iron on a solu-
tion of sulphuric acid presents a non-reversible reaction, then we may by this means
obtain hydrogen in a very compressed state, and compressed hydrogen will not act on
solutions of sulphates of the above-named metals. This is verified in reality as far as
was possible in the experiments to keep up the compression or pressure of the hydro-
gen. Those metals which do not evolve hydrogen with acids, on the contrary, should, at
least at an increase of pressure, be displaced by hydrogen. And in fact Brunner showed
that gaseous hydrogen displaces platinum and palladium from the aqueous solutions of
their chlorine compounds, but not gold, and Beketoff succeeded in showing that silver
and mercury, under a considerable pressure, are separated from the solutions of certain
of their compounds by means of hydrogen. Keaction already commences under H pres-
sure of six atmospheres, if a weak solution of silver sulphate be taken ; with a stronger
solution a much greater pressure is required, however, for the separation of the silver.

15 For the same reason, many metals in acting on solutions of the alkalis displace
hydrogen. Aluminium acts particularly clearly in this respect, because its oxide gives a
soluble compound with alkalis. For the same reason tin, in acting on hydrochloric acid,
evolves hydrogen, and silicon does the same with hydrofluoric acid. It is evident that
in such cases the sum of all the affinities plays a part ; for instance, taking the action of
zinc on sulphuric acid, we have the affinity of zinc for oxygen (forming zinc oxide, ZnO),
the affinity of its oxide for sulphuric anhydride, S05 (forming zinc sulphate, ZnSO4), and
the affinity of the resultant salt, ZnSO4, for water. It is only the first-named affinity that
acts in the reaction between water and the metal, if no account is taken of those forces
(of a physico-mechanical character) which act between the molecules (for instance, the
cohesion between the molecules of the oxide) and those forces (of a chemical character)
which act between the atoms forming the molecule, for instance, between the atoms of
hydrogen giving the molecule H2 containing two atoms. I consider it necessary to
remark, that the hypothesis of the affinity or endeavour of heterogeneous atoms to enter
into a common system and in harmonious movement (i.e., to form a compound molecule)
must inevitably be in accordance with the hypothesis of forces inducing homogeneous
atoms to form complex molecules (for instance, H2), and to build up the latter into
solid or liquid substances, in which the existence of an attraction between the homo-
geneous particles must certainly be admitted. Therefore, those forces which bring about
solution must also be taken into consideration. These are all forces of one and the same
series, and in this may be seen the great difficulties surrounding the study of mole-
cular mechanics and its province — chemical mechanics.

14 The representation given above of the cause of the easy action of iron or zinc on
sulphuric acid, naturally forms a hypothesis which explains only what is observed.
It is only at first sight that this hypothesis exhibits any similarity to the hypothesis of
predisposing affinity which reigned in past times. According to that, it was supposed that
reaction takes place (and hydrogen is evolved) by reason of the affinity for the sulphuric-
acid of the oxide of zinc which might be produced, and that decomposition could
not take place without this. The influence of a force in respect to a substance \shirh lias
not been produced, but which is capable of being formed, is not clear. In the repre-
sentation introduced by me, it is acknowledged that zinc already acts on water by

124 PRINCIPLES OF CHEMISTRY

for this purpose ; from it the hydrogen is displaced by many metals with
incomparably greater facility than directly from water, and such a
displacement is accompanied by the evolution of a large amount of
heat.15 By the action of zinc or iron on sulphuric acid, hydrogen is
evolved, because the metal replaces it. When the hydrogen in sulphuric
acid is replaced by a metal, a substance is obtained which is called a
salt of sulphuric acid or a sulphate. Thus, by the action of zinc on
sulphuric acid, hydrogen and zinc sulphate, ZiiSO^, are obtained.
The latter is a solid substance, soluble in water. In order that the
action of the metal on the acid should go on regularly, and to the end,
it is necessary that the acid should be diluted with water, which dis-
solves the salt as it is formed ; otherwise the salt covers the metal,
and hinders the acid from attacking it. Usually the acid is diluted
with from three to five times its volume of water, and the metal is
covered with this solution. In order that the metal should act
rapidly on the acid, it should present a large surface, so that a maxi-
mum amount of the reacting substances may come into contact in a
given time. For this purpose the zinc is used as strips of sheet zinc,
or in the granulated form (that is, zinc which has been poured from a
certain height, in a molten state, into water). The iron should be in
the form of wire, nails, filings, or cuttings.

The usual method of obtaining hydrogen is as follows :— A certain
quantity of granulated zinc is put into a double- necked, or Woulfe's,
bottle. Into one neck a funnel is placed, reaching to the bottom of
the bottle, so that the liquid poured in may prevent the hydrogen from

itself, even at the ordinary temperature, but that the action is limited by small
masses and only proceeds at the surface. In reality, zinc, in the form of a very
fine powder, or so called ' zinc dust/ is capable of decomposing water with the
formation of oxide (hydrated) and hydrogen. The oxide formed acts 011 sulphuric acid,
water then dissolves the salt produced, and the action continues because one of the
products of the action of water on zinc, zinc oxide, is removed from the surface. One
might naturally imagine that the reaction does not proceed directly between the metal
and water, but between the metal and the acid, but such a simple representation, which
we shall cite afterwards, hides the mechanism of the reaction, and does not permit of its
actual complexity being seen.

15 According to Thomsen the reaction between zinc and a very weak solution of
sulphuric acid evolves about 38,000 calories (zinc sulphate beinjj; formed) per (55 parts
by weight of zinc ; and 56 parts by weight of iron — which combine, like (55 parts by
weight of zinc, with 16 parts by weight of oxygen — evolve about 25,000 calories (forming
ferrous sulphate, FeSO4). Paracelsus observed the action of metals on acids in the
seventeenth century; but it was not until the eighteenth century that Lemery
determined that the gas which is evolved in this action is a particular one which differs
from air and is capable of burning. Even Boyle confused it with air. Cavendish
determined the chief properties of the gas discovered by Paracelsus. At first it was
called 'inflammable air'; later, when it was recognised that in burning it gives water,
it was called hydrogen, from the Greek words for water and generator.

THE COMPOSITION <>F WATKK. HYDROGEN

125

escaping through it. The gas escapes through a special gas-conducting
tube, which is firmly tixctl. by a cork, into the other neck, and which
ids in a water bath (fig. 20), under the orifice of a glass cylinder full

FIG. 20.— Apparatus for the preparation of hydrogen from zinc and sulphuric acid.

of water.16 If sulphuric acid be now poured into the W.oulfe's bottle,
it will soon be seen that bubbles of a gas are evolved, which is hydrogen.

lti As laboratory experiments with gases require a certain preliminary knowledge, we
will describe certain practical methods for the preparation and collection of gases.
When in laboratory practice an intermittent supply of hydrogen (or other gas which is
evolved without the aid of heat) is required the apparatus represented in fig. 21 is the

FIG. 21.— A very convenient apparatus for the preparation of gases obtained without heat. It may
also replace an aspirator or gasometer.

most convenient. It consists of two bottles, having orifices at the bottom, in which
corks with tubes are placed, and these tubes are connected by an india-rubber tube
(sometimes furnished with a spring clamp). Zinc is placed in one bottle, and dilute sul-

1-26

PRINCIPLES OF CHEMISTRY

The first part of the gas evolved should not be collected, as it is
mixed with the air originally in the apparatus. This precaution

phuric acid in the other. The neck of the former is closed by a cork, which is fitted with
a gas-conducting tube with a stop-cock. If the two bottles are put in communication
with each other and the cock be opened, the acid will flow to the zinc and evolve hydro-
gen. If the cock be closed, the hydrogen will force out the acid from the bottle contain-
ing the zinc, and the action will cease. Or the vessel containing the acid may be placed
at a lower level than that containing the zinc, when all the liquid will flow into it, and in
order to start the action^the acid vessel may be placed on a higher level than the other,
and the acid will flow to the zinc. Such an arrangement presents the simplest form of a
continuously-acting apparatus, which is of great use in chemical work. It can also be
employed for collecting gases (as an aspirator or gasometer).

In laboratory practice, however, other forms of apparatus are generally employed for

FIG. 22. — Constant-acting aspirator. The tube d should be long (over 32 feet).

exhausting, collecting, and holding gases. We will here cite the most usual forms. An
aspirator usually consists of a vessel furnished with a stop-cock at the bottom. A stout

THE COMPOSITION <>K WATKK. HYDROGEN

127

should he taken in the preparation of all gases. Time must be allowed
for the gas evolved to displace all the air from the apparatus, Other-
cork, through which u glass tube passes, is fixed into the neck of this vessel. If the
vessel I)*-' tilled u}) with wnter to the cork and the bottom stop-cock be opened, then the
water will run out and draw gas in. For this purpose the glass tube is connected with
the apparatus from which it is desired to pump out or exhaust the gas.

Tib aspirator represented in fig. 22 may be recommended for its continuous
action. It consists of a tube tl which widens out at the top, the lower part being long
and narrow. In the expanded upper portion c, two tubes are sealed ; one, e, for drawing
in the gas. whilst the other, b, is connected to the water supply //*. The amount of water
supplied through the tube b must be less than the amount which can be carried off by
the tube d. Owing to this the water in the tube d will flow through it in cylinders
alternating with cylinders of gas, which will be thus carried away. The gas which is drawn
through may be collected from the end of the tube rf, but this form of pump is usually
employed where the air or gas aspirated is not to be collected. If the tube d is of con-
siderable length, say 40 ft. or more, a very fair vacuum will be produced, the amount of
which is shown by the gauge g ; it is often used for filtering under reduced pressure, as
shown in the figure. If water be replaced by mercury, and the length of the tube d be
greater than 760 mm., the aspirator may be employed as an air-pump, and all the air
may be exhausted from a limited space ; for instance, by connecting g with a hollow
sphere.

Gasholders are often used for collecting and holding gases. They are made of glass,
copper, or tin plate. The usual form is shown in fig. 23. The lower vessel B is made
hermetically tight — i.e., impervious to
gases — and is filled with water. A funnel
is attached to this vessel (on several sup-
ports). The vessel B communicates with
the bottom of the funnel by a stop-cock
b and a tube a, reaching to the bottom of
the vessel B. If water be poured into the
funnel and the stop-cocks a and b opened,
the water will run through a, and the air
escape from the vessel B by b. A glass
tube / runs up the side of the vessel B, with
which it communicates at the top and bot-
tom, and shows the amount of water and
gas the gasholder contains. In order to fill
the gasholder with a gas, it is first filled
with water, the cocks a, b and e are closed,
the nut d unscrewed, and the end of the tube
conducting the gas from the apparatus in
which it is generated is passed into d. As
the gas fills the gasholder, the water runs
out at d. If the pressure of a gas be not
greater than the atmospheric pressure and
it be required to collect it in the gasholder, ^=j
then the cock e is put into communication g
with the space containing the gas. Then, ^H
having opened the orifice d, the gasholder
acts like an aspirator; the gas will pass
through e, and the water run out at d. If

Fig. 23.— Gasholder.

the cocks be closed, the gas collected in the gasholder may be easily preserved and trans-
ported. If it be desired to transfer this gas into another vessel, then a gas-conducting
tube is attached to e, the cock a opened, b and d closed, and then the gas will pass out
at e, owing to its pressure in the apparatus being greater than the atmospheric pressure

l-2s

wi-c in tc-nnu; tin- ci >nil nist il );lit v < »t the hydrogen an explosion inav
occur from the formation of detonating ^as (the mixture of the oxygon
of t he air ^ it h the hydr« i^en ).'"

Hydrogen, which i>> contained in water, and which therefore can
he obtained from it. i> also Contained in manv oilier substances, 's and
may be obtained from Them. A-> examples of this, it may l»e men-
tioned (l)that a mixture of formate of sodium. < ' 1 1 Na< ) .,, and canst ic
soda. Xa 1 1 <>. when heated to redness, forms sodium carbonate. Na.,('< ).,.
and hydrogen. II., : ''•' i'2) tiiat a number of organic -ub.-tances are
deci iinj M isei 1 at a red heat, tormin^ hvdroi^en, amon^ other leases, and
thus it is that hydrogen is contiiined in ordinary liu'htinu' u'as.

( 'harcoal itself liberate- hydrogen from steam at a hiudi tempera -
ture : -" but the reaction which here takes place i- distinguished by a
certain complexity, and will therefore be considered later.

cylinder or tla-k \villi the i;-a>. it i- tilled \vitli water ami inverted in tin1 funnel, and
tin -t ' >| >-<•'>(•]< - // and '/ opened. Then water will run I hrouiih n, and the uM- \\"ill e-cape
from the gasholder into tin- cylinder thromjh //.

'' Wlirn it i-; I'ci pi ircd l<> pri-parc hydroj/pn in larj^v quantities for lillinLT lialloon.-.
cnjipcr vc>sds or \voodcii casks lined \villi lead are eniployed : they are tilled with -crap
i ron . over which dilute sui ] >hu ri<- acid is poured. The hydn i^'eii general ed from a numl per
i it ca >K'~ i> ca rried t hrou^'h lead pi pe- into special ca>l\'- contain 1 1 in' wat er M n order to cool

the ua-1 and lii in order to remove acid fuincsl. To avoid loss of ^a- all the point-

are made hermetic, ill v ti-'ht with a paste of piaster or tar. In order to till his ^i;_'ant ic
halloon lot' i2.".iHio ciihic metre, capacity i. (iit'fard. in ISjS, constriu-tcd a coinjilicale.l
apparatn- f"!' •_;'! \'inur a cont iniious supply of hydrogen, in \\hic!i a mixture of sulphuric
acid and water wa- continually run into \c-sel- containiiiL; iron, and from which the
solution of iron sulphate formed \\a- continually drawn ot't. \\heii coal ua-. ex-
tracted fromcoal. isemjiloyed 1'. >r tilliiiLT halloo us it should lie a> li^lit. or a > rich in hydrogen.
a~ po^-ilde. I''or this reason, only the la-t portion- of the ^as comin.u from the retort-
are collected, and. hcside- thi-. M i- then sometimes passed through red-hot vesseU. in

urdei' to decomjiosc the liydrocarl - a- much as possible; charcoal i- deposited in the

red-hot vosels. and hydros-ell remain- as «rus. Coal -'a- may he yet furt her enriched
in hydro-'en. and couseinient 1\ rendered lighter, hy passing it o\er an ignited mixture ot
charcoal and lime.

i- oi the metal-, only a very few comhine with hydrogen i for exam).le. sodium).
;11,d j_rj\,. >iil»staiices which are ea-il\ <leco)np,,sed. Of the iion-melals, tin- halo-

tahle. wli : ' lho-e ol l.roniine ami iodine are easily dec po.ed. e-pecialh the

Iiydro'_'en compound- ot different coinpo-ition .,,,,] proper! ie-. Imt they are all le-s -talili
1),,,,, '/.ater. The numliei' "t the carl. on compound- of hydrogen is e 'moil-, l.ut tln-ri

hy«lr..'_'i'-n. at a ivd heal.

I , ••,,,, , •. |, . , , , i i : , ei | nation ( 'Na !l( ) NallO CNa .< >.-, II . may \,<

, ... ,.,,., | , . ,. ||,, -,, , , mpo ition o| ,-opp. r , arhoiiate or mercurx oxid.

,.r,-fo|'e I'ictet f, !. I ide 11-i- ot i ! t o oht a i 11 1 1 \ < 1 1'o^el I 1 1 1 1 d e r ;J lea 1 | .1 e -> l| re

< The reaction li.-tw 'h, \ a n< I n peri MM 1 . •• I team i-adoiihleoue— tliat i-. thi-n

,,a\ IM lol'ined eilhel- i nl.oiii' o\lde.< (I a ccol'd ill- t o t 1 ie e. I lla t ioll I I ,< ) (' II . C'Ol, O]

THE COM POSITION OF WATER, HYDRO* i FA

The properties <>f ]ii/<Ii-<>>/i'n.— Hydrogen presents us with an example
of a gas which at first sight does not differ from air. It is not sur-
prising, therefore, that Paracelsus, having discovered that an aeriform
substance is obtained by the action of metals on sulphuric acid, did not
quite determine its difference from air. In fact, hydrogen, like air, is
colourless, and has no smell ; 21 but a more intimate acquaintance
with its properties proves it to be entirely different from air. The first
sign which distinguishes hydrogen from air is its combustibility. This
property is so easily observed that it is the one to which recourse is
usually had in order to recognise hydrogen, if it is evolved in a re-
action, although there are many other combustible gases. But before
speaking of the combustibility and other chemical properties of hydro-
gen, we will first describe the physical properties of this gas, as we did
in the case of water. It is easy to show that hydrogen is one of the
lightest gases.22 If passed into the bottom of a flask full of air,

carbonic anhydride C0.2 (according to the equation 2H..O + C = 2H.> + CO.2), and the result-
ing mixture is called water-gas ; we shall speak of it in describing the oxides of carbon.

-1 Hydrogen obtained by the action of zinc or iron on sulphuric acid generally smells
of hydrogen sulphide (like rotten eggs), which it contains in admixture. As a rule such
hydrogen is not so pure as that obtained by the action of an electric current or of sodium
on water. The impurity of the hydrogen depends on the impurities contained in the
zinc, or iron, and sulphuric acid, and on secondary reactions which take place simul-
taneously with the main reaction. Thus iron sulphide gives hydrogen sulphide
(FeS + HoSO4 = HoS + FeSO4). However, the hydrogen obtained in this manner may be
easily freed from the impurities it contains : some of them — namely those having acid
properties — are absorbed by caustic soda, and therefore may be removed by passing the
hydrogen through a solution of this substance ; another series of impurities is absorbed
by a solution of mercuric chloride ; and, lastly, a third series is absorbed by a solution of
potassium permanganate. The hydrogen may be dried by passing it over sulphuric acid
or calcium chloride. The substances serving for purifying the hydrogen are either
placed in Woulfe's bottles, or in tubes containing pumice stone moistened with the
purifying agent. The surface of contact is then greater, and the purification proceeds
more rapidly. If it be desired to procure completely pure hydrogen, it is sometimes
obtained by the decomposition of water (previously boiled to expel all air, and mixed
with pure sulphuric acid), by the galvanic current. Only the gas evolved at the negative
electrode is collected. Or else, an apparatus like that which gives detonating gas is used,
only the positive electrode being immersed under mercury containing zinc in solution.
The oxygen which is evolved at this electrode then immediately, at the moment of its
evolution, combines with the zinc, and this compound dissolves in the sulphuric acid and
forms zinc sulphate, which remains in solution, and therefore the hydrogen generated
will be quite free from oxygen.

'-'- An inverted beaker is attached to one arm of the beam of a rather sensitive
balance, and its weight counterpoised by weights in the pan attached to the other arm.
If the beaker be then filled with hydrogen it rises, owing to the air being replaced
by hydrogen. Thus, at the ordinary temperature of a room, a litre of air weighs
about 1'2 grams, and on replacing the air by hydrogen a decrease in weight of about 1
irrum per litre is obtained. Moist hydrogen is heavier than dry — for aqueous vapour
is nine times heavier than hydrogen. In filling balloons it is usually calculated that (it
being impossible to have perfectly dry hydrogen or to obtain it quite free from air)
the lifting force is equal to 1 kilogram ( = 1,000 grams) per cubic metre ( = 1,000 litres).
VOL. I. K

130

PRINCIPLES OF CHEMISTRY

hydrogen will not remain in it, but, owing to its lightness, rapidly
escapes and mixes with the atmosphere. If, however, a cylinder whoso
orifice is turned downwards be filled with hydrogen, it will not escape,
or, more correctly, it will only slowly mix with the atmosphere. This
may be demonstrated by the fact that a lighted taper sets fire to the
hydrogen at the orifice of the cylinder, and is itself extinguished inside
the cylinder. Hence hydrogen, being itself combustible, does not
support combustion. The great lightness of hydrogen is taken advan-
tage of for balloons. Ordinary coal gas, which is often also used for
the same purpose, is only about twice as light as air, whilst hydrogen is
•I 4^ times lighter than air. A very simple experiment with soap bubbles
very well illustrates the application of hydrogen for filling balloons.
Charles, of Paris, showed the lightness of hydrogen in this way, and con-
structed a balloon filled with hydrogen almost simultaneously with Mont-
golfier. One litre of hydrogen23 at 0° and 760 mm. pressure weighs

-3 The density of hydrogen in relation to the air has been determined by accurate
experiments. The first determination, made by Lavoisier, was not entirely exact ; taking
the density of air as unity, he obtained 0'0769 for that of hydrogen — that is, hydrogen as
thirteen times lighter than air. Later determinations have corrected this figure, the
most accurate determinations being due to Thomsen, who obtained the figure 0*0698 ;
Berzelius and Dulong, who obtained 0'0688 ; and Dumas and Bunseii. who obtained
0-06945. But the most exact determination of all is, without doubt, due to Regnault.
He took two spheres of considerable capacity, which cm en hied equal volumes of air
(thus avoiding the necessity of any correction for weighing them in air). Both spheres
were attached to the scale pans of a balance. One was sealed up, and the other first
weighed empty and then full of hydrogen. Thus, knowing the weight of the hydrogen
filling the sphere, and the capacity of the sphere, it was easy to rind the weight of a litre
of hydrogen ; and, knowing the weight of a litre of air at the same temperature and
pressure, it was easy to calculate the density of hydrogen. Regnault, by these experi-
ments, found the average density of hydrogen to be 0'06926 in relation to air, or including
the necessary corrections 0'06949.

In this book I shall always refer the densities of all gases to hydrogen, and not
to air ; therefore, for the sake of clearness, I will cite the weight of a litre of dry pure
hydrogen in grams at a temperature t° and under a pressure H (measured in millimetres
of mercury at 0°, in long. 45°). The weight of a litre of hydrogen

1

= 0-08958 x x __
760 1 -

gram.
I) -008(57*

For aeronauts it is very useful to know, besides this, the weight of the air at different
heights, and I therefore insert the adjoining table, constructed on the basis of Glaisher's

Pressure

760 m.m.

700 „

650 „

600 ,.

550

500

450

400

350

300

250

Temperature

15° C.

11-0°

7'6°

4'3°

+ 1-0°

- 2'4°

- 5-8°

- 9-1°

-12-5°

-15-9°

-19-2°

ICotatan

Height

\Vci-litc.f the :ur

60 p.c.

0 ni-tves 12'22 kilos.

64 „

<;<><> „ 1141 ,

64 „

1300 „

1073 ,

T.

£

63 „

1960 „

1003 ,

~%

(52 „

2660 „

',.:;! ,

58 „

3420 „

s.-,7 ,

-

52 .,

4250 „

7si :

"^

44 „

5170 703 ,

B

:!(•, ., 61!»0 „ i-,-24 ,

\

27 „ 7:-!i'>0 „ ."-I-2 ,

18 „ S7-20 „ 4.17 „ 1

THE n>M POSITION OF WATER, HYDROGEN 131

O08957S gram ; that is, hydrogen is almost 1-U (more exactly, 14-43)
times lighter than air. It is the lightest of all gases. The small density
of hydrogen determines many remarkable properties which it shows ;
thus, hydrogen flows exceedingly rapidly from fine orifices, its molecules
(Chap. I.) being endued with the greatest velocity of movement.24 At
pressures somewhat higher than the atmospheric pressure, all other
gases exhibit a greater compressibility and co-efficient of expansion than
they should according to the laws of Mariotte and Gay-Lussac ; whilst
hydrogen, on the contrary, is less compressed than should follow from
the law of Mariotte,'2"' and with a rise of pressure it expands slightly

data, for the temperature and moisture of the atmospheric strata in clear weather. All
the figures are given in the metrical system — 1000 millimetres = 39'37 inches, 1000 kilo-
grams = '220 l-:',:',7"> 11 >s., 1000 cubic metres = 35316'5 cubic feet. The starting temperature
at the earth's surface is taken as = 15° C., its moisture 60 p.c., pressure 760 millimetres.
The pressures are taken as indicated by an aneroid barometer, assumed to be corrected
at the sea level and at long. 45°.

Although the figures of this table are calculated with every possible care from average
data, yet they can only be taken for an elementary judgment of the matter, for in every
separate case the conditions, both at the earth's surface and in the atmosphere, will differ
from those here taken. In calculating the height to which a balloon can ascend, it is
evident that the density of gas in relation to air must be known.' This density for
ordinary coal gas is from 0'6 to 0'35, and for hydrogen with its ordinary contents of
moisture and air from O'l to 0'15.

Hence, for instance, it may be calculated that a balloon of 1000 cubic metres capacity
filled with pure hydrogen, and weighing (the envelope, tackle, people, and ballast) 727
kilograms, will ascend to a height of not much more than 4250 metres.

24 If a cracked flask be filled with hydrogen and its neck immersed under water or
mercury, then the liquid will rise up into the flask, owing to the hydrogen passing
through the cracks about 3'8 times quicker than the air is able to pass through these
cracks into the flask. The same thing may be better seen if, instead of a flask, a tube
whose end is closed by a porous substance, such as graphite, unglazed earthenware, or a
gypsum plate, be employed.

25 According to Boyle and Mariotte's law, for a given gas at a constant temperature the
volume decreases by as many times as the pressure increases; that is, this law requires
that the product of the volume v and the pressure p for a given gas should be a constant
quantity: pv — C, a constant quantity which does not vary with a change of pres-
sure. In reality this equation does very nearly and exactly express the observed rela-
tion between the volume and pressure, but only within comparatively small variations
of pressure, density, and volume. If these variations be in any degree considerable, the
quantity /tv proves to be dependent on the pressure, and it either increases or diminishes
with an increase of pressure. In the former case the compressibility is less than it
should be according to Mariotte's law, in the latter case it is greater. We will call the
.tii -4 case a positive discrepancy (because then d (pv) VZ (p) is greater than zero), and the
second case a negative discrepancy (because then d (pv) /d (p) is less than zero). Deter-
minations made by myself, M. L. Kirpicheff, and Hemilian showed that all known gases
at low pressures, when considerably rarefied, present positive discrepancies. On the
other hand it appears from the researches of Cailletet, Natterer, and Amagat that all
gases under great pressures (when the volume obtained is 500-1000 times less than
iimler the atmospheric pressure) also present positive discrepancies. Thus under a pres-
>nre of 2700 atmospheres air is compressed, not 2700 times, but only 800, and hydrogen
1000 times. Hence the positive kind of discrepancy is, so to say, normal to gases. And
this is easily understood. Did a gas follow Mariofcte's law, or were it compressed to a

K 2

132 PRINCIPLES OF CHEMISTRY

less than at the atmospheric pressure.26 However, hydrogen, like-
air and many other gases which are permanent at the ordinary tem-

greater extent than is shown by this law, then under great pressures it would attain a
density greater than that of solid and liquid substances, which is in itself improbable and
even impossible by reason of the fact that solid and liquid substances are themselves but
little compressible. For instance, a cubic centimetre of oxygen at 0D and under the at-
mospheric pressure weighs about 0-0014 gram, and at a pressure of 3000 atmospheres
(this pressure is attained in guns) it would, if it followed Mariotte's law, weigh 4'2 grams —
that is, would be about four times heavier than water — and at a pressure of 10000 atmo-
spheres it would be heavier than mercury. Besides this, positive discrepancies are pro-
bable in the sense that the molecules of a gas themselves must occupy a certain volume.
Admitting that Mariotte's law only applies to the intermolecular space still we find the
necessity of positive discrepancies. If we designate the volume of the molecules of a gas
by 6 (like Van der Waals, see Chap. I. note 34), then it must be expected that^> (v — b) = C.
Hence pv~C + bp, which expresses a positive discrepancy. Supposing that for hydrogen
j>y = 1000, at a pressure of one metre of mercury, according to the results of Regnault 's,
Amagat's, and Natterer's experiments, we obtain b as approximately 0'7 to 0*9.

Thus the increase of pv with the increase of pressure must be considered as the
normal law of the compressibility of gases. Hydrogen presents such a positive compres-
sibility at all pressures, for it presents positive discrepancies from Mariotte's law, accord-
ing to Regnault, at all pressures above the atmospheric pressure. Hence hydrogen is,
so to say, a sample gas. No other gas behaves so simply with a change of pressure. All
other gases at pressures from 1 to 30 atmospheres present negative discrepancies— that
is, they are then compressed to a greater degree than should follow from Mariotte's law,
as was shown by the determinations of Regnault, which were verified when repeated by^
myself andBoguzsky. Thus, for example, on changing the pressure from 4 to 20 metres
of mercury — that is, on increasing the pressure five times — the volume only decreased
4'93 times when hydrogen was taken, and 5'06 when air was taken.

The discrepancies from the law of Boyle and Mariotte for considerable pressures
(from 1 to 3000 atmospheres) are well expressed (for constant temperatures) by the
above-mentioned formula of Van der Waals (Chap. I. Note 34) ; Clausius' formula is more
closely approximate, but as it and Van der Waals' formula also do not in any way express
the existence of positive discrepancies from the law at low pressures, and as, accord-
ing to the above-mentioned determinations made by myself, Kirpicheff, and Hemilian and
verified (by two methods) by K. D. Kraevitch, they are proper to all gases (even to those
which are easily compressed into a liquid state, such as carbonic and sulphurous anhy-
drides) ; therefore these formulae, whilst accurately interpreting the phenomena of con-
densation and even of liquefaction, do not answer in the case of a high rarefaction of
gases — that is, to that instance where a gas approaches to a condition of maximum dis-
persion of its molecules, and perhaps presents a passage towards the substance termed
' luminiferous ether ' which fills up interplanetary and interstellar space. If we suppose
that gases are rarefiable to a definite limit only, having attained which they (like solids)
do not alter in volume with a decrease of pressure, then on the one hand the passage of
the atmosphere at its upper limits into a homogeneous ethereal medium becomes com-
prehensible, and on the other hand it would be expected that gases would, in a state of
high rarefaction (i.e., when small masses of gases occupy large volumes, or when furthest
removed from a liquid state) present positive discrepancies from Boyle and Mariotte's law.
Our present acquaintance with this province of highly rarefied gases is most limited, and
its further development promises to elucidate much in respect to natural phenomena. To-
the three states of matter (solid, liquid, and gaseous) it is evident a fourth must be yet
added, the ethereal or ultra-gaseous (as Crookes proposed), understanding by tins
matter in its highest possible state of rarefaction.

26 The law of Gay-Lussac states that all gases in all conditions present one coefficient
of expansion 0'00367 ; that is, when heated from 0° to 100° they expand like air;
namely, a thousand volumes of a gas measured at 0° will occupy 1367 volumes at 100°.

THE COMPOSITION OF WATER, HYDROGEN 133

perature, does not pass into a liquid state under a very consider-
able pressure,-7 but is compressed into a lesser volume than would

Regnault, about 1850, showed that Gay-Lussac's law is not entirely correct, and that
different gases, and also one and the same gas at different pressures, have not quite the
same coefficients of expansion. Thus the expansion of air between 0° and 100° is 0'367
under the ordinary pressure of one atmosphere, and at three atmospheres it is 0'371, the
expansion of hydrogen is 0'366, and of carbonic anhydride 0'37. Regnault, however, did
not directly determine the change of volume between the 0° and 100°, but measured the
variation of tension with the change of temperature ; but as gases do not entirely follow
Mariotte's law, therefore the change of volume cannot be directly judged by the variation
of tension. The investigations carried on by myself and Kayander, about 1870, showed
the direct variation of volume on heating from O3 to 100°. These investigations confirmed
Regnault's conclusion that Gay-Lussac's law is not entirely correct, and further showed
(1) that the expansion per volume from 0° to 100 J under a pressure of one atmosphere,
for air -0-368, for hydrogen = 0'367, for carbonic anhydride = 0'373, for hydrogen bromide
= 0'386, &c. ; (2) that for gases which are more compressible than should follow
from Mariotte's law the expansion by heat increases with the pressure — for example,
for air at a pressure of three and a half atmospheres, it equals 0'371, for carbonic
anhydride at one atmosphere it equals 0'373, at three atmospheres 0'389, and at eight
.atmospheres 0'413 ; (3) that for gases which are less compressible than should follow
from Mariotte's law, the expansion by heat decreases with an increase of pressure' —
for example, for hydrogen at one atmosphere 0'367, at eight atmospheres 0'369, for air at
a quarter atmosphere 0"370, at one atmosphere 0'368 ; and hydrogen like air (and all
gases) is less compressed at low pressures than should follow from Mariotte's law (air
at higher pressures than the atmospheric pressure gives a contrary result), as investiga-
tions made by myself, aided by Kirpicheff and Hemilian, showed. Hence, hydrogen,
starting from zero to the highest pressures, exhibits a gradually, although only slightly,
varying coefficient of expansion, whilst for air and other gases at the atmospheric and
higher pressures, the coefficient of expansion increases with the increase of pressure, so
long as their compressibility is greater than should follow from Mariotte's law. But
when at considerable pressures, this kind of discrepancy passes into the normal (see Note
25), then the coefficient of expansion of all gases decreases with an increase of pressure,
as is seen from the researches of Amagat. The difference between the two coefficients
of expansion, for a constant pressure and for a constant volume, is explained by these
relations. Thus, for example, for air at a pressure of one atmosphere the true coefficient
of expansion (the volume varying at constant pressure) = 0'00368 (according to Mende-
leeff and Kayander) and the variation of tension (at a constant volume, according to
Regnault) =0'00367.

27 Permanent gases are such as cannot be liquefied by an increase of pressure alone.
With a rise of temperature, all gases and vapours become permanent gases. As we shall
afterwards learn, carbonic anhydride becomes a permanent gas at temperatures above
31°, and at lower temperatures it has a maximum tension, and may be liquefied by
pressure alone.

The liquefaction of gases, accomplished by Faraday (see Ammonia) and others, in
the first half of this century, showed that a number of substances are capable, like water,
of taking all three physical states, and that' there is no essential difference between
vapours and gases, the only distinction being that the boiling points (or the temperature
at which the tension =760 mm.) of liquids lie above the ordinary temperature, and those
of liquefied gases below, and consequently a gas is a superheated vapour, or vapour
heated above the boiling point, or removed from saturation, rarefied, having a lower
tension than that maximum which is proper to a given temperature and substance. We
will here cite, as we did for water (p. 54), the maximum tensions of certain liquids and
gases at various temperatures, because they may be taken advantage of for obtaining
constant temperatures by changing the pressure at which boiling or the formation of

134 PRINCIPLES OF CHEMISTRY

follow from Marietta's law.28 From tins it may be concluded
that the absolute boiling point of hydrogen, and of gases resembling

saturated vapours takes place. The temperatures (according to the air thermometer)
are placed on the left, and the tension in millimetres of mercury (at 0 ) on the right,
hand side of the equations. Carbon bisulphide, CSg, 0° = 127'9; 10° = 198'5; 20° = 298*1;
30° = 431-6; 40° = 617'5; 50° = 857'1. Chlorobenzene, CCH5C1, 70° = 97'9; 80° = l-il'8;
90° = 208'4; 100° = 292'8; 110° = 402-6; 120° = 54t2'.s; 13u- = 71iK). Aniline, C6H7N,
150° = 283-7; 160° = 887-0; 170° = 515-6; 180° = 677"2; LS.V =771-5. Methyl sulicylate,
C8H8O5, 180° = 249-4; 1900 = 330'9; 200° = 432'4; 210C = 557'5; 220C = 710"2; 224': =77i»".».
Mercury, Hg, 300° = 246'8 ; 310° = 304'9 ; 3200 = 373'7 ; 3300 = 454'4; 340° = 548-6;
350° = 658'0; 359° = 770'9. Sulphur, S, 395° = 300; 423° = 500; 443° =-700 ; 452° = 800 ;
459° = 900. These figures (Ramsay and Young) show the possibility of fixing con-
stant temperatures in the vapours of boiling liquids. The tension of liquefied
gases is expressed in atmospheres. Sulphurous anhydride, SO>£ — bOc = 0'4 ; —20° = 0*6;
-10° = 1; 0° = 1'5; +10° = 2'3; 20° = 3'2; 30°=5'3. Ammonia, NH5, -40° = 0'7;
-30° = 1-1; -20° = l-8; -10° = 2'8; 0° = 4'2; + 10° = 6'0; 20° = 8'4. Carbonic anhydride,
CO2, -115° = 0-033 ; -80° = 1; — 70° = 2'1; -600 = 3'9 ; -50° = 6'8 ; -40° = 10; -20° = 23;
0° = 35; +10° = 46; 20°=58. Nitrous oxide, N2O, -125° = 0'033 ; -92° = 1; -80° = 1'9;
-50° = 7-6; -20° = 23-1; 00 = 36'1; +20° = 55'3. Ethylene, CoH4, -140° = 0'033;
-130° = 0-1; -103° = 1; -40° = 13; -1° = 42. Air, -191° = 1; -15~8° = 14; -140° = 39.
Nitrogen, N2, -203° = 0'085; -193° = 1; -160° = 14; -146° = 32. The methods of
liquefying gases (by pressure and cold) will be described under ammonia, nitrous oxide,
sulphurous anhydride, and in later footnotes. We will now turn our attention to the
fact that the evaporation of volatile liquids, under various, and especially under lowr
pressures, gives an easy means for obtaining low temperatures. Thus liquefied carbonic
anhydride, under the ordinary pressure, reduces the temperature to - 803, and when it
evaporates in an atmosphere rarefied (in an air-pump) to 25 mm. ( = 0'033 atmospheres)
the temperature, judging by the above-cited figures, falls to -115° (Dewar). Even the
evaporation of liquids of common occurrence, under low pressures easily attainable in an
air-pump, may produce low temperatures, which may be again taken advantage of for ob-
taining still lower temperatures. Water boiling in a vacuum becomes cold, and under
a pressure of less than 4'5 mm. it freezes, because its tension at 0° is 4'5 mm. A
sufficiently low temperature may be obtained by forcing fine streams of air through
common ether, or liquid carbon bisulphide, CS2, or methyl chloride, CH3C1, and other
similar volatile liquids. In the adjoining table are given, for certain gases, (1) the
number of atmospheres necessary for their liquefaction at 15°, and (2) the boiling points
of the resultant liquids under a pressure of 760 mm.

C2H4 NaO CO., H.,S AsH3 NH*3 HC1 CH3C1 CLX, SO,

(1) 42 31 52 10 8 7 25 4 4 3

(2) -103° -92° -80° -74° -58° -38° -35° -24° -21° -10°

28 Natterer's determinations (1851-1854), together with Amagat's results (1880-1888),
show that the compressibility of hydrogen, under high pressures, may be expressed by
the following figures : —

p. 1 100 1000 2500

v I 0-0107 0-0019 0-0013

2W 1 1-07 1-9 3-25

s 0-11 10-3 58 85

where p = the pressure in metres of mercury, v = the volume, if the volume taken under
a pressure of 1 metre =1, and s the weight of a litre of hydrogen at 20° in grams. If
hydrogen followed Mariotte's law, then under a pressure of 2500 metres, one litre would
contain not 85, but 265, grams. It is evident from the above figures that the weight of
a litre of the gas approaches a limit as the pressure increases, which is doubtless the
density of the gas when liquefied, and therefore the weight of a litre of liquid
hydrogen will probably be near 100 grams (density about O'l, being less than that of all
other liquids).

THE COMPOSITION OF WATER, HYDROGEN 135

it,2<J lies very much below the ordinary temperature ; that is, that the
liquefaction of this yas is only possible at low temperatures, and under

2) Cagniard de Latour, on heating ether in a closed tube to about 190°, observed that
at this temperature the liquid is transformed into vapour occupying the original volume
— that is, having the same density as the liquid. The further investigations made by
Drion and myself, showed that every liquid has such an absolute boiling point, above which
it cannot exist as a liquid and is transformed into a dense gas. In order to grasp the true
signification of this absolute boiling temperature, it must be remembered that the liquid
state is characterised by a cohesion of its particles which does not exist in vapours and
gases. The cohesion of liquids is expressed in their capillary phenomena (the breaks
in a column of liquid, drop formation, and rise in capillary tubes, &c.), and the product of
the density of a liquid into the height to which it rises in a capillary tube (of a definite
diameter) may serve as the measure of the magnitude of cohesion. Thus, in a tube of
2 mm. diameter, water at 15° rises (the height being corrected for the meniscus) 14'8mm.,
and ether at t° to a height 5'35 — 0'028 tc mm. The cohesion of a liquid is lessened by
heating, and therefore the capillary heights are also diminished. It has been shown
by experiment that this decrement is proportional to the temperature, and hence by the
aid of capillary observations we are able to form an idea that at a certain rise of
temperature the cohesion may become = 0. For ether, according to the above formula,
this would happen at 191°. If the cohesion disappear from a liquid it becomes a gas,
for cohesion is the only point of difference between these two states. A liquid in
evaporating and overcoming the force of cohesion absorbs heat. Therefore, the absolute
boiling point was defined by me (1861) as that temperature at which (a) a liquid cannot
exist as a liquid, but forms a gas which cannot pass into a liquid state under any
pressure whatever ; (b) cohesion = 0; and (c) the latent heat of evaporation = 0.

These ideas were but little spread until Andrews (1869) explained the matter from
another aspect. Starting from gases, he discovered that carbonic anhydride can-
not be liqnefied by any degree of compression at temperatures above 81°, whilst at
lower temperatures it can be liquefied. He called this temperature the critical tem-
perature. It is evident that it is the same as the absolute boiling point. We shall after-
wards designate it by tc. At low temperatures a gas which is subjected to a pressure
greater than its maximum tension (Note 27) is completely transformed into a liquid,
which, in evaporating, gives a saturated vapour which possesses this maximum tension ;
whilst at temperatures above tc the pressure to which the gas is subjected may increase
indefinitely. However, under these conditions the volume of the gas does not change
indefinitely but approaches a definite limit (see Note 28) — that is, it resembles in this
respect a liquid or a solid which is altered but little in volume by pressure. The
volume which a liquid or gas occupies at tc is termed the critical volume, which corre-
sponds with the critical pressure, which we will designate byjpc and express in atmo-
spheres. It is evident from what has been said that the discrepancies from Mariotte
and Boyle's law, the absolute boiling point, the density in liquid and compressed
gaseous states, and the properties of liquids, must all be intimately connected together.
We will consider these relations in one of the following notes. At present we will
supplement the above observations by the values of tc and pc for certain liquids and
gases which have been investigated in this respect —

„

p.c.

t.c. p.c.

N2 - 14f,

88

H2S + 108°

92

CO - 140°

39

C2N2 + 124° 62

O.2 - 119°

50

NH3 + 181° 114

CH4 - 100°

50

CH3C1 + 141°

78

NO - 93°
CsHt + 10°

71
51

SO2 + 155°
C5H10 + 192°

79
84

CO* + 82°

77 C4H10O + 193°

40

N20 + 53°

75

CHC13 + 268°

55

C2Ho + 87°

68

CS., + 278°

78

HCf H 52°

86

C6H6 + 292°

60 .

136

PRINCIPLES OF CHEMISTRY

great pressures.30 This conclusion was verified (1879) by tire ex-
periments of Pictet and Cailletet.31 They compressed gases at a

30 This conclusion was arrived at by me in 1870 (Ann. Phys. Chem. 141, 023).

31 Pictet, in his researches, effected the direct liquefaction of many gases which up to
that time had not been liquefied. He employed the apparatus used for the manufacture
of ice on a large scale, employing the vaporisation of liquid sulphurous anhydride
which may be liquefied by pressure alone. This anhydride is a gas which is transformed
into a liquid at the ordinary temperature under a pressure of several atmosphere-
Note 27), and boils at —10° at the ordinary atmospheric pressure. This liquid, like all
others, boils at a lower temperature under a diminished pressure, and by continually
pumping out the gas which comes off by means of a powerful air-pump its boiling point
falls as low as —75°. Consequently, if we on the one hand force liquid sulphurous
anhydride into a vessel, and on the other hand pump out the gas from the same vessel
by powerful air-pumps, then the liquefied gas will boil in the vessel, and cause the tempera-
ture in it to fall to — 7S3. If a second vessel is placed inside this vessel, then another
gas may be easily liquefied in it at the low temperature produced by the boiling liquid
sulphurous anhydride. Pictet in this manner easily liquefied carbonic anhydride, COo
(at —60° under a pressure of from four to six atmospheres). This gas is more refractory
to liquefaction than sulphurous anhydride, but for this reason it gives on evaporating a
still lower temperature than can be attained by the evaporation of sulphurous anhydride.
A temperature of — 80° may be obtained by the evaporation of liquid carbonic anhydride at
a pressure of 760 mm., and in an atmosphere rarefied by a powerful pump the temperature
falls to —140°. By employing such low temperatures, it was possible, with the aid of
.pressure, to liquefy the majority of the other gases. It is evident that special pumps
which are capable of rarefying gases are necessary to reduce the pressure in the
chambers in which the sulphurous and carbonic anhydride boil ; and that, in order to
re-condense the resultant gases into liquids, special force pumps are required for pumping
the liquid anhydrides into the refrigerating chamber. Thus, in Pictet's apparatus
(fig. 24), the carbonic anhydride was liquefied by the aid of the pumps E F, which com-

FIG. 24. — General arrangement of the apparatus employed by Pictet for liquefying gases.

THE COMPOSITION OK WATKI;. HYDROGEN 137

very low temperature, and then allowed them to expand, either by
directly decreasing the pressure or by allowing them to escape into the
air, by which means the temperature fell still lower, and then, just as
steam when rapidly rarefied3- deposits liquid water in the form of a

pressed the gas (;lt il pressure of 4-6 atmospheres) and forced it into the tube K,
vigorously cooled by being surrounded by boiling liquid sulphurous anhydride, which
was condensed in the tube C by the pump B, and rarefied by the pump A. The
liquefied carbonic anhydride flowed down the tube K into the tube H, in which it was
subjected to a low pressure by the pump E, and thus gave a very low temperature of
about 140°. The pump E carried off the vapour of the carbonic anhydride, and conducted it
to the pump F, by which it was again liquefied. The carbonic anhydride thus made an
entire circuit — that is, it passed from a rarefied vapour of small tension and low tempera-
ture into a compressed and cooled gas, which was transformed into a liquid, which
again vaporised and produced a low temperature.

Inside the wide inclined tube H, where the carbonic acid evaporated, was placed a
second and narrow tube M containing hydrogen, which was evolved in the vessels L
from a mixture of sodium formate and caustic soda (CHOoNa + NaHO^Na^COs + Ho).
This mixture gives hydrogen on heating the vessel L. This vessel and the tube M were
made of thick copper, and could withstand great pressures. They were, besides, her-
metically connected together and closed up. Thus the hydrogen which was evolved had
no outlet, accumulated in a limited space, and its pressure increased in proportion to
the amount of it evolved. The magnitude of this pressure was recorded on a metallic
manometer E attached to the end of the tube M. As the hydrogen in this tube was sub-
mitted to a very low temperature and a powerful pressure, there were all the necessary con-
ditions for its liquefaction. When the pressure in the tube H became steady — i.e., when
the temperature had fallen to — 140 J, and the manometer R indicated a pressure of 650
atmospheres in the tube M — then this pressure did not rise with a further evolution of
hydrogen in the vessel L. This served as an indication that the tension of the vapour of
the hydrogen had attained a maximum corresponding with —140°, and that consequently
all the excess of the gas was condensed to a liquid. Pictet convinced himself of this
by opening the cock N, when the liquid hydrogen rushed out from the orifice. But, on
leaving a space where the pressure was equal to 650 atmospheres, and coming into contact
with air under the ordinary pressure, the liquid or powerfully-compressed hydrogen
expanded, began to boil, absorbed still more heat, and became still colder. In doing so
a portion of the liquid hydrogen, according to Pictet, passed into a solid state, and did
not fall in drops into a vessel placed under the outlet N, but as pieces of solid matter,
which struck against the sides of the vessel like shot and immediately vaporised.
Thus, although it was impossible to see and keep the liquefied hydrogen, still it was
admitted that it passed not only into a liquid, but also into a solid, state, because Pictet
in his experiments obtained other gases which had not previously been liquefied,
especially oxygen and nitrogen, in a liquid and solid state. Pictet supposed that liquid
and solid hydrogen have the properties of a metal, like iron.

3- At the same time (1879) as Pictet was working on the liquefaction of gases in
Switzerland, Cailletet, in Paris, was occupied en the same subject, and his results,
air hough not so convincing as Pictet's, still showed that the majority of gases, previously
unliquefied, were capable of passing into a liquid state. Cailletet subjected gases to a
pressure of several hundred atmospheres in thin glass tubes (fig. 25) ; he then cooled
the compressed gas as far as possible by surrounding it with a freezing mixture; a
•cock was then rapidly opened for the outlet of mercury from the tube containing the gas,
which consequently rapidly and vigorously expanded. This rapid expansion of the gas
would produce great cold, just as the rapid compression of a gas evolves heat and causes
a rise in temperature. This cold was produced at the expense of the gas itself, for in
rapidly expanding its particles were not able to absorb heat from the walls of the
tube, and in cooling a portion of the expanding gas was transformed into liquid. This

138

PRINCIPLES OF CHEMISTKY

fog, hydrogen in expanding forms a fog, thus indicating its passage into
a liquid state. But as yet it has been impossible to preserve this
liquid, even for a short time, to determine its properties, notwithstanding
the employment of a temperature of — 200° and a pressure of 200 atmo-
spheres,33 although by these means the gases of the atmosphere may be
kept in a liquid state for a long time. This is naturally dependent
on the fact that the absolute boiling point of hydrogen lies lower than
that of all other known gases, which is related to the extreme lightness
of hydrogen.34

was seen from the formation of cloud-like drops, like a fog, which rendered the gas opaque.
Thus Cailletet proved the possibility of the liquefaction of gases, but lie did not isolate
the liquids. The method of Cailletet allows the passage of
gases into liquids being observed with greater facility and
simplicity than Pictet's method, which requires a very
complicated and expensive apparatus.

The methods of Pictet and Cailletet were afterwards
improved by Olszewski, Wroblewski, Dewar, and others.
In order to obtain a still lower temperature they employed
liquid ethylene or nitrogen instead of carbonic acid gas,
whose evaporation at low pressures produces a much lower
temperature (to —200°). They also improved on the
methods of determining such low temperatures, but the
methods were not essentially altered ; they obtained nitro-
gen and oxygen in a liquid, and nitrogen even in a solid,
state, but no one has yet succeeded in seeing hydrogen in
a liquid form.

55 The investigations of C. Wroblewski in Cracow
clearly proved that Pictet could not have obtained liquid
hydrogen in the interior of his apparatus, and that if he
did obtain it, it could only have been at the moment of its
outrush due to the fall in temperature following its sud-
den expansion. Pictet calculated that he obtained a tem-
perature of — 140°, but in reality it hardly fell below — 120°,
judging from the latest data for the vaporisation of car-
bonic anhydride under low pressure. The diffei*ence lies
in the method of determining low temperatures. Judging
from other properties of hydrogen (see Note 34), one would
think that its absolute boiling point lies far below - 120°,
and even ~140° (according to the calculation of Sarrau, on
the basis of its compressibility, at - 174°). But even at -200°
(if the methods of determining such low temperatures be correct) hydrogen does not give
a liquid even under a pressure of several hundred atmospheres. However, on expan-
sion a fog is formed and a liquid state attained, but the liquid does not separate.

54 After the conception of the absolute temperature of ebullition (tc, note 211) had
been worked out (about 1870), and its connection with the deviations from Mariotte's law
had become evident, and especially after the liquefaction of permanent gases, general
attention was turned to the development of the fundamental conceptions of the gaseous
and liquid states of matter. Some investigators directed their energies to the further
study of vapours (for instance, Ramsay and Young), gases (for instance, Amagat), and
liquids (for instance, Zaencheffsky, Nadeschdin, and others), especially to liquids near tc
and pc ; others (for instance, Konovaloff and De Haen) endeavoured to discover the rela-
tion between liquids under ordinary conditions (removed from tc and pc) and gases,

forli°uef In*'* ases

THE COMPOSITION OF WATER, HYDK< Kil-N IB'J

Although a substance which passes with great difficulty into a
liquid state by the action of physico-mechanical forces, hydrogen loses

while a third class of investigators (Van der Waals, Clausius, and others ), starting from the
already generally-accepted principles of the mechanical theory of heat and the kinetic
theory of gases, and having made the self-evident proposition of the existence in jj
of those forces which clearly act in liquids, deduced the connection between the properties
of one and the other. It would be out of place in an elementary handbook like the
present to enunciate the whole mass of conclusions arrived at by this method, but it is
necessary to give an idea of the results of Van der Waals' considerations, for they explain
the gradual uninterrupted passage from a liquid into a gaseous state in the simplest
form, and, although the deduction cannot be considered as complete and decisive (see
note 25), nevertheless it penetrates so deeply into the essence of the matter that its
signification is not only reflected in a great number of physical investigations, but also in
the province of chemistry, where instances of the passage of substances from a gaseous
to a liquid state are so common, and where the very processes of dissociation, decomposi-
tion, and combination must be identified with a change of physical state of the partici-
pating substances.

For a given quantity (weight, mass) of a definite substance, its state is expressed
by three variables— volume v, pressure (elasticity, tension) p, and temperature t.
Although the compressibility — [i.e., d(v)d(p)] — of liquids is small, still it is clearly ex-
pressed, and varies not only with the nature of liquids but also with their pressure and
temperature (at tc the compressibility of liquids is very considerable). Although gases,
according to Mariotte's law, with small variations of pressure, are uniformly compressed,
nevertheless the dependence of their volume v on t and p is very complex. The same
applies to the coefficient of expansion [ = d(v)d(t), or d(p)d(t)], which also varies with
t and_p, both for gases (see Note 26), and for liquids (at tc it is very considerable, and
often exceeds that of gases, 0'00367). Hence the equation of state must include three
variables — v, p, and t. For a so-called perfect (ideal) gas, or for inconsiderable variation
of density, the elementary expression pv = Ra(t + at), or pv — R (273 + 2) should be
accepted, where R is a constant varying with the mass and nature of a gas, as expressing
this dependence, because it includes in itself the laws of Gay-Lussac and Mariotte, for at
a constant pressure the volume varies proportionally to 1 + at, and when t is constant
the product of tv is constant. In its simplest form the equation may be expressed thus :

where T denotes what is termed the absolute temperature, or the ordinary temperature
+ 273- that is, T-2 + 273.

Starting from the supposition of the existence of an attraction or internal pressure
(expressed by a) proportional to the square of the density (or inversely proportional to
the square of the volume), and of the existence of a volume or length of path (expressed
by b) of gaseous molecules, Van der Waals gives for gases the following more complex
equation of state : —

(p+ a } (v -6) = 1+0-003672 ;
V 9* J

if at 0° under a pressure ^ = 1 (for instance, under the atmospheric pressure), the volume
(for instance, a litre) of a gas or vapour be taken as 1, and therefore v and b be expressed
by the same units as p and a. The deviations from both the laws of Mariotte and Gay-
Lussac are expressed by the above equation. Thus, for hydrogen a must be taken as
infinitely small, and 6 = 0'0009, judging by the data for 1000 and 2500 metres pressure
(Note 28). For other permanent gases, for which (Note 28) I showed (about 1870) from
Regnault's and Natterer's data, a decrement of pv, followed by an increment, which was .
confirmed (about 1880) by fresh determinations made by Amagat, this phenomena may
be expressed in definite magnitudes of a and b (although Van der Waals' formula is not
applicable for minimum pressures) with sufficient accuracy for contemporary require-
ments. It is evident that Van der Waals' formula can also express the difference of the

140 PRINCIPLES OF CHEMISTRY

its gaseous state (that is, its elasticity, or the physical energy of its
molecules, or their rapid progressive movement) with comparative ease

coefficients of expansion of gases with a change of pressure, and according to the
methods of determination (Note 26). Besides this, Van der Waals' formula shows that

at temperatures above 273 ( a — 1\ only one actual volume (gaseous) is possible,

whilst at lower temperatures, by varying the pressure, three different volumes— liquid,
gaseous, and partly liquid partly saturated-vaporous — are possible. It is evident that

the above temperature is the absolute boiling point — that is, (tc) = 273 f ~ — 1 J . It is

found under the condition that all three possible volumes (the three roots of Van der
Waals' cubic equation) are then similar and equal (vc = Sb). The pressure in this case

(we) = a 9. These ratios between the constants a and b and the conditions of critical
276

state — i.e. (tc) and (pc) — give the possibility of determining the one magnitude from the
other. Thus for ether (Note 29), (tc}= 193°, (*p) = 40, from whence a = 0'0307, 6 = 0'00533.
From whence (t>c) = 0'016. That mass of ether which at a pressure of one atmosphere at
0° occupies one volume — for instance, a litre — occupies, according to the above- mentioned
condition, this critical volume. And as the density of the vapour of ether compared with
hydrogen = 37, and a litre of hydrogen at 0° and under the atmospheric pressure weighs
0-089(5 grams, then a litre of ether vapour weighs 3'32 grams ; therefore, in a critical
state (at 193° and 40 atmospheres), 3'32 grams occupy 0*016 litres, or 16 c.c. ; therefore 1
"gram occupies a volume of about 5 c.c., and the weight of 1 c.c. of ether will then be 0'21.
According to the investigations of Kamsay and Young (1887), the critical volume of ether
was approximately such at about the absolute boiling point, but the compressibility of
the liquid is so great that the slightest change of pressure or temperature acts consider-
ably on the volume. ^But the investigations of the above savants gave another indirect
demonstration of the true composition of Van der Waals' equation. They also found for
ether that the isochords, or the lines of equal volumes, are generally straight lines if the
temperatures and pressures vary. For instance, the volume of 10 c.c. for 1 gram of ether
corresponds with pressures (expressed in metres of mercury) equal to 0'185£ — 8'3 (for
instance, at 180° and 21 metres pressure, at 280° and 34'5 metres pressure). The recti-
linear form of the isochord (then v — & constant quantity) is a direct result of Van der
Waals' formula.

When, in 1883, I demonstrated that the specific gravity of liquids decreases in propor-
tion to the rise of temperature [S, = S0-K£ or S,= S0 (1-Kf)], or that the volumes
increase in inverse proportion to the binomial 1 — K£, that is, V/ = V0 (1 — Ktf)"1, where K
is the modulus of expansion, which varies with the nature of the liquid (an exactitude of
the same kind as that by which for gases the volumes increase proportionately to the
binomial l + at), then, in general, not only does a connection arise between gases and
liquids with respect to a change of volume, but also it would appear possible, by availing
oneself of Van der Waals' formula, to judge, from the phenomena of the expansion of
liquids, as to their transition into vapour, and to connect together all the principal pro-
perties of liquids, which up to this time had not been considered to be in direct dependence.
Thus Thorpe and Riicker found that 2(f c) + 278 = 1/K, where K is the modulus of expan-
sion in the above-mentioned formula. For example, the expansion of ether is expressed
with sufficient accuracy from 0° to 100° by the equation S< = 0'786 (1-0'00154£), or V<
= 1 (1 — 0'00154£), where 0'00154 is the modulus of expansion, and therefore (tc) = lS8°, or
by direct observation 193°. For silicon tetrachloride, SiCl4, the modulus equals 0'00186,
. from whence (£c) = 231°, and by experiment 280°. On the other hand, D. P. Konovoloff,
admitting that the external pressure p in liquids is insignificant when compared with the
internal (a in Van der Waals' formula), and that the work in the expansion of liquids is
proportional to their temperature (as in gases), directly deduced, from Van der Waals'
formula, the above-mentioned formula for the expansion of liquids, Vt=-l/ (1 — Kt), and

TIIK COMPOSITION OF WATKK. HYDROGEN 141

under the influence of chemical attraction,3"' which is not only shown
from the fact that hydrogen and oxygen (two permanent gases) form
liquid water, but also from many phenomena of the absorption of
hydrogen.

Hydrogen is vigorously condensed by certain solids ; for example,
by charcoal and by spongy platinum. If apiece of freshly-ignited char-
coal be introduced into a cylinder full of hydrogen standing in a
mercury bath, then the charcoal absorbs as much as twice its volume
of hydrogen Spongy platinum condenses still more hydrogen. But
l><illadium, a grey metal which occurs with platinum, absorbs more
hydrogen than any other metal. Graham showed that when heated to
a red heat and cooled in an atmosphere of hydrogen, palladium retains
as much as 600 volumes of hydrogen. When once absorbed it retains
the hydrogen at the ordinary temperature, and only parts with it when
heated to a red heat.30 This capacity of certain dense metals for the
absorption of hydrogen explains the property of hydrogen of passing
through metallic tubes.37 It is termed occlusion, and presents a

also the magnitude of the latent heat of evaporation, cohesion, and compressibility under
pressure. In this way Van der Waals' formula embraces the gaseous, critical, and liquid
states of substances, and shows the connection between them. On this account, although
Van der Waals' formula cannot be considered as perfectly general and accurate, yet it is
not only very much more exact i\i&npv = RT but is also more comprehensive, because
it applies to both gases and liquids. Further research will naturally give further prox-
imity to truth, and will show the connection between composition and the constants
(a and b) ; but a great scientific progress is seen in this form of the equation of
state.

Clausius (in 1880), taking into consideration the variability of a, in Van der Waals'
formula, with the temperature, gave the following equation of state : —

Sarrau applied this formula to Amagat's data for hydrogen, and found a = 0'0551,
c = — 0-00043, b = G'00089, and therefore calculated its absolute boiling point as — 174°, and
(pc] = 99 atmospheres. But as similar calculations for oxygen ( — 105°), nitrogen ( — 124°),
and marsh gas ( — 76°) gave t c higher than it really is, therefore the absolute boiling point
of hydrogen must lie below — 174°.

55 This and a number of similar cases clearly show how great are the internal
chemical forces compared with physical and mechanical forces.

36 The capacity of palladium to absorb hydrogen, and in so doing to increase in
volume, may be easily demonstrated by taking a sheet of palladium varnished on one
side, and using it as a cathode. The hydrogen which is evolved by the action of the
current is retained by the unvarnished surface, as a consequence of which the sheet curls
up. By attaching a pointer (for instance, a quill) to the end of the sheet this bending
effect is rendered strikingly evident, and on reversing the current (when oxygen will be
evolved and combine with the absorbed hydrogen, forming water) it may be shown that
on losing the hydrogen the palladium regains its original fo'rm.

37 Deville discovered that iron and platinum become pervious to hydrogen at a red
heat. He speaks of this in the following terms : — ' The permeability of such homogeneous
substances as platinum and iron is quite different from the passage of gases through
such non-compact substances as clay and graphite. The permeability of metals depends

142 PRINCIPLES OF CHEMISTRY

similar phenomenon to solution ; it is based on the capacity of metals
of forming unstable easily dissociating compounds38 with hydrogen
similar to those which salts form with water.

At the ordinary temperature hydrogen very feebly and rarely enters
into chemical reaction. The capacity of gaseous hydrogen for reaction
becomes evident only under a change of circumstances — by compression,
heating, or the action of light, or at the moment of its evolution. How-
ever, under these circumstances it combines directly with only a very
few of the elements. Hydrogen combines directly with oxygen, sulphur,
carbon, potassium, and certain other elements, but it does not combine
directly with either the majority of the metals or with nitrogen, phos-
phorus, ifcc. Compounds of hydrogen with certain elements on which
it does not act directly are, however, known ; they are not obtained by
a direct method, but by reactions of decomposition, or of double decom-
position, of other hydrogen compounds. The property of Irj'drogen of
combining with oxygen at a red heat determines its combustibility.
We have already seen that hydrogen easily takes fire, and that it then

- on their expansion, brought about by heat, and proves that metals and alloys have a
certain porosity.' However, Graham proved that it is only hydrogen which is capable of
passing through the above-named metals in this manner. Oxygen, nitrogen, ammonia,
and many other gases, only permeate through in extremely minute quantities. Graham
showed that at a red heat about 500 c.c. of hydrogen pass per minute through a surface
of one square metre of platinum I'l mm. thick, but that with other gasea the amount
transmitted is hardly perceptible. Indiarubber has the same capacity for allowing the
transference of hydrogen through its substance (see Chap. III.), but at the ordinary tem-
perature one square metre, 0'014 mm. thick, transmits only 127 c.c. of hydrogen per
' minute. In the experiment on the decomposition of water by heat in porous tubes, the
clay tube may be exchanged for a platinum one with advantage. Graham showed that
by placing a platinum tube containing hydrogen under these conditions, and surrounding
it by a tube containing air, the transference of the hydrogen may be observed by the
decrease of pressure in the platinum tube. In one hour almost all the hydrogen (97 p.c.)
had passed from the tube, without being replaced by air. It is evident that the occlusion
and passage of hydrogen through metals capable of occluding it are not only intimately
connected together, but are dependent on the capacity of metals to form compounds of
various degrees of stability with hydrogen — like salts with water.

58 Palladium, as it appeared on further investigation, gives a definite compound,
PdoH (see further) with hydrogen ; but what was most instructive was the investigation
of sodium hydride, Na.2H, which clearly showed that the origin and properties of such
compounds are in entire accordance with the conceptions of dissociation. In the chapter
devoted to sodium we shall therefore speak more fully of this substance.

Being a gas which is difficult to condense, hydrogen is little soluble in water ami
other liquids. At 0° a hundred volumes of water dissolve 1'9 volumes of hydrogen, and
alcohol 6'9 volumes measured at 0° and 760 mm. Molten iron absorbs hydrogen, but in
solidifying, it expels it. The solution of hydrogen by metals is to a certain degree
based on its affinity for metals, and must be likened to the solution of metiils in mercury
and to the formation of alloys. In its chemical properties hydrogen, as we shall see
later, has much of a metallic character. Pictet (see Note 81) even affirms that liquid
hydrogen has metallic properties. The metallic properties of hydrogen are also evinced
in the fact that it is a good conductor of heat, which is not the case with other gases
(Magnus).

THE COMPOSITION OF AVATKR. II VI'IK >< , KN 143

burns with a pale — that is, non-luminous — flame.39 Hydrogen does not
combine with the oxygon of the atmosphere at the ordinary tempe-
rature ; but this combination takes place at a red heat,40 and is accom-
panied by the evolution of much heat. The product of this combination
is \vater — that is, a compound of oxygen and hydrogen. This is the
xy/^/^.v/'x i>f water, and we have already noticed its analysis or decom-
position into its component parts. The synthesis of water may be very
easily observed if a cold glass bell jar be placed over a burning hydrogen
Ha me, and, better still, if the hydrogen flame be lighted in the tube of
a condenser. The water will condense in drops as it is formed on the
walls of the condenser and trickle down.41

Light does not aid the combination of hydrogen and oxygen, so
that a mixture of these two gases does not change when exposed to the
action of light ; but an electric spark acts just like a flame, and this is
taken advantage of for inflaming a mixture of oxygen and hydrogen, or
detonating gas, inside a vessel, as will be explained in the following
chapters. As hydrogen (and oxygen also) is condensed by spongy
platinum, by which a rise of temperature ensues, and as platinum acts
by contact (p. 38), therefore hydrogen also combines with oxygen,
under the influence of platinum, as Dobereiner showed. If spongy
platinum be thrown into a mixture of hydrogen and oxygen, an explo-
sion takes place. If a mixture of the gases be passed over spongy
platinum, combination also ensues, and the platinum becomes red-hot.42

50 If it be desired to obtain a perfectly colourless hydrogen flame, it must issue from
a platinum nozzle, as the glass end of a gas-conducting tube imparts a yellow tint to the
Hume, owing to the presence of sodium in the glass.

40 Let us imagine that a stream of hydrogen passes along a tube, and let us mentally
divide this stream into several parts, consecutively passing out from the orifice of the
tube. The first part is lighted — that is, brought to a state of incandescence, in which
state it combines with the oxygen of the atmosphere. A considerable amount of heat is
e\ -nlved in the combination. The heat evolved then, so to say, ignites the second part of
hydrogen coming from the tube, and, therefore, when once ignited, the hydrogen con-
tinues to burn, if there be a continual supply of it, and if the atmosphere in which it
l)n rns be unlimited and contains oxygen.

41 The combustibility of hydrogen may be shown by the direct decomposition of water
by sodium. If a pellet of sodium be thrown into a cup containing water, then it floats
on the water and evolves hydrogen, which may be lighted. The presence of sodium imparts
;i yellow tint to the flame. If potassium be taken, the hydrogen bursts into flame of
itself, because sufficient heat is evolved in the reaction for the ignition and inflammation
of the hydrogen. The flame is rendered violet by the potassium. If sodium be thrown
not on water, but on an acid, it will evolve more heat, and the hydrogen will then also
burst into flame. These experiments must be carried on with caution, as sometimes
towards the end a mass of sodium oxide (Note 8) is produced, and flies about; therefore
it is best to cover the vessel in which the experiment is carried on.

'- This property of spongy platinum is made use of in the so-called hydrogen cigar-
light. It consists of a glass cylinder or beaker, inside which there is a small lead stand
i which is not acted on by sulphuric acid), on which a piece of zinc is laid. This zinc is
covered by a bell, which is open at the bottom and furnished with a cock at the top.

144 PRINCIPLES OF CHEMISTRY

Although gaseous hydrogen does not act directly43 on many sub-
stances, yet in a nascent state reaction often takes place. Thus, for
instance, water on which sodium amalgam is acting contains hydrogen
in a nascent state. The hydrogen is here evolved from a liquid, and at
the first moment of its formation it must be in a condensed form.44

Sulphuric acid is poured into the space between the bell and the sides of the outer glass
cylinder, and will thus compress the gas in the bell. If the cock of the cylinder be
opened the gas will escape by it, and will be replaced by the acid, which, coining into
contact with the zinc, evolves hydrogen, and it will escape through the cock. If the
cock be closed, then the hydrogen evolved will increase the pressure of the gas in the
bell, and thus again force the acid into the space between the bell and the walls of the
outer cylinder. Thus the action of the acid on the zinc may be stopped or started at
will by opening or shutting the cock, and consequently a stream of hydrogen may be
always turned on. Now, if a piece of spongy platinum be placed in this stream, the
hydrogen will take light, because the spongy platinum becomes hot in condensing the
hydrogen and inflames it. The considerable rise in temperature of the platinum depends,
among other things, on the fact that the hydrogen condensed in its pores comes into
contact with previously absorbed and condensed atmospheric oxygen, with which hydrogen
combines with great facility in this form. In this manner the hydrogen cigar-light gives
a stream of burning hydrogen when the cock is open. In order that it should work
regularly it is necessary that the spongy platinum should be quite clean, and it is best
enveloped in a thin sheet of platinum foil, which protects it from dust. In any case,
after some time it will be necessary to clean the platinum, which may be easily done by
boiling it in nitric acid, which does not dissolve the platinum, but clears it of all
dirt. This imperfection has given rise to several other forms, in which an electric
spark is -made to pass before the orifice from which the hydrogen escapes. This is
arranged in such a manner that the zinc of a galvanic element is immersed when
the cock is turned, or a small coil giving a spark is put into circuit on turning the
hydrogen on.

45 Under conditions the same as those in which hydrogen combines with oxygen it is
also capable of combining with chlorine. A mixture of hydrogen and chlorine explodes
on the passage of an electric spark through it, or on contact with an incandescent sub
stance, and also in the presence of spongy platinum ; but, besides this, the action of light
alone is enough to bring about the combination of hydrogen and chlorine. If a mixture
of equal volumes of hydrogen and chlorine be exposed to the action of sunlight, com-
plete combination rapidly ensues, accompanied by a report. Hydrogen does not combine
directly with carbon, neither at the ordinary temperature nor by the action of heat and
pressure. But if an electric current be passed through carbon electrodes at a short
distance from each other (as in the elecric light or voltaic arc), so as to form an electric
arc in which the particles of carbon are carried from one pole to the other, then, in the
intense heat to which the carbon is subjected in this case, it is capable of combining
with hydrogen. A peculiar-smelling gas, called acetylene, C.,H..>, is thus formed from
carbon and hydrogen.

44 There is another explanation for the facility of the reactions which proceed at the
moment of separation. We shall afterwards learn that the molecule of hydrogen contains
two atoms, H2, but there are elements the molecules of which only contain one atom —
for instance, mercury. Therefore, every reaction of gaseous hydrogen must be accom-
panied by the dissolution of that bond which exists between the atoms forming a mole-
cule. At the moment of evolution, however, it is supposed that free atoms exist, and
for this reason, according to the hypothesis, act energetically. This hypothesis is not
borne out by facts, and the conception of hydrogen being condensed at the moment of
its evolution is more natural, and is in accordance with the fact (Note 12) that com-
pressed hydrogen displaces palladium and silver (Brunner, Beketoff) — that IP, acts as at
the moment of its evolution.

THE COMPOSITION OF WATER, HYDROGEN 145

In this condensed form it is capable of reacting on substances on which
it does not act in a gaseous state. There is a very intimate and evident
relation between the phenomena which take place in the action of
spongy platinum and the phenomena of the action in a nascent state.
The combination of hydrogen with aldehyde may be taken as an ex-
ample. Aldehyde is a volatile liquid with an aromatic smell, boiling at
21°, soluble in water, and absorbing oxygen from the atmosphere, and
in this absorption forming acetic acid — the substance which is found in
ordinary vinegar. If sodium amalgam be thrown into an aqueous
solution of aldehyde, the greater part of the hydrogen evolved combines
with the aldehyde, forming alcohol — a substance which is also soluble
in water, which forms the principle of all spirituous liquors, boils at 78°,
and which contains the same amount of oxygen and carbon as aldehyde,
but more hydrogen. The composition of aldehyde is C2H,0, and of
alcohol C2H6O. Reactions of substitution or displacement of metals
by hydrogen at the moment of its evolution are particularly nume-
rous.4"'

Metals, as we shall afterwards see, are in many cases able to replace
each other ; they also, and in some cases still more easily, replace and
are replaced by hydrogen. We have already seen examples of this in
the formation of hydrogen from water, sulphuric acid, ttc. In all these
cases the metals sodium, iron, or zinc displace the hydrogen which occurs
in these compounds. Hydrogen may be displaced from many of its
compounds by metals by exactly the same method as it is displaced

45 When, for instance, an acid and zinc are added to a salt of silver, the silver is
reduced ; but this may be explained as a reaction of the zinc, and not of the hydrogen at
the moment of its evolution. There are, however, examples to which this explanation
is entirely inapplicable ; thus, for instance, hydrogen, at the moment of its evolution,
easily takes up oxygen from its compounds with nitrogen if they be in solution, and
converts the nitrogen into its combination with hydrogen. Here the nitrogen and hydrogen,
so to speak, meet at the moment of their evolution, and in this state combine together.

It is evident from this that the elastic gaseous state of hydrogen fixes the limit of its
energy : hinders it from entering into those combinations of which it is capable. In the
nascent state we have hydrogen which is not in a gaseous state, and its action is then
much more energetic. This is rendered very clear from the conception of chemical
energy, because the process of passing into a gas requires a certain amount of heat, and
consequently absorbs a certain amount of work. If gaseous hydrogen is produced, it
shows that there are already conditions sufficient for the transmission of heat to the
hydrogen evolved in order to convert it into a gas. It is evident at the moment of evo-
lution that heat, which would be latent in the gaseous hydrogen, is transmitted to its
molecules, and consequently they are in a state of potential, and can hence act on many
substances.

Let us here remark the circumstance, which will be clearly understood from what has
been said above, that hydrogen condensed in the pores of certain metals, like palladium
and platinum, acts as a reducing agent on many substances. It will afterwards be
understood that substances containing much hydrogen, and easily parting with it, can
also act vigorously in effecting a reduction.

VOL. I. L

146 PRINCIPLES OF CHEMISTRY

from water ; so, for example, hydrochloric acid, which is formed
directly by the combination of hydrogen with chlorine, gives hydrogen
by the action of a great many metals, just as sulphuric acid does.
Potassium and sodium also displace hydrogen from its compounds with
nitrogen ; it is only from its compounds with carbon that hydrogen is
not displaced by metals. Hydrogen, in its turn, is able to replace
metals ; this is accomplished most easily on heating, and with those
metals which do not themselves displace hydrogen. If hydrogen be
passed over the compounds of many metals with oxygen at a red heat,
it takes up the oxygen from the metals and displaces them just
as it is itself displaced by metals. If hydrogen be passed over the
compound of oxygen with copper at a red heat, then metallic copper
and water are obtained — CuO-fH2=H2O + Cu. This kind of double
decomposition is called reduction with respect to the metal, which is
thus reduced to a metallic state from its combination with oxygen.
But it must be recollected that all metals do not displace hydrogen
from its compound with oxygen, and, conversely, hydrogen is not able
to displace all metals from their compounds with oxygen ; thus it does
not displace potassium, calcium, or aluminium from their compounds
with oxygen. If the metals be arranged in the following series :
K, Na, Ca, Al . . . . Fe, Zn, Hg . . . . Cu, Pb, Ag, Au, then
the first are able to take up oxygen from water — that is, displace
hydrogen — whilst the last do not act thus, but are, on the contrary,
reduced by hydrogen — that is, have, as is said, a less affinity for
oxygen than hydrogen, whilst potassium, sodium, calcium have more.
This is also expressed by the amount of heat evqlved in the act of
combination with oxygen, and is shown by the fact that potassium and
sodium and other similar metals evolve heat in decomposing water : but
copper, silver, and the like do not do this, because in combining with
oxygen they evolve less heat than hydrogen does, and therefore it hap-
pens that when hydrogen reduces these metals heat is evolved. Thus,
for example, if 16 grams of oxygen combine with copper, 38000 units of
heat are evolved ; and when 16 grams of oxygen combine with hydrogen,
forming water, 69000 units of heat are evolved ; whilst 23 grams of
sodium, in combining with 16 grams of oxygen, evolve 100000 units of
heat. This example clearly shows that chemical reactions which pro-
ceed directly and unaided evolve heat. Sodium decomposes water and
hydrogen reduces copper, because they are exothermal reactions, or
those which evolve heat ; copper does not decompose water, because
such a reaction would be accompanied by an absorption (or secretion)
of heat, or belongs to the class. of endothermal reactions, in which heat
is absorbed ; and such reactions do not generally proceed directly,

Till: COMPOSITION OF WATER, HYDROGEN 147

although they may take place with the aid of energy (electrical, ther-
mal, &c.) borrowed from some foreign source."1

The reduction of metals by hydrogen is taken advantage of for
determining the exact composition of water by weight. Copper oxide is
usually chosen for this purpose. It is heated to redness in hydrogen,
and the quantity of water thus formed is determined, then the quantity
of oxygen which occurs in it is found from the loss in weight of the
copper oxide. This loss will depend on the fact that the oxygen has
entered into the water. The copper oxide must be weighed immediately
before and after the experiment. The difference shows the weight of
the oxygen which entered into the composition of the water formed.
In this manner only solids have to be weighed, which is a very great
gain in the accuracy of the results obtained.47 Dulong and Berzelius
(1819) were the first to determine the composition of water by this
method, and they found that water contains 88'91 of oxygen and 11*09
of hydrogen in 100 parts, or 8-008 parts of oxygen per one part of
hydrogen. Dumas (1842) improved on this method,48 and found that

46 Several numerical data and reflections bearing on this matter are enumerated in
Notes 7, 9, and 11. It must be observed that the action of iron or zinc on water, or, con-
versely, of hydrogen on the oxides of iron or zinc, forms a reversible reaction, which
proceeds in one or the other direction, according to which is removed from the sphere of
action ; the hydrogen or the water act according to which is present in a predominating
mass. The influence of mass is clearly evinced in this case. . But the reaction
CuO + H.2 = Cu + HoO is not reversible ; the difference between the degrees of affinity is
very great in this case, and, therefore, as far as is at present known, no hydrogen is
evolved even in the presence of a large excess of water. It is to be further remarked,
that under the conditions of the dissociation of water, copper is not oxidised by water, most
probably because the oxide of copper itself is decomposable by heat.

47 This determination may be carried on in an apparatus like that mentioned in Note
13 of Chapter I.

48 We will proceed to describe Dumas' method and results. For this determination
pure and dry copper oxide is necessary. Dumas took a sufficient quantity of copper
oxide for the formation of 50 grams of water in each determination. As the oxide of
copper was weighed before and after the experiment, and as the amount of oxygen con-
tained in water was determined by the difference between these weights, it was essential
that no other substance besides the oxygen forming the water should be evolved from
the oxide of copper during its ignition in hydrogen. It was necessary, also, that the
hydrogen should be perfectly pure, and free not only from traces of moisture, but from
any other impurities which might dissolve in the water or combine with the copper and
form some other compound with it. The bulb containing the oxide of copper (fig. 26),
and which was heated to redness, should be quite free from air, as otherwise the oxygen
in the air might, in combining with the hydrogen passing through the vessel, form water
in addition to the oxygen of the oxide of copper. The water formed should be entirely
absorbed in order to accurately determine the quantity of the resultant water. The
hydrogen was evolved in the three-necked bottle. The sulphuric acid, for acting on the zinc,
is poured through funnels into the middle neck. The hydrogen evolved in the Woulfe's
bottle passes through U tubes, in which it is purified, to the bulb, where it comes into
contact with the copper oxide, forms water, and reduces the oxide to metallic copper;
the water formed is condensed in the second bulb, and any passing off is absorbed in the
second set of U tubes. This is the general arrangement of the apparatus. The bulb

L 2

148

PRINCIPLES OF CHEMISTRY

water contains 12 '575 parts of hydrogen per 100 parts oxygen, that is —
7-990 parts of oxygen per 1 part of hydrogen, and therefore it is usually

with the copper oxide is weighed before and after the experiment. The loss in weight,
shows the quantity of oxygen which went into the composition of the water formed,

the weight of the latter being
shown by the gain in weight of
the absorbing apparatus. Know-
ing the amount of oxygen in the
water formed, we also know the
quantity of hydrogen contained
in it, and consequently we deter-
mine the composition of water by
weight. This is the essence of the
determination. We will now turn
to particulars. In one neck of the
three-necked bottle there is placed
a tube immersed in mercury. This
serves as a safety-valve to pre-
vent the pressure inside the ap-
paratus becoming too great from
the rapid evolution of hydrogen.
Did the pressure rise to any con-
siderable extent, the current of
gases and vapours would be very
rapid, and, as a consequence, the
hydrogen would not be perfectly
purified, or the water be entirely
absorbed in the tubes placed for
this purpose. In the third neck
of the Woulfe's bottle there is a
tube leading the hydrogen to the
purifying apparatus, consisting
of eight U tubes, destined for the
purification and testing of the hy-
drogen. The hydrogen, evolved
by zinc and sulphuric acid, is
purified by passing it first through
a tube full of pieces of glass moist-
ened with a solution of lead ni-
trate, next through silver sul-
phate; the lead nitrate retains
sulphuretted hydrogen, and ar-
seniuretted hydrogen is retained
by the tube with silver sulphate.
Caustic potash in the next U tube
retains any acid which might
come over. The two follow-
ing tubes are filled with lumps of
dry caustic potash in order to ab-
sorb any carbonic anhydride and
moisture which the hydrogen
might contain. The next two tubes
are, to completely dry the gas,
filled with a powder of phosphoric

THE COMPOSITION OF WATER, HYDROGEN 149

received that -i rater contains eight parts by weight of oxygen per one part
/ii/ /n •!(//! f. of hydrogen. By whatever method water be obtained, it will

anhydride, intermingled with lumps of pumice-stone. They are immersed in a freezing
mixture. The small U tube contains hygroscopic substances, and is weighed before the
experiment : this is in order to know whether the hydrogen passing through still retains
any moisture. If it does not, then the weight of this tube will not vary during the
whole experiment, but if the hydrogen evolved still retains moisture, the tube will in-
crease in weight. The copper oxide is dropped into the bulb, which is, previous to the
experiment, dried with the copper oxide during a long period of time. The air is
then exhausted from it, in order to weigh the oxide of copper in a vacuum and to
avoid making any correction for weighing in air. The bulb is made of infusible glass,
that it may be able to withstand a lengthy (20 hours) exposure to a red heat without
changing in form. The weighed bulb is only connected with the purifying apparatus after
the hydrogen has already passed through for a long time, and after experiment has shown
that the hydrogen passing from the purifying apparatus is pure and does not contain
any air. When the bulb is connected with the purifying apparatus, its cock is opened
and the hydrogen fills the bulb. The drawn-out end of the bulb is joined by an india-
rubber tube with the second bulb, in which the water formed is condensed. When this
connection is made, the thread binding up the india-rubber tube is untied, and then the
hydrogen can pass freely through the apparatus. On passing from the condensing bulb
the gas and vapour enter into an apparatus for absorbing the last traces of moisture.
The first U tube contains pieces of ignited potash, the second and third tubes phosphoric
anhydride or pumice-stone moistened with sulphuric acid. The last of the two is
employed for determining whether all the moisture is absorbed, and is therefore weighed
separately. The final tube only serves as a safety-tube for the whole apparatus, in order
that the external moisture should not penetrate into it. The glass cylinder contains
sulphuric acid, through which the excess of hydrogen passes; it enables the rate at
which the hydrogen is evolved to be judged, and whether its amount should be decreased
or increased.

When the apparatus is set up it must be seen that all its parts are hermetically tight
before commencing the experiment. When the previously weighed parts are joined up
together and the whole apparatus put into communication, then the bulb containing the
copper oxide is heated with a spirit lamp (reduction does not take place without the aid
of heat), and the reduction of the copper oxide then takes place, and water is formed,
which condenses in the absorbing apparatus. When nearly all the copper oxide is re-
duced the lamp is removed and the apparatus allowed to cool, the current of hydrogen
being kept up all the time. When cool, the drawn-out end of the bulb is fused up, and
the hydrogen remaining in it is exhausted, in order that the copper may be again weighed in
a vacuum. The absorbing apparatus remains full of hydrogen, and would therefore present
a less weight than if it were full of air, as it was before the experiment, and, therefore,
having disconnected the copper oxide bulb, a current of dry air is passed through it until
the gas passing from the glass cylinder is quite free from hydrogen. The condensing
bulb and the two tubes next to it are then weighed, in order to determine the quantity of
water formed. Dumas repeated this experiment many times. The average result was
that water contains 1253'3 parts of hydrogen per 10000 parts of oxygen. Making a
correction for the amount of air contained in the sulphuric acid employed for producing
the hydrogen, Dumas obtained the average figure 1251'5, between the extremes 1247 -2
a n< I 1256-2. This proves that per 1 part of hydrogen water contains 7'9904 parts of
oxygen, with a possible error of not more than 7^, or 0'08, in the amount of oxygen per
1 part of hydrogen.

Erdmann and Marchand, in eight determinations, found that per 10000 parts of
oxygen water contains an average of 1252 parts of hydrogen, with a difference of from
1258-5 to 1248-7 ; hence per 1 part of hydrogen there would be 7'9952 of oxygen, with an
error of at least 0'05, because, taking the figure 1258'5, the amount of oxygen per 1
part of hydrogen would be 7'944.

150 PKIXCIPLES OF CHEMISTRY

always present the same composition. Whether it be taken from nature
and purified, or whether it be obtained from hydrogen by oxidation, or
whether it be separated from any of its compounds, or obtained by some
double decomposition — it will in every case contain one part of hydrogen
and eight parts of oxygen. This is because water is a definite chemical
compound. Detonating-gas, from which it may be formed, is a simple
mixture of oxygen and hydrogen, although a mixture of the same
composition as water. All the properties of both constituent gases are
preserved in detonating-gas. Either one or the other gas may be
added to it without destroying its homogeneity. The fundamental
properties of oxygen and hydrogen are not found in water, and neither
of the gases can be added to it. But they may be evolved from it. In
the formation of water there is an evolution of heat ; for the decom-
position of water heat is required. All this is expressed by the words,
Water is a definite chemical compound of hydrogen with oxygen. Tak-
ing the symbol of hydrogen, H, as expressing a unit quantity by weight
of this substance, and by expressing 16 parts by weight of oxygen by O,
we can express all the above statements by the chemical symbol of
water, H0O. As only definite chemical compounds are denoted by
formulae, having denoted the formula of a compound substance, we
express by it the entire series of conceptions which are connected with the
representation of a definite compound, and, at the same time, the quan-
titative composition of the substance by weight. Further, as we shall
afterwards see, formulae express the volume of the gases contained in a
substance. Thus the formula of water shows that it contains two volumes
of hydrogen and one volume of oxygen. Besides which, we shall learn
that the formula expresses the density of the vapour of a compound,
and on this, as we have seen, many properties of substances depend.
This vapour density, as we shall learn, also determines the quantity of
a substance entering into a reaction. Thus the letters H2O tell
the chemist the entire history .of the substance. This is an inter-
national language, which endows chemistry with a simplicity, clear-
ness, stability, and trustworthiness founded on the investigation of the
laws of nature.

Reiser (1888), in America, by employing palladium hydride, and by introducing
various new precautions for obtaining accurate results, found the composition of water
to be 15'95 parts of oxygen per 2 of hydrogen.

Certain of the latest determinations of the composition of water are hardly less exact
than the analysis made by Dumas, and always give less than 8, and on the average
7'98, of oxygen per 1 part of hydrogen. At present, therefore, the atomic weight of
oxygen is taken as 15'96. However, this figure is not to be entirely depended on, and
for ordinary accuracy it may be considered that O = 16.

151

CHAPTER III

OXYGEN AXD THE CHIEF ASPECTS OF ITS SALINE COMBINATIONS.

ON the earth's surface there is no other element which is so widely dis-
tributed as oxygen in its various compounds.1 It makes up eight-ninths
of the weight of water, which occupies the greater part of the earth's
surface. Nearly all earthy substances and rocks consist of compounds
of oxygen with metals and other elements. Thus, the greater part of
sand is formed of silica, SiO2, which is a compound of oxygen with silicon,
and contains 53 p.c of oxygen ; clay contains water, alumina (formed of
aluminium and oxygen), and silica. It may be considered that earthy
substances and rocks contain up to one-third of their weight of oxygen ;
animal and vegetable substances are also very rich in oxygen. With-
out counting the water present in them, plants contain up to 40, and
animals up to 20 p.c. by weight of oxygen. Thus, oxygen compounds
predominate on the earth's surface, and form about one-half of the
whole of the solid and liquid matters of the earth's crust. Besides
this, a portion yet remains free, and is contained in admixture with
nitrogen in the atmosphere, forming about one-fourth of its mass, or
one-fifth of its volume.

Being so widely distributed in nature, oxygen plays a very im-
portant part in it, for a number of the phenomena which take place
before us are mainly dependent on it. Animals breathe air in order
to obtain only oxygen from it, the oxygen entering into their
respiratory organs (the lungs of human beings and animals, the gills of
fishes, and the trochae of insects) ; they, so to say, drink in air in order
to absorb the oxygen. The oxygen of the air (or dissolved in water)
passes through the membranes of the respiratory organs into the blood,
is retained in it by the blood corpuscles, is transmitted by their
means to all parts of the body, aids their transformations, bringing

1 As regards the interior of the earth, it probably contains far less oxygen compounds
than the surface, judging by the accumulated evidences of the earth's origin, of mete-
orites, of the earth's density, &c., as set forth in the fourth chapter of my work on the
' Naphtha Industry,' 1877, in speaking of the origin of naphtha.

152 PEINCIPLES OF CHEMISTRY

about chemical processes in them, and chiefly extracting carbon from
them in the form of carbonic anhydride, the greater part of which
passes into the blood, is dissolved by it, and is thrown off by the lungs
during the absorption of the oxygen. Thus, in the process of respiration
carbonic anhydride (and water) is given off, and the oxygen of the air
absorbed, by which means the blood is changed from a dark-red
venous to a bright-red arterial blood. The cessation of this process causes
death, because then all those chemical processes, and the consequent
heat and work which the oxygen introduced into the system brought
about, ceases. For this reason suffocation and death ensue in a vacuum,
or in a gas which does not contain free oxygen (which does not support
combustion). If an animal be placed in an atmosphere of free oxygen,
then at first its movements are very active and a general invigoration is
remarked, but a reaction soon sets in, and perhaps death may ensue.
The oxygen of the air, when it enters the lungs, is diluted with four
volumes of nitrogen, which is not absorbed into the system, and there-
fore the blood absorbs but a small quantity of oxygen from the air,
whilst in an atmosphere of pure oxygen a large quantity of oxygen
would be absorbed, which would produce a very rapid change of all parts
of the organism, and destroy it. From what has been said, it will be
understood that oxygen may be employed in respiration, at least for a
limited time, when the respiratory organs suffer under certain forms of
suffocation and impediment to breathing.2

The combustion of organic substances — that is, substances which
make up the composition of plants and animals- — proceeds in the
same manner as the combustion of many inorganic substances, such as
sulphur, phosphorus, iron, &c., from the combination of these sub-
stances with oxygen, as was described in the Introduction. The de-
composition, rotting, and similar transformations of substances, which

2 It is evident that the partial pressure (see Chap. II.) acts in respiration. The researches
of Paul Bert showed this with particular clearness. Under a pressure of one-fifth of an at-
mosphere consisting of oxygen only, animals and human beings remain under the ordinary
conditions of the partial pressure of oxygen, but organisms cannot support air rarefied to one-
fifth, for then the partial pressure of the oxygen falls to one-twenty-fifth of an atmosphere.
Even under a pressure of one-third of an atmosphere the regular life of human beings is im-
possible, by reason of the impossibility of respiration (of the decrease of solubility of oxygen
in the blood), owing to the small partial pressure of the oxygen, and not from the mechani-
cal effect of the decrease of pressure. Paul Bert illustrated all this by many experiments,
some of which he conducted on himself. This explains, among other things, the discom-
fort felt in the ascent of high mountains or in balloons when the height reached exceeds
eight kilometres, and at pressures below 250 mm. (Chap, II. note 23). It is evident that
an artificial atmosphere has to be employed in the ascent to great heights, just as in sub-
marine work. The cure by compressed and rarefied air which is practised in certain ill-
nesses is based partly on the mechanical action of the change of pressure, and partly on
the alteration in the partial pressure of the respired oxygen.

OXVHKN AND JTS SALINE COMBINATIONS 158

proceed around us, are also very often dependent on the action of the
oxygen of the air, and also reduce it from a free to a combined state.
The majority of the compounds of oxygen are, like water, very stable,
and do not give up their oxygen under the ordinary conditions of nature.
As these processes are taking place everywhere, therefore the amount
of free oxygen in the atmosphere should decrease, and this decrease
should proceed somewhat rapidly. This is, in fact, observed where
combustion or respiration proceeds in a closed space. Animals suffocate in
a closed space because in consuming the oxygen the air remains unfit for
respiration. In. the same manner combustion, in time, ceases in a closed
space, which may be proved by a very simple experiment. An ignited
.substance — for instance a piece of burning sulphur — has only to be placed
in a glass flask, which is then closed with a stout cork to prevent the
access of the external air ; combustion will proceed for a certain time,
so long as the flask contains any free oxygen, but it will cease, although
there still remain unburnt sulphur, when all the oxygen of the enclosed
air has combined with the sulphur. From what has been said, it is
evident that regularity of combustion or respiration requires a con-
stant renewal of air — that is, that the burning substance or respiring
animal should have access to a fresh supply of oxygen. This is attained
in human habitations by having many windows, outlets, and ventilators,
and by the current of air produced by tires and stoves. As regards the
air over the entire earth's surface, its amount of oxygen hardly decreases,
because in nature there is a process going on which renews the supply
of free oxygen. Plants, or rather their leaves, during daytime 3 — that is,
under the influence of light — evolve free oxygen. Thus the loss of
oxygen which occurs in consequence of the respiration of animals and of
combustion is made good by plants. If a leaf be placed in a bell jar con-
taining water, and carbonic anhydride (because this gas is absorbed and
oxygen evolved from it by plants) be passed into the bell, and the whole
-apparatus be placed in sunlight, then oxygen will accumulate in the
bell jar. This experiment was first made by Priestley at the end of the
last century. Thus the life of plants on the earth not only serves for
the formation of food for animals, but also for keeping up a constant
percentage of oxygen in the atmosphere. In the long period of the life of
the earth that equilibrium has been attained between the processes ab-

•" A t night, without the action of light, without the absorption of that energy which
is required for the decomposition of carbonic anhydride into free oxygen and carbon,
which is retained by the plants, they breathe like animals, absorbing oxygen and evolving
carbonic anhydride. This process also goes on side by side with the reverse process in
daytime, but then it is far feebler than that which gives oxygen. This observation is a
necessary consequence of an aggregate of data referring to the physiological processes of
plants.

154 PRINCIPLES OF CHEMISTRY

sorbing and envolving oxygen, by which a definite quantity of free
oxygen is preserved in the entire mass of the atmosphere.4

Free oxygen may be obtained by one or another method from all
the substances in which it occurs. Thus, for instance, the oxygen of
many substances may be transferred into water, from which, as we
have already seen, oxygen may be obtained.5 We will first consider
the methods of extracting oxygen from air as being a substance every-
where distributed. The separation of oxygen from it is, however,
hampered by many difficulties.

From air, which contains a mixture of oxygen and nitrogen, the
nitrogen alone cannot be removed, because it has 110 inclination to
combine directly or readily with any substance ; and although it does
combine with certain substances (boron, titanium), these substances com-
bine simultaneously with the oxygen of the atmosphere.6 However,

4 The earth's surface is equal to about 510 million square kilometres, and the mass of
the air (at a pressure of 760 mm.) on each kilometre of surface is about 10 J thousand millions
of kilograms, or about 10^ million tons ; therefore the whole weight of the atmosphere
is about 5100 million million ( = 51xl014) tons. Consequently there are about 2 x 1015
tons of free oxygen in the earth's atmosphere. The innumerable series of processes
which absorb a portion of this oxygen are compensated for by the plant processes. Count-
ing that 100 million tons of vegetable matter, containing 40 p.c. of carbon, formed from
carbonic acid, are produced (and the same process proceeds in water) per year on the 100
million square kilometres of dry land (ten tons of roots, leaves, stems, &c. per hectare, or
Y£O of a square kilometre), we find that the plant life of the dry land gives about 100,000
tons of oxygen, which is an insignificant fraction of the entire mass of the oxygen of
the air.

5 The extraction of oxygen from water may evidently be accomplished by two pro-
cesses : either by the decomposition of water into its constituent parts by the action of a
galvanic current (Chap. II.), or by means of the removal of the hydrogen from water.
But, as we have seen and already know, hydrogen enters into direct combination with very
few substances, and then only under special circumstances ; whilst oxygen, as we
shall soon learn, combines with nearly all substances. Only gaseous chlorine (and
especially, fluorine) is capable of decomposing water, taking up the hydrogen from it,
without combining with the oxygen. Chlorine is soluble in water, and if an aqueous
solution of chlorine, so-called chlorine water, be poured into a flask, and this flask be
inverted in a basin containing the same chlorine water, then we shall have an apparatus
by means of which oxygen may be extracted from water. At the ordinary temperature,
and in the dark, chlorine does not act on water, or only acts very feebly ; but under
the action of direct sunlight chlorine decomposes water, with the evolution of oxygen.
The chlorine then combines with the hydrogen, and gives hydrochloric acid, which dis-
solves in the water, and therefore free oxygen only will be separated from the liquid:
and it will only contain a small quantity of chlorine in admixture, which can be easily
removed by passing the gas through a solution of caustic potash, which retains the
chlorine.

6 A difference in the physical properties of both gases cannot be here taken advantage
of, because they are very similar in this respect. Thus the density of oxygen is 1(5, and
of nitrogen 14 times greater than the density of hydrogen, and therefore porous vessels
cannot be here employed — the difference between the times of their passage through a
porous surface would be too insignificant.

Graham, however, succeeded in enriching air in oxygen by passing it through india-

OXYGEN AND ITS SALINE COMBINATIONS

155

oxygen may be separated from air by causing it to combine with sub-
stances which may be easily decomposed by the action of heat, and, in

rubber. This may be done in the following way : — A common india-rubber cushion, E

(Fig. 27), is taken, and its orifice hermetically connected with an air-pump, or, better

still, a mercury aspirator (the Sprengel pump is designated by the letters A, c, B). "When

the aspirator (Chap. II. note 16)

pumps out the air, which will be

seen by the mercury running

out in an almost uninterrupted

stream, and from its stand-

ing at near the barometric

height, then it may be clearly re-

marked that gas passes through

the india-rubber. This is also

seen from the fact that bubbles

of gas continually pass along with

the mercury. A small pressure

of air may be constantly kept

up in the cushion by pouring

mercury into the funnel A, and

screwing up the cock c, so that

the stream flowing from it be

small, and then a portion of the

air passing through the india-

rubber will be carried along

with the mercury. This air may

be collected in the cylinder B.

Its composition proves to be

about 42 volumes of oxygen with

57 volumes of nitrogen, and one

volume of carbonic anhydride,

whilst ordinary air contains

only 21 volumes of oxygen in

100 volumes. A square metre of

india-rubber surface (of the usual

thickness) passes about 45 c.c. of

such air per hour. This experi-

ment clearly shows that india-

rubber is permeable to gases.

This may, by the way, be ob-

served in common toy balloons

filled with coal-gas. They fall

after a day or two, not be-

cause there are holes in them,

but because air penetrates into,

and the gas from, their interior,

through the surface of the india-

rubber of which they are made. The rate of the passage of gases through india-

rubber does not, as Mitchell and Graham showed, depend on their densities, and con-

sequently its permeability is not determined by orifices. It more resembles dialysis

— that is, the penetration of liquids through colloid surfaces. Equal volumes of gases

penetrate through india-rubber in periods of time which are related to each other as

follows : — carbonic anhydride, 100 ; hydrogen, 247 ; oxygen, 582 ; marsh gas, 688 ; carbonic

oxide, 1220 ; nitrogen, 1858. Hence nitrogen penetrates more slowly than oxygen, and

carbonic anhydride more quickly than other gases. 2' 556 volumes of oxygen and

Fra> 27.-Graham's apparatus for the decomposition of air
by pumping it through india-rubber.

-o lining. u'ive up the oxygen absorbed that is, l»v making use of re-
versible react io] i v. 'llms. ft>i' instance, the oxv^'en < »f t he at niosphere
may In- made to oxidise sulphurous anhydride, S( )., (bypassing directly
over ignited spongy platinum), and to form sulphuric1 anhydride, or
sulphur trioxide. S( );j : and this su list a net1 (which is a solid and volatile,
and therefore, easily separated from the nitrogen and sulphurous
anhydride), l»y heating again, gives oxvgen and sulphurous anhydride.
Caustic >oda or lime extracts (absorbs) the sulphurous anhydride from
this mixture, whil>t the oxygen is not absorbed, and thu> it is isolated
from the air. < hi a lar^e scale in works, as we >hall afterwards see,
sulphurous anhydride is transformed into hydrate of .-ulphuric tri oxide,
or sulphuric acid. H._,S(),: if this is made to fall in drops on reddiot
flagstones, water, sulphurous anhydride, and oxygen are obtained.
The oxygen i^ ra-ilv isolated from this mixture bv parsing the gases
over lime. The extraction of oxvgen from oxide of mercury
(Priestley, Lavoisier;, which is obtained from meivurv and the oxvgen
ot the atmosphere, is also a reversible reaction bv which oxygen mav be
obtained from the atmosphere. So also, bv passing diy air through a
red-hot tube containing barium oxide, it is made- to combine with tin
oxygen of the air. \}y this reaction the so-called barium peroxide.
I»a< ) „ is formed from the barium oxide I>a() and -at a higher tempe-
rature the former evolves the absorbed oxygen, and leaves the bari
oxide originally taken.7

[f the process of dialv-i- 1"' repealed on the ;iir \vlucl
i i ndia- ruhl icr. then a mixture coiitainin.;' i'.,"> p.c. l>y volnnn
uiy lie thouudit that the cause ol this phen. nneiimi i- the ah
.11 -,, Chap. 1 1 of :_:ases l,y india-ruhher and the ,.\ ,,liH imi of the -a-
iim : and. indeed, india rnld'cr does ali-orli ptsex, especially carlimiii
metal-, especially mi an increase of temperature. al>-<>H> ^ases, as \va-
ipter. (iraham called the ahove method of the decompositi. i

; Ti e preparation ot oxygen li\ tin- method, uhidi is due to !'.nn-.en. i- conducted ii
.1 porcelain tnl.e. whicli i- placed in a stove heated liv charcoal, -o that it- emU project

pn dried ; ed in the tnl.e. one end of \\ Inch i> cmuiecteil \\ ith a pair ci

:. d< . Mr keeping up a current of air tliroii-h it. The air is previmish

pa ed t h)-oii. _d i .1 -olnt ion i ,f can-.! ic pota-h. to renio\-e all t race- it carl ionic anhydride

a .1 ' er> caret, ill;, rii-ie,] t . H' 1 1 1C hy d 1M t e 1 5a I I ,< ) , doe> not - i S e t he p, ToX ide I. At;

• • :, -on tin , de ot liai-iuni ah-orlis o.xy_ren from the air. so that the j.ra!

• • • im-t i-nl i'el\ of nil i-o-en. Wln-n I he a li~orpt ion cea--es, t he ail

i • - • Thekn inn i,\i.|e i-, ciiiix erled into pei'ovide under tlie~e circumstances

' i- all o I'll all. .lit one part of o\\ -en l.\ Wei-lit. When tin

,1, ..,,,•,,: '. ., . , ., , .... elided, a em-k with a •ja-cmidiictiii'_' tiihe istixiM

<>XY<;KN AND ITS SALINE COMBINATIONS 157

( >.\ygen is evolved with particular ease by a whole series of un-
stable oxygen compounds, of which we will proceed to take a general
survey, remarking that many of these reactions, although not all, belong
to the number of reversible reactions ; 8 so that in order to ob-
tain many of these substances (for instance, potassium chlorate) rich
in oxygen, recourse must be had to indirect methods (see Intro-
duction), with which we shall become acquainted in the course of this
book.

1. The- compounds of oxygen with certain metals, and especially
with the so-called noble metals — that is, mercury, silver, gold, and
platinum — having been once obtained, retain their oxygen at the ordi-
nary temperature, but part with it at a red heat. The compounds are
solids, generally amorphous and infusible, and are easily decomposed by
heat into the metal and oxygen. We have seen an example of this in
speaking of the decomposition of mercury oxide. Priestley, in 1774,
obtained pure oxygen for the first time by heating mercury oxide by
means of a burning-glass, and clearly showed its difference from air.
He showed its characteristic property of supporting combustion ' with
remarkable vigour,' and named it dephlogisticated air.

into the other end, and the heat of the stove is increased to a bright-red heat (800°). At
this temperature the barium peroxide gives up all that oxygen which it acquired at a dark-
red heat — i.e., about one part by weight of oxygen is evolved from twelve parts of barium
peroxide. After the evolution of the oxygen there remains the barium oxide which was
originally taken, so that air may be again passed over it, and thus the preparation of oxygen
from one and the same quantity of barium oxide may be repeated many times. Oxygen
has been procured one hundred times from one mass of oxide by this method ; all the neces-
sary precautions being taken, as regards the temperature of the mass and the removal of
moisture and carbonic acid from the air. Unless these precautions be taken, the mass
of oxide soon spoils.

As oxygen may become of considerable technical use, from its capacity for giving
high temperatures and intense light in the combustion of substances, its preparation
directly from air by practical methods forms a problem whose solution many investi-
gators continue to work at up to the present day. The most practical method is that of
Tessie du Motoy. It is based on the fact that a mass of equal weights of manganese
peroxide and caustic soda at an incipient red heat (about 850°) absorbs oxygen from air,
with the separation of water, according to the equation MnO.j + 2NaHO + O = Na.»MnO4
+ H._,O. If superheated steam, at a temperature of about 450°, be then passed through
the mixture, the manganese peroxide and caustic soda orginally taken are regenerated, and
the oxygen held by them is evolved, according to the reverse equation Na.^MnO4
+ H.jO = MnOo + '2NaHO + O. This mode of preparing oxygen may be repeated for an
infinite number of times. The oxygen in combining separates out water, and steam,
acting on the resultant substance, evolves oxygen. Hence all that is required for the
preparation of oxygen by this method is fuel and the alternate cutting off the supply of
air and steam.

8 Even the decomposition of manganese peroxide is reversible, and it may be re-
ol.tained from that suboxide (or its salts), which is formed in the evolution of oxj'gen
(Chap. XI. note 6). The compounds of chromic acid containing the trioxide CrO5 in
evolving oxygen give chromium oxide, Cr.,O3, but they re-form the salt of chromic acid
when heated at a red heat in air with an alkali.

ir>8

-. Tin- SUOM nice- called !„ •!-<>. i-'nl' x'-' eyohe oxygen at a e;reater o
le>s heat (and also by the action of many acids). They usually contaii
nift;ils combined with a laruv quantity of oxygen. Peroxides arc tin
hiu'he-t oxides df certain metals ; those metals \vhidi form them irene
rally Lfive seyeral compounds with oxygen. Those of tin- lowe-t decree
of oxidation, containing the least amount of oxygen, are generally sub
stances which are capable of easily reacting on acids for instance
with sulphuric, acid. Such low oxides art4 called bases. Peroxide:
contain more oxygen than the ba-es formed by the same metals. Fo
example, lead oxide contains 7'1 parts of oxygen in 1 < >< I parts, and i:
basic, but lead peroxide contains ]'.}•'.} parts of oxygen in lou parts
^^<t ii'/<i in x>< jn'i'n.i'n.Ji' is a similar substance, which is a solid of a darl-
colour, and occurs in nature. It is employed in the manufacture!
under the name of black oxide of manganese (in (lerman. ' Braunstein,
the pyrolusite of the mineralogist). Peroxides are able to eyolyt
oxygen at a more or less elevated temperature. They do not then pan
with all their oxygen, but with only a portion of it. and are c<>nyerte<
into a lower oxide or base. Thus, for example, lead peroxide, on heat-
in U\ u'ives oxygen and lead oxide. The decomposition of this peroxidt
proceeds Somewhat easily on heating, eyen in a glass vessel, but manga-
nese peroxide <»nl\" exolyes oxygen at a strong red heat, and therefore
oxygen can only be obtained from it in iron, or other metallic, or clay
yessels. This used to be the method for obtaining oxygen. .Man^'anest!
peroxide only ]>arts with one-third of its oxygen (accordiiiiLj to the
equation .">.M n().,= M n:(( ), + ().,), whilst t wo-tliirds remain in the solid
substance which forms the residue, from the heating. Metallic peroxides
are also capable of eyolvimj; oxygen on heating with sulpliuric acid.
'J'hey then e\'ol\e so much oxygen as is in excess of that necessary tor
the formation of the base, the latter reacting on the >ulplmric acid
forming a compound (salt) with it. Thus barium peroxide, when
heated with sulphuric acid, forms ox vgen and barium oxide, which gives
a compound with sulphuric acid which is termed barium sulphate
(BaO.J+H^SOI^l>aS()1-f-H./)-r-O). This reaction usually proceeds
with irreater ease than the decomposition of peroxides by heat
a lone. Kor t lie purposes of experiment powdered man^am-M- peroxide is
usually taken and mixed with strong sulphuric acid in a lla.-d<. and the
apparatti- set up a- -ln>wn in Fig. L'S. rl'he gas \\-hicli is e\ol\ed is

OXYGEN AND ITS SALINK COMBINATIONS

159

passed through a Woulfe's bottle containing a solution of caustic potash,
to purify it from carl ionic anhydride and chlorine, which accompany the
evolution of oxygen from commercial manganese peroxide, and the ua- is
not collected until a thin smouldering taper placed in front of the escape
orifice bursts into flame, which shows that the gas coming off is oxygen.
By this method of decomposition of the manganese peroxide by sul-

FIG. 28.— Preparation of oxygen from manganese peroxide and sulphuric' acid. The gas evolved
is passed through a Woulfe's bottle containing caustic potash.

phuric acid there is evolved, not, as in heating, one-third, but one-half
of the oxygen contained in the peroxide (Mn02 + H2S04 = MnS04 -f
HoO + O) — that is, from 50 grams of peroxide about 7i grams, or
about 5^ litres, of oxygen,10 whilst by heating only about 3^ litres are
obtained. The chemists of Lavoisier's time generally obtained oxygen
by heating manganese peroxide. Now there are more convenient
methods known.

3. A third source to which recourse may be had for obtaining
oxygen is represented in acids and salts containing much oxygen, and
which are capable, by parting with a portion or all of their oxygen,
of being converted into other compounds (lower products of oxida-
tion) which are more difficultly decomposed. These acids and salts
(like peroxides) evolve oxygen either on heating alone, or when
heated with some other substance. Sulphuric acid may be taken
as an example of an acid which is decomposed by the action of heat
alone,11 for it breaks up at a red heat into water, sulphurous anhydride,

10 Scheele, in 1785, discovered the method of obtaining oxygen by treating manganese
peroxide with sulphuric acid.

11 All acids rich in oxygen, and especially those whose elements form lower oxides,
evolve oxygen either directly at the ordinary temperature (for instance, ferric acid), or on
hciiting (for instance, nitric, manganic, chromic, chloric, and others), or if basic lou.-r
oxides are formed from them, by heating with sulphuric acid. Thus the salts

160 PRINCIPLES OF CHEMISTRY

and oxygen, as was mentioned before. Priestley, in 1772, and Scheele,
somewhat later, obtained oxygen by heating nitre to a red heat. The
best examples of the formation of oxygen by the heating of salts is given
in jwtassium chlorate, or Berthollet's salt, so called after the French
chemist who discovered it. Potassium chlorate is a salt composed of
the elements potassium, chlorine, and oxygen, KC103. It occurs as-
transparent colourless plates, is soluble in water, especially in hot
water, and resembles common table salt in some of its physical properties;
it melts on heating, and in melting begins to decompose, evolving oxygen
gas. This decomposition ends in all the oxygen being evolved from
the potassium chlorate, potassium chloride being left as a residue, accord-
ing to the equation KC1O3=KC1 + O3.12 This decomposition proceeds
at a temperature which allows of its being conducted in a vessel
made of glass. However, in decomposing, the molten potassium
chlorate swells up and boils, and gradually solidifies, so the evolution of
the oxygen is not regular, and the glass vessel may crack. In order
to overcome this inconvenience, the potassium chlorate is crushed
and mixed with a powder of a substance which is incapable of com-
bining with the oxygen evolved, and which is a good conductor of heat.
Usually it is mixed with manganese peroxide.13 The decomposition of
the potassium chlorate is then considerably facilitated, and proceeds at
a lower temperature (because the entire mass is then better heated,
both externally and internally), without swelling up, and is therefore
more convenient than the decomposition of the salt alone. This
method for the preparation of oxygen is very convenient ; it is generally
employed when a small quantity of oxygen is required. Further, potas-
sium chlorate is easily obtained pure, and it evolves much oxygen. 100
grams of the salt give as much as 39 grams, or 30 litres, of oxygen.
This method is so simple and easy,14 that a course of practical chemistry

of chromic acid (for instance, potassium dichromate, K.)Cr.)O7) give oxygen with
sulphuric acid ; first potassium sulphate, K.^SO.^ is formed, and then the chromic acid set
free gives a sulphui'ic acid salt of the lower oxide, Cr.,05.

12 This reaction is not reversible, and is exothermal — that is, it does not absorb heat,
but, on the contrary, evolves 9713 calories per molecular weight KC1O5, equal to 122
parts of salt (according to the determination of Thomsen, who burnt hydrogen in a
calorimeter either alone or with a definite quantity of potassium chlorate mixed with
oxide of iron). It does not proceed at once, but first forms perchlorate, KCIO^ (see
Chlorine and Potassium). It is to be remarked that potassium chloride melts at 788°,
potassium chlorate at 372°, and potassium perchlorate at 010°.

13 The peroxide does not evolve oxygen in this case. It may be replaced by many oxides
— for instance, by oxide of iron. It is necessary to take the precaution that no combustible
substances (such as bits of paper, splinters, sulphur, &c.) fall into the mixture, as they
might cause an explosion.

14 The decomposition of a mixture of melted and well-crushed potassium chlorate

()XV(iKN AND ITS SALINE COMBINATIONS 101

is often commenced by the preparation of oxygen by this method, and
of hydrogen by the aid of zinc and sulphuric acid, all the more as
thi'M- -uses enable many interesting and striking experiments to be
made.15

A solution of bleaching powder, which contains calcium hypo-
chlorite, CaCl2O2, evolves oxygen when gently heated with the ad-
dition of a small quantity of certain oxides — for instance, cobalt
oxide, which in this case acts by contact (see Introduction). Of
itself, a solution of bleaching powder does not evolve oxygen when
heated, but it oxidises the cobalt oxide to a higher degree of oxidation ;
this higher oxide of cobalt in contact with the bleaching powder, decom-
poses into oxygen and lower oxidation products, and the resultant lower
oxide of cobalt with bleaching powder again gives the higher oxide,
which again gives up its oxygen, and so on.16 The calcium hypo-
chlorite is here decomposed according to the equation CaCl2O2 =
CaCl2 + O2. In this manner a small quantity of cobalt oxide17 is
sufficient for the decomposition of an indefinitely large quantity
of bleaching powder.

with powdered manganese peroxide proceeds at so low a temperature (the salt does not
melt) that it may be effected in an ordinary glass flask. As the reaction is exothermal, the
decomposition of potassium chlorate with the formation of oxygen may probably be
accomplished, under certain conditions (for example under contact action), at very low
temperatures. Substances mixed with the potassium chlorate probably act partially in
this manner.

15 Many other salts evolve oxygen by heat, like potassium chlorate, but they only
part with it either at a very strong heat (for instance, common nitre) or else are un-
suited for use on account of their cost (for instance, potassium manganate), or evolve
impure oxygen at a high temperature (for instance, zinc sulphate at a red heat gives
a mixture of sulphurous anhydride and oxygen), and are not therefore used in prac-
tice.

1(1 Such is, at present, the only possible method of explaining the phenomenon
of contact action. In many cases, as here, it is supported by observations based on facts.
Thus, for instance, it is known, as regards oxygen, that often two substances rich in
oxygen retain it so long as they are separate, but directly they come into contact
free oxygen is evolved from both of them. Thus, an aqueous solution of hydrogen
peroxide (containing twice as much oxygen as water) acts in this manner on silver oxide
(containing silver and oxygen). This reaction takes place at the ordinary temperature,
and the oxygen is evolved from both compounds. To this class of phenomena may be
also referred the fact that a mixture of barium peroxide and potassium manganate with
water and sulphuric acid evolves oxygen at the ordinary temperature. It would seem
that the essence of phenomena of this kind is entirely and purely a property of
contact ; the distribution of the atoms is changed by contact, and if the equilibrium be
unstable it is destroyed. This is especially clear for substances which change exother-
inally — that is, for those reactions which are accompanied by an evolution of heat. The
decomposition CaCLO.^ = CaCL> + O.2 belongs to this class (like the decomposition of
potassium chlorate).

17 Generally a solution of bleaching powder is alkaline (contains free lime), and, there-
VOL. I. M

Ki-J

'1 i'1 /'/'"/"/'//. s «/ ti.i-i/t/i //.' s — It is a ] lerma iH'iii o-a> -tliat is, it can-
not In- liquefied by pressure at the ordinary t empcrat ure, and further,
i- only li<|Uelird with difficulty (although more easily than hydrogen) at
temporal ures below — 1 '_'<)•. because this i~> its absolute boiling point.
As its critical pressure ''•' is about ')() atmospheres, it can lie easily
1 ii] netied HIM ler prosnres ureat pr than -^ ' atmospheres at temperat ures
belo\y — 1 L!U . 1 Met cT obtained liquid oxygen at — 1 I1 ' . l>v employing a
pre»ure above 100 atmospheres. According t() I*ewar, the density <>f
«»\y^cn in a critical stale is ()•().") (\\-atcr=l ), Inn it. like all (•ilici1 sub-
>taiH't'.s in tliis Mate.-'" varies considerablv in clen>itv \\'itli a clianicc <>t'
] ircs>tn-f and t cnijieraturc, and therefore inanv in \ cst i^'ators \vlio made
their observations tiudei1 hi^li ]>ressiii'c-> ^i\-e a ^i-eatei- density, as much
as I'l. ( >.\yu'e]i, like all u'ase^, is transparent, and like the majority of
u'a>e>, colourless. It has no smell or taste, which is evident from the
tact of it-- lieinu1 a component of air. The weight of one cubic centi-
metre in grains at U and 7(50 mm. pressure is (i-(K)l l^'.is Drains, and a
litre weighs Tll'liS u'rams : it is therefore ^li^htly denser than air.
Its den.Mtv in respect to air=l'10">(), and in ropect to hydrogen =1(5
(more exactly 1 .V'.ir,).-'1

'" IT musl lir rcninrl^fd that in all tin- above-cited rea
may lie prevented by the adinixtuve of substances ca]i
example, charcoal, many carbon (organic] conipoiiiiiU. ^n

lower oxi.lation product's, ^c-. These substances absorb the oxygen evolved, coinhiiu-
with it . a nd a coin] lound containin^r oxygen, but not free oxygen, is formed. Thus, if a

Lin-e of potassium chlorate and charcoal be heated, no oxyp-n is obtained, but
an explo-ion takes place from the rapid formation of ^rases rr-nlt in;j from the com-
bination of the oxygen of the pota--inm chloi-jite \\ith the charcoal.

The oxygen ol)tained by any of the abo\ c-descrilied methods is rarely -pure. It

chloride, which retain- the water. IJesides this, the oxygen nearly always contain--

-.a. f carbonic anhy<lride. and very <.f'ten small trace- of chlorine. The oxygen may

b'- treed troni the^e impni'itie-. by pas, MIL;' it tlii'oii'jh a solution of caustic potash.

Tin- i- done in \Voiflte's bottle-, a- was described in the la-t chapter. If the potassium

te be dr\ and pure, it -i\e, almosl pure oxygen. However, if the oxygen be

• •'• loi- i-e.piration in ca-es of sickness, it should be wa-hed b\ passing it thr«.n-h a

-olntion ol caustic alkali and through water. The be-t wa\ to obtain pure oxygen

d rertly. i N, take pot a- in in perch lorale i K('lO.,i. which can be well pnrilie.l and then

pun o \ \ • j en on heating.

' ' i i ice riling the absolute boil in;/ poii it. critical prr-siir*-. and on t he critical state in
i . -'I. -ee Chaji II. Note ii'.l and :', \ .

.In<l-in- IVom -ah.it ha been -,,id in Not- ::i of the ia-t clui|it«-r. and also from the
re-ult , of direct ob.ervation, if i- evidenl that all • nb-t a nee i n a crit ieal -t ale ha\ e a

• i . and I hat thev are \ er\ compn lile.

• A uatercon i-t ot 1 volume o| oxygen and -2. -, . .In me- ol hydrogen, and contain*

IT, part- b\ weight of oxy-en per '2. part-- b\ Wei'jhl ol hydrogen, it therefore alread\
. ~ from thi . that o\ \-en i Hi time- denser than h\dro'_'en. ( 'on \erselv. the com

OXYGEN AND ITS SALINE COMBINATIONS 163

In its chemical properties oxygen is remarkable from the fact that
it very easily— and, in a chemical sense, vigorously — reacts on a number
of substances, forming oxygen compounds. However, only a few
substances and mixtures of substances (for example, phosphorus, copper
with ammonia, decomposing organic matter, aldehyde, pyrogallol with
an alkali, <kc.) combine directly with oxygen at the ordinary
temperature, whilst many substances easily combine with oxygen at a
red heat, and often this combination presents a rapid chemical reaction
accompanied by the evolution of a large quantity of heat. Every
reaction which takes place rapidly, if it be accompanied by so great an
evolution of heat as to produce incandescence, is termed combustion.
Thus combustion ensues when many metals are plunged into chlorine,
or oxide of sodium or barium into carbonic anhydride, or when a spark
falls on gunpowder. A great many substances are combustible in
oxygen, and, owing to its presence, in air also. In order to start
combustion it is generally necessary22 that the combustible substance
should be brought to a state of incandescence. When once started —
i.e., when once the incandescent portion of the substance begins to
combine with oxygen — then combustion will proceed uninterruptedly
until either all the combustible substance or all the oxygen is consumed.
The continuation of the process does not require the aid of fresh
external heat, because sufficient heat23 is evolved to raise the tempeTa-
ture of the remaining parts of the combustible substance to the required

position of water by weight may be deduced from the densities of hydrogen and oxygen,
and the volumetric composition of water. This kind of mutual and opposite correction
is a method which strengthens the practical data of the exact sciences, whose
conclusions require, above all things, the greatest possible exactitude and variety of
corrections.

It must be observed that the specific heat of oxygen at constant pressure is 0'2175,
consequently it is to the specific heat of hydrogen (8'409) as 1 is to 15'6. Hence, the
specific heats are inversely proportional to the weights of equal volumes. This signifies
that equal volumes of both gases have (nearly) equal specific heats — that is, they require
an equal quantity of heat for raising their temperature by 1°. We shall afterwards con-
sider the specific heat of different substances more fully, and we will not, therefore, linger
over it at present.

Oxygen, like the majority of difficulty-liquefiable gases, is but slightly soluble
in water and other liquids. At the ordinary temperature, 100 volumes of water dissolve
about 3 volumes of oxygen, or more exactly, at 0° 4'1 vols., at 10° 8'3, and at 20° 3*0
(measuring the volumes at the same temperature as the water). From this it is evident
that water standing in air must absorb— i.e., dissolve — oxygen. This oxygen serves for
the respiration of fishes. Fishes cannot exist in boiled water, because it does not contain
the oxygen necessary for their respiration (see Chap. I.).

-- ( Vrtain substances (with which we shall afterwards become acquainted), however,
inflame of themselves in air ; for example, impure phosphuretted hydrogen, silicon,
hydride, zinc ethyl, and pyrophorus (very finely divided iron, &c.).

-•"' If so little heat is evolved that the adjacent parts are not heated to the tempera-
ture of combustion, then combustion will cease.

M 2

164

PEINCIPLES OF CHEMISTRY

degree. Examples of this are familiar to all from every-day experience.
Combustion proceeds in oxygen with greater rapidity, and is accom-
panied by a more powerful incandescence, than in ordinary air. This
may be demonstrated by a number of very convincing experiments. If
a piece of charcoal, attached to a wire and previously brought to red-
heat, be plunged into a flask full of oxygen, it rapidly burns at a white
heat — i.e., it combines with the oxygen, forming a gaseous product of
combustion called carbonic anhydride, or carbonic acid gas. This is the
same gas that is evolved in the act of respiration, for charcoal is one of
the substances which is obtained by the decomposition of all organic
substances which contain it, and in the process of respiration part of the

constituents of the body, so to speak, slowly
burn. If a piece of burning sulphur be laid on
a small cup attached to a wire and be placed
in a flask full of oxygen, then the sulphur,
which burns in air with a very feeble flame,
burns in the oxygen with a violet flame,
which, although pale, is much larger than
in air. If the sulphur be exchanged for a
piece of phosphorus,24 then, unless the phos-
phorus be heated, it combines very slowly
with the oxygen ; but, if heated, although
on only one spot, it burns with a very bril-
liant white flame, which is unbearable to
the sight. In order to heat the phosphorus

inside the flask, the most simple way is to bring a red-hot wire into con-
tact with it. Before the charcoal can burn, it must be brought to a state
of incandescence. Sulphur also will not burn under 100°, whilst phos-
phorus inflames at 40°. Phosphorus which has been already lighted in air
cannot so well be introduced into the flask, because it burns very rapidly
and with a large flame in air. If a small lump of metallic sodium be put
in a small cup made of lime,25 melted, and inflamed,26 then it burns very
feebly in air. But if burning sodium be immersed in oxygen, the

FIG. 29.— Mode of burning sul
phur, phosphorus, sodium, &c.
in oxygen

24 The phosphorus must be dry ; it is usually kept in water, as it oxidises in air. It
should be cut under water, as otherwise the freshly-cut surface oxidises. It must be dried
carefully and quickly by wrapping it in blotting-paper. If damp, it splutters in burning.
A small piece should be taken, as otherwise the iron spoon will melt. In this and the
other experiments on combustion, water should be poured over the bottom of the vessel
containing the oxygen, to prevent it from cracking. The cork closing the vessel should not
fit tightly, otherwise it may fly off with the spoon and burning substance, owing to the
expansion due to the heat of the combustion.

25 An iron cup will melt with sodium in oxygen.

26 In order to rapidly heat the lime crucible with the sodium, they are heated in the
flame of a blow-pipe described in Chap. VIII.

OXYGEN AND ITS SALINE r< >.M IM NATIONS 165

combustion is invigorated and is accompanied by a brighter yellow

flame. Metallic magnesium, which burns brightly in air, continues to

burn with still greater vigour in oxygen, forming a white powder,

which is a compound of magnesium with oxygen (magnesium oxide ;

magnesia). A strip of iron or steel does not

burn in air, but an iron wire or steel spring

may be easily burnt in oxygen. A much

larger piece of iron might naturally be burnt

if it only were convenient to heat it to the

required degree.27 The combustion of steel

or iron in oxygen is not accompanied by a

flame, but sparks of oxide fly in all directions

from the burning portions of the iron.28

In order to demonstrate by experiment
the combustion of hydrogen in oxygen, a gas-
conducting tube, bent so as to form a con-

..,.,,,, ,. , . . FIG. 30.— Mode of burning a steel

vemeiit jet, is led from the vessel evolving spring in oxygen.

hydrogen. The hydrogen is first set light

to in air, and then the gas-conducting tube is let down into a 'flask
containing oxygen. The combustion in oxygen will be similar to
that in air ; the flame remains pale, notwithstanding the fact that its
temperature rises considerably. It is instructive to remark that oxygen
may burn in hydrogen, just as hydrogen in oxygen. In order
to show the combustion of oxygen in hydrogen, a tube bent vertically
upwards and ending in a fine orifice is attached to the stop-cock of a
gas holder full of oxygen. Two wires, placed at such a distance from

-7 In order to burn a watch spring, a piece of tinder (or paper soaked in a solution of
nitre, and dried) is attached to one end. The tinder is lighted, and the spring is then
plunged into the oxygen. The burning tinder heats the end of the spring, the heated
part burns, and in so doing heats the further portions of the spring, which thus entirely
burns if enough oxygen is present.

28 The sparks of rust are produced by reason of the volume of the oxide of iron being
nearly twice that of the volume of the iron, and as the heat evolved is not sufficient to en-
tirely melt the oxide or the iron, the particles must be torn off and fly about. Similar
sparks are formed in the combustion of iron, in other cases also. We saw the combustion
of iron filings in the Introduction. In the welding of iron small iron splinters fly off in all
directions and burn in the air, as is seen from the fact that whilst flying through the air
they remain red hot, and also because, on cooling, they are seen to be no longer iron, but
a compound of it with oxygen. The same thing takes place when the hammer of a gun
strikes against the flint. Small scales of steel are heated by the friction, and glow and
burn in the air. The combustion of iron is still better seen by taking it as a very fine
powder, such as is obtained by the decomposition of certain of its compounds — for
instance, by heating Prussian blue, or by the reduction of its compounds with oxygen by
hydrogen ; when this fine powder is strewn in air, it burns by itself, even without being
previously heated (it forms a pyrophorus). This obviously depends on the fact that the
powder of iron presents a larger surface of contact with air than an equal weight in a
compact form.

each other as hi allow tin- passage of a constant series of sparks from a
Ividimkorii"s coil, are lixed in from of the oriiicc of the tube. This is
in order to ignite the oxvgen. which nia\' also he done bv udtach-
HIL; tinder round the ontire. and burning it. \\hen tlie wires are
arranged about the onl'.ee of the tube, and a series of sparks passes
! i» •; wren I IK 'in. 1 1 it MI an in \ ert ed ( because of the lightness of the hydro-
uvn i jar full of hydrogen is placed over the gas-conducting tube.
\\'hen the jarco\ers the orilice of the gas-conducting tube (and not
1 iff ore. as otherwise an explosion mi^ht take place ) the cock of the gaso-
meter is opened, and the oxvgen tlows into the hvdro^en and is set liidit
t«> by the sparks. The tlanie obtained is similar to that formed bv the
combustion of hydrogen in oxygen.'-"-' I'Yom this it is evident that tlie
tiaine is the localitv where the oxygen combines with the hydrogen,
then-fore a tlanie of burning oxygen can be obtained as well as a tlanie
of 1 lurninu' hvdrogen.

If. instead of hvdrogen. anv other combustible gas be taken -for
example, ordinary coal gas then the phenomenon of combustion will
be exactly the same, onlv a bright llame will be obtained, and the
products nf combustion will be different. However, as lighting gas
contains a considerable amount of free and combined hydrogen, it will.
also form a considerable (juantitv of water in its combustion.

If hvdrogen be mixed with o.xyuvn in the proportion in which thev
form water -i.e., if two volumes of hydrogen be taken for each
\olume of OXVLMMI then the mixture will he the same as that obtained
bv the decomposition of watt r bv a galvanic1 current detonating

• • | !•]• nicnl may l.c coinliictcd witlmut tin- \viiv-. if tin- liytlr..Lrrii In- li-lilrd in
lir. ,.| :, ryliiul.T. .MM! ,n I in- ,:iin,. !ini»' tin- rylin.l.T !>!• hrmi-'lit <.V<T tin- cud of a

>|. f 1,-ii in; i.\\-.-n. iiixl tin- i.llu-r with n -a-lit.ld.-r lull nf liyiln^i-n.

i! : . llh In (| )-, ,_cii i- li-litcil. .iinl a c ...... nut! la nip i: las-. ta|'ci-iii'_r

• • ' i . . jilai > 'i ''.< i tin' (-'irk. 'I'lii- liydr.i-i'ii nuil nin^ In Imi-n in-^iiic the
. • ,|i . . a! ! In • •• • |" •!!-•• n! tin1 n\\ jcn. It I In- ciirrcnl n| d\\;.'t'ii I"1 lln-n lit 1 !•• li\ litt !<•

; ,i . . Tl ;:
al . IHI!\ 1 ii- i ni Tea

I. id Ml li\di'i'ji , id !
and : can ea il\ I »< ,r.i\

IM tin- in^iitiicifiit siipi'lv nl' u.\\ LTi-ii. the IliiiiH-

i| I'.ir . \ , • I, i I 1 1 1 1 -I in 1 1 1 - . a 1 1 < I ! I U • 1 1 f. -a | »| '« 'a r> a t

,\\ i,f i..\\ -i-ii In- a.-_-ain im-n-iiM-il, the llaim- rr-
ll.iini- max In- mad.' ;<> a|>|u-ar at one nr the
di-i-n-a e nf tile elllTelil nl •_• a -, in list In- I>V

hi-ii Li- slin\\n liu\\ air Imnis in an at nn>>jihere
ai ill'- lam _dass is mil nl a -ja- eniiil)Ust ilile

OXYGEN AND ITS SALINE COMBINATIONS

167

an electric spark, because the spark heats the space through which it
passes, and acts consequently in a manner similar to ignition by means
of contact with an incandescent or burning substance. In fact, instead
of a spark a fine wire simply
may be taken, and an elec-
tric current passed through
it to bring it to a state of
incandescence ; in this case
there will be no sparks, but
the gases will inflame if the
wire be fine enough to be-
come red hot by the passage
of the current. Cavendish
made this experiment on the
ignition of detonating gas,
at the end of the last cen-
tury, in the apparatus shown
in fig. 31. Ignition by the
aid of the electric spark is
convenient, for the reason
that it may then be brought
about in a closed vessel,
and hence chemists still em-
ploy this method when it is FIG. 31.— Cavendish's apparatus for exploding detonatin
. . . gas. The bell jar standing in the bath is filled wit

required to ignite a mixture
of oxygen with a combus-
tible gas in a closed vessel.
For this purpose they now,
especially since Bunsen's
time,30 employ an eudiometer.

It consists of a thick glass tube graduated along its length in milli-
metres (for indicating the height of the mercury column), and
calibrated for a definite volume (weight of mercury). Two plati-
num wires are fused into the upper closed end of the tube, as
shown in fig. 32. They must be hermetically sealed into the tube,
so that there be no aperture left between them and the glass.31

50 Now, a great many other different forms of apparatus, sometimes designed for
special purposes, are employed in the laboratory for the investigation of gases. Detailed
descriptions of the methods of gas analysis, and of the apparatus employed, must be
looked for in works on analytical and applied chemistry.

31 In order to test this, the eudiometer is filled with mercury, and its open end
inverted into mercury. If there be the smallest orifice at the wires, the external air will
enter into the cylinder and the mercury will fall, although not rapidly if the orifice
be very fine.

a mixture of two volumes of hydrogen and one volume of
oxygen, and the thick glass vessel A is then screwed
into it. The air is first pumped out of this vessel, so
that when the stop-cock c is opened, it becomes filled
with detonating gas. The stop cock is then re-closed,
and the explosion produced by means of a spark from
a Leyden jar. After the explosion has taken place the
stop-cock is again opened, and the water rises into the
vessel A.

168

PRINCIPLES OF CHEMISTRY

By the aid of the eudiometer we may not only determine the volu-
metric composition of water,32 and the quantitative contents of oxygen

•I'!' 5- The eudiometer is used for determining the composition of combustible

gases. A detailed account of gas analysis would be out of place in this work
(see Note 30), but, as an example, we will give a short description of the deter-
mination of the composition of water by the eudiometer.

Pure and dry oxygen is first introduced into the eudiometer. When the
eudiometer and the gas in it acquire the temperature of the surrounding
atmosphere — which is recognised by the fact of the meniscus of the mercury
not altering its position during a long period of time — then the heights at
which the mercury stands in the eudiometer and in the bath are observed.
The difference (in millimetres) gives the height of the column of mercury in
the eudiometer. It must be reduced to the height at which the mercury
would stand at 0° and deducted from the atmospheric pressure, in order to
find the pressure under which the oxygen is measured (see Chap. I. Note 29).
The height of the mercury also shows the volume of the oxygen. The tem-
perature of the surrounding atmosphere and the height of the barometric
column must also be observed, in order to know the temperature of the oxy-
gen and the atmospheric pressure. When the volume of the oxygen has been
measured, pure and dry hydrogen is introduced into the eudiometer, and the
volume of the gases in the eudiometer again measured. They are then ex-
ploded. This is done by a Leyden jar, whose outer coating is connected by
a chain with one wire, so that a spark passes when the other wire, fused into
the eudiometer, is touched by the terminal of the jar. Or else an electrophorus
is used, or, better still, a Ruhmkorff's coil, which has the advantage of work-
ing equally well in damp or dry air, whilst a Ley Jen jar or electrical machine
does not act in damp weather. Further, it is necessary to close the lower
orifice of the eudiometer before the explosion (for this purpose the eudio-
meter, which is fixed in a stand, is firmly pressed down from above on to a piece
of india-rubber placed at the bottom of the bath), as otherwise the mercury
and gas would be thrown from the apparatus by the explosion. It must
also be remarked that to ensure complete combustion the proportion between
the volumes of oxygen and hydrogen must not exceed twelve volumes of
hydrogen to one volume of oxygen, or fifteen volumes of oxygen to one
volume of hydrogen, because no explosion will take place if one of the gases
be in great excess. It is best to take a mixture of one volume of hydrogen
with several volumes of oxygen. The combustion will then be complete. It is
FIG. 32.— evident that water is formed, and that the volume (or tension) is diminished,
Eudiometer. 8O thati on opening the end of the eudiometer the mercury will rise in it.
But the tension of the aqueous vapour is now added to the tension of the
gas remaining after the explosion. This must be taken into account (Chap. I. Note 1).
If there remain but little gas, the water which is formed will be sufficient for its satura-
tion with aqueous vapour. This may be learnt from the fact that drops of water are
visible on the sides of the eudiometer after the mercury has risen in it. If there be none,
a certain quantity of water must be introduced into the eudiometer. Then the number
of millimetres expressing the pressure of the vapour corresponding with the tempera-
ture of the experiment must be subtracted from the atmospheric pressure at which the
remaining gas is measured, otherwise the result will be inaccurate.

This is essentially the method of the determination of the composition of water which
was made for the first time by Gay-Lussac1 and Humboldt with sufficient accuracy.
Their determinations led them to the conclusion that water consists of two volumes of
hydrogen and one volume of oxygen. Every time they took a greater quantity of oxygen,
the gas remaining after the explosion was oxygen. When they took an excess of hydro-
gen, the remaining gas was hydrogen ; and when the oxygen and hydrogen were taken in

N AND ITS SALINE COMBINATIONS 169

in aiiyj;< but also make a number of experiments explaining the
phenomenon of combustion.

Thus, for example, it may be demonstrated, by the aid of the
eudiometer, that for the ignition of detonating gas a definite temperature
is required. If the temperature be below that required, combination
will not take place, but if at any spot within the tube it rises to the
temperature of inflammation, then combination will ensue at that spot,
and evolve enough heat for the ignition of the adjacent portions of the
detonating mixture. If to 1 volume of detonating gas there be added
10 volumes of oxygen, or 4 volumes of hydrogen, or 3 volumes of
carbonic anhydride, then we shall not obtain an explosion by passing
a spark through the diluted mixture. This depends on the fact that
the temperature falls with the dilution of the detonating gas by another
gas, because the heat evolved by the combination of the small quantity
of hydrogen and oxygen brought to incandescence by the spark is not
only transmitted to the water proceeding from the combination, but
also to the foreign substance mixed with the detonating gas.34 The
necessity of a definite temperature for the ignition of detonating gas is
also seen from the fact that pure detonating gas explodes in the presence
of a red-hot iron wire, or of charcoal so feebly incandescent as to be
hardly distinguishable by day light, but with a lower degree of in-
candescence there is not any explosion. It may also be brought about
by rapid compression, when, as is known, heat is evolved.3'"1 Experi-
ments made in the eudiometer showed that the ignition of detonating
gas takes place at a temperature between 450° and 5000.36

exactly the above proportion neither one nor the other remained. The composition of
water was thus definitely confirmed.

55 Concerning this application of the eudiometer, see the chapter on nitrogen.

31 Thus \ volume of carbonic oxide, an equal volume of marsh gas, two volumes of
hydrogen chloride or of ammonia, and six volumes of nitrogen or twelve volumes of air
added to one volume of detonating gas, prevent its explosion.

"" If the compression be brought about slowly, so that the heat evolved succeeds in
passing to the surrounding space, then the combination of the oxygen and hydrogen does
not take place, even when the mixture is compressed by 150 times ; for the gases are not
heated. If paper soaked with a solution of platinum (in aqua regia) and sal ammoniac
be burnt, then the ash obtained contains very finely-divided platinum, and in this form
it is best fitted for setting light to hydrogen and detonating gas. Platinum wire requires
to be heated, but platinum in so finely divided^ a state as it occurs in this ash inflames
hydrogen, even at — 203. Many other metals, such as palladium, iridium, and gold, act
with a slight rise of temperature, like platinum ; charcoal, like the majority of finely
divided substances, inflames detonating gas at 850°, but mercury, at its boiling point,
• I..,. s not inflame detonating gas. All data of this kind show that the explosion of
detonating gas presents one of the many cases of contact phenomena.

58 From the very beginning of the diffusion of the idea of dissociation, it might have
been imagined that reversible reactions of combination (the formation of Ho and O
belongs to this number) start at the same temperature as that at which dissociation
begins. And so it is in many cases, but not always, as may be seen from the facts (1) that

170

PRINCIPLES OF CHEMISTRY

The combination of hydrogen with oxygen is accompanied by the
evolution of a very considerable amount of heat ; according to
the determinations of Favre and Silbermann*1 1 part by weight of
hydrogen in forming water evolves 34462 units of heat. Many of the
most recent determinations are very near this figure, so that it may be
taken that in the formation of 18 parts of water (H2O) there are
evolved 69 major calories, or 69000 units of heat.38 If the specific heat

at 450-560 J, when detonating gas explodes, the density of aqueous vapour not only
does not vary (and it hardly varies at higher temperatures, probably because the amount
of the products of dissociation is small), but there are not, as far as is yet known, any
traces of dissociation ; (2) that under the influence of contact the temperature at which
combination takes place falls even to the ordinary temperature, when water and similar
compounds naturally are not dissociated and, judging from the data communicated by
D. P. Konovaloff (Introduction, Note 39) and others, it is impossible to escape the phe-
nomena of contact ; all vessels, whether of metal or glass, show the same influence as
spongy platinum although to a much less degree. The phenomena of contact, judging
from the mass of the data referring to it, must be especially sensitive in reactions which
are powerfully exothermal, and the explosion of detonating gas is of this kind.

57 The amount of heat evolved in the combustion of a known weight (for instance, 1
gram) of a given substance is determined by the rise in temperature of water, to which
the whole of the heat evolved in the combustion is transmitted. A calorimeter, for
example, that shown in fig. 33, is employed for this purpose. It consists of a thin (in
order that it may absorb less heat), polished (that it should transmit a minimum of heat)
metallic vessel, surrounded by down (c), or some other bad conductor of heat, and an outer
metallic vessel. This is necessary in order that the least possible amount of heat should
be lost from the vessels ; nevertheless, there is always a certain loss, whose magnitude

is determined by preliminary experiment (by taking
warm water, and determining its fall in temperature
after a definite period of time) as a correction for the
results of observations. The water to which the heat
of the burning substance is transmitted is poured
into the vessel. The stirrer g allows of all the layers
of water being brought to an equal temperature, ;m<l
the thermometer serves for the determination of the
temperature of the water. The heat evolved p;is>c>,
naturally, not to the water only, but to all the parts (A
the apparatus. The quantity of water corresponding
with the whole amount of those objects (the vessels,
tubes, &c.) to which the heat is transmitted is pre-
viously determined, and in this manner another most
important correction is made in the calorimetric deter-
minations. The combustion itself is carried on in the
vessel a. The ignited substance is introduced through
the tube at the top, which closes tightly. In fig. 3&
the apparatus is arranged for the combustion of a gas,
introduced by a tube. The oxygen required for the
combustion is led into a by the tube <?, and the pr<>-

PIG. 33. — Favre and Silbermann's calo- ducts of combustion either remain in the vessel a (if

"volve'd ifcombSln.^ "" ^ li(luid or solid), or escape by the tube/ into an n1MKua-

tus in which their quantity and properties can easily

be determined. Thus the heat evolved in combustion passes to the walls of the vessel a,

and to the gases which are formed in it, and these transmit it to the water of the

calorimeter.

58 This quantity of heat corresponds with the formation of liquid water at the ordinary

OXYGEN AND ITS SALINE COMBINATIONS 171

of aqueous vapour (0'4S) remained constant from the ordinary tempera-
ture to tJutf of /'-/tick the combustion of detonating gas takes place (but

temperature from detonating gas at the same temperature. If the water be as vapour
the heat evolved = 58 major calories; if asice = 70'4 major calories. A portion of this
heat is due to the fact that 1 vol. of hydrogen and £ vol. of oxygen give 1 vol. of aqueous
vapour — that is to say, contraction ensues — and this evolves heat. This quantity of heat
may be calculated, but it cannot be said how much is expended in the tearing apart of
the atoms of oxygen from each other, and therefore, strictly speaking, we do not know
the quantity of heat which is evolved in the combination of hydrogen with oxygen ;
although the number of units of heat evolved in the combustion of detonating gas is
accurately known.

The construction of the calorimeter and even the method of determination vary
considerably in different cases. The greatest number of calorimetric determinations were
made by Berthelot and Thomsen. They are given in their works Essai de mecanique
cJiiiniquc fonilea sur la thcrmucltimie, by M. Berthelot, 1879 (2 vols.), and thermo-
chcmische Untersnchu)it/en, by J. Thomsen, 1886 (4 vols.). The student must refer to
works on theoretical and physical chemistry for a description of the elements and methods
of thermochemistry, into the details of which it is impossible to enter in this work, all
the more so because, as has been shown of late, both the theoretical side of this subject
and its practical methods are still in an elementary state of development, and must be
subjected to improvement in many aspects before thermochemical study can be of that
enormous utility to chemical mechanics which was expected from it at the time of the
appearance of the first researches in its province. One of the originators of thermo-
chemistry was a member of the St. Petersburg Academy of Sciences, Hess. Since 1870
a mass of researches have appeared in this province of chemistry, especially in France
and Germany, after the leading works of the French Academician, Berthelot, and the
Copenhagen professor, Thomsen. Among Russians, Beketoff, Luginin, Cheltzoff, Chroust-
choff, and others are known by their thermo-chemical researches. The present epoch_of
thermochemistry, in the absence of a steadfast foundation (and the principle of maximum
work cannot be counted as such), must be considered rather as a collective one, wherein
the material of facts is amassed, and the first consequences arising from them are noticed.
In my opinion three essential circumstances prevent the possibility of extracting any
exact consequences, of importance to chemical mechanics, from the amassed and already
immense store of thermochemical data : (1) The majority of the determinations are con-
ducted in weak aqueous solutions, and, the heat of solution being known, are referred to
the substances in solution ; yet there is much (Chap. I.) which forces one to consider that
in solution water does not play the simple part of a diluting medium, but of itself
acts independently in a chemical sense on the substance dissolved. (2) The other chief
portion of thermochemical determinations is conducted by the ignition of substances
at high temperatures, and as yet we do not know the specific heat of many substances
at these temperatures. (3) Physical and mechanical changes (decrease of volume, diffu-
sion, and others) inevitably proceed side by side with chemical changes, and for the pre-
sent it is impossible, in a number of cases, to distinguish the thermal effect of the one
and the other kind of change. It is evident that the one kind of change (chemical) is essen-
tially inseparable and incomprehensible without the other (mechanical and physical) ; and
therefore it seems to me that thermochemical data will only acquire their true meaning
when the connection between the phenomena of both kinds (on the one hand chemical
and atomic, and on the other hand mechanical and molecular or between entire masses)
is explained more clearly and fully than is the case at present. As there is no
doubt that the simple mechanical contact, or the action of heat alone, on substances some-
times causes an evident and always a latent (incipient) chemical change — that is, a
different distribution or movement of the atoms in the molecules — it follows that purely
chemical phenomena are inseparable from physical and mechanical phenomena. This is
because the atomic relations forming the essence of the chemical relations of a substance
are not observable, and at present are incomprehensible, without the molecular relations

172 PEINCIPLES OF CHEMISTRY

most probably it increases), were the combustion concentrated at one
point39 (but it occurs as a flame), were there no loss from radiation and
heat conduction, and, chiefly, did dissociation not take place — that is, did
not a state of equilibrium between the hydrogen, oxygen, and water come
about — then it would be possible to calculate the temperature of tlie flame
of detonating gas. It would then be 100000.40 In reality it is very
much lower, but it is nevertheless higher than the temperature attained
in furnaces and flames, and reaches up to 2000°. The explosion of
detonating gas is explained by this high temperature, because the
aqueous vapour formed must occupy a volume at least 5 times greater
than that occupied by the detonating gas at the ordinary temperature.
Detonating gas emits a sound, not only as a consequence of the
commotion which occurs from the rapid expansion of the heated vapour,
but also because it is immediately followed by a cooling effect, the
conversion of the vapour into water, and a rapid contraction.41

forming the essence of the physical relations, and even without the relations of the entire
masses of molecules evincing themselves in purely mechanical relations, inasmuch as
an individual atom is something unreal and fantastic. A mechanical change may be
imagined without a physical change, and a physical without a chemical change (although
such a representation would be artificial), but it is impossible to imagine a chemical
change without a physical and mechanical one, for without them we should not perceive
it, and through them we attain it. There was a time when the province of physics
embraced the whole of chemistry and mechanics. In the present day they have been de-
veloped independently and been isolated from each other, but in the future a fresh conjunc-
tion is imminent, and is heralded by the laws of the conservation of matter and of energy.
59 The flame, or locality where the combustion of gases and vapours is accomplished,
is a complex phenomenon, ' an entire factory,' as Faraday says, and therefore we will
consider flame in some detail in one of the following notes.

40 If 34500 units of heat are evolved in the combustion of 1 part of hydrogen, and
this heat is transmitted to the resulting 9 parts by weight of aqueous vapour, then we
find that, taking the specific heat of the latter as 0'475, each unit of heat raises the
temperature of 1 part by weight of aqueous vapour 2'1° and 9 parts by weight (2'l-*-9)
0-23° ; hence the 34500 units of heat raise its temperature 7935°. If detonating gas is
converted into water in a closed space, then the aqueous vapour formed cannot expand,
and therefore, in calculating the temperature of combustion, the specific heat at a con-
stant volume must be taken into consideration ; it is 0'36 for aqueous vapour. This
figure gives a still higher temperature for the flame. In reality it is much lower, but the
results given by different observers are very contradictory (from 1700° to 2400°), the
discrepancies depending on the fact that flames of different sizes are cooled by radiation
to a different degree, but mainly on the fact that the methods and apparatus (pyro-
meters) for the determination of high temperatures, although they enable relative
changes of temperature to be judged, are of little use for determining their absolute
magnitude. By taking the temperature of the flame of detonating gas as 2000°, I give,
I think, the average of the most trustworthy determinations.

41 It is evident that not only hydrogen, but every other combustible gas, will give an
explosive mixture with oxygen. For this reason coal-gas mixed with air explodes
when the mixture is ignited. The pressure obtained in the explosions serves as the
motive power of gas engines. In this case advantage is taken, not only of the pressure
produced by the explosion, but also of that contraction which takes place after the
explosion. On this is based the construction of several motors, of which Lenoir's was

nXYCKN AND ITS SALINE COMBINATIONS

173

Mixtures of hydrogen and of various other gases with oxygen
are taken advantage of for obtaining high temperatures. By the
aid of such high temperatures metals like platinum may be melted
on a large scale, which cannot be
done in furnaces heated with char-
coal and fed by a current of air. The
burner, shown in fig. 34, is constructed
for the application of detonating gas
to the purpose. It consists of two
brass tubes, one fixed inside the other,
as shown in the drawing. The internal
central tube C C conducts oxygen, and
the outside, enveloping, tube E' E' con-
ducts hydrogen. Previous to their
egress the gases do not mix together,
so that there can be no explosion inside
the apparatus. When this burner is
in use C is connected with a gasholder
containing oxygen, and E with a gas
holder containing hydrogen (or some-
times C0al-£as). The flow of the FlG- 34-~ Safety burner for detonating gas,.

described in text.

gases can be easily regulated by

the stop-cocks O H. The flame is shortest and evolves the greatest
heat when the gases burning are in the proportion of 1 volume of
oxygen to 2 volumes of hydrogen. The degree of heat may be easily
judged from the fact that a thin platinum wire placed in the flame
easily melts. By placing the burner in the orifice of a hollow piece
of lime, a crucible A B is obtained in which platinum may be easily
melted, even in large quantities if the current of oxygen and
hydrogen be sufficiently great (Deville). The flame of detonating gas
may also be used for illuminating purposes. It is by itself very pale,
but owing to its high temperature it may serve for rendering infusible
objects incandescent, and at the very high temperature produced by the
detonating gas the incandescent substance gives a most intense light.
For this purpose lime, magnesia, or oxide of zirconium are used, as they
are not fusible at the very high temperature evolved by the detonating
gas. A small cylinder of lime placed in the flame of detonating gas,
if regulated to the required point, gives a very brilliant white

formerly, and Otto's is now, the best known. The explosion is usually produced by coal-
g.is and air, but of late the vapours of combustible liquids (kerosene, benzene) are
also being employed in place of gas (Chap. IX.). In Lenoir's engine a mixture of coal-
KJIS and air is ignited by means of sparks from a RuhmkorfF s coil, but in the most recent
marl lines the gases are ignited by the direct action of a gas jet.

1 , 1 PRINCIPLES OK CHEMISTRY

liu'ht. v. Inch \vas at one time proposed for illuminating lighthouses.
At present in the majority of cases electric light, osving to its constancy
and other advantages, has replaced it for this purpose. The light
produced bv lime in detonating gas is called the /JrHnimond fif/Jtf or

The above cases form examples of the combustion of (dements in
oxvgen, but exactly similar phenomena are observed in the conJiuxtion
of ••itiii j:ini //'/\. So, for instance, the solid, colourless, shmv substance,
naphthalene. (',,,! Fs, burns in air with a smoky (lame, whilst in oxygen
it continues to burn with a very brilliant llame. Alcohol, oil. and
other substances burn brilliantly in oxygen on conducting the oxygen
by a tube to the flame of lamps burning these substances. A high
temperature is thus evolved, which is sometimes taken advantage of
in chemical practice.

I n order to understand why combustion in oxvgen proceeds more
rapidlv. and is accompanied by a more1 intense heat etl'ect, than com-
bustion in air.it must be recollected that air is oxvgen diluted with
nitrogen, which does not support combustion, and therefore fewer par-
ticles of oxygen flow to the surface of a substance burning in air than
when burning in pure oxygen. The chief reason of the intensity of com-
bustion in oxygen is the high temperature acquired by the substance
burning in it. Let us consider as an example the combustion of sulphur
in air and in oxygen. If 1 gram of sulphur burns in air or oxvgen it
evolves in either case I'L'oO unitsof heat /.''., evolves sufficient heat for
heating i' •_'•"><> grams of water 1" ('. This heat is first of all transmitted
to th'- sulphurous anhydride, HO.,, formed by the combination of sulphur
with oxygen. In its combustion 1 gram of sulphur forms '2 grams
of sulphurous anhydride /'.'., the sulphur combines \\ith 1 gram of
o\vg('H. In order that 1 gram of sulphur should have access to 1 gram
of oxvgen in air. it is necessary that .">••! grams of nitrogen should
simultaneously reach the sulphur, because air contains seventy-seven
parts of nitrogen (by weight) per twenty-three parts of oxvgen. Thus
in the combustion of 1 gram of sulphur, the L'l'on units of heat are
t ransmitl od to '2 grains of sulphurous oxide and toat least .">• I grams of
nitrogen. As (Hr>f) units of heat are required to raise 1 gram of
sulphurous anhydride I ('., therefore L' grams require <)•.'}] units. So
also .">• 1 grains of nitrogen require •"»• I '/ O'L'll or O-S."> unitsof heat,
and therefore in order to raise both gases 1 ( '. n-.°, I 4- n-s:l or I'll
units of heat are required, lint as the combustion of the sulphur
evohcs '-'.I'-"'1' units of heat, therefore the gases miuht be heated (if

their sjH-citic heats remained constant) to ~ or 1D71 < '. That

OXYUKN AND ITS SALINE COM HI NATIONS 175

is, the maximum possible temperature of the flame of the sulphur
burning in air will be 1974° C. In the combustion of the sulphur
in oxygen the heat evolved (2250 units) can only pass to the '1 grains
of sulphurous anhydride, and therefore the highest possible tempera-
ture of the flame of the sulphur in oxygen will be =~- or 7L'">s .

O'ol

In the same manner it may be calculated that the temperature of char-
coal burning in air cannot exceed 2700°, while in oxygen it may attain
10100° C. For this reason the temperature in oxygen will always be
higher than in air, although (judging from what has been said re-
specting detonating gas) neither one nor the other temperature will
nearly approach the theoretical quantities.

Among the phenomena accompanying the combustion of certain
substances, the phenomenon of flame attracts attention. Sulphur,
phosphorus, sodium, magnesium, naphthalene, cvrc., burn like hydro-
gen with a flame, whilst in the combustion of other substances no
flame is observed, as, for instance, in the combustion of iron and
of charcoal. The appearance of flame depends on the capacity of the
combustible substance to yield gases or vapours at the temperature of
combustion. At the temperature of combustion, sulphur, phosphorus,
sodium, and naphthalene pass into vapour, whilst wood, alcohol, oil, &c.,
are decomposed into gaseous and vaporous substances. The com-
bustion of gases and vapours forms flames, and therefore a flame is
composed of the hot and incandescent gases and vapours produced by co/n-
bustion. It may be easily proved that the flames of such non-volatile
substances as wood contain volatile and combustible substances formed
from them, by placing a tube in the flame and drawing air from
it with an aspirator. Besides the products of combustion, com-
bustible gases and liquids, previously in the flame as vapours, collect in
the aspirator. For this experiment to succeed— -i.e., in order to really
extract combustible gases and vapours from the flame — it is necessary
that the suction tube should be placed inside the flame. The com-
bustible gases and vapours can only remain unburnt inside the flame,
for at the surface of the flame they come into contact with the oxy.vvn
of the air and burn.42 Flames are of different degrees of

42 Faraday proved this by a very convincing experiment on a candle flame. It one
arm of a bent glass tube be placed in a candle flame above the wick in tin1 dark pert ion
of the flame, then the products of the partial combustion of the stearin will pass up the
tube, condense in the other arm, and collect in a flask placed under it iti-. '•'•'» a- heavy
white fumes which burn when lighted. If the tube be raised into the upper lumi-
nous portion of the flame, then a dense black smoke which will not inflame aeeiimulates
in the flask. Lastly, if the tube be let down until it touches the wick, then little
but stearic acid condenses in the flask.

176

PRINCIPLES OF CHEMISTRY

brilliancy, according to whether solid incandescent particles occur in
the combustible gas or vapour, or not. Incandescent gases and
vapours emit but little light by themselves, and therefore give a paler

flame.43 If a flame does not
contain solid particles it is
transparent, pale, and emits
but little light.44 The flames
of burning alcohol, sulphur,
and hydrogen are of this kind.
A pale flame may be rendered
luminous by placing fine par-
ticles of solid matter in it.
Thus, if a very fine platinum
wire be placed in the pale
flame of burning alcohol— or,
better still, of hydrogen — then
the flame emits a bright light.
This is still better seen by sift-
ing the powder of an incom-
bustible substance, such as
fine sand, into the flame, or
by placing a bunch of asbestos
threads in it. Every brilliant
flame always contains some
kind of solid particles, or at least some very dense vapour. The flame
of sodium burning in oxygen has a brilliant yellow colour, from the
presence of particles of solid sodium oxide. The flame of magnesium
is brilliant from the fact that in burning it forms solid magnesia, which
becomes white hot, and similarly the brilliancy of the Drummond light
is due to the heat of the flame raising the solid non-volatile lime to a
state of incandescence. The flames of a candle, wood, and similar sub-
stances are brilliant, because they contain particles of charcoal or soot.
It is not the flame itself which is luminous, but the incandescent soot
it contains. These particles of charcoal which occur in flames may be
easily observed by introducing a cold object, like a knife, into the

Fw. 35.— Faraday's experiment for investigating the
different parts of a caudle flame.

43 All transparent substances which transmit light with great ease (that is, which
absorb but little light) are but little luminous when heated ; so also substances which
absorb but few heat rays, when heated transmit few rays of heat.

44 There is, however, no doubt but that very heavy dense vapours or gases under
pressure (according to the experiments of Frankland) are luminous when heated, be-
cause, as they become denser they approach a liquid or solid state. Thus detonating
gas when exploded under pressure is brightly luminous.

oXYGKN AND ITS SALINH COMBINATIONS

177

flame.1' The particles of charcoal burn at the outer surface of the
flame if the supply of air be sufficient, but if the supply of air that is,
of oxygen — be insufficient for their combustion the flame smokes, because
these unconsumed particles of charcoal are carried off by the current
of air.4(J

45 If hydrogen gas be passed through a volatile liquid hydrocarbon — for instance,
through benzene (the benzene maybe poured directly into the vessel in which hydrogen is
generated) — then its vapour burns with the hydrogen and gives a very bright flame,
because the resultant particles of carbon (soot) are powerfully ignited. Benzene, or
platinum gauze, introduced into a hydrogen flame may be employed for illuminating
purposes.

46 Inflames the separate parts may be distinguished with more or less distinctness.
That portion of the flame whither the combustible vapours or gases flow, is not
luminous because its temperature is still too low for the process of combustion to take
place in it. This is the space which in a candle surrounds the wick, or in a gas jet
is immediately above the orifice from which the gas escapes. In a candle the com-
bustible vapours and gases which are formed by the action of

heat on the melted tallow or stearin, rise in the wick, and
are heated by the high temperature of the flame. By the
action of the heat, the solid or liquid substance is here, as
in other cases, decomposed, forming products of dry dis-
tillation. These products occur in the central portion of the
flame of a candle. The air travels to the flame from the
outside, and is not able to intermix with the vapours and
gases in all parts of the flame ; consequently, in the outer
portion of the flame the amount of oxygen flowing to it
will be greater than in the interior portions of the flames.
But, owing to diffusion, the oxygen, naturally together with
nitrogen, flowing to the combustible substance penetrates
inside the flame, when the combustion takes place in
ordinary air. The combustible vapours and gases combine
with this oxygen, evolve a considerable amount of heat, and
bring about that state of red heat which is so necessary
both for keeping up the combustion and also for the uses
to which the flame is applied. Passing from the colder
envelope of air to the interior of the flame, to the source of
the combustible vapours (for instance, the wick), we evidently
first traverse layers of high temperature, and then
layers of lower and lower temperature, in which the com-
bustion is less complete, owing to the limited supply of
oxygen.

Thus, yet unburnt products of the decomposition of
organic substances occur in the interior of the flame. But flamJG^ie p^fon (? contains
there is always free hydrogen in the interior of the flame, even the vapours and products of
when oxygen is introduced there, or when a mixture of ^e^^he'combustionTias coni-
hydrogen and oxygen burns, because the temperature menced, and particles of carbon
evolved in the combustion of hydrogen or the carbon of are emitted : and in the pale

zone B the combustion is corn-
organic matter is so high that the products of combustion pieted.

are themselves partially decomposed — that is, dissociated —

at this temperature. Hence, in a flame a portion of the hydrogen and of the oxygen
which might combine with the combustible substances must always occur in a free
state. If a hydrocarbon burns, and we imagine that a portion of the hydrogen occurs in
a free state, then a portion of the carbon must also occur in the same form in
VOL. I. N

ITS

thi- '- oh-erved in reality in the coinliUstioii of various h\ drocai'hon-. ('harcoal, or
the soot of a common flame, proceeds from the di ciation ol ,.. janic -nh-tances con-
tained in the tlame. The majority oMiydrocarl.on-. e-pedallv those containing much
stance, naphthalene— hum even in oxyp-n. w;th -eparatioiiof soot. The
hums, hul tin- carhoii n I. . t.] lly so. It is this free

, , i \\ 1 1 id i causes the 1 ir ill iancy of the flat e. That tl • • of the flame contains

. .vhich is still ca] ' • 1 illowiiiLT experi-

t ; A portion of the ibises may In willidrax i 1-y an ' the central portion

e flame of carhonic oxide, wliicli is comhu-t ihle in air. For thi- purpo-e Deville
pa-sed water tlmm-h a metallic tuhe haviiiL' a ti ie lateral or:tice. which i- placed in the
flame. As the water parses alon- the tnhe the -a -e- oi the l!a me enter it . they are
•, mipled l.y . ylinders of water pas-ine' alon- the ml,,., and are carried off with it into
• [< for their investigation. It appears that all portion- of the flame obtained
l,v the cond'Ustion of a mixture of earl • tain a portion of this

ture -till unl.urnt. The re-earch.-s o! [Vville and i'.u i wed that in the

-ion of a mixture of hydi'op-u and of c • vp-n in a closed

space, complete comliustioii sometimes does iiof take place immediately. It two
volumes of hydro-en and one volume of oxyp-n he enclosed in a closed -pace, then mi
explosion the pressure doe- not attain thai ma-mtude which it would were there
lediate and c-om]ilete comhustion. It may he calculated that in this ca-e the
pressure should attain twenty-six atmospheres. In reality, it has heen shown l>y
•\ experiment that in the explosion ol hydro-en and oxy-jvn the pressure does
n ; exceed nine and a-lialt atnifis]>heres.

This may he explained l.y the fact that, in : ;,< • .!•• of the oxyp-n

does not all nl once comhine with the ci I • The amount of apis

l,urnt may even l.e determined from the pressure produced in i'- coinliiist ion, knmvin^
tin- heat evolved in it- comhustion and the specific heat of all the resultant and partici-
pating -uhstances. and hence the tempera! lire of condni-t ion. and therefore also the
pressure which may he evolved a- a eoiise<|Uence of that ri-e of temperature which pro-
ceed- from the evolution of heat. It appeal's 1 hat in t his <-a -e onlv one-third of the pises

ilie at the I e|n] icra t lire e\'i 'I Veil ill

tioiiof the remaining mas-, which i- capaMi of luiruin--:'. The admi\ture of carhonic

interferes in the -ame manner. Thi- shows thai e\er\ portion of a tlame nin-t contain
hydro-'eu. hydrocarhoiis. carl ionic anhydride, and \\ ater. < '•<<.••• , ijiii'iit ly. /'/ /x tin i'«x>-il>lr

' me. A ci itii n i in

ffe rent ] ' . In thi pace differ, •,,••!,, ompoiienl parts are

Vel\ nliji cted to c.niihiisl ion. ' - il under iln '.'.<\< nee of adjacenl

,,|,jec1-. and c, mhii-tion onlv end- v,ln re ti:,- llame , nd . I' the coinlin-t ion coidd he

,.Mi,c,-n1ra1ed at - -pot. then the temperature /. o d,| I,,- in,- parahlv hii/ln-r llian it is

,Ul,lel' t lie a el i la I ci re 1 1 In -t a lice . ||ellce 't I not to he \\ o|i(|i I'ed at that -moke and soot
| „.,.., 1 1 -e t I'on i -.. hat ha ln-.-n aid a 1 - e, e complete com I ill ' >1 take place in-tan-

OXYCKX AM) ITS SALINK COM HINATH >NS 17'.)

inconsiderably. This may either proceed from the fact that ih»-
reaction of the substance (for example, tin, mercury, lead at a high
temperature, or a mixture of pyrogallol with caustic potash at the
ordinary temperature) evolves but little heat, or that the hoat
evolved is transmitted to good conductors of heat, like metals, or that
the combination with oxygen takes place so slowly that the heat
evolved succeeds in passing to the surrounding objects. Combustion
is only a particular, intense, and evident case of combination with
oxygen. Respiration is also an act of combination with oxygen ;
it also serves, like combustion, for the development of heat by
those chemical processes which are its consequences (the trans-
formation of oxygen into carbonic anhydride). Lavoisier enun-
ciated this in the clear expression, ' respiration is slow combus-
tion.'

Reactions of slow combination of substances with oxygen are
termed oxidations. Combination of this kind (and also combustion)
often results in the formation of acid substances, and hence the
name oxygen (Sauerstoff). Combustion is only rapid oxidation.
Phosphorus, iron, and wine may be taken as examples of substances
which slowly oxidise in air at the ordinary temperature. If such a
substance be left in contact with a definite volume of air or oxygen, it
little by little absorbs the oxygen, as may be seen by the decrease in
volume of the gas. This slow oxidation is, as a rule, rarely accom-
panied by a sensible evolution of heat ; but an evolution of heat really
occurs, only it is not apparent to our senses, owing to the inconsider-
able rise of temperature which takes place ; this is owing to the
slow rate of the reaction and to the transmission of the heat formed as
radiant heat, <fcc. Thus, in the oxidation of wine and its transformation
into vinegar by the usual method of its preparation, the heat evolved
cannot be observed because it extends over whole weeks, but in the
so-called rapid process of the manufacture of vinegar, when a large
quantity of wine is comparatively rapidly oxidised, the evolution of
heat is quite apparent.

Such slow processes of oxidation are always taking place in nature
by the action of the atmosphere. Dead organisms and the substances
obtained from them — such as bodies of animals, wood, wool, grass, &c. —

temperature. If they vary (as Berthelot and Vieille affirm), the portion of a substance
which remains unburnt on explosion cannot be calculated from the pressure, and there-
fore the quantitative side of the subject should be considered as doubtful. But the quali-
tative side of the subject cannot be subject to doubt, because the dissociation of the
products of combustion at high temperatures is proved clearly by the most varied
experiments.

x •_'

180 PRINCIPLES OF CHEMISTRY

are especially subject to this action. They rot and putrefy — that is,
their solid matter is transformed into gases, under the influence of
moisture, and atmospheric oxygen, and often under the influence of
other organisms, such as moulds, worms, micro-organisms (bacteria), and
suchlike. These are processes of slow combustion, of slow combination
with oxygen. Everyone knows that manure rots and evolves heat,
that stacks of damp hay, damp flour, straw, &c., become heated and
are changed in the process.47 In all these transformations there are
formed the same chief products of combustion as are contained in
smoke ; the carbon gives carbonic anhydride, and the hydrogen
water. Hence these processes require oxygen just like combustion.
This is the reason why the entire prevention of access of air hinders
these transformations,48 and an increased supply of air accelerates them.
The mechanical treatment of arable lands by the plough, harrow, and
other similar means has not only the object of facilitating the spread
of roots in the ground, and of making the soil more permeable to water,
but it also serves to facilitate the access of the air to the component
parts of the soil ; as a consequence of which the organic remains of
soil rot — so to speak, breathe air and evolve carbonic anhydride.
One acre of good garden land in summer evolves more than six tons
of carbonic anhydride.

It is not only vegetable and animal substances which are subject ta
slow oxidation in the presence of water. The very metals are rusted
under these conditions. Copper very easily absorbs oxygen in the
presence of acids. Many metallic sulphides (for example, pyrites) are
very easily oxidised with access of air and moisture. Thus processes
of slow oxidation proceed throughout nature.

There are many elements which do not, under any circumstances,,
combine directly with gaseous oxygen ; nevertheless their compounds
with oxygen may be obtained. Platinum, gold, iridium, chlorine,
and iodine are examples of such elements. In this case recourse is
had to a so-called indirect method — i.e., the given substance is-

47 Cotton waste (it is used in factories for cleaning machines from lubricating oil)
soaked in oil and lying in heaps is self-combustible, being oxidised by the air.

48 "When it is desired to preserve a supply of vegetable and animal food, the access of
the oxygen of the atmosphere (and also of the germs of organisms borne in the air)
is often prevented. For this reason articles of food ai-e often kept in hermetically closed
vessels, from which the air is withdrawn ; vegetables are dried and soldered up while hot
in tin boxes ; sardines are immersed in oil, &c. The removal of water from substances is
also sometimes resorted to with the same object (the drying of hay, corn, fruits), as also
is saturation with substances which absorb oxygen (such as sulphurous anhydride),
which hinder the growth of organisms forming the first cause of putrefaction, as in
processes of smoking, embalming, and in the keeping of fishes and other animal sj
mens in spirit, &c.

(>XV(iKN AND ITS SALINE CCLMI51NATX >NS 181

combined with another element, and by a method of double decom-
position this element is replaced by oxygen, or a substance is taken
which easily evolves oxygen, and is brought into contact with the given
substance. The oxygen then acts at the moment of its evolution. If
the conditions are such that the substance to be oxidised is liberated
at the same moment, then oxidation proceeds with greater ease.
(The explanation of this phenomenon was given in the last chapter.)
It must be remarked that substances which do not directly combine
with oxygen, but form compounds with it by an indirect method, often
readily lose the oxygen which was absorbed by them by double decomposi-
tion or at the moment of its evolution. Such, for example, are the com-
pounds of oxygen with chlorine, nitrogen, and platinum, which evolve
oxygen on heating. They, like other substances which easily evolve
oxygen on heating, may serve as a means for obtaining oxygen, or for
oxidation. They, in the presence of substances which are capable of
combining with oxygen, are decomposed, give up their oxygen to them,
and may thus be themselves employed for indirect oxidation. In this
respect oxidising agents, or those compounds of oxygen which are em-
ployed in chemical and technical practice for transf erring oxygen to
other substances, are especially remarkable. The most important
among these is nitric acid or aquafortis — a substance rich in oxygen,
and capable of evolving it when heated, and which easily oxidises a great
number of substances. Thus nearly all metals and organic substances
containing carbon and hydrogen are more or less oxidised when heated
with nitric acid. If strong nitric acid be taken, and a piece of burning
charcoal be immersed in the acid, it continues to burn, the combustion
proceeding in this case at the expense of the oxygen contained in
the liquid nitric acid. Chromic acid acts like nitric acid ; alcohol
burns when mixed with it. Although the action is not so marked,
even water may oxidise with its oxygen. Sodium is not oxidised in
perfectly dry oxygen at the ordinary temperature, but it burns very
easily in water and aqueous vapour. Charcoal can burn in carbonic
anhydride — a product of combustion— forming carbonic oxide. Mag-
nesium burns in the same gas, separating carbon from it. Generally,
combined oxygen can pass from one compound to another.

The products of combustion or oxidation — and in general the definite
compounds of oxygen — are termed oxides. Some oxides are not capable
of combining with other oxides— or combine with only a few, and then
form unstable compounds with the evolution of very little heat ;
others, on the contrary, enter into combination with very many other
oxides, and in general have remarkable chemical energy. The oxides
incapable of combining with others, or only showing this quality in a

small degree, are termed t ndijj'' r< it' <>,i-'i<l<-s. Such a re the peroxides, of
Nvhich mention li;is before brcli made.

1 he class (it oxides capable of entering into mutual combination
we Nvill term sit/ hi'' <>.<•></>, •>•. Thev t'all into two chief i^roups at least,
a- regards t he mo-t extreme members. Tin- members of one group do not
combine with each other, l>ut combine with the members of the other
u;roup. As representative <»f one group niav lie taken the oxides of
the metals, magnesium, sodium, calcium. iVc. Representatives of the
othi-r uToup are the oxides formed by the non-metals, sulphur, phos-
phorus, earbon. If we take, for instance, the oxide of calcium or
lime, and bring it into contact with oxides of the second ^roup, there
ensues very readv combination. rJ"lius. for instance, if \\-e mix calcium
oxid'- \\~ith oxide of phosphorus, thev combine \\ith i^reat tacilitv. with
the evolution of much heat. If we pass the vapour of sulphuric an-
hvdride. obtained by the combination of sulphurous oxide with oxv^en,
over pieces of lime heated to redness, then the sulphuric anhydride is
absorbed by the lime, with the formation of a substance called
calcium sulphate. I he oxides of the first kind, which contain
metals, are termed imxir a. r, <!,.-< iii' Imws. Lime is a familiar example
of this class. The oxides of the second group, which are capable of
combining with the bases, are termed a ithi/<lri< '• x <>f ///•• <iri</s or <n'n1
a. i-iil, N. Sulphuric anhydride, S( ). , may l>e taken as a type of the
v;roup. It is foi'iued by the combination of sulphur with oxygen : by
the addition ot a fresh (juantitv of oxx'gen to the above-mentioned
sulphurous anhvdride, S( ).,. b\- passing it and oxvgen o\'er incandescent
sponu;v platinum. ( 'arbonic anliydride loften termed "carbonic acid,'
(.'O.,). [ihosphoric anliydride, sulphurous anhydride, are all acid oxides,
fur thev can combine \\iih such oxides as lime or calcium oxide,
magnesia or magnesium oxide, .MgO, soda or sodium oxide. Na.,<),
iV ' ' .

It a i;]\eii element form one basic oxide, it is termed the n.i-i<li : for
example, calcium oxide, magnesium OXlile. potassium oxide. Some
indill'ereiit oxid«-s ai'e also called 'oxides ' it' i he\ ha \ e not t lie projiert ies
of peroxides. ;ind ai 1 he same time do not si IONS' the properties of acid
anh vd rides tor mst a nee. carbonic oxide, ot which men t ion has already
been made. If an clou'-lit forms t NS'o basic oxides (or t NS o indlHerent
oxide- not haxin^ the characteristics of a peroxide) then that of the
lower degree of ox ida t ion i- ca lied a stilio.i'i<li that is. su box ides contain
Ies- ox Vgen than oxides. 'Mills. \s hen copper Is hea t ed to 1'ei I ness 111 a
furnai-e it increases in \vei^ht and absorbs oxvifen, until for '»•"> pails
of copper thel'e is absorbed not more than > pa rt - of ox \gen b\- NS'eigllt,
tormiiiL;' a red muss. NS'hich is suboxide ot copper : but if the roasting

OXYGEN AM) ITS SALINK Co.MIHNATIoNs 183

be prolonged, and tin- draught of air be increased, 63 parts of copper
absorb 16 parts of oxygen, and form black oxide of copper. Some-
times to distinguish between the degrees of oxidation a change of
suffix is made in the oxidised element — ic oxide naming the higher
degree of oxidation, and — ous oxide the lower degree. Thus ferrous
oxide and ferric oxide are the same as suboxide of iron and oxide of
iron. This nomenclature is convenient in some cases, but cannot
always be employed. If an element forms one anhydride only, then it
is named by an adjective formed from the name of the element made to
end in — ic and the word anhydride. When an element forms two
anhydrides, then the suffixes — ous and — ic are used to distinguish
them : — ous signifying less oxygen than — ic ; for example, sulphurous
and sulphuric anhydrides.49 When several oxides are formed from the
same element, the prefixes mon, di, tri, tetra are used, thus : chlorine
monoxide, chlorine dioxide, chlorine trioxide, and chlorine tetroxide
or chloric anhydride.

Chemical transformations of the oxides themselves are rarely
accomplished, and in the few cases where they are subject to such
changes a particularly important part is played by their combinations
with water. The majority of, if not all, basic and acid oxides combine
with water, either by a direct or an indirect method forming hydrates
— that is, such compounds as split up into water and an oxide of the
same kind only. We already know that many substances are cap-
able of combining with water. Oxides possess this property in the
highest degree. We have already seen examples of this (Chap. I.)
in the combination of lime, and of sulphuric and phosphoric anhydrides,
with water. Hence the results of such combination are basic and acid
hydrates. Acid hydrates are called acids, because they have an acid

49 It must be remarked that certain elements form oxides of all three kinds — i.e.,
indifferent, basic, and acid ; for example, manganese forms manganous oxide, manganic
oxide, peroxide of manganese, red oxide of manganese, and manganic anhydride, although
some of them are not known in a free state but only in combination. It is, then, always to be
remarked that the basic oxide contains less oxygen than the peroxides, and the peroxides
less than the acid anhydride. Thus they must be placed in the following general normal
order with respect to the amount of oxygen entering into their composition — (1) basic
oxides, suboxides, and oxides; (2) peroxides; (8) acid anhydrides. The majority of
elements, however, do not give all three kinds of oxides, some giving only one degree
of oxidation. It must further be remarked that there are oxides fonned by the combina-
tion of acid anhydrides with basic oxides, or, in general, of oxides with oxides. For
every oxide having a higher and a lower degree of oxidation, it might be said that the in-
termediate oxide was formed by the combination of the higher with the lower oxide. But this
is not true in all cases— for instance, when the oxide under consideration forms a whole
series of independent compounds — for oxides which are really formed by the combination
of two other oxides do not give such independent compounds, but in many >
decompose into the higher and lower oxides.

184 PRINCIPLES OF CHEMISTRY

taste when dissolved in water (or saliva, for then only can they act on
the palate). Vinegar, for example, has an acid taste because it contains
acetic acid dissolved in water. Sulphuric acid, of which we have made
mention many times, because it is the acid of the greatest importance
both in practical chemistry and for its technical applications, is really
a hydrate formed by the combination of sulphuric anhydride with
water. Besides their acid taste, dissolved acids or acid hydrates have
the property of changing to red the blue colour of certain vegetable
dyes. Of these dyes litmus is particularly remarkable and much used.
It is the blue substance extracted from certain lichens, and is used for
dyeing tissues blue ; it gives a blue infusion with water. This
infusion, on the addition of an acid, changes from blue to red.5n

Basic oxides, in combining with water, form hydrates, of which,
however, very few are soluble in water. Those which are soluble in
water have an alkaline taste like that of soap or of water in which ashes
have been boiled, and are called alkalis. Further, alkalis have the

50 Blotting or unsized paper, soaked in a solution of litmus, is usually employed for
detecting the presence of acids. This paper is cut into strips, and is called lest paper ;
when dipped into acid it immediately turns red. This is a most sensitive reaction, and
may be employed for testing for the least traces of acids. If 10000 parts by weight of water
be mixed with 1 part of sulphuric acid, the coloration is distinctly perceptible, and it is
quite distinguishable on the addition of ten times more water. Certain precautions
must, however, be taken in the preparation of such very sensitive litmus paper. Litmus
is sold in lumps. Take, say, 100 grams of it ; pound it, and add it to cold pure water in
a flask. Shake and decant the water. Kepeat this three times. This is done to wash
away easily-soluble impurities, especially alkalis. Transfer the washed litmus to a
flask, and pour in (!00 grams of water, heat, and allow the hot infusion to remain for
some hours in a warm place. Then filter, and divide the filtrate into two parts. Add a
few drops of nitric acid to one portion, so that a faint red tinge is obtained, and then
mix the two portions. Add spirit to the mixture, and keep it thus in a stoppered bottle
(it soon spoils if left open to the air). This infusion may be employed directly ; it reddens
in the presence of acids, and turns blue in the presence of alkalis. If evaporated, a
solid muss is obtained which is soluble in water, and may be kept unchanged for any
length of time. The test paper may be prepared as follows : — Take a strong infusion of
litmus, and soak blotting-paper with it ; dry it, and cut it into strips, and use it as test-
paper for acids. For the detection of alkalis, the paper must be soaked in a solution
of litmus just reddened by a few drops of acid ; if too much acid be taken, the paper will
not be sensitive. Such acids as sulphuric acid colour litmus, and especially its infusion,
a brick-red colour, whilst more feeble acids, such as carbonic, give a faint red-wine tinge.
Test-paper of a yellow colour is also employed ; it is dyed by an infusion of turmeric roots
in spirit. In alkalis it turns brown, but regains its original hue in acids. Many blue
and other vegetable colouring matters may be used for the detection of acids and alkalis ;
for example, infusions of cochineal, violets, log-wood, &c. Certain artificially-prepared
substances and dyes may also be employed. Thus rosolic acid, C2oH1(jO3, and
phenolphthale'm, C..>0H14O4, are colourless in an acid, and red in an alkaline, solution.
Cyanine is also colourless in the presence of acids, and gives a blue coloration with
alkalis. These are very sensitive tests. Their behaviour in respect to various acids,
alkalis and salts sometimes gives the means of distinguishing substances from each,
other.

OXYGES AM) ITS SALINE COMIMNATInNs IS;")

property of restoring the blue colour to litmus which has been reddened
by the action of acids. The hydrates of the oxides of sodium and
potassium, NaHO and KHO, are examples of basic hydrates easily
soluble in water. They are true alkalis, and are termed caustic, because
they act very powerfully on the skin of animals and plants. Thus
NaHO is called ' caustic ' soda.

Thus, the saline oxides are capable of combining together and with
water. Water itself is an oxide, and not an indifferent one, for it can,
as wre have seen, combine with basic and acid oxides ; it is a represen-
tative of a whole series of saline oxides, intermediate oxides, capable of
combining with both basic and acid oxides. There are many such
oxides, which, like water, combine with basic and acid anhydrides — for
instance, the oxides of aluminium and tin, &c. From this it may be
concluded that all oxides might be placed, in respect to their capacity
for combining with one another, in one uninterrupted series, at one
extremity of which would stand those oxides which do not combine
with the bases — that is, the alkalis— while at the other end would be
the acid oxides, and in the interval those oxides which combine with
one another and .with both the acid and basic oxides. The further
apart are the members of this series the more stable are the compounds
they form together, the more energetically do they act on each other,
the greater the quantity of heat evolved in their reaction, and the
clearer is their saline chemical character.

We said above that basic and acid oxides combine together, but
rarely react on each other ; this depends on the fact that the majority
of them are solids or gases — that is, they occur in the state least prone
to chemical reaction. The gaseo-elastic state is with difficulty destroyed,
because it necessitates overcoming the elasticity proper to the gaseous
particles. The solid state is characterised by the immobility of its
particles ; whilst chemical action requires contact, and hence a dis-
placement and mobility. If solid oxides be heated, and especially if
they be melted, then reaction proceeds with great ease. But such a
change of state rarely occurs in nature or in practice. In a few furnace
processes only is this the case. For example, in the manufacture of
glass, the oxides contained in it combine together in a molten state.
But when oxides combine with water, and especially when they form
hydrates soluble in water, then the mobility of their particles increases
to a considerable extent, and their reaction is greatly facilitated. Re-
action then takes place at the ordinary temperature — easily and rapidly ;
so that this kind of reaction belongs to the class of those which take
place with unusual facility, and are, therefore, very often taken advan-
tage of in practice, and also have been and are going on in nature at

186 PRINCIPLES OF CHEMISTRY

every step. We will now consider the reactions of oxides in the state
of hydrates, not losing sight of the fact that water is itself an oxide
with definite properties, and has. therefore, no little influence on the
course of those changes in which it takes part.

If we take a definite quantity of an acid, and add an infusion of
litmus to it, it turns red ; the addition of an alkaline solution does not
at once alter the red colour of the litmus, but on adding more and
more of the alkaline solution a point is reached when the red colour
changes to violet, and then the further addition of a fresh quantity of
the alkaline solution changes the colour to blue. This change of the
colour of the litmus is a consequence of the formation of a new com-
pound. This reaction is termed the saturation or neutralisation of
the acid by the base, or vice versa. The solution in which the acid
properties of the acid are saturated by the alkaline properties of the
base is termed a neutral solution. Such a solution, although derived
from the mixture of a base with an acid, does not, however, exhibit
either the acid or basic reaction on litmus, yet it preserves many other
signs of the acid and alkali. It is observed that in such a definite
admixture of an acid with an alkali, besides the change in the colour
of litmus, there is a heating effect — i.e., an evolution of heat — which is
alone sufficient to prove that there was chemical action. And, indeed,
if the resultant violet solution be evaporated, there separates out, not
the acid nor the alkali originally taken, but a substance which has
neither acid nor alkaline properties, but is usually solid and crystal-
line, having a saline appearance ; this is a salt in the chemical sense of
the word. Hence it is derived from the reaction of an acid on
an alkali, and through a definite relation between the acid and
alkali. The water here taken for solution plays no other part than
merely facilitating the progress of the reaction. This is seen from the
fact that the anhydrides of the acids are able to combine with basic
oxides, and give the same salts as do the acids with the alkalis or
hydrates. Hence, a salt is a compound of definite quantities of an
acid with an alkali. In the latter reaction, water is separated out if
the substance formed be the same as is produced by the combination of
anhydrous oxides together.51 Examples of the formation of salts from
acids and bases are easily observed, and are very often applied in

51 That water really is separated in the reaction of acid 011 alkaline hydrates, ni;iy In-
shown by taking some other intermediate hydrate — for instance, alumina — instead of
water. Thus, if a solution of alumina in sulphuric acid be taken, it will have, like the
acid, an acid reaction, and will therefore colour litmus red. If, on the other hand, a
solution of alumina in an alkali — for instance, potash — be taken, it will have an alkaline
reaction, and will turn red litmus blue. On adding the alkaline to the acid solution
until neither an alkaline nor an acid reaction is produced, a salt is formed, consisting of

OXYGEN AND ITS SALINE O OIlilN.XTK >NS 187

practice. If we take, for instance, insoluble nui^m-sium oxide, it i>
easily dissolved in sulphuric acid, and on evaporation ^m-s a saline
substance, bitter, like all the salts of magnesium, and familiar to
all under the name of Epsom salts, used as a purgative. If a solu-
tion of caustic soda — which is obtained, as we >a\\ , by the action of
water on sodium oxide — be poured into a flask in which charcoal has
been burnt ; or if carbonic anhydride, which is produced under so many
circumstances, be passed through a solution of caustic soda, then sodium
carbonate or soda, Na2C(X, is obtained, of which we have spoken several
times, and which is prepared on a large scale and often used in manu-
factures. This reaction is expressed by the equation, 2NaHO + CO2 =
Na2CO3 + H.)O. Thus, the various bases and acids form an innumer-
able number of different salts.52 Salts constitute an example of definite
chemical compounds which, both in the history and practice of science,

sulphuric anhydride and potassium oxide. In this, as in the reaction of hydrates, an
intermediate oxide is separated out — namely, alumina. Its separation will be very
evident in this case, as alumina is insoluble in water, whilst its compounds with the
acid and alkali, like the compound of an alkali with an acid — i.e.,& salt— are soluble
in water, and therefore on mixing the solutions of alumina in an acid and an alkali, it is
precipitated as a gelatinous hydrate.

5- The mutual interaction of hydrates, and their capacity of forming salts, may .be
taken advantage of for determining the character of such hydrates as are insoluble in
water. Let us imagine that a given hydrate, whose chemical character is unknown, is
insoluble in water. It is therefore impossible to test its reaction on litmus. It is then
mixed with water, and an acid — for instance, sulphuric acid — is added to the mixture. If
the hydrate taken be basic, reaction will take place, either directly or by the aid of
heat, with the formation of a salt. In certain cases, the resultant salt is soluble in
water, and this will at once show that combination has taken place between the
insoluble basic hydrate and the acid, with the formation of a soluble saline substance. In
those cases where the resultant salt is insoluble, still the water loses its acid reaction,
and therefore it may be ascertained, by the -addition of an acid, whether a given
hydrate has a basic character, like the hydrates of oxide of copper, lead, &c. If
the acid does not act on the given insoluble hydrate (at any temperature), then
it has not a basic character, and it should be tested as to whether it has an acid
character. This is done by taking an alkali, instead of the acid, and by observing
whether the unknown hydrate then dissolves, or whether the alkaline reaction dis-
appears. Thus it may be proved that hydrate of silica is acid, because it dissolves in
alkalis and not in acids. If it be a case of an insoluble intermediate hydrate, then it
will be observed to react on both the acid and alkali. Hydrate of alumina is an
instance in question, which is soluble both in caustic potash and in sulphuric acid.
But it must be remarked that intermediate oxides, in an anhydrous state, often
evince great resistance to the formation of saline compounds. Thus alumina or
aluminium oxide, in the anhydrous form in which it is met with in nature, and which
forms a crystalline substance, is insoluble in this form both in solutions of alkalis and
of acids. In order to convert it into a soluble form, it must be ground into a fine
powder and fused together with certain acid compounds, which are unchanged by
heat, such as acid potassium sulphate.

The degree of affinity or chemical energy proper to oxides and their hydrates is very
dissimilar ; some extreme members of the series have it to a great extent. When acting
on each other they evolve a large quantity of heat, and when acting on intermediate
hydrates they also evolve heat to a considerable degree, as we saw in the coinbi-

188

are most often cited a> confirming the conception of definite chemical
compounds. Indeed, all the indications of a definite chemical combina-
tion are clearlv seen in the formation and properties of >alts. Thus,
>alts are produced with a definite proportion of oxides, heat is evoked
in their formation/'3 and the character of the oxides and manv of their
physical properties are hidden in salts. Thus, when gaseous carbonic
anhydride combines with a base to form a solid >alt, the elasticity of
the u'as <|iiite disappears in its passage into the salt.'"'1

Judging tVom the above, a salt i^ a compound of basic and

nation ot lime and sulphunc anhydride with water. When extreme oxides combine they
lornistable salt-, which are ditticiiltlv decompo-ed. and often show characteristic proper-
ties. The compounds of the intermediate oxides with each other, or even with basic and
acid oxides, present a very different case. However much alumina we may dissolve
in sulphuric arid, we cannot saturate the acid properties of the sulphuric acid, the
resulting solution will always have an acid reaction. So aUo. whatever quantity of
alumina is dissolved in an alkali, the resultiiiLT solution will always present an alkaline
reaetii ui.

' In order to pve an idea of the quantity of heat evolved in the formation of salts,
I append a table of data tor rrry dilute arjncun* milittimix of acids and alkalis, accord-
ing to the determinations of Berthelot and Thonisen. The li-nres are pven in major
cal' -ries— t hat is. in thousands of units of heat. Hence. l'.» -'rams of sulphuric acid.
H SO., taken in a dilute aqueous solution, when mixed with such an amount of a weak
solution of call-tic soda, NallO. that a neutral salt i- form-d iwheii all the hvdn»_reii of
the acid i- replaced by the sodium), evolves l.">sUi) units of heat. A star signifies the
formation of an insoluble salt.

l .So4 II NO ,

Xallo . . ir.-s 1:5-7 M-o . l.vt; i:;-s

Kilo . . . ir.'T i:j-.s Fe() . }•!•:> ld'7 i?)

NH- . . . }{•:, \>i-:> '/.uO . . 11-7 i»-s

Ca<) . . . l.'.-t; ]:;•;) l°«-'-< • :''7 :'';l

'l'he-e (i '.Hires cjiinidt l>e considered as the heat of ueiit ralisat ion. l)ecause tin- water
here plays an important part. Thus, for instance, sulphuric acid and caustic soda in
dis-olviii'_' in water, evolve very much heat, and the result a ni -odium sulphate very little ;
con~c(| ui-iil I \ . the he;il e\iil\ed iii an a nliyilroiis -tale \\ill lie dit't'erent from (hat in a

1 1 vd rated state. Those acids wliich are not eiierj.fel ic in coiiiliiniiiir \\ it li the sal iiian-

titv of alkali- as i- reqiiired for the formation of normal -alts of sulphuric or nitric

acid- alwavs, howe\er. i:ive less heat. |-'or example, wit li caustic soda: carbonic acid

_;•.. Kr-J. liydi-ocyanic ii".i. 1 1 yd ro-eii sulphide :',".l. And as I'eelile liases (for example,

I-'e ( )- al-o evuh'c less heal tlian lliose uhich an- more powerful, so a certain ueiieral

correlation l.etwecn theniiochemical data and tlie conception of (lie measure of affinity

-hous itself here. ,i, in other cases i.srr Chap. II.. Note 7 1. which does not, however, -,'ive

• i on f..r juduriii^i »l the measure i.f the aftinit\ wliich l.inds the elements o) salts

h\ the heal of | he formation ,,| salts I,, dilute solutions. This is rendered especially

M the fact tli.it water is alile to decompose mam salts. ;<nd is -eparated in their

(>XV(iKN AM> ITS SALINE COMBINATIONS 189

acid oxides, or the result of the action of hydrates of these cl;i
on each other, with separation of water. But salts may be obtained
by other methods. Let us not forget that basic oxides are formed
by metals, and acid oxides often by non-metals. But metals and
non-metals are capable of combining together, and a salt is frequently
formed by the oxidation of such a compound. For example, iron very
easily combines with sulphur, forming iron sulphide (as we saw in the
Introduction) ; this in air, and especially moist air, absorbs oxygen,
with the formation of the same salt as may be obtained by the combina-
tion of the oxides of iron and sulphur, or of the hydrates of these
oxides. Hence, it cannot be said or supposed that a salt contains
the principles of the oxides, or that a salt must necessarily contain two
kinds of oxides in itself. The same conclusion may be arrived at by
investigating the different other methods of the formation of salts —
thus, for instance, many salts enter into double decomposition with the
metals, in which case the acting metal replaces that which originally
occurred in the salt. As we saw in the Introduction, iron, when placed
in a solution of copper sulphate, separates out the copper, and forms
an iron salt. Thus, the derivation of salts from oxides, is only
one of the methods of their preparation, there being many others,
and, therefore, it cannot be affirmed that a salt is simply the compound
of two oxides. We saw, for instance, that in sulphuric acid it was
possible to replace the hydrogen by zinc, and that by this means zinc
sulphate was formed ; so likewise the hydrogen in many other acids
may be replaced by zinc, iron, potassium, sodium, and a whole series of
similar metals, corresponding salts being obtained. The hydrogen in
the water of the acid, in this case, is exchanged for a metal, and a salt
is obtained from the hydrate. In this sense of a salt it may be said,
that a salt is an acid in which hydrogen is replaced by a metal. Such
a definition will be much more exact than that previously given, for it
refers directly to elements and not to their compounds with oxygen.
It shows that a salt and an acid are essentially compounds of the same
series, with the difference that the latter contains hydrogen and the
former a metal. Such a definition is still more exact than the first
definition of salts in respect to its referring likewise to those acids
which do not contain oxygen, and, as we shall afterwards learn, there
is a series of such acids. Such elements as chlorine and bromine form

ciating, evolves carbonic anhydride. The same gas, when dissolved in solutions of salts,
acts in one or the other manner (see Chap. II., Note 88). Here it is seen what a successive
series of relations exists between compounds of a different order, between sub-
stances of different degrees of stability. Were solutions distinctly separated from
chemical compounds, we should not be able to see those natural transitions which exist
in reality.

190 PRINCIPLES OF CHEMISTRY

compounds with hydrogen, in which the hydrogen may be replaced by
a metal forming substances which, in their reactions and external
characters, resemble the salts formed from oxides. Table salt, NaCl,
is an example of this. It may be obtained by the replacement of hydro-
gen in hydrochloric acid, HC1, by the metal sodium, just as sulphate
of sodium, NaaSO.,, may be obtained by the replacement of hydrogen
in sulphuric acid, H.2SO4, by sodium. The exterior appearance of the
resulting products, their neutral reaction, and even their saline taste,
show their mutual resemblance ; as the acid reaction, the property of
saturating bases, the capacity of exchanging their hydrogen for some
metal, and the acid taste, show the common properties belonging to
hydrochloric and sulphuric acids.

To the fundamental properties of salts yet another must be added —
namely, that they are more or less decomposed by the action of a galvanic
current. The results of this decomposition are very different, accord-
ing to whether the salt be taken in a fused or dissolved state. But

O

the decomposition may be so represented, that the metal appears at the
electro-negative pole (like hydrogen in the decomposition of water, or
its mixture with sulphuric acid), and the remaining parts of the salt
appear at the electro- positive pole (where the oxygen of water appears).
If, for instance, an electric current acts on an aqueous solution of sodium
sulphate, then the sodium appears at the negative pole, and oxygen
and the anhydride of sulphuric acid at the positive pole. But in the
solution itself the result is different, for sodium, as we know, decom-
poses water with evolution of hydrogen, forming caustic soda ; conse-
quently hydrogen will be evolved, and caustic soda appear at the
negative pole : while at the positive pole the sulphuric anhydride
immediately combines with water and forms sulphuric acid, and there-
fore oxygen will be evolved and sulphuric acid formed round this
pole.55 In other cases, when the metal separated is not able to decom-
pose water, it will be deposited in a free state. Thus, for example, in
the decomposition of copper sulphate, copper separates out at the
cathode, and oxygen and sulphuric acid appear at the anode, and
if a copper plate be attached to the positive pole, then the oxygen
evolved will oxidise the copper, and the oxide of copper will dissolve in
the sulphuric acid which is formed around this pole ; hence the copper
will be dissolved at the positive, and deposited at the negative, pole —

55 This kind of decomposition maybe easily observed by pouring a solution of sodium
sulphate in a U-shaped tube and inserting electrodes in both branches- If the solution
lae coloured with an infusion of litmus, it will easily be seen that it turns blue round the
electro-negative pole, owing to the formation of sodium hydroxide, and red at the
electro-positive pole, from the formation of sulphuric acid.

oXVdKN AND ITS SALINK ( '( >.M l',I NATI< >NS 191

that is, a transfer of copper from the positive to the negative pole
ensues. The galvanoplastic art (electrotyping) is based on this
principle/"1 Therefore the most radical and general properties of salts
(including also such salts as table salt, which contains no oxygen) may
be expressed by representing the salt as composed of a metal M and a
haloid X — that is, by expressing the salt by MX. In common table
salt the metal is sodium, and the haloid an elementary body, chlorine.
In sodium sulphate, Na2SO4, sodium is again the metal, but the
complex group, S04, is the haloid. In sulphate of copper, CuSO4, the
metal is copper, and the haloid the same as in the preceding salt.
Such a representation of salts expresses with great simplicity the
capacity of every salt to enter into saline double decompositions with
ntlicr salts ; consisting in the mutual replacement of the metals in the
salts. This exchange of their metals forms the fundamental property
of salts. If there be two salts with different metals and haloids, and
they be in solution or fusion, or any other manner, brought into con-
tact, then the metals of these salts will always partially or wholly
exchange places. If we designate one salt by MX, and the other by
NY, then we either partially or wholly obtain from them new salts,
MY and NX. Thus we saw in the Introduction, that on mixing
solutions of table salt, NaCl, and silver nitrate, AgNO3, a white
insoluble precipitate of silver chloride, AgCl, is formed, and a new salt,
sodium nitrate, NaNO3, is obtained in solution. If the metals of salts
exchange places in reactions of double decomposition, it is clear that
metals themselves, taken in a separate state, are able to act on salts, as
zinc evolves hydrogen from acids, and as iron separates copper from
copper sulphate. When, to what extent, and which metals displace each
other, and how the metals are distributed between the haloids, all this we
will discuss later on, guided by those reflections and deductions which
Berthollet introduced into the science at the beginning of this cen-
tury.

According to the above observations, an acid is nothing more than
a salt of hydrogen. Water itself may be looked on as a salt in which

56 In other cases the decomposition of salts by the electric current may be accom-
panied by much more complex results. Thus, when the metal of the salt is capable of a
higher degree of oxidation, such a higher oxide may be formed at the positive pole by
the oxygen which is evolved there. This takes place, for instance, in the decomposition
of salts of silver and manganese by the galvanic current, peroxides of these metals being
formed. If the metal separated at the negative pole acts on a salt occurring in the
solution, then it may do so at this pole, and in this manner the phenomena of the action
of a current on a salt are in many cases rendered remarkably complicated. But all the
phenomena as yet known may be expressed by the above law — that the current decom-
poses salts into metals, which appear at the negative pole, and into the remaining com-
ponent parts, which appear at the positive pole.

the hydrogen is combined with either oxygen or the aqueous radicle,
Oil : water will then he 11<>H, and alkalies or basic hydrates, MOIL
The group ()H. or the <-HJU.''<>/IS rftdiclt^ otherwise called Jti/dro.vyl, may
be looked on as a haloid like the chlorine in table salt, not only because
the element ( '1 and the group OH very often change places, and com-
bine with one and the same element, but also because free chlorine is
very similar in many respects and reactions to peroxide of hydrogen,
which is the same in composition as the aqueous radicle, as we shall after-
wards see. Alkalis and basic hydrates are also salts consisting of a
metal and hydroxyl— for instance, caustic soda, XaOH ; this is therefore
termed sodium Jiydroride. According to this view, nc.'nl xa/f* arc1 those
in which a portion only of the hydrogen is replaced bv a metal, and a
portion of the hydrogen of the acid remains. Thus sulphuric (H.,SO, |
acid with sodium not only gives the normal salt Xa^SO,. hut also an
acid salt, XallSO,. A //r/x/r wilt is one in which the metal is com-
bined not only with the haloids of acids, but also wit h the aqueous radicle
of basic hydrates -for example, bismuth gives not only a normal salt
of nitric acid, .l>i(X"( ):i)s, but also basic salts like I>i(< >H )L)(XO.<). As
basic and acid salts corresponding with the oxygen acids contain
hydrogen and oxygen, they are therefore able to part with these as
water and to give anhydro-salts, which it is evident will be equal to
compounds of normal salts with anhydrides of the acids or with bases.
Thus the above-mentioned acid sodium sulphate corresponds with
the anhydro-salt, Xa._,S._,O7, equal to L'XallSO,, less H2O. The loss
of water is here, and frequently in other cases, brought about bv
heat alone, and therefore such salts are frequently termed />yro-saffts
for instance, the preceding is sodium pyrosulphate (Xa.jS.^O-), or it may
be regarded as the normal salt X"a._,SO} -(- sulphuric anhydride, SOV
l)i,nl,Ii' salts are those which contain either two metals, 1\ A1(S( ),).,. or
two haloids/'7

••' The abo\e-enunciated generalisation of the conception of sa It s as compound- of
the metal- (simple, or compound like ammonium. N 1 1 , i, wit h the haloids i simple, like
impound, like cyanogen, CN. or the radicle of sulphuric acid, SO,), capable

'

lata respecting salts, was only formed little by little after a succession of

UK 1st \ a I'led

1

t

Salt- belong to the class of substances which have Ion- been known in \
hen-fore were ^udied in maiiv respects from very far back. At lirst, liowi

ii,ni\ artificial -all- during the latter half of the seventeenth eenturv. I'p 1

iractice. and

ler prepared
ii that time
•h \\ e have

1

aid icr's s-ilt

In iwed t heir

ction <,n \e -el able dye-., -till he c. ni t oi i n d e<l ii i a n y -a 1 1 s with acids i b\ the wa

V. we oll^llt,
be replaced

<>XY<JKN AND ITS sALINK COMBINATIONS 199

Inasmuch as oxygen compounds predominate in nature, it should
be expected, from what has been said above, that the occurrence of
salts, rather than of acids or bases, would be most frequent in nature,
for the latter on meeting, especially under the medium of the all-per-

by metals — that is, it is the hydrogen of an acid). Baume disputed Rouelle's opinion
concerning tin- subdivision of salts, contending that normal salts only are true salts, ami
that basic salts are simple mixtures of normal salts with bases and acid salts with acids,
considering that washing alone could remove the base or acid from them. Rouelle, in tin-
middle of the last century, however, rendered a great service to the study of salts and
the diffusion of knowledge respecting this class of compounds in his attractive lectures.
He, like the majority of the chemists of that period, did not employ the balance in his
researches, but satisfied himself with purely qualitative data. The first quantitative
researches on salts were carried on by Wenzel about this time. He was the director of
the Freiburg mines, in Saxony. Wenzel studied the double decomposition of salts, and
he observed that in the double decomposition of neutral salts a neutral salt was always
obtained. He proved, by a method of weighing, that this is due to the fact that the satura-
tion of a given quantity of a base requires such relative quantities of different acids as are
capable of saturating every other base. Having taken two neutral salts — for example,
sodium sulphate and calcium nitrate — let us mix their solutions together. Double
decomposition takes place, because the almost insoluble calcium sulphate is formed.
However much we might add of each of the salts, the neutral reaction will still be pre-
served, consequently the neutral character of the salts is not destroyed by the inter-
change of metals ; that is to say, that quantity of sulphuric acid which saturated the
sodium is sufficient for the saturation of the calcium, and that amount of nitric acid
which saturated the calcium is enough to saturate the sodium contained in combination
with sulphuric acid in sodium sulphate. Wenzel was even convinced that matter does
not disappear in nature, and on this principle he corrects, in his Doctrine of Affinity,
the results of his experiments when he remarked that he obtained less than he had origi-
nally taken. Although Wenzel deduced the law of the double decomposition of salts
quite correctly, he did not determine those quantities in which acids and bases act on
each other. This was done quite at the end of the last century by Richter. He deter-
mined the quantities by weight of the bases which saturate acids and of the acids which
saturate bases, and he obtained comparatively correct results, although his conclusions
were not correct, for he states that the quantity of a base saturating a given acid varies
in arithmetical progression, and the quantity of an acid saturating a given base in geo-
metrical progression. Richter studied the deposition of metals from their salts by other
metals, and observed that the neutral reaction of the solution is not destroyed by this
exchange. He also determined the quantities by weight of the metals replacing one
another in salts. He showed that copper displaces silver from its salts, and that zinc
displaces copper and a whole series of other metals. Those quantities of metals which
were capable of replacing one another were termed equivalents.

Richter's teaching found no followers, because, although he fully believed in the dis-
coveries of Lavoisier, yet he still held to the phlogistic reasonings which rendered his
expositions very obscure. The works of the Swedish savant Berzelius freed the facts
discovered by Wenzel and Richter from the obscurity of former conceptions, and led to
their being explained in accordance with Lavoisier's views, and in the sense of the law
of multiple proportions which had already been discovered by Dalton. On applying to
salts those conclusions which Berzelius arrived at by a whole series of researches of re-
markable accuracy, we are obliged to acknowledge the following law of equivalents —
oni' part by weight of hydrogen in an acid is replaced by the corrcsjHHtditiy i-<jnir<ilriif
irr'ujht of any metal ; and, therefore, when metals replace each other their weights are in
the same ratio as their equivalents. Thus, for instance, one part by weight of hydrogen
is replaced by 28 parts of sodium, 89 parts of potassium, 12 parts of magnesium, 20 parts
VOL. I. O

194 PRINCIPLES OF CHEMISTRY

vading water, form salts. And, indeed, salts are found everywhere
in nature. In animals and plants they occur, although in but small

of calcium, 28 parts of iron, 108 parts of silver, 33 parts of zinc, &c. ; and thei'efore, if zinc
replaces silver, then 33 parts of zinc will take the place of 108 parts of silver, or 33 parts
of zinc will be substituted by 23 parts of sodium, £c.

The doctrine of equivalents would be precise and simple did every metal only give
one oxide or one salt. It is rendered complicated from the fact that many metals form
several oxides, and consequently offer different equivalents in their different degrees of
oxidation. For example, there are oxides containing iron in which its equivalent is
28— this is in the salts formed by the suboxide ; and there is another series of salts
in which the equivalent of iron equals 18| — which contain less iron, and conse-
quently more oxygen, and correspond with a higher degree of oxidation — ferric
oxide. It is true that the former salts are easily formed by the direct action of
metallic iron on acids, and the latter only by a further oxidation of the compound
formed already ; but this is not always so. In the case of copper, mercury, and
tin, under different circumstances, there are formed salts which correspond with
different degrees of oxidation of these metals, and many metals have two equivalents
in their different salts — that is, in salts corresponding with the different degrees of
oxidation. Thus it is impossible to endow every metal with one definite equivalent
weight. Therefore the conception of equivalents, while playing an important part
from an historical point of view, appears, with a fuller study of chemistry, to be but an
incidental conception, subordinate to a higher one, with which we shall afterwards
become acquainted.

The fate of the theoretical views of chemistry was for a long time bound up with
the history of salts. The clearest representation of this subject dates back to
Lavoisier, and was very severely developed by Berzelius. This representation is called
the binary theory. All compounds, and especially salts, are represented as consisting
of two parts. Salts are represented as a compound of a basic oxide (a base) and an
acid (that is, an anhydride of an acid, then termed an acid), whilst hydrates are repre-
sented as compounds of anhydrous oxides with water. They employed such an expres-
sion not only to denote the most usual method of formation of these substances (which
would be quite true), but also to express that internal distribution of the elements by
which they proposed to explain all the properties of these substances. They supposed
copper sulphate to contain two most intimate component parts— copper oxide and
sulphuric anhydride. This is an hypothesis. It arose from the so-called electro-chemical
hypothesis, which supposed the two component parts to be held in mutual union,
because one component (the anhydride of the acid) has electro-negative properties, and
the other (the base in salts) electro-positive. Both parts are attracted together, like
substances having opposite electrical charges. But as the decomposition of salts in a
state of fusion by an electric current always gives a metal, therefore the representation
of the constitution and decomposition of salts, called the hydrogen theory of acids, is
more probable than that considering salts as made up of a base and an anhydride of
an acid. But the hydrogen theory of acids is also a binary hypothesis, and docs not
even contradict the electro-chemical hypothesis, but is rather a modification of it.
The binary theory dates from Kouelle and Lavoisier, the electro-chemical representation
was developed with great power by Berzelius, and the hydrogen theory of acids is due
to Davy and Liebig.

These hypothetical representations simplified and generalised the study of a com-
plicated subject, and gave support to arguments, but when salts were in question it
was equally convenient to follow one or the other of these hypotheses. But these
theories were brought to bear on all other substances, on all compound substances.
Those holding the binary and electro-chemical hypotheses searched for two anti-polar
component parts, and endeavoured to express the process of chemical reactions by electro-
chemical and similar differences. If zinc replaces hydrogen, they concluded that it is

OXYGEN AND ITS SALINE r< >M 111 NAT 1« >.\- 195

amount, because, as forming the last stage of chemical reaction, they
are capable of only a few chemical transformations, the energy of the
elements being evolved (passing into heat) both in the formation of
oxides and in their mutual combinations ; hence in salts there re-
mains but little energy. Organisms are bodies in which a series of
uninterrupted, varied, and active chemical transformations proceed,
whilst salts, which only enter into double decompositions between
each other, are incapable of such changes. But organisms always
contain salts. Thus, for instance, bones contain calcium phosphate,
the juice of grapes, potassium tartrate (cream of tartar), certain
lichens, calcium oxalate, and the shells of mollusca, calcium car-
bonate, &c. As regards water and soil, portions of the earth in
which the chemical processes are less active, they are full of salts.
Thus the waters of the oceans, and all others (Chap. I.), abound in
salts, and in the soil, in the rocks of the earth's crust, in the up-
heaved lavas, and in the falling meteorites the salts of silicic acid, and

more electro-positive than hydrogen, whilst they forgot that hydrogen may, under different
circumstances, displace zinc — for instance, at a red heat. Chlorine and oxygen were con-
sidered as being of opposite polarity to hydrogen because they easily combine with it, whilst
one and the other are capable of replacing hydrogen, and, what is very characteristic, in
the replacement of hydrogen by chlorine in carbon compounds, not only does the
chemical character often remain unaltered, but even the external form remains un-
changed, as Laurent and Dumas demonstrated. These considerations undermine the
binary theory, and especially the electro-chemical system. An explanation of known
reactions then began to be sought for not in the difference of the polarity of the
different substances, but in the joint influences of all the elements on the properties of
the compound formed. This is the reverse of the preceding hypotheses.

This reversal was not, however, limited to the destruction of the tottering founda-
tions of the preceding theory ; it projected a new doctrine, and laid the foundation for
the whole contemporary direction of our science. This doctrine may be termed the
unitary theory — that is, it is such as strictly acknowledges the joint influences of the ele-
ments in a compound substance, denies the existence of separate and contrary components
in them, regards copper sulphate, for instance, as a strictly definite compound of copper,
sulphur, and oxygen ; then seeks for compounds which are analogous in their properties,
and, placing them side by side, endeavours to express the influence of each element on
the united properties of its compound. In the majority of cases it arrives at systems of
consideration similar to those which are obtained by the above-mentioned hypotheses
but in certain special cases the conclusions of the unitary theory are in entire opposition
to the binary theory and its consequences. Cases of this kind are most often met with
in the consideration of compounds of a more complex nature than salts, especially
organic compounds containing hydrogen. But it is not in this revolution from an
artificial to a natural system, important as it is, that the chief service and strength of
the unitary doctrine lies. By a simple review of the vast store of data regarding the
reactions of typical substances, it succeeded from its first appearance in establishing a
new and important law, it introduced a new conception into science — namely, the
conception of molecules, with which we shall soon become acquainted. The deduction
of the law and of the conception of molecules has been verified by facts in a number of
cases, and was the cause of the majority of chemists of our times deserting the binary
theory and accepting the unitary theory, which forms the basis of the present work.
Laurent and Gerhardt must be looked on as the propagators of this doctrine.

o 2

: often form mountain chainsand wliole t hickn----e- of
-T rat a. t he-e consi.-t in n" of calcium earhonate, ( 'a< '* > .

I is \\ . • have -een o\v;_feii in a free -tate and in various compounds
of dill'crent decree- of stahilitv, from, the un-tahle salts, like I'.ert hollet '-
-•dt and nitre, to the most -laMe silicon compound-, -udi as exist in
granite. \\"e -,aw an entirely -imilar gradation of -lability in the com

Is of \\ater and of h vdro^en. In all it s aspect s oxygon, as an
element, a- a -uK-tance. remain- the same in it-elf in the nio-t varied
cliemical -tate-. just a- a -uhstance mav appear in dill'erent physical
1 a_;'u i'ei;at e ) -tales, l»ut our notion of the immense varietv of the
chemical >tates in which oxygen can occtii1 would nor lie completely

underst 1 if we did not make ourselves acquainted with it in the

Toi-m in \\hich it occur- in ozone and peroxide ot hydrri^en. In the-e
it i- nio-t active, its eiier^v seem- to ha\c inci'ea-ed. 'Then the fre-h
a-jiect- of chemical correlation-, ,-md the \ariet\- of the form- in which
in uter can appear, -land out (dearly. \Ve will therefore consider these
' wo -uhstances some\\-]iat in detail.

197

CHAPTER IV

OZONE AND HYDROGEN PEROXIDE. DALTON's LAW

VAN-MARUM, during the last century, observed that oxygen in a glass
tube, when subjected to the action of a series of electric sparks, acquired
a peculiar smell and the property of combining with mercury at the
ordinary temperature. This was afterwards confirmed by a number of
fresh experiments. Even in the simple revolution of an electrical
machine, when electricity diffuses into the air or passes through it, the
peculiar and characteristic smell proper to ozone, proceeding from
the action of the electricity on the oxygen of the atmosphere, is
recognised. In 1840 Prof. Schonbein, of Basle, turned his attention
to this odoriferous substance, and showed that it is also formed,
with the oxygen evolved at the positive pole, in the decomposition of
water by the action of a galvanic current ; in the oxidation of phos-
phorus in damp air, and also in the oxidation of a number of
substances, in consequence of which it is found in the atmosphere,
although it is distinguished for its instability and capacity for oxidis-
ing other substances. The characteristic smell of this substance (which
is always mixed with unaltered oxygen) gave it its name, from the Greek
o£w, ' to emit an odour.' Schonbein pointed out the characteristic pro-
perties of ozone, and especially its power of oxidising many substances,
even silver, acting like oxygen, but with this difference — that there are
a number of substances on which oxygen does not act at the ordinary
temperature, whilst ozone does so very energetically. It will be
enough to point out, for instance, that it oxidises silver, mercury,
charcoal, and iron with great energy at the ordinary temperature. It
might be thought that ozone was some new substance, simple or com-
pound, as it was at first supposed to be ; but careful observations
made in this direction have long led to the conclusion that ozone is
nothing but oxygen altered in its properties. This is most strikingly
proved by the complete transformation of oxygen containing ozone into
ordinary oxygen when it is passed through a tube heated to 250°.
Further, at a low temperature pure oxygen gives ozone when electric

IDS PKJNClPLKs ol- CHEMISTKY

-park- are passed through it ( Marii^nac and I >e l;i Rive). Hence it is
proved, by a method fur its preparation from oxygen and by a method of
its transformal inn into ox v^'en (synthesis and analysis), that oxone is tliat
same oxygen with which we art- already acquainted, nnly endowed with
particular properties and in a particular state. However, l»v whatever
method it IK' obtained, the anmiint of it contained in the oxygen is
inconsiderable, general Iv only a few fractions of a per cent., rarelv
'2 percent., and only under verv propitious circumstances as much as
l'<> per cent. The reason of this must be looked for first in the fact
that "•_"/" lit it* foi-inatioii from o.i'ij<i<'n dltxorb* If-nt. If any substance
be burnt in a calorimeter at the expense of o/onised oxygen, then more
heat is evolved than when it is burnt in ordinary oxygen, and Berthelot
showed that this ditl'erence is very lar^e namely, L'Uo'OU heat units
correspond with every forty-eiuht parts by weight of oxone. This
Minifies that the transformation of fort y-eii^ht parts of oxvgen into
nxone is accompanied by the absorption of this quantity of heat, and
that i In- reverse process evolves this quantity of heat. Therefore the
passage of oxone into oxygen should take place easily (as an exother-
mal reaction), like combustion : and this is pmved by the fact that at
•_'."»(> oxone cnlirelv disappears, foi-minn' oxyuvn. Anv rise of tem})era-
ture may thus brini;- about the breaking u]> of oxone. and as a rise of
temperature take.-j jilace in th(> action of an electrical discharge,
therefor** there are in an electric discharge the conditions both for the
preparation of oxone and for its destruction. Hence it is clear that
the transformation of oxygen into oxone, <>* >> /'ft't'i'siti/f i'>'<i<'t ton,
has a limit when a state of equilibrium rs arrived at between the
products of the i \\ ( i opposite reactions, that the phenomena of this
transformation accord \\ith t he phenomena of ili.<ot<n*mti<ni, and that a
fall of temperature should aid the format ion of a l;irg<- quantity of
oxone.1 Furiher. it is evident, from what ha- l»cen said, that the best
way of preparing oxone is not by electric sparks.- which raise the

p., nr]ii ii.n. rli..lm-,.,] l,y in-' a-. t,,r ku-k II- 1-7- MnHitritr Srii-ntijiijui-\l>\

, |l,i : I \I |f, |-1 l.ssil . wlui-ll >h..\V(..i lllilt tllr passil.UV .-I ;l -llclll

:;n i|, t., f.ii i • IH.-I .,] all i,, i ln< dclcrniinal .HI- nt' < 'li;i|>|»ui- and Haute

. ] --H . •/, |,M IMIIIK! lli. i' .il ii ! i -in] I. •]•..! nr.' i.i' -li.*. :i -liflil ili-rliar-T CdllVt-rtcd -Jl I p.c.

liil-1 .it 'JH i! \\.i- - iui|".--.il.lr t....l.la:n IIPHV 1 h. in 1- )'.<•.. an. hit lull

• \ ., , , , • , , n-ii i • . in. i\ In- (.lit; I I'itlirr l>\ aii ..rdiuarv i-lcrlrii-al inacliiiu-.

' ' Unit

OZONE AND HYDROGEN PEROXIDE — 1) ALTON'S LAW 199

temperature, but by the employment of a continual discharge or
flow of electricity — that is, to transform the oxygen by the action
of a silent discharge.3 For this reason all ozomsers (which are of
most varied construction), or forms of apparatus for the preparation of
ozone from oxygen (or air) by the action olf electricity, now usually
consist of conductors (sheets of metal — for instance, tinfoil — or a solution
of sulphuric acid with chromic acid, &c.) separated by thin glass
surfaces placed at short distances from each other, and between which

FIG. 37.— Siemens' apparatus for preparing ozone by means of a silent discharge.

the oxygen or air to be ozonised is introduced and subjected to the
action of a silent discharge.4 Thus in Siemens' apparatus (fig. 37) the

5 A silent discharge is such a combination of opposite statical (potential) electricities
as takes place (generally between large surfaces) regularly, without sparks, slowly, and
quietly (as in the dispersion of electricity). The discharge is only luminous in the dark ;
there is no observable rise of temperature, and therefore a larger amount of ozone is
formed. But, nevertheless, on continuing the passage of a silent discharge through
ozone it is destroyed. For the action to be observable a large surface is necessary, and
consequently a powerful source of electrical potential. For this reason the silent dis-
charge is best produced by a Ruhmkorff coil, as the most handy means of obtaining a
considerable potential of statical electricity with the employment of the comparatively
feeble current of a galvanic battery.

4 V.Sabo'n (ijijifirattm was one of the first constructed for ozonising oxygen bymeanB
•of a silent discharge (and it is still one of the best). It is composed of a number (twenty
and more) of long, thin capillary glass tubes closed at one end. A platinum wir-
tending along their whole length, is introduced into the other end of each tube, and this
end is then fused up round the wire, the end of which protrudes outside the tube.
The protruding ends of the wires are arranged alternately in two sides in such a manner
that on one side there are ten closed ends and ten wires. A bunch of such tubes (forty
should make a bunch of not more than 1 c.m. diameter) is placed in a glass tube, and
the ends of the wires are connected into two conductors, and are fused to the ends of the
surrounding tube. The discharge of a Ruhmkorff coil is passed through these cuds of
the wives, and the dry air or oxygen to be ozonised is passed through the tube. If
oxygen be passed through, ozone is obtained in large quantities, and free' from oxides of

200 PKINCIPLES OF CHEMISTRY

exterior of the tube a and the interior of the tube b c are coated with
tinfoil and connected with the poles of a source of electricity (with the
terminals of a Ruhmkorff's coil). A silent discharge passes through the
thin walls of the glass cylinders a and b c over all their surfaces, and
consequently, if oxygen be passed through the apparatus by the tube d,
fused into the side of «, it will be ozonised in the annular space between
a and b c. The ozonised oxygen escapes by the tube e, and may be
introduced into any other apparatus.5

The properties of ozone obtained by such a method'' distinguish it
in many respects from oxygen. Ozone very rapidly decolorises indigo,
litmus, and many other dyes by oxidising them. Silver is oxidised by
it at the ordinary temperature, whilst oxygen is not able to oxidise
silver even at high temperatures ; a bright silver plate rapidly turns

nitrogen, which are partially formed when air is acted on. It is remarked that at low
temperatures ozone is formed in large quantities. As ozone is acted on by corks and
india-rubber, the apparatus should be made entirely of glass. With a powerful Ruhmkorff
coil and forty tubes the ozonation is so powerful that the gas, when passed through a,
solution of iodide of potassium, not only sets the iodine free, but even oxidises it into
potassium iodate, so that in five minutes the gas-conducting tube is choked up with
crystals of the insoluble iodate.

5 In order to connect the ozoniser with any other apparatus it is impossible to make
use of india-rubber, mercury, or cements, etc., because they are themselves acted on by,
and act on, ozone. All connections must, as was first proposed by Brodie, be hermetically
closed by sulphuric acid, which is not acted on by ozone. Thus, a cork is passed over
the vertical end of a tube, over which a wide tube passes so that the end of the first tube
protrudes aboA'e the cork ; mercury is first poured over the cork (to prevent its being
acted on by the sulphuric acid), and then sulphuric acid is poured over the mercury.
The protruding end of the first tube is covered by the lower end of a third tube immersed
in the sulphuric acid.

6 The above-described method is the only one which has been well investigated. The
admixture of nitrogen, or even of hydrogen, and especially of silicon fluoride, appears to
aid the formation and preservation of ozone. Amongst other methods for preparing
ozone we may mention the following : — 1. In the action of oxygen on phosphorus at the
ordinary temperature a portion of the oxygen is converted into ozone. At the ordinary
temperature a stick of phosphorus, partially immersed in water and partially in air in a
large glass vessel, causes the air to acquire the odour of ozone. It must further be
remarked that if the air be left for long in contact with the phosphorus, or without the
presence of water, the ozone formed is destroyed by the phosphorus. 2. By the action
of sulphuric acid on peroxide of barium. If the latter be covered with strong sulphuric
acid (the acid, if diluted with only one-tenth of water, does not give ozone), then at a low
temperature the oxygen evolved contains ozone, and in much greater quantities than
that in which ozone is obtained by the action of electric sparks or phosphorus. 3. Ozone
may also be obtained by decomposing strong sulphuric- acid by potassium inanimate,
especially with the addition of barium peroxide. Gorup-Besanez stated (but it requires
confirmation) that ozone is formed in the slow evaporation of large quantities of water.
In the near proximity of salt-gardens (salterns) the atmosphere is considerably richer in
ozone than in the surrounding neighbourhood. In connection with this is the fact that
the air of the sea-shore is rich in ozone. Ozone is also stated to be formed in the
ordinary process of the respiration of plants. This is, however, denied by many to be
the case.

OZONE AM) HYDROGEN PEROXIDE— DAI/TON'S LAW 201

black (from oxidation) in. ozonised oxygen. It is rapidly absorbed by
mercury, forming oxide ; it transforms the lower oxides into higher — for
instance, sulphurous anhydride into sulphuric, nitrous oxide into
nitric, arsenious anhydride (As.2O3) into arsenic anhydride (As.205) <fcc. 7
But what is especially characteristic in ozone is the decomposing action
it exerts on potassium iodide. Oxygen does not act on it, but ozone
passed into a solution of potassium iodide liberates iodine, whilst the
potassium is obtained as caustic potash, which remains in solution,
2KI + H2O + 0=2KHO-|-l2. As the presence of minute traces of
free iodine may be discovered by means of starch paste, with which it
forms a very dark blue coloured substance, a mixture of potassium
iodide with starch paste will detect the presence of very small traces of
ozone.8 Ozone is destroyed or converted into ordinary oxygen not
only by heat, but also by long keeping, especially in the presence of
alkalis, peroxide of manganese, chlorine, tkc.

Hence ozone, although it has the same composition as oxygen, differs

7 Ozone takes up the hydrogen from hydrochloric acid ; the chlorine is set free, and
can dissolve gold. Chromium and iodine are directly oxidised by ozone, but not by oxygen,
and so also with a number of other substances. Ammonia, NH5, is oxidised by ozone into
ammonium nitrite (and nitrate), 2NH5 + O3 = NH4NO^ + H2O, and therefore a drop of
ammonia, on falling into the gas, gives a thick cloud of the salts formed. Ozone converts
lead oxide into peroxide, and suboxide of thallium (which is colourless) into oxide (which
is brown), so that this reaction is made use of for discovering the presence of ozone.
Lead sulphide, PbS, is converted into sulphate, PbSO4, by ozone. A neutral solution of
manganese sulphate gives a precipitate of manganese peroxide, and an acid solution may
be oxidised into permanganic acid, HMnO4. With respect to the oxidising action of ozone
on organic substances, it may be mentioned that with ether, C4H10O, ozone gives ethyl
peroxide, which is capable of decomposing with explosion (according to Berthelot), and is
decomposed by water into alcohol, 2C.>HeO, and hydrogen peroxide, EUO.j.

8 This reaction is the one usually made use of for detecting the presence of ozone.
In the majority of cases paper is soaked in solutions of potassium iodide and starch.
Such ozonometrical or iodised starch-paper when damp turns blue in the presence of ozone,
and the tint obtained varies considerably, according to the length of tune it is exposed and
to the amount of ozone present. The amount of ozone in a given gas may even to a
certain degree be judged by the shade of colour acquired by the paper, if preliminary
tests be made.

Test-paper for ozone is prepared in the following manner: — One gram of neutral
potassium iodide is dissolved in 100 grams of distilled water; 10 grams of starch are
then shaken up in the solution, and the mixture is boiled until the starch is converted
into a jelly. This jelly is then smeared over blotting-paper and left to dry. The colour
of iodised staivh-paper is changed not only by the action of ozone, but of many other
oxidisers ; for example, by the oxides of nitrogen and hydrogen peroxide. Houzeau pro-
posed soaking common litmus-paper with a solution of potassium iodide, which in the
presence of iodine would turn blue, owing to the formation of K.HO. In order to find if
the blue colour is not produced by an alkali (ammonia) in the gas, a portion of the papt-r
is not soaked in the potassium iodide, but moistened with water ; this portion will then
also turn blue if ammonia be present. A reagent for distinguishing ozone from hydrogen
peroxide with certainty is not known, and therefore these substances in very small quan-
tities (for instance, in the atmosphere) may easily be confounded.

202 PRINCIPLES OF CHEMISTRY

from it in stability, and by the fact that it oxidises a number of sub-
stances very energetically at the ordinary temperature. In this
respect ozone resembles the oxygen of certain unstable compounds, or
oxygen at the moment of its liberation.

In ordinary oxygen and ozone we see an example of one and the
same substance, in this case an element, appearing in two states. This
indicates that the properties of a substance, and even of an element,
may vary without its composition varying. Very many such cases
are known. Such cases of a chemical transformation which determines
a difference in the properties of one and the same element are termed
isomerism. The cause of isomerism evidently lies deep within the
essence of the nature of a substance, and its investigation has already
led to a number of results of unexpected importance and of immense
scientific significance. It is easy to understand the difference between
substances containing different elements or the same elements in
different proportions. That a difference should exist in the latter
case necessarily follows, if, as our knowledge compels us, we admit
that there is a radical difference in the simple bodies or elements.
But when the quality and quantity of the elements (the composition)
in a substance are the same and yet its properties are different,
then it becomes clear that the conceptions of the elements and of the
composition of compounds, alone, are insufficient for the expression of
all the diversity of the properties of the matter of nature. Something
else, still more profound and internal than the composition of sub-
stances, must, judging from isomerism, determine the properties and
transformation of substances.

On what is the isomerism of ozone with oxygen, and the peculiarities
of ozone, dependent ? In what, besides the store of energy, which in its
way expresses the peculiarities of ozone, resides the causes of its difference
from oxygen 1 These questions for long occupied the minds of investi-
gators, and were the motive for the most varied, exact, and accurate
researches, which were chiefly directed to the study of the volumetric
relations exhibited by ozone. In order to acquaint the reader with the
previous researches of this kind, I cite the following from a memoir by
Soret,in the * Transactions of the French Academy of Sciences ' for 1866 :

4 Our present knowledge of the volumetric relations of ozone may be
expressed at the present time in the following manner :

'1. " Ordinary oxygen in changing into ozone under the action of
electricity shows a diminution in volume." This was discovered by
Andrews and Tait.

' 2. " In acting on ozonised oxygen with potassium iodide and other
substances capable of being oxidised, we destroy the ozone, but the

AND HYDEOGEN PEROXIDE— DALTON'S LAW 203

volume of the gas remains unchanged." Indeed, the researches of
Andrews, Soret, v. Babo, and others showed that the quantity of oxygen
absorbed by the potassium iodide is equal to the original contraction of
the volume of the oxygen — that is, in the absorption of the ozone the
volume of the gas remains unchanged. From this it might be imagined
that ozone, so to say, does not occupy any room — is indefinitely
•dense.

'3. "By the action of heat ozonised oxygen increases in volume,
and is transformed into ordinary oxygen. This increase in volume
corresponds with the quantity of oxygen which is given up to the
potassium iodide in its decomposition " (the same observers).

' 4. These indubitable experimental results lead to the conclusion
that ozone is denser than oxygen, and that ozone in its oxidising
action gives off that portion of its substance which distinguishes it by
its density from ordinary oxygen.'

If we imagine (says Weltzien) that n volumes of ozone consist of n
volumes of oxygen combined with m volumes of the same substance, and
that ozone in oxidising gives up m volumes of oxygen and leaves n
volumes of oxygen gas, then all the above facts can be explained ;
otherwise it must be supposed that ozone is indefinitely dense. ' In
order to determine the density of ozone (we again cite Soret) recourse
cannot be had to the direct determination of the weight of a given
volume of the gas, because ozone cannot be obtained in a pure state.
It is always mixed with a very large quantity of oxygen. It was
necessary, therefore, to have recourse to such substances as would
absorb ozone without absorbing oxygen and without destroying the
ozone. Then the density might be deduced from the decrease of
volume produced in the gas by the action of this solvent in comparison
with the quantity of oxygen given up to potassium iodide. Advantage
must also be taken of the determination of the increase of volume
produced by the action of heat on ozone, if the volume previously
occupied by the ozone before heating be known.' Soret found two such
substances, turpentine and oil of cinnamon. ' Ozone disappears in the
presence of turpentine. This is accompanied by the appearance of a
dense vapour, which fills a vessel of small capacity (0- 14 litre) to such an
extent that it is impenetrable to direct sun-rays. On then leaving the
vessel at rest, it is observed that the cloud of vapour settles ; the
clearing is first remarked at the upper portion of the vessel, and the
brilliant colours of the rainbow are seen on the edge of cloud of
vapour.' Oil of cinnamon — that is, the volatile or odoriferous substance
of the well-known spice, cinnamon — gives under similar circumstances
the same kind of vapours, but they are much less voluminous. On

sure. ,vc.) and making a scries of coi 1 1 j uirat i ve determinations. Sort-t
obtaintMl tli'- t'i .l!n\\ in-- result : two volumes »>t' ozone capable of beiim
dissolved, when df>iroved (1»\- heating a wire to a rc«l heat by a
galvanic current) increase by one volume. Hence it is evident thai in
the formation "t ozone three volumes of oxvu'en u'ive two volumes of
ozone tliai is. it- density (referred to hydrogen ) = _ I .

The observations and determinations of Soret sho\\-ed that ozone is
hea\'ier than oxygen, and e\'en than carbonic anhydride (because
o/.oni>ed oxyu'cn parses from tine orifices more slowly than oxygen
and than its mixtures with carbonic anhydride), although lighter than
chlorine (it flows more rapidly from such orifices than chlorine), and
they al>o indicated that n\nn>' /x on* and " hftff tini'-* d'liy-i' th<in
<>.'->j'/' a . which mav be expressed bv designating a molecule of oxygen
by ( )., and of ozone bv ( ^ \ and which likens ozone to compound sub-
stances'1 formed by oxygen, as. for instance. CO.,, SO,. ()()._,. XO.,, iVc.
This explain- the chief dillerence> between ozone and oxygen, and the
cause of the i>oineri-m. and at the same time leads one to expect "
'hat ozone, as a uas \\'hich is denser than ox\-u'<'ii, would be liquefied

070>7E AND HYDROGEN PEROXIDE— DALTON'S LAW 205

much more easily. This was actually shown to be the case, in 1880, by
Chappuis and Hautefeuille in their researches on the physical properties
of oxygen. Its absolute boiling point is about — 106°, and consequently
compressed and refrigerated ozone when rapidly expanded gives drops,
is liquefied. Liquid and compressed u ozone is blue. In dissolving in
water ozone partly passes into oxygen. Ozone violently explodes when
suddenly compressed and heated, changing into ordinary oxygen, and
evolving, like all explosive substances,12 that heat which distinguishes
it from oxygen.

Thus, judging by what has been said above, ozone should be
formed in nature not only in the many processes of oxidation which
go on, but also by the condensation of atmospheric oxygen. The
significance of ozone in nature has often arrested the attention of
observers. There is a series of ozonometrical observations which show
the different amounts of ozone in the air at different localities, at
different times of the year, and under different circumstances — for
instance, on the appearance of epidemics. But the observations made
in this direction cannot be considered as sufficiently exact, because the
methods in use for determining ozone were not quite accurate. It is
however indisputable 13 that the amount of ozone in the atmosphere is
subject to variation ; that the air of dwellings contains no ozone (it dis-
appears in oxidising organic matter) ; that the air of fields and forests
always contains ozone, or substances (peroxide of hydrogen) which act
like it ; that the amount of ozone increases after storms ; and that
miasms, £c., are destroyed by ozonising the atmosphere. It may be
imagined that the influence exerted by ozone on animal life is due to
the fact that it easily oxidises organic substances, and miasms are
formed of organic substances and the germs of organisms, which are
easily changed and oxidised. Indeed, many miasms — for instance,

conditions, evidently be less capable of passing into a state of gaseous movement, should
sooner attain a liquid state, and have a greater cohesive force.

11 The blue colour proper to ozone may be seen through a tube one metre long con-
taining oxygen 10 p.c. ozonised. The density of liquid ozone has not, as far as I am
aware, been determined.

12 All explosive bodies and mixtures (gunpowder, detonating gas, &c.) evolve heat in
exploding (in giving a greater number of molecules from one molecule, and sometimes
several substances from one substance, as in the explosion of nitro-compounds ; see later) —
that is, the reactions which accompany explosions are exothermal. In this manner
ozone in decomposing evolves latent heat, although generally heat is absorbed in
decomposition. This shows the meaning and cause of explosion.

13 In Paris it has been found that the further from the centre of the town the greater
the amount of ozone in the air. The reason of this is evident : in a city there are many
conditions for the destruction of ozone. This is why we distinguish country air as being
fresh. In spring the air contains more ozone than in autumn ; the air of fields more than
the air of towns.

206 PRINCIPLES OF CHEMISTRY

the volatile substance of decomposing organisms — are clearly destroyed
or changed not only by ozone, but also by many powerfully oxidising
substances, such as chlorine with water, potassium permanganate, and
the like.14

Thus in ozone we see (1) the capacity of elements (and it must
be all the more marked in compounds) of changing in properties with-
out altering in composition ; this is termed isomerism ; 15 (2) the
capacity of elements for arranging themselves in molecules of 'different
densities ; this forms a special case of isomerism called polymerism •
(3) the capacity of oxygen for appearing in a still more intense and
energetic chemical state than that in which it occurs in ordinary
gaseous oxygen ; and (4) the formation of unstable equilibria, or
chemical states, which are expressed both by the ease with which ozone
acts as an oxidiser and in its capacity for decomposing with explo-
sion.16

Hydrogen peroxide. — Many of those properties which we have seen
in ozone belong also to a peculiar substance containing oxygen and
hydrogen, and called hydrogen peroxide, or oxygenated water. This
substance was discovered in 1818 by Thenard. When heated it is
decomposed into water and oxygen, evolving as much oxygen as is
contained in the water remaining after the decomposition. That
portion of oxygen by which hydrogen peroxide differs from water be-
haves in a number of cases just like the active oxygen in ozone, which
distinguishes it from ordinary oxygen. In H2O2, and in O3, one atom
of oxygen acts in a powerfully oxidising manner, and on separating out

14 The oxidising action of ozone may be taken advantage of for technical ends ; for
instance, for destroying colouring matters. It has even been employed for bleaching
tissues and for the rapid preparation of vinegar, although these methods have not yet
received wide application.

15 Isomerism in elements is termed allotropism.

16 A number of substances resemble ozone in one or another of these respects. Thus
cyanogen, C.^N.^, nitrogen chloride, &c., decompose with an explosion and evolution of
heat. Nitrous anhydride, N./)3, forms a blue liquid like ozone, and in a number of cases
oxidises like ozone. Bed phosphorus is to white phosphorus, in a certain sense, what
oxygen is to ozone, and in other respects the reverse ; this is also a case of allotropism.
Thus a chemical analogy is diffused in different and most varied directions, and it is only
after an acquaintance with the diverse relations of substances that an idea can be formed
of the complexity of chemical changes, whilst their general system is still wanting; that
is to say, there is nothing analogous to and explaining the correlation of liquid to
gaseous substances. But there is reason to think that in this case also an explanation
will arise with the accumulation of data, as we see from the fact that the conception of
dissociation explained in the simplest manner a number of chemical relations which
without it were not at all clear. It should be here observed that the transition
between oxygen and ozone under the conditions of a silent discharge forms a reversible
reaction which is subject to the conception of dissociation, whilst, exempt from the
conditions of a silent discharge, the passage of ozone into oxygen is not reversible, and
forms an instance of decomposition in the strictest sense.

o/n NE AND HYDROGEN PEROXIDE — DALTnN'S LAW 207

it leaves H20 or O2, which do not act so sharply, although they still
contain oxygen.17 Both contain the oxygen in a compressed state, so
to speak, and when freed from pressure by the forces (internal) of the
elements in another substance, this oxygen is easily evolved, and there-
fore acts like oxygen at the moment of its liberation. Both substances
in decomposing, with the separation of a portion of their oxygen, evolve
heat, while an absorption of heat is usually required for decomposi-
tion.

Hydrogen peroxide is formed under many circumstances by com-
bustion and oxidation, but in very limited quantities ; thus, for instance,
it is sufficient to shake up zinc with sulphuric acid, or even with water,
to remark the formation of a certain quantity of hydrogen peroxide in
the water.18 From this cause, probably, a series of diverse oxidation
processes are accomplished in nature, and, according to Prof. Schone, of
Moscow, hydrogen peroxide occurs in the atmosphere, although in vari-
able and small quantities, and probably its formation is connected with
ozone, with which it has much in common. The usual case of the
formation of hydrogen peroxide, and the means by which it may be in-

17 It is evident that there is a want of words here for distinguishing oxygen, O, as an
ultimate element, from oxygen, Oo, as & free element. It should be called oxygen gas, did
not habit and the length of the expression render it inconvenient.

18 Schiinbein states that the formation of hydrogen peroxide is to be remarked in every
oxidation in water or in the presence of aqueous vapour. According to Struve, hydrogen
peroxide is contained in snow and in rain-water, arid its formation, together with ozone
and ammonium nitrate, is even probable in the processes of respiration and combustion.
A solution of tin in mercury, or liquid tin amalgam, when shaken up in water containing
sulphuric acid gives rise to the formation of hydrogen peroxide, whilst iron under the
same circumstances does not give rise to its formation. The presence of small quantities
of hydrogen peroxide in these and similar cases is recognised by many reactions
Amongst them, its action on chromic acid in the presence of ether is very characteristic.
Hydrogen peroxide converts the chromic acid into a higher oxide, Cr2O7, which is of a
dark-blue colour, and dissolves in ether. This ethereal solution is to a certain degree
stable, and therefore the presence of hydrogen peroxide may be recognised by mixing
the liquid to be tested with ether and adding several drops of a solution of chromic acid.
On shaking the mixture the ether dissolves the higher oxide of chromium which is
formed, and acquires a blue colour. The formation of hydrogen peroxide in the combus-
tion and oxidation of substances containing or evolving hydrogen must be understood in
the sense of the conception, to be considered later, of molecules occupying equal volumes
in a gaseous state. At the moment of its evolution a molecule H.> combines with a mole-
cule O2 and gives H3O2. As this substance is unstable, a large proportion of it is
decomposed, a small amount only remaining unchanged. If it is obtained, water is easily
formed from it ; this reaction evolves heat, and the reverse action is not very pro-
bable. Direct determinations show that the reaction H2O2 = H2O + O evolves 22000 heat
units. From this it will be understood how easy is the decomposition of hydrogen
peroxide, as well as the fact that a number of substances which are not directly
oxidised by oxygen are oxidised by hydrogen peroxide and by ozone, which also evolves
heat on decomposition. Such a representation of the origin of hydrogen peroxide has
been developed by me since 1870. In recent times Traube has pronounced a similar
opinion.

208 PRINCIPLES OF CHEMISTRY

directly obtained,111 is by the double decomposition of an acid and the
peroxides of certain metals, especially those of potassium, calcium, and
barium.20 Among these peroxides, that of barium is the most
conveniently obtained, it being enough, as we saw when speaking of
oxygen (Chap. III.), to heat the anhydrous oxide of barium to a red heat
in a current of air or oxygen ; or, better still, to heat it with potassium
chlorate, and then to wash away the potassium chloride also formed.21
Barium peroxide gives hydrogen peroxide by the action of acids in the
cold.22 The process of decomposition is very clear in this case ; the
hydrogen of the acid replaces the barium of the peroxide, a barium salt
of the acid being formed, while the hydrogen peroxide formed by the

19 The formation of hydrogen peroxide from barium peroxide by a method of double
decomposition is an instance of a number of indirect methods of prepa/Tafaon. A sub-
stance A does not combine with B, but AB is obtained from AC in its action on HP (see
Introduction) when CD is formed. Water does not combine with oxygen, but as a hydrate
of acids it acts on the compound of oxygen with barium oxide, because this oxide gives a
salt with an acid anhydride ; or, what is the same, hydrogen with oxygen does not directly
form hydrogen peroxide, but when combined with a haloid (for example, chlorine), under
the action of barium peroxide, BaOo, it leads to the formation of a salt of barium and H._,(\>.
It is to be remarked that the passage of barium oxide, BaO, into the peroxide, BaO,>, is
accompanied by the evolution of 121000 heat units per 16 parts of oxygen by weight
combined, and the passage of H.,O into the peroxide H._>O._> does not proceed directly,
because it would be accompanied by the absorption of 22000 units of heat by 10 parts
by weight of oxygen combined. Barium peroxide, in acting 011 an acid, evidently evolves
less heat than the oxide, and it is this difference of heat that is absorbed in the hydrogen
peroxide. Its energy is obtained from the energy evolved in the formation of the salt of
barium.

20 Peroxides of lead and manganese, and other analogous peroxides (see Chapter III.,
Note 9), do not give hydrogen peroxide under these conditions, but yield chlorine
with hydrochloric acid.

21 The impure barium peroxide obtained in this manner may be easily purified. For
this purpose it is dissolved in a dilute solution of nitric acid. There will always remain
a certain quantity of an insoluble residue, from which the solution is separated by filtra-
tion. The solution will contain not only the compound of the barium peroxide, but also
a compound of the barium oxide itself, a certain quantity of which always remains un-
combined with oxygen. The acid compounds of the peroxide and oxide of barium are
easily distinguishable by their stability. The peroxide gives an unstable compound, and
the oxide a stable salt. By adding an aqueous solution of barium oxide to the resultant
solution, the whole of the peroxide contained in the solution may be precipitated as a
pure aqueous compound. The first portions of the precipitate will consist of impurities —
for instance, oxide of iron. The barium peroxide separates out, and is collected on a
filter and washed ; it then forms a substance having an entirely definite composition,
BaOo,8H2O, and is very pure. Pure hydrogen peroxide should always be prepared from
such purified barium peroxide.

22 In the cold, strong sulphuric acid with barium peroxide gives ozone; when diluted
with a certain amount of water it gives oxygen (see Note 6), and hydrogen peroxide is
only obtained by the action of very weak sulphuric acid. The acids hydrochloric,
hydrofluoric, carbonic, and hydrosilicofluoric, and others, when diluted with water also
give hydrogen peroxide with barium peroxide. Professor Scho'ne, who investigated
hydrogen peroxide with great detail, showed that it is formed by the action of many of
the above-mentioned acids on barium peroxide.

0/ONE AND HYDROGEN PEROXIDE — D ALTON'S LA\V 209

barium peroxide remains in solution.23 The reaction is expressed
by the equation BaO2 + H2SO4=H2O2 + BaSO4. It is best to take a
weak cold solution of sulphuric acid and to almost saturate it with
barium peroxide, so that a small excess of acid remains; insoluble
barium sulphate is formed. A more or less dilute aqueous solution
of hydrogen peroxide is obtained. This solution may be concentrated
in a vacuum over sulphuric acid. In this way the water may even be
entirely evaporated from the solution of the hydrogen peroxide ; only
in this case it is necessary to work at a low temperature, and not to
keep the peroxide for long in the rarefied atmosphere, as otherwise it
decomposes.24

When pure, hydrogen peroxide is a colourless liquid, without smell,
and having a very unpleasant taste — such as belongs to the salts of
many metals— the so-called ' metallic ' taste. Water held in zinc vessels
has this taste, which is probably due to its containing hydrogen peroxide.
The tension of the vapour of hydrogen peroxide is less than that of
aqueous vapour ; this enables its solutions to be concentrated in a
vacuum. The specific gravity of anhydrous hydrogen peroxide is 1'455. _^ ,
Pure hydrogen peroxide decomposes, with the evolution of oxygen, when
heated even to 20° (by the action of light ?). But the more dilute its
aqueous solution the more stable it is. Very weak solutions may be
distilled without the hydrogen peroxide decomposing. It decolorises
solutions of litmus and turmeric, and acts in a similar manner on many
colouring matters of organic origin (for which reason it is employed for
bleaching tissues).

Many substances decompose hydrogen peroxide, forming water and
oxygen, without apparently suffering any change. In this case sub-
stances in a state of fine division evince an incomparably quicker action

23 With the majority of acids, that salt of barium which is formed remains in solution ;
thus, for instance, by employing hydrochloric acid, hydrogen peroxide and barium chloride
remain in solution. Complicated processes would be required to obtain pure hydrogen
peroxide from such a solution. It is much more convenient to take advantage of the
action of carbonic anhydride on the pure hydrate of barium peroxide. For this purpose
the hydrate is stirred up in water, and a rapid stream of carbonic anhydride is passed
through the water. Barium carbonate, insoluble in water, is formed, and the hydrogen
peroxide remains in solution, so that it may be separated from the carbonate by filtering
only. On a large scale hydrofluosilicic acid is employed, because its barium salt is also
insoluble in water.

24 Hydrogen peroxide may be extracted from very dilute solutions by means of ether,
which dissolves it, and when mixed with it the hydrogen peroxide may even be distilled.
A solution of hydrogen peroxide in water may be enriched by cooling it to a low tempera-
ture, when the water crystallises out — that is, is converted into ice — whilst the hydrogen
peroxide remains in solution, as it only freezes at very low temperatures. It must be
observed that hydrogen peroxide, in a strong solution in a pure state, is exceedingly
unstable even at the ordinary temperature, and therefore it must be preserved in vessels
always kept cold, as otherwise it evolves oxygen and forms water.

VOL. I. P

•210

than compact masses, from which it is evident that the action is here
li:isp<l on contact i .-•• • Introduction). It is enough to hrinij hydrogen
peroxide into contact \\ith charcoal, e/old, the peroxide of manganese
or lead, the alkalis, metallic silver, ami platinum, to bring about the
above decomposition.*'1 l>e>ide> which, livdro^eii |>ero\ide forms water
and part- with it> oxv^'en \\ith uivat ea-e to a number of substances
which arc capable of being oxidised or of combining \\'ith oxygen, and
in this respect i- very like ozone and ot her /,mr, /_•/'/// ,,.,'i<lis' /-x.'-'1 To
the numlier of contact phenomena, which are so natural to hydrogen
peroxide, as a substance which is unstable and ea.-ilv decomposable with
the evolution of heat, must be referred the following- -that in the pre-
xeiice ot many substances containing oxvu'en it evolves, not only its own
oxvgen. but also that of the substances which are brought into contact
with it that i-. /'/ <'<•(* In <> r<-<l//<-i/ir/ indnm-r. It behaves thus wit h
ozone, the oxide^ of silver, mercury, gold and platinum, and lead
peroxide. The oxvgen in these -ubstances is not stable, and therefore
the feeble inllueiice of contact is enough to destroy its position.

i'i --t a IT] i. ci • rt a in (it tin1 r<i/ttli/t /<' i>r contact jiliriK 'iiu-na

timi. wliiNt. h.. Wfv.-r.it (l..«.sii..t iiltrr tin- S.TH-S of c-Iimip-s
liens ..nly. I'n.f.-sscr Scli.".n.- of tin- 1 '.•! mtT-ky . \i-a.lnny.
i idy cxjilainc.l a nninli.T of i1. -act ions of h\ driven peroxide \vliich prrviou-ly \\ •'!•<•

• ,.1 und'-r-i 1. Tim-, for instance, lie showed iliat witli liydrop-n jicroxidc. alkali^ "'ivc

IH I'M id. - n: the alkaliin' metals, \\hirli cMinliinc \\-ith the remaining hydrc.Li'i'ii pci'Mxiilf.

un-talilc f nil] Mimd- \vhirharc easily decoiii]ioscd. and therefore allcali- evince

,i di-ciiin]in iT.talvtii i Iliience mi - ihilimis of hydrogen |>eroxide. Only acid >«'ln-

• |.ei'Mxide. and tht-n ..nly dilute ones, can !..• pre-erved well.
:"" // '•'//"'-. a- a -nil-:.-, nee conlaiiiin^ nnirh Mxvufen ( namely. Id ]iart> to

ar-eiKc. CM • ' in.- intu cal. ide, the oxides of x.inc and cMjiper into jx-roxides ;

I p.irt- wit en 1 m\ suljiliide-. I-MII\ ei'tin.L; th.'in iiit.. sulphates, iVc. So. lur

, vain],!. . / . iTl I !; cli 1, ad r-lllpllide. I'l.S. into \\hite lead .lllphate. I'I, SO,. CM]. per

• . -I i er ^nlphate. and -o MH. The iv-iMrali.ii i of ..Id ..il pa i nt in--- 1 1\ liydrM-vn
:. I. i- lia-i .1 MII this a li-iii. Oil-, IMHI'S are n-nally admixed with wliite lead, and in

it | true. M| 1 inie. This i- ]>art ly

: .. to tin ilphn elli-d li\.h.i/.n cMiitained in (he air. \\hi.-h acts MM white lead.

i.-ad ulj.hi.l.'. whieh i- Mark. The intermixture .,| the I, lark colMiir darkens the

r. t. In i-I. i tureuitha ..luli..ii <.f h\.lr,,-.-n |.i-r..\i«l.-. tin- lilai-k li-atl stilpliiilr

nf il.. i i il-i i !....• ....,: , . M them. HxdiM-en pn-Mxidr oxidisi-s
with it »•" '1 ' nl.:, MM. . Tim it , mpM.-,,-, h\ dri.. die acid, sett in^ the iodine

fl'. e and CM|,\r-f1 tin I.T ; il I. I'MinpM-es -Illplmretted

in in <•:• art lh< • er. t! ihe ; iilplmr Ire... Starch paste with

, did. t. I i. u. ,r. direr! , ; , (,,. , ,f huln.p.,, in the entire

:d,-ence .,f tree ... id : Lilt I he a dd i t i..| , , ,| ;, s Ilia 1 1 . | Ua 11 t it V of in .)! Illphate I- feel i \ itl'i.ill

,, i- ,,f l..,id aci lal •• J,, 1 hi' m:\tnr. i . : .. .!i . 1 1 I . i r , , i , r, -i •. i ,!,,,-],, .,, t ),,. p;l.,t,.. This i:, a very
,..'.' •.•:•:]• ,• •;• ••• h\drM . . , d-.. the test with i-hroniii- iiriil

i.lirl fill, i • '• N"1 -

OZONE AND HYDROGEN PEROXIDE — D ALTON'S LAW '211

Hydrogen peroxide, especially in a concentrated form, in contact with
these substances, evolves an immense quantity of oxygen, so that an
explosion takes place and an exceedingly powerful evolution of heat is
observed if hydrogen peroxide in a concentrated form be made to fall
in drops upon these substances in dry powder. An exactly similar de-
composition takes place in dilute solutions.27

Just as a whole series of metallic compounds, and especially the
oxides and their hydrates, correspond with water, so also there are
many substances analogous to hydrogen peroxide. Thus, for instance,
calcium peroxide is related to hydrogen peroxide in exactly the same
way as calcium oxide or lime is related to water. In both cases the
hydrogen is replaced by a metal — namely, by calcium. But it is most
important to remark that the nearest approach to the properties of
hydrogen peroxide is afforded by a non-metallic element, chlorine ; its
action on colouring matters, its capacity for oxidising, and for evolving
oxygen from many oxides, is analogous to that exhibited by hydrogen
peroxide. Even the very formation of chlorine is closely analogous to the
formation of peroxide of hydrogen ; chlorine is obtained from manganese
peroxide, MnO2, and hydrochloric acid, HC1, and hydrogen peroxide from
barium peroxide, BaO2, and the same acid. The result in one case is
essentially water, chlorine, and manganese chloride ; and in the other
case there is produced barium chloride and hydrogen peroxide. Hence
water + chlorine corresponds with hydrogen peroxide, and the action
of chlorine in the presence of water is analogous to the action of
hydrogen peroxide. This analogy between chlorine and hydrogen
peroxide is expressed in the conception of an aqueous radicle, which
(Chap. III.) has been already mentioned. This aqueous radicle (or
hydroxyl) is that which is left from water if it be imagined as deprived
of half of its hydrogen. According to this method of expression, caustic
soda will be a compound of sodium with the aqueous radicle, because it
is formed from water with the evolution of half the hydrogen. This is
expressed by the following formulae : water, H2O, caustic soda, NaHO,

27 To explain the phenomenon an hypothesis has been put forward by Brodie, Clausius,
•and Schonbein which supposes ordinary oxygen to be an electrically neutral substance,
composed of, so to speak, two electrically opposite aspects of oxygen — positive and negative.
It is supposed that hydrogen peroxide contains one kind of such polar oxygen, whilst in
the oxides of the above-named metals the oxygen is of opposite polarity. It is supposed
that in the oxides of the metals the oxygen is electro-negative, and in hydrogen
peroxide electro-positive, and that on the mutual contact of these substances ordinary
neutral oxygen is evolved as a consequence of the mutual attraction of the oxygens of
opposite polarity. Brodie admits the polarity of oxygen in combination, but not in an
uncombined state, whilst Schonbein supposes uncombined oxygen to be polar also, con-
sidering ozone as electro-negative oxygen. The supposition of the oxygen of ozone being
other than that of hydrogen peroxide is contradicted by the fact that in acting on barium
peroxide strong sulphuric acid forms ozone, and dilute acid forms hydrogen peroxide.

p 2

212 PRINCIPLES OF CHEMISTRY

just as hydrochloric acid is HC1 and sodium chloride NaCl. Hence the
aqueous radicle HO is a compound radicle, just as chlorine, Cl, is a
simple radicle. They give hydrogen compounds, HHO, water, and HC1,
hydrochloric acid ; sodium compounds, NaHO and NaCl, and a whole
series of analogous compounds. Free chlorine in this sense will be
C1C1, and hydrogen peroxide HOHO, which indeed expresses its
composition, because it contains twice as much oxygen as water.

Thus in ozone and hydrogen peroxide we see examples of very
unstable, easily decomposable (by time, spontaneously, and on contact)
substances, full of the energy necessary for change,28 capable of
being easily reconstructed (in this case decomposing with the evolu-
tion of heat) ; therefore they are examples of unstable chemical
equilibria. If a substance exists, it signifies that it already presents a
certain form of equilibrium between those elements of whicli it is built
up. But chemical, like mechanical, equilibria exhibit different degrees
of stability or solidity.29

28 The lower oxides of nitrogen and chlorine and the higher oxides of manganese
are also formed with the absorption of heat, and therefore, like hydrogen peroxide, act in
a powerfully oxidising manner, and are not formed by the same methods as the majority
of other oxides. It is evident that, being endowed with a richer store of energy (acquired
in combination or absorption of heat), such substances, compared with others poorer
in energy, will exhibit the greatest diversity of cases of chemical action with other sub-
stances.

29 If the point of support of a body lies in a vertical line below the centre of gravity, the
equilibrium is entirely unstable. If the centre of gravity lies below the point of support,
the state of equilibrium is very stable, and a vibration may take place about this posi-
tion of stable equilibrium, as in a pendulum or balance, which ends in the body passing
to its position of stable equilibrium. But if, keeping to the same mechanical example,
the body be supported not on a point, in the geometrical sense of the word, but on a
small plane, then the state of unstable equilibrium may be preserved, unless destroyed
by external influences. Thus a man stands upright supported on the plane, or several
points of the surfaces of his feet, having the centre of gravity above the points of support.
Vibration is then possible, but it is limited, otherwise on passing outside the limit of
possible equilibrium another more stable position is attained about which vibration
becomes more possible. A prism immersed in water may have several more or less
stable positions of equilibrium. It is the same with the atoms in molecules. Some
molecules present a state of more stable equilibrium than others. Hence from this simple
comparison it will be already clear that the stability of molecules may vary considerably,
that one and the same elements, taken in the same number, may give isomerides of different
stability, and, lastly, that there may exist states of equilibria which are so unstable, so
ephemeral, that they will only arise under particularly special conditions — such, for
example, as certain hydrates mentioned in the first chapter (see Notes 57, (57, and others).
And if in one case the instability of a given state of equilibrium is expressed by its
instability with a change of temperature or physical state, then in other cases it is
expressed by the case of decomposition under the influence of contact or of the purely
chemical influence of other substances. However clearly the greater or less stability
of the elementary structure of substances be depicted to us in these general considera-
tions, still at present there is no possibility of presenting them in a sufficiently con-
crete form to enable purely mechanical conceptions to be applied to them; that is,
to subject them to mathematical analysis, and to master the subject to such an extent

OZOM- AND HYUKOGEN PEROXIDE— DALTON'S LAW 213

Besides this, hydrogen peroxide indicates another side of the subject
which is not less important, and is much clearer and more general.

Hydrogen unites with oxygen in two degrees of oxidation : water
or hydrogen oxide, and oxygenated water or hydrogen peroxide ; for a
given quantity of hydrogen the peroxide contains twice as much oxygen
as does water. This is a fresh example confirming the correctness of
the law of multiple proportions, of which we have already made men-
tion in speaking of the water of crystallisation of salts. Now we can
formulate this law with entire clearness — the law of multiple propor-
tions. If two radicles A, and B (either simple or compound substances),
unite together to form several compounds, AnBOT, A^Br . . . ., then
having expressed the compositions of all these compounds in such a ivay
that the quantity (by weight or volume) of one of the component parts
will be a constant quantity A, it will be observed that in all the compounds
AB((, AB,, . ... the quantities of the other component part, B, will
always be in commensurable relation : generally in simple multiple
proportion — that is, that a : b . . ., or m/nis to r/q as whole numbers,
for instance as 2 : 3 or 3 : 4. . . .

The analysis of water shows that in 100 parts by weight it contains
11-112 parts by weight of hydrogen and 88*888 of oxygen, and the
analysis of peroxide of hydrogen shows that it contains 94-112 parts of
oxygen to 5 -888 parts of hydrogen. In this the analysis is expressed,
as analyses generally are, in percentages ; that is, it gives the amounts
of the elements in a hundred parts by weight of the substance. The
direct comparison of the percentage compositions of water and hydrogen
peroxide does not give any simple relation. But such a relation is
immediately observed if we calculate the composition of water and of
hydrogen peroxide, having taken either the quantity of oxygen or the
quantity of hydrogen as a constant quantity — for instance, as unity. The
most simple proportions show that in water there are contained eight
parts of oxygen to one part of hydrogen, and in hydrogen peroxide
sixteen parts of oxygen to one part of hydrogen ; or one-eighth part of
hydrogen in water and one- sixteenth part of hydrogen in hydrogen
peroxide to one part of oxygen. Naturally, the analysis does not give
these figures with absolute exactness — it gives them within a certain
degree of error — but they approximate, as the error diminishes, to that
limit which is here given. The comparison of the quantities of hydrogen
and oxygen in the two substances above named, taking one of the com-
ponents as a constant quantity, gives an example of the application of

as to foretell the degree of stability of different chemical states of equilibrium. The
commencement of elementary generalisations has been apprehended in only a few

214 PRINCIPLES OF CHEMISTRY

the law of multiple proportions, because water contains eight parts and
hydrogen peroxide sixteen parts of oxygen to one part of hydrogen, and
these figures are commensurable and are in simple proportion as 1 : 2.

An exactly similar multiple proportion is observed in the composition
of all other well-investigated definite chemical compounds,30 and there-
fore the law of multiple proportions is accepted in chemistry as the
starting point from which other considerations are judged.

The law of multiple proportions was discovered at the very
beginning of this century by John Dalton, of Manchester, in investigat-
ing the compounds of carbon with hydrogen. It appeared that two
gaseous compounds of these substances — marsh gas, CH4, and olefiant
gas, C2H4, contain for one and the same quantity of hydrogen quanti-
ties of carbon which stand in multiple proportion ; namely, marsh gas
contains relatively half as much carbon as olefiant gas. Although the
analysis of that time was not exact, and did not give Dalton results
in complete accordance with truth, still the accuracy of this law,
recognised by Dalton, was confirmed by further more accurate investiga-
tions. On establishing the law of multiple proportions, Dalton gave a
hypothetical explanation for it. This explanation is based on the
atomic theory of matter. In fact, the law of multiple proportions is
understood with unusual ease by admitting the atomic structure of
matter.

50 When, for example, any element forms several oxides, they are subject to the
law of multiple proportions. For a given quantity of the non-metal or metal the
quantities of oxygen in the different degrees of oxidation will stand as 1 : 2, or as 1 : 3, or
as 2 : 3, or as 2 : 7, and so on. Thus, for instance, copper combines with oxygen in at
least two proportions, forming the oxides found in nature, and called the suboxide and
the oxide of copper, Cu2O and CuO ; the oxide contains twice as much oxygen as the sub-
oxide. Lead also presents two degrees of oxidation, the oxide and peroxide, and in the
latter there is twice as much oxygen as in the former, PbO and PbO.,>. The substance
known under the name of minium, and which is somewhat widely used as a red paint,
is only a mixture of the mutual compounds of these oxides, which is proved not only by
the inconstancy of its composition, but also by the fact that reagents capable of extract-
ing the oxide of lead, especially acids, do actually extract it and leave lead peroxide.
When a base and an acid are capable of forming several kinds of salts, normal, acid, basic,
and anhydro-, it is found that they also clearly exemplify the law of multiple proportions.
This was demonstrated by Wollaston soon after the discovery of the law in question. We
saw in the first chapter that salts show different degrees of combination with water of
crystallisation, and that they obey the law of multiple proportions. And, more than
this, the indefinite chemical compounds existing as solutions may, as we saw in the same
chapter, be brought under the law of multiple proportions by the hypothesis that solu-
tions are unstable hydrates formed according to the law of multiple proportions, but
occurring in a state of dissociation. By means of this hypothesis the law of multiple
proportions becomes still more general, and all the aspects of chemical compounds are
subject to it. The direction of the whole contemporary state of chemistry was deter-
mined by the discoveries of Lavoisier and Dalton. By bringing indefinite compounds
also under the law of multiple proportions we arrive at that unity of chemical conceptions

M/oNK AND HYDROGEN PEROXIDE— DA LT< >.Vs LAW 215

The essence of the atomic theory is that matter is supposed to con-
sist of an agglomeration of small and indivisible parts — atoms — which do
not fill up the whole space occupied by a substance, but stand apart
from each other, as the sun, planets, and stars do not fill up the whole
space of the universe, but are at a distance from each other. The form and
properties of substances are determined by the position of their atoms in
space and by their state of movement, while the phenomena accomplished
by substances are understood as redistributions of the relative positions
of atoms and changes in their movement. The atomic representation of
matter arose in very ancient times,31 and up to recent times was at strife
with the dynamical hypothesis, which considers matter as only a mani-
festation of forces. At the present time, however, the majority of
scientific men uphold the atomic hypothesis, although the present con-
ception of an atom is quite different from that of the ancient

which was impossible so long as definite compounds were separated from indefinite by a
sharp line of demarcation.

51 Leucippus, Democritus, and especially Luoretius, in the classical ages, repre-
sented matter as made up of atoms — that is, of parts incapable of further division. The
geometrical impossibility of such an admission, as well as the conclusions which were
deduced by the ancient atomists from their fundamental propositions, prevented other
philosophers from following them, and the atomic doctrine, like very many others, lived,
without being ratified by fact, in the imaginations of its followers. Between the present
atomic theory and the doctrine of the above-named ancient philosophers there is naturally
a remote historical connection, as between the doctrine of Pythagoras and Copernicus,
but they are essentially profoundly different. For us the atom is indivisible, not in
the geometrical abstract sense, but only in a physical and chemical sense. It would be
better to call the atoms indivisible individuals. The Greek atom = the Latin individual,
according to both the sum and sense of the words, but historically these two words are
endowed with a different meaning. The individual is mechanically and geometrically
divisible, but only indivisible in a definite sense. The earth, the sun, a man or fly
are individuals, although geometrically divisible. Thus the atoms of contemporary
science, indivisible in a physico-chemical sense, form those units which are concerned in
the investigation of the natural phenomena of matter, just as a man is an indivisible unit in
the investigation of social relations, or as the stars, planets, and luminaries serve as units
in astronomy. The formation of the vortex hypothesis, in which, as we shall afterwards
see, atoms are entire whirls mechanically complex, although physico-chemically indivisible,
already shows that the scientific men of our time in holding to the atomic theory have
only borrowed the word and form from the ancient philosophers, and not the essence of
their atomic doctrine. It is erroneous to imagine that the contemporary conceptions of
the atomists are nothing but the repetition of the metaphysical reasonings of the
ancients. As a geometrician in reasoning about curves represents them as formed of a
sum total of straight lines, because such a method enables him to analyse the subject
under investigation, so the scientific man applies the atomic theory as a method
of analysing the phenomena of nature. Naturally there are people now, as in ancient
times, and as there always will be, who apply reality to imagination, and therefore
there are to be found atomists of extreme views ; but it is not in their spirit that we
should acknowledge the great services rendered by the atomic doctrine to all science,
which, while it has been essentially independently developed, is, if it be desired to
reduce all ideas to the doctrines of the ancients, a union of the ancient dynamical and
atomic doctrines.

•2ir>

philo-nphers. Now. an atom i- regarded ratlier a- an isolate or
which i> indivisible by physical :v- and chemical forces, \\-hilst the atom
of the ancients \\as mechanically and uvometricallv indi\ i-iMe. \\ hen
I 'alt on ( 1 v| ' 1 ) discovered t he la w of mult iple propoi t ions, he pronounced
himself in favour of the atomic doctrine, because it enables this law to
Ke very easily understood. If the divisibility of everv element has a
limit, namely the atom, then the atoms ot element- are the extreme
limits of all di visibilit v. and t hey ditl'er from each other in t heir nat ure,
and the tormation ot a compound trom elementary matter must consist
in the a-'uTe-'at ion of several different atoms into one whole or system
of atoms, now termed mi.rti1'?''* or ///'-/<*•///,>•. As atoms can onlv com-
bine in their entire masses, n i- evident that not onlv the law of defi-
nite coi M posit ion. but a 1 so i hat of multiple proport ions, must apply to the
combination of atoms with one another ; for one atom of a substance
can combine with one. two. or three atoms of another substance, or in
iM'iier;i 1 one. t wo. i h ive atoms ( if one siib-t anee are able t < » combine with
one. t wo. or t hive atoms of a not her : this b 'inu' the essence of the law
of multiple proportions. Chemical and physical data are verv well
explained by the aid of the atomic theory. The displacement ot one
element by another follows the law of equivalency. In this case one
or several atom- of a Lnven element take the p'ace < if one or several
a t oms ot another element in its compounds. I he at on is of di lie rent
-ubstances can lie mixed together in the same sense a- sand can be
1 1 1 1 xed v. 1 1 1 1 da v. "I I ie\- do not unite i n' o one \\ hole /.<•.. t here is not a
perfect blendiiiLf in the one or other case, but only a juxtaposition, a
homogeneous whole beinu' formt d from individual parts. This is
the tir-t and mo.-i simple form of applying the atomic theory to the
explanation of chemical phenomena.'1'*

, ini iili-i';iilv clc;irl\ -yiiilinlixcd tin- ilitTci-i'iifc dt' tlu-ir iijiininM fi'inn

- . . . . • .-M,.;. -lit . NM\\ inil\ tin' iii(li\i(lu;ils nt ihi' clcinclits. iiuli-

| ,-!n .... ;nv t.Tliicil iiti.ni-, and 111.' indlN idlliiU nt' emu

. . .

. ••',,. , . - l|lll.l>x,T\illllr. i||\ i-il,l(..illl(l

: lili- tn lllldi'l--t:iui| i-itlitT I i'_'ht en-

|,., , . . ,| I i,,. i nt liM-rllil Ilii-il I. ]ill\ -iriil. ur rllf|llii-;ll

! . . ' ' i-j||i-llt ill ;'nilli;iU nlllv. hilt to IIS till1 ^Illilllr-t

|,|, ... • ',,, ir , •,.•,'•• nii-t inli. 'I'llll- iii"! i- 'li li;i- lici-i Ulic il d i)irc|it mil
, , | ] | In ci,iii-i-|>t i<>ii «i in.it N-I-. .Did 1 111 - lilt-. |iri'|i;irnl t I it • ^rn Hi I id fur t hr

t . . , ] , | ( | ,, . 1 1 -. 1 1 j i i 1 h \ i . ' ' . i 'I llii' i nil -t it lit ii ill nl 1 1 i.i I! i T. Ill thr ;i 1 1 >lli lc 1 1 1 1 'i >1'V

. , m,ivci ' - "i hfiisfiih L.,di«-. \\ Mli n - -mi . pl.mi't-, mid incti-nrH. i-nilucd witli ever-

OZONE AND Jiyi>i;<x;KN I'Ki;< >X I DK — DAI/fuN's LAW '217

A certain number of atoms n of an element A in combining with
several atoms m of another element B give a compound AnBm, each
molecule of which will contain the atoms of the elements A and B in
this ratio, and therefore the compound will present a definite composition,
expressed by the formula AnBm, where A and B are the weights of the

lusting force of motion, forming molecules as the heavenly bodies form systems, like
the solar system, which molecules are only relatively indivisible in the same way as the
planets of the solar system are inseparable, and stable and lasting as the solar system is
lasting. Such a representation, without necessitating the absolute indivisibility of
atoms, expresses all that science can require for an hypothetical representation of the
constitution of matter. In closer proximity to the dynamical hypothesis of the constitu-
tion of matter is the oft-times revived vortex hypothemt. Descartes first endeavoured
to raise it ; Helmholtz and Thomson gave it a fuller and more modern form ; many
scientific men applied it to physics and chemistry. The idea of vortex rings serves
tis the starting point of this hypothesis; these are familiar to all as the rings of
tobacco smoke, and may be artificially obtained by giving a sharp blow to the sides of a
cardboard box having a circular orifice and filled with smoke. Phosphine, as we shall
see later on, when bubbling from water always gives very perfect vortex rings in a still
atmosphere. In such rings it is easy to observe a constant circular motion about their
axes, and to remark the stability the rings possess in their motion of translation. This
unchangeable maps, endued with a rapid internal motion, is likened to the atom. In a
medium deprived of friction, such a ring, as is shown by theoretical considerations of the
subject from a mechanical point of view, would be perpetual and unchangeable. The
rings are capable of grouping together, and combining, being indivisible, remain
indivisible. The vortex hypothesis has been established in our times, but it has not
been fully developed ; its application to chemical phenomena is not clear, although
not impossible ; it does not satisfy a doubt in respect to the nature of the space existing
between the rings (just as it is not clear what exists between atoms, and between the
planets), neither does it tell us what is the nature of the moving substance of the ring,
und therefore for the present it only presents the germ of an hypothetical conception of
the constitution of matter, consequently, I consider that it would be superfluous to
speak of it in greater detail. However, the thoughts of investigators are now (and
naturally will be in the future), as they were in the time of Dalton, often turned to the
question of the limitation of the mechanical division of matter, and the atomists have
searched for an answer in the most diverse spheres of nature. I select one of the
methods tried, which does not in any way refer to chemistry, in order to show how closely
all the provinces of natural science are bound together. Wollaston proposed the inves-
tigation of the atmosphere of the heavenly bodies as a means for confirming the
existence of atoms. If the divisibility of matter be infinite, then air must extend
throughout the entire space of the heavens as it extends all over the earth by its elasticity
and diffusion. If the infinite divisibility of matter be admitted, it is impossible that any
portion of the whole space of the universe can be entirely void of the component parts of
our atmosphere. But if matter be divisible up to a certain limit only — namely, up to the
atom — then there can exist a heavenly body void of an atmosphere ; and if such a body
be discovered, it would serve as an important factor for the acceptation of the validity of
the atomic doctrine. The moon has long been considered as such a luminary, and this
circumstance, especially from its proximity to the earth, has been cited as the best proof
of the validity of the atomic doctrine. This proof is apparently (Poisson) deprived of
some of its force from the possibility of the transformation of the component parts of
our atmosphere into a solid or liquid state at immense heights above the earth's surface,
where the temperature is exceedingly low ; but a series of researches (Poule) has shown
that the temperature of the heavenly space is, comparatively, not so very low, and is
attainable by experimental means, so that at the low existing pressure the liquefaction

218 PRINCIPLES OF CHEMISTRY

atoms and m and n their relative number. If the same elements A and
B, in addition to AMBm. also yield another compound A,.B<;, then by-
expressing the composition of the first compound by AlirBmr (and this
is the same composition as AHBm), and of the second compound by
AruB5n, we have the law of multiple proportions, because for a given

of gases cannot be expected. Therefore the absence of an atmosphere about the moon,
if it were not subject to doubt, would be counted as a forcible proof of the atomic
theory. As a proof of the absence of a lunar atmosphere, it is cited that the moon,
in its independent movement between the stars, when eclipsing a star — that is, when
passing between the eye and the star — does not show any signs of refraction at its
edge ; the image of the star does not alter its position in the heavens on approach-
ing the moon's surface, consequently there is no atmosphere on the moon's surface
capable of refracting the rays of light. Such is the conclusion by which the absence of
a lunar atmosphere is acknowledged. But this conclusion is most feeble, and there are
even facts in exact contradiction to it, by which the existence of a lunar atmosphere
may be proved. The entire surface of the moon is covered with a number of mountains,
having in the majority of cases the conical form natural to volcanoes. The volcanic
character of the lunar mountains was confirmed in October 1866, when a change was
observed in the form of one of them (the crater Linnea). These mountains must be on
the edge of the lunar disc. Seen in profile, they screen one another and interfere with
-making observations on the surface of the moon, so that when looking at the edge of
the lunar disc we are obliged to make our observations not on the moon's surface, but
at the summits of the lunar mountains. These mountains are higher than those on
our earth, and consequently at their summits the lunar atmosphere must be exceed-
ingly rarefied even if it possess an observable density at the surface. Knowing the mass of
the moon to be eighty-two times less than the mass of the earth, we are able to approxi-
mately determine that our atmosphere at the moon's surface would be about twenty-
eight times lighter than it is on the earth, and consequently at the very surface of the
moon the refraction of light by the lunar atmosphere must be very slight, and at the
heights of the lunar mountains it must be imperceptible, and would be lost within the
limits of experimental error. Therefore the absence of refraction of light at the edge of
the moon's disc cannot yet plead in favour of the absence of a lunar atmosphere. There
is even a series of observations obliging us to admit the existence of this atmosphere,
These researches are due to Sir John Herschel. This is what he writes : — ' It has often
been remarked that during the eclipse of a star by the moon there occurs a peculiar
optical illusion; it seems as if the star before disappearing passed over the edge of the
moon and is seen through the lunar disc, sometimes for a rather long period of time. I
myself have observed this phenomenon, and it has been witnessed by perfectly trust-
worthy observers. I ascribe it to optical illusion, but it must be admitted that the star
might have been seen on the lunar disc through some deep ravine on the moon.' Geniller,
in Belgium (1856), following the opinion of Kassine, Eiler, and others, gave an explana-
tion to this phenomenon ; he considers it due to the refraction of light in the valleys of
the lunar mountains which occur on the edge of the lunar disc. In fact, although
these valleys do not probably present the form of straight ravines, yet it may sometimes,
happen that the light of a star is so refracted that its image might be seen, notwith-
standing the absence of a direct path for the light-rays. He then goes on to remark
that the density of the lunar atmosphere must be variable in different parts, owing to
the very long nights on the moon. On the dark, or non-illuminated, portion, owing to
these long nights, which last thirteen of our days and nights, there must be excessive cold,
and hence a denser atmosphere, while, on the contrary, at the illuminated portion the
atmosphere must be much more rarefied. This variation in the temperature of the
different parts of the moon's surface explains also the absence of clouds, notwithstanding
the possible presence of air and aqueous vapour, on the visible portion of the moon. The

OZONE AND HYDROGEN PEROXIDE— DALTON'S LAW 219

quantity of the first element, A,.,,, there occur quantities of the second
element bearing the same ratio to each other as mr is to qn ; and as //>,
r, q, and n are whole numbers, therefore their products are also whole
numbers, and this is also expressed by the law of multiple proportions.
Consequently the atomic theory is in accordance with and evokes the
first laws of definite chemical compounds : the law of definite composi-
tion and the law of multiple proportions.

So, also, is the relation of the atomic theory to the third law of definite
chemical compounds, the law of reciprocal combining weights, which is as
follows : — If a certain weight of a substance C combine with a weight
ft of a substance A, and with a weight b of a substance B, then, also, the
substances A and B will combine together in quantities a and b (or in
multiples of them). This should be the case from the conception of atoms.
Let A, B, and C be the weights of the atoms of the three substances, and
for simplicity of reasoning let combination proceed in the quantity of one
atom. It is evident that if the substance gives AC and BC, then the
substances A and B will give a compound AB, or their multiple, A,tBTO.

Sulphur combines with hydrogen and with oxygen. Sulphuretted
hydrogen contains thirty-two parts by weight of sulphur to two parts
by weight of hydrogen, which is expressed by the formula H2S. Sulphur
dioxide, SO2, contains thirty-two parts of sulphur and thirty-two parts of
oxygen, and therefore we conclude, from the law of combining weights,
that oxygen and hydrogen will combine in the proportion of two parts
of hydrogen and thirty -two parts of oxygen, or multiple numbers of
them. And we have seen this to be the case. Hydrogen peroxide
contains thirty-two parts of oxygen, and water sixteen parts, to two
parts of hydrogen ; and so it is in all other cases. This consequence of
the atomic theory is in accordance with nature, with the results of
analysis, and is one of the most important laws of chemistry. It is a law,
because it indicates the relation between the weights of substances enter-
ing into chemical combination. Further it is an eminently exact law,
and not an approximate one. The law of combining weights is a law
of nature, and by no means an hypothesis, for let the entire theory of
atoms be cast down, still the laws of multiple proportions and of com-
bining weights will remain, inasmuch as they deal with facts. They
may be guessed at from the sense of the atomic theory, and historically

presence of an atmosphere round the sun and planets, judging from astronomical observa-
tions, may be considered as fully proved. On Jupiter and Mars there may be even
distinguished bands of clouds. Thus the atomic doctrine, admitting a finite mechanical
divisibility only, must be, as yet at least, only accepted as a means, similar to that means
which a mathematician employs when he breaks up a continuous curvilinear line into a
number of straight lines. There is a simplicity of representation in atoms, but there is
,110 absolute necessity to have recourse to them. The conception of the individuality of
the parts of matter exhibited in chemical elements only is necessary and^trustworthy.

220 PRINCIPLES OF CHEMISTRY

the law of combining weights is intimately connected with this theory ;
but they are not identical, but only connected, with it. The law of
combining weights is formulated with great ease, and is an immediate
consequence of the atomic theory, without it, it is even difficult to under-
stand. Data for its evolution existed previously, but it was not seen
until those data were interpreted by the atomic theory. Such is the
property of hypotheses. They are indispensable to science ; they bestow
an order and simplicity which are difficultly attainable without their
aid. The whole history of science is a proof of this. And therefore
one may boldly say that it is better to hold to an hypothesis which may
afterwards prove untrue than to have none at all. Hypotheses facilitate
scientific work and render it uniform. The search for truth, like the
plough of the husbandman, helps forward the Avork of the labourer,
regulates it, and forces him to think of the further improvement both
of the work itself and of its implements.

221

CHAPTER V

NITROGEN AND AIR

GASEOUS nitrogen forms about four-fifths (by volume) of the atmo-
sphere ; consequently the air contains an exceedingly large mass of it.
Whilst entering in so considerable a quantity into the composition of
air, nitrogen does not seem to play any active part in the atmosphere,
the chemical action of which is mainly dependent on the oxygen it con-
tains. But this is not an entirely correct idea, because animal life
cannot exist in pure oxygen, in which animals pass into an abnormal
state and die ; and the nitrogen of the air, although slowly, forms
diverse compounds, many of which play a most important part in
nature, especially in the life of organisms. However, neither plants
nor animals directly absorb the nitrogen of the air, but take it up
from already prepared nitrogenous compounds ; further, plants are
nourished by the nitrogenous substances contained in the soil and water,
and animals by the nitrogenous substances contained in plants and in
other animals. Atmospheric electricity is capable of aiding the passage
of gaseous nitrogen into nitrogenous compounds, as we shall afterwards
see, and the resultant substances are carried to the soil by rain, where
they serve for the nourishment of plants. Plentiful harvests, fine
crops of hay, vigorous growth of trees — other conditions being equal —
are only obtained when the soil contains ready prepared nitrogenous
compounds, consisting either of those which occur in air and water, or
of the residues of the decomposition of other plants or animals (as
in manure). The nitrogenous substances contained in animals have
their origin in those substances which are formed in plants. Thus
the nitrogen of the atmosphere is the origin of all the nitrogenous
substances occurring in animals and plants, although not directly so,
but after first combining with the other elements of air.

The nitrogenous compounds which enter into the composition of
plants and animals are of primary importance ; no vegetable or animal
cell — that is, the elementary form of organism — exists without con-
taining a nitrogenous substance ; organic life, before all, evinces itself in

\ he-e nitrogenous substances. Tin- germs, seeds, and those parts by
which cells multiply themselves abound in nitrogenous substances; the
--tun total of the phenomena \\hidi are proper to organisms depend,
before all. on the chemical properties of the nitrogenous substances
which enter into their eoinposit ion. It is enough, tor instance, t o point
out the fact that vegetable and animal organisms, dearly distinguish-
able as such, are characterised by a dilierent degree of energy in their
nature, and at the same time by a difference in the amount of nitro-
genous substances they contain. In plants, which compared with
animals possess but little activity, being incapable of independent move-
ment, iVc.. the amount of nitrogenous substances is very much less than
in animals, who-e tissues are almost exclusively formed of nitrogenous
substances. It is remarkable that the nitrogenous parts of plants,
chietlv of the lower orders, sometimes present both iorms and properties
which approach to those of animal organisms : tor example, the xoo-
spores of seaweeds, or those parts by means of which the latter multiply
themselves. These xoospores on leaving the seaweed in many respects
re-einble the lower orders of animal life, having, like the latter, the pro-
perty of moving. They also approach the animal kingdom in their com-
jio.-ition, their outer coat containing nitrogenous matter. IMrectlv the
xoospore becomes covered with that non-nitrogenous or cellular coating
which is proper to all the o'-dinarv cells of plants, it loses all re-
semblance to an animal organism and becomes a small plant. 1 1 may be
thought from this that the cause of the diH'erence in the vital processes
of animals and plants is the different amount of nitrogenous substances
'hey contain. Tho-e nitrogenous elements which occur in plants and
animals apperta in to the series of exceedingly coin] 'lex and very change-
able chemical compounds: their elementary composition alone shows
this; besides nitrogen, they contain carbon, hydrogen, oxygen, and
-ulphur. I'eing distinguished by a very great instability under many
condition-; in uhidi other compounds remain unchanged, these sub-
-tance.- are titled for those perpetual changes which form the first con-
dition of \ii;il activity. These complex and changeable nitrogenous
, lances of the or^ani-m are called jir<>t<'nl unhxtn uri-n. The white
(if egg- is a familiar example of such a substance. They are also
contained in the lle-h of animals, the curdy dements of milk, the
glutinous matter of \\heaten Hour, or so called gluten, \\hidi forms the
diicf component < if macan >ni. ive.

Ni'r'>Lfen occur- in the earth crust, in compounds either forming
the remain- of pl:i ni - ;i i id a nimals, or derived t rom the n 1 1 rogen ot the
atmosphere as a con-djuence of it.- combination with the other com-
ponent pan- of the air. It \-^ not found in other forms in the earths

NITROGEN* AND AIR 223

crust ; so that nitrogen must be considered, in contradistinction to
oxygen, as an element which is purely superficial, and does not extend
to the depths of the earth.1

Nitrogen is liberated in a free state in the decomposition of the
nitrogenous organic substances entering into the composition of
organisms — for instance, on their combustion. All organic substances
burn when heated to redness with oxygen (or substances readily yielding
it, such as oxide of copper) ; the oxygen combines with the carbon,
sulphur, and hydrogen, and the nitrogen is evolved in a free state,
because at a high temperature it does not form any stable compound,
but remains free. Carbonic anhydride and water are formed from the
carbon and hydrogen respectively, and therefore to obtain pure
nitrogen it is necessary to remove the carbonic anhydride from the
gaseous products obtained. This may be done very easily by the action
of alkalis— 4»r instance, caustic soda. The amount of nitrogen in
organic substances is determined by a method founded on this.

It is also very easy to obtain nitrogen from air, because oxygen
combines with many substances. Either phosphorus or metallic copper
are usually employed for removing the oxygen from air, but, naturally,
a number of other substances may also be used. If a small saucer 011
which a piece of phosphorus is laid be placed on a cork floating on water,
and the phosphorus be lighted, and the whole be covered with a glass
bell jar, then the air under the jar will be deprived of its oxygen, and
nitrogen only will remain, owing to which, on cooling the water will
rise to a certain extent in the bell jar. The same object (procuring
nitrogen from air) is attained much more conveniently and perfectly
when air is passed through a red-hot tube containing copper filings.
At a red heat, metallic copper combines with oxygen and gives a black-
powder of copper oxide. If the layer of copper be sufficiently long and
the current of air slow, all the oxygen of the air will be absorbed, and
nitrogen alone will pass from the tube.2

1 The reason why there are no other nitrogenous substances within the earth's mass
beyond those which have come there with the remains of organisms, and from the air
with rain-water, must be looked for in two circumstances. In the first place, in the in-
stability of many nitrogenous compounds, which are liable to break up with the forma-
tion of gaseous nitrogen ; and in the second place in the fact that the salts of nitric acid,
forming the product of the action of air on many nitrogenous and especially organic
compounds, are very soluble in water, and on penetrating into the depths of the earth
(with water) give up their oxygen. The result of the changes of the nitrogenous organic
substances which fall into the earth is without doubt frequently, if not always, the forma-
tion of gaseous nitrogen. Thus the gas evolved from coal always contains much nitrogen
(together with marsh gas, carbonic anhydride, and other gases).

2 Copper (best as shavings, which present a large surface) absorbs oxygen, forming
CuO, at the ordinary temperature in the presence of solutions of acids, or, better still, in

224 PRINCIPLES OF CHEMISTRY

Nitrogen may also be procured from many of its compounds ivitk
oxygen* and hydrogen^ but the best fitted for this purpose is a saline
mixture containing, on the one hand, a compound of nitrogen with
oxygen, termed nitrous anhydride, N2O3, and on the other hand,
ammonia, NH3 — that is, a compound of nitrogen with hydrogen. By
heating such a mixture the oxygen of the nitrous anhydride combines
with the hydrogen of the ammonia, forming water, and gaseous nitrogen
is evolved, 2NH.< + N203 = 3H2O -f- N4. Nitrogen is procured by
this method in the following manner : — A solution of caustic potash is
saturated with nitrous anhydride, by which means potassium nitrite is
formed. On the other hand, a solution of hydrochloric acid saturated
with ammonia is prepared ; a saline substance called sal-ammoniac,
NH4C1, is thus formed in the solution. The two solutions thus pre-
pared are mixed together and heated. Reaction takes place according
to the equation KNO, + NH4C1 == KC1 + 2H2O -f N2. This reaction
proceeds in virtue of the fact that potassium nitrite and ammonium
chloride are salts which, on interchanging their metals, give potassium
chloride and ammonium nitrite, NH4NO2, which breaks up into water
and nitrogen. This reaction does not take place without the aid of
heat, but it proceeds very easily at a moderate temperature. Of the
resultant substances, the nitrogen only is gaseous, the potassium chloride
is non-volatile, and is left behind in the vessel in which the solutions
are heated. Pure nitrogen may be obtained by drying the resulting
gas and passing it through a solution of sulphuric acid (to absorb, a
certain quantity of ammonia which is evolved in the reaction).

Nitrogen is a gaseous substance which does not much differ in
physical properties from air ; its density, referred to hydrogen, is
approximately equal to 14 — that is, it is slightly lighter than air ; one
litre of nitrogen weighs 1-256 grams. Nitrogen mixed with oxygen,

the presence of a solution of ammonia, when it forms a bluish-violet solution of oxide
of copper in ammonia. Nitrogen is very easily procured by this method. A flask
is filled with copper shavings and closed with a cork furnished with a funnel and stop-
cock. A solution of ammonia is poured into the funnel, and caused to slowly drop upon
the copper. If at the same time a current of air be slowly passed through the flask
(from a gasholder), then all the oxygen will be absorbed from it and the nitn^ni
will pass from the flask. It should be washed with water to retain any ammonia that
may be carried off with it.

3 The oxygen compounds of nitrogen (for example, NoO, NO, NO2) are decomposed
at a red heat by themselves, and under the action of red-hot copper, sodium. A.V., they
give up their oxygen to the metals, leaving the nitrogen free. According to Meyer and
Langer (1885), nitrous oxide, N2O, decomposes below 900°, although not completely, whilst
the decomposition of nitric oxide, NO, does not start at 1200°, but is complete at 1700°.

4 Chlorine and bromine (in excess), as well as bleaching powder (hypochlorites). take
up the hydrogen from ammonia, NH5, leaving nitrogen. Nitrogen is best procured from
ammonia by the action of a solution of sodium hypobromite on solid sal-ammoniac.

NITIMMIKX AND A IK 225

which is slightly heavier than air, forms air. It is a gas which, like
oxygen and hydrogen, is difficultly liquefied, and but little soluble in
water and other liquids. Its absolute boiling point5 is about — 140°;
above this temperature it is not liquefiable by pressure, and at lower
temperatures it remains a gas at a pressure of 50 atmospheres. Liquid
nitrogen boils at — 193°, so that it may be employed as a source of great
cold. At about —203°, in vaporising under a decrease of pressure,
nitrogen solidifies into a colourless snow-like mass. Nitrogen does not
burn, does not support combustion, is not absorbed by any of the re-
agents used in gas analysis, at least at the ordinary temperature — in a
word, it presents a whole series of negative chemical properties ; this is
expressed by saying that this element has no energy for combination.
Although it is capable of forming compounds both with oxygen and
hydrogen as well as with carbon, yet these compounds are only formed
under particular circumstances, to which we will directly turn our atten-
tion. At a red heat nitrogen combines with boron, titanium, and silicon,
forming very stable nitrogenous compounds,6 whose properties are
entirely different from those of nitrogen with hydrogen, oxygen, and
carbon. However, the combination of nitrogen with carbon, although
it does not take place directly between the elements at a red heat, yet
proceeds with comparative ease by heating a mixture of charcoal with
an alkaline carbonate, especially potassium carbonate or barium carbo-
nate, to redness, carbo-nitrides or cyanides of the metals being formed ;
for. instance, K2CO3 + 4C + N, = 2KCN + 3CO.7

Nitrogen is found with oxygen in the air, but they do not readily
combine. Cavendish, however, in the last century, showed that nitrogen
combines with oxygen under the influence of a series of electric sf>arks.
Electric sparks in passing through a moist8 mixture of nitrogen and
oxygen — for instance, through air — cause these elements to combine,

5 See Chapter II. note 29.

6 The combination of boron with nitrogen is accompanied by the evolution of suffi-
cient heat to raise the mass to redness; titanium combines so easily with nitrogen that it
is difficult to obtain it free from that element. It is a remarkable and instructive fact
that the compounds of nitrogen with these non-volatile elements are very stable, and
are themselves non-volatile. Probably in this case the physical state of the substance
with which the nitrogen combines, and ,the state in which the nitrogenous substance is
obtained, evinces its influence. Thus carbon (C = 12) with nitrogen gives cyanogen, C;>N2,
which is gaseous and very unstable, and whose molecule is not large, whilst boron (B = ll)
forms a nitrogenous compound which is solid, non- volatile, and very stable. Its compo-
sition, BN, is essentially like that of cyanogen, but its molecular weight is probably
greater.

7 This reaction, as far as is known, does not proceed beyond a certain limit, probably
because cyanogen, CN, itself breaks up into carbon and nitrogen.

8 Fremy and Becquerel took dry air, and observed the formation of brown vapours of
oxides of nitrogen on the passage of sparks.

VOL. I. Q

funning reddish-lirowii fumes of oxides of nit ro^vn.'' which form with
\\ ater a co]n| on i n 1 ci i] it am 1 1 iu' nit ro;_;e n, oxygen, and hydrogen namely,
nitric acid.1" N 1 1 < ' ... The presence of t lie lat ter is easily reco^iii.M'd, not
only ir.'iii it-- I'IM Menu iu' litmus paper, Itut al>o Irom its acting as a
l"'\verful oxidistT even <if inci'cuvv. Conditions similai1 to these occur
111 nature, during a thunderstonn or in otlier eleetneal discharges
accomplished in the atmosphere* whence it mav lie understood that
air and rain-water always contain traces of nitric acid.11

Further observations >ho\\ed that under the inlluence of electrical
discharges,1- silent as \vell as witli sparks, nitrogen is able to enter into

'If.: inn of one vohniH- of uitvo^'ii and hmrteen vohunes of hydni^en be burnt,

then water and a ci 'i i -it li Tal ilf quantity of nitric acid arc formed. It may lie partly due
in quantity ''t' nitric acnl i- produced in ilic -low oxidation of nitro-
genous -uh.-taiices :n an excess (it air. '1 hi- is especially facilitated by the presence of
ii ii the nitric aci<l t'tinned can combine. It a iralvanic current lie passed
'. ter < ontainin^ the nitre-en ard oxygen of the air in solution, then the hydro-
_,•!! and oxygen -••' free combine uith the iiitni^reii. furniiiiL;- aninioiiia and nitric a -id.

Wh.-n cop]n r is oxidised at the expense of the air at the ordinary temperature in the
pre-ence of ainiiionia. oxygen is ah-orbcd. not only for coinliinat imi with the coppei-. l»ut.
a Ui i fi ir the fi irmatii ai of nit ric acid.

The coinl ination of iiitroL.f<'ii with nxy^cn, even, for example, \>y the action of electric

-parks, i- not accompanied I >y an explosion or rapid comliinat ion. a> in i he action of > parks

. of oxy_'en and hydrogen. 'Diis i<; explained hv the fact that heat is not

evolved iii tin- combination of nitro-en with oxygen, lnit is a'l-orhed an exjieiiditure of

eiieru'v is reijuiri d. there is no evolution ol eiiei'LTy. In lact. there will not he the trans-

mi~-ion in particle to |i.-irticle which occur- in the explosion of detonating pis.

!-'.acli -pur!: will aid the formation of a certain quantity of the compound of oxygen and

• . J.iil ill not excite the same in the nei^hltotiriiiLr particles. I n ot her \vor.U. t he

comliinat ion of hydro-en \vit h oxyuren i- an exothermal reaction, and the ci >m hi nation , ,f

The! ol the explosion of detoiiatin- pis if it he /// r.rrv.s'.s are especially

: • in of nitroureii. If a mixture ol t wo volumes of detoniitiiiLT -:as

, . - air he exploded, then one-tenth ot the air is con\ erted into nitric acid.

-, i id con , . • ft er the expl.i-ion has taken place there remain^ only nine tent lis of the

• n. If a larje pri 'pori ion of air he taken for instance, four

• ,\o \ ohm ie- di detonat in;_f ufns then it he tempera tun- of t he explosion

i ii remains iiiicliaiiLTed. and no n it ric acid is formed.

'I . uletohe .,b<ened in making n^eof the eudiometer na ui.-ly. t ha! to weaken

e of the ex ] oil l|o| leS-, th.ill all eqll.il Volume (.1 air should he added to the

, , . (i llieo-.her hand, a lar-e exceNsmust ii.it he taken, as no explosion

: . , n ..hide, uh,, li.as ue-hall afterward- learn, in 'he pivsenceof waterand

,,:,:v/, ,, , ., • c M d. l',\ the aci : |, m ,| char-e Hinni-li air hotli oxide- ,,f

• , lor d. Inil with a leehle ,|i char-e Hi,, formation

I III. e-o| tro-el, are not prodllc. .1. h\ wllirll fact Merthelot

n 'I'),, ,,'•;• e .e id c. uita ed 111 the nd. stream water < ha pt ,-r I . n, ,| ,. -J ,. wells. A'c..
proe. ed i . Irid. •,,,-,.•,., com|,oiuiils which ha\e

fa.l!t ,: .: to ,. ,t. r. ' /.

!-' Thi cu. i eri. • cuit;, t'-r r. ad i ' • • •• n. \\ hi. h under normal condition- is

. •; to lln d. : that under tin- influence of an electric discharge gaseous

AND AIR 227

many reactions with hydrogen itself and with many hydrocarbons ;
although these reactions cannot be effected by exposure to a red heat.
Thus, for instance, a series of electric sparks passed through a mixture
of nitrogen and hydrogen causes them to combine and/orm ammonia**
or nitrogen hydride, NH3, composed of one volume of nitrogen and
three volumes of hydrogen. This combination is limited to the forma-
tion of 6 per cent, of ammonia, because ammonia is decomposed,
although not entirely (f^o) by electric sparks. This signifies that
under the action of electric sparks the reaction NH3 = N" -t 3H is
reversible, consequently it is a dissociation, and in it a state of equili-
brium is arrived at. The equilibrium may be destroyed by the addition
of gaseous hydrochloric acid, HC1, because with ammonia it forms a solid
saline compound, sal-ammoniac, NH4C1, which (being formed from a
gaseous mixture of 3H, N, and HC1) fixes the ammonia. The re-
maining mass of nitrogen and hydrogen, under the action of the sparks,
-again forms ammonia, and in this manner solid sal-ammoniac is obtained
to the end by the action of a series of electric sparks on a mixture of
gaseous N, H3, and HC1.14 Berthelot (1876) showed that under the
action of a silent discharge many non-nitrogenous organic sub-
stances (benzene, CGH6, cellulose in the form of paper, resin, glucose,
C6H10O5, and others) absorb nitrogen and form complex nitrogenous
compounds, which are capable, like albuminous substances, of evolving
their nitrogen as ammonia when heated with alkalis.15

nitrogen changes in its properties ; if not permanently like oxygen (electrolysed oxygen or
•ozone does not react on nitrogen, according to Berthelot), it may be temporarily at the
moment of the action of the discharge, just as some substances under the action of heat are
durably affected (that is, when once changed remain so — for instance, mercuric oxide is
decomposed, white phosphorus passes into red, &c.), whilst others are only temporarily
altered (the dissociation of S6 into S2 or of sal-ammoniac into ammonia and hydrochloric
acid). Such a proposition is favoured by the fact of nitrogen giving two kinds of spectra,
with which we shall afterwards become acquainted. It may be that the molecules N2
tin 'ii give less complex molecules, N containing one atom. Probably under a silent
discharge the molecules of oxygen, O2, are partly decomposed and the individual atoms
O combine with O.>, forming ozone, O3.

15 This reaction, discovered by Chabrie and investigated by Thenard, was only rightly
understood when Deville applied the principles of dissociation to it.

14 The action of nitrogen on acetylene (Berthelot) resembles this reaction. A mixture
of these gases under the influence of a silent discharge gives hydrocyanic acid, CoH-j + N3
-=2CNH. This reaction cannot proceed beyond a certain limit because it is reversible.
* 15 Berthelot successfully employed electricity of even feeble potential in these experi-
ments, which fact led him to think that in nature, where the action of electricity takes
place very frequently, a part of the complex nitrogenous substances may proceed from
the gaseous nitrogen of the air by this method.

As the nitrogenous substances of organisms play a very important part in them
(organic life cannot exist without them), and as the nitrogenous substances introduced
into the soil are capable of invigorating its crops (naturally in the presence of the
other nourishing principles required by plants), therefore the question of the means
of converting the atmosphericV nitrogen into the nitrogenous compounds of the soil, or

Q2

228 ' PRINCIPLES OF CHEMISTRY

By such and, it may be, other similar indirect methods does gaseous
nitrogen yield its primary compounds, in which form it enters into
plants, and is elaborated in them into complex albuminous substances.
But, starting from a given compound of nitrogen with hydrogen or
oxygen, we may, without the aid of organisms, obtain, as will after-
wards be partially indicated, most diverse and complex nitrogenous
substances, which cannot by any means be formed directly from gaseous
nitrogen. In this we see an example not only of the difference between
an element in the free state and an intrinsic element, but also of those
circuitous or indirect methods by which substances are formed in nature.
The discovery, prognostication, and, in general, study of such indirect
methods of the preparation and formation of substances forms one of
the existing problems of chemistry. From the fact that A does not
act at all on B, it must not be concluded that a compound AB is not
to be formed. The substances A and B contain atoms which occur in
AB, but their state, the nature of their movement and union, may not
be- at all that which is required for the formation of AB, and in this
substance, although it contains the same elements in mass and quality
of them as in A and B, yet their chemical state may be as different as
the state of the atoms of oxygen in ozone and in water. Thus free
nitrogen is inactive ; but in its compounds it very easily enters into
changes and is distinguished by great activity. An acquaintance with
the compounds of nitrogen confirms this. But, before entering on this
subject, let us consider air as a mass containing free nitrogen.

Judging from what has been already stated with respect to water,
oxygen, ozone, and nitrogen, it will be evident that atmospheric air™

into assimilable nitrogen capable of being absorbed by plants and of forming com-
plex (albuminous) substances in them, forms a question of great theoretical and prac-
tical interest. The artificial (technical) conversion of the atmospheric nitrogen into
nitrogenous compounds, notwithstanding repeated trials, cannot yet be considered as
fulfilled in a practical, remunerative manner, although its possibility is already evident.
Electricity will probably aid in solving this problem of great practical importance. When
the theoretical side of the question is further advanced, then without doubt an advan-
tageous means will be found for the manufacture of nitrogenous substances from the
nitrogen of the air ; and this is needed, before all, for the agriculturist, to whom nitro-
genous fertilisers form an expensive item, and are more important than all other manures.

One thousand tons of farmyard manure do not generally contain more than four tons
of nitrogen in the form of complex nitrogenous substances, and this amount of nitrogen
is contained in twenty tons of ammonium sulphate, therefore the action evinced by the
mass of manure in respect to the introduction of nitrogen may be produced by sin a II
quantities of artificial nitrogenous fertilisers. Over 160000 tons of guano are imported
into Europe from South America, because guano (the excrement of sea and other birds)
contains many nitrogenous compounds which are required by the agriculturist.

16 Under the name of atmospheric air the chemist and physicist understand ordinary
air containing nitrogen and oxygen only, notwithstanding that the other component parts
of air have a very important significance for the vitality of the earth's surface. That air

MTKodKN AND AIR 229

contains a mixture of several gases and vapours. Some of them are
met with in it in nearly constant proportions, whilst others, 011 the
contrary, are very variable in their amount. The chief component
parts of air, placed in the order of their relative amounts, are the
following : nitrogen, oxygen, aqueous vapour, carbonic anhydride, nitric
acid, salts of ammonia, ozone, hydrogen peroxide, and complex nitro-
genous substances. Besides these, air generally contains water, as spray ^
drops, and snow, and particles of solids, perhaps of cosmic origin in
certain instances but in the majority of cases proceeding from the
mechanical translation of solid particles from one locality to another by
the wind. These small solid and liquid particles (having a large sur-
face and little weight) hang in air as solid matter hangs in turbid
water ; they often settle on the surface of the earth, but the air is never
-entirely free from them, because they are never in a state of complete
rest. Then, air not unfrequently contains incidental traces of various
substances, as everyone knows by experience. These incidental sub-
stances sometimes belong to the order of those which act injuriously
(miasmas), the germs of lower organisms — for instance, of moulds — and
to the class of carriers of infectious diseases.

In the air of the diverse countries of the earth, at different longitudes
and at different altitudes above its surface, on the ocean or on the dry
land — in a word, in the air of most diverse localities of the earth — the
oxygen and nitrogen are everywhere in a constant ratio. This is,
moreover, self-evident from the fact that the air constantly diffuses
(intermixes in virtue of the internal movement of the gaseous particles)
and is put in a state of movement and intermixed by the wind, and
therefore it is equalised in its composition over the entire surface of the

is so represented in science is based on the fact that only the two above-named com-
ponents are met with in air in a constant quantity, whilst the others are variable. The
solid impurities may be separated from air required for chemical or physical research
by simple filtration through a long layer of cotton- wool placed in a tube. Organic im-
purities are removed by passing the air through a solution of potassium permanganate.
The carbonic anhydride contained in air is absorbed by alkalis — best of all, soda lime,
which in a dry state in porous lumps absorbs it with exceeding rapidity and complete-
ness. Aqueous vapour is removed by passing the air over calcium chloride, strong sul-
phuric acid, or phosphoric anhydride. Air thus purified is accepted as containing only
nitrogen and oxygen, although in reality it still contains a certain quantity of hydrogen
.and hydrocarbons, from which it may be purified by passing over copper oxide heated to
redness. The copper oxide then oxidises the hydrogen and hydrocarbons — it burns them,
forming water and carbonic anhydride, which may be removed as above described. Such
purified air differs in many respects from ordinary air. Thus, for instance, it does not
support plant life. When it is said that in the determination of the density of gases the
weight of air is taken as unity, then it is understood to be such air, containing only
nitrogen and oxygen. It is a litre of such air that weighs 1'298 grams at 0° and 700 mm.
pressure at Jong. 4S3, and T294 grams at St. Petersburg.

230 PRINCIPLES OF CHEMISTRY

earth. In those localities where the air is subject to change, being in a
more or less enclosed space, or, at least, an unventilated space, it may
alter very considerably in its composition. For this reason the air of
dwellings, cellars, and wells, in which there are substances absorbing
oxygen, contains less of this gas, whilst the air on the surface of
standing water, abounding in the lower orders of plant life evolving
oxygen, holds an excess of this gas.17 The constant composition of
air over the whole surface of the earth has been proved by a number
of most careful researches.18

17 As a proof of the fact that certain circumstances may actually change the composi-
tion of air, it will be enough to point out that the air contained in the cavities of glaciers
(of permanent mountain ice) contains only up to 10 p.c. of oxygen. This depends on the
fact that at low temperatures oxygen is much more soluble in snow-water and snow than
nitrogen. When shaken up with water the composition of air should change, because
the water dissolves an unequal quantity of oxygen and nitrogen. We have already seen
(Chapter I.) that the air boiled off from water saturated at about 0° contains about thirty-
five volumes of oxygen and sixty-five volumes of nitrogen, and we have considered the
reason of this. It is remarkable that the solubility of oxygen and nitrogen in water de-
creases so uniformly with the temperature that the proportion of oxygen and nitrogen held
in an aqueous solution remains almost constant at the most varied temperatures. This ex-
plains the circumstance that the air over the sea (especially arctic) is, as certain observers
have found, poorer in oxygen than on dry land — the water dissolves more oxygen than
nitrogen. The difference does not, however, exceed 0'3 p.c., and sometimes does not exist.

18 The analysis of air by weight conducted by Dumas an'1 Boussingault in Paris, and
which they repeated many times between April 27 and September 22, 1841, under various,
conditions of weather, showed that the amount by weight of oxygen only varies between
22'89 p.c. and 23*08 p.c., the average amount being 23'07 p.c. Brunner, at Bern in Switzer-
land, and Bravais, at Faulhorn in the Bernese Alps, at a height of two kilometres above
the level of the sea, made analyses of the air at the same season of the year as Dumas,,
and found that the composition of the air at these places did not exceed the limits deter-
mined for Paris. Marignac at Geneva, Lewy at Copenhagen, and Stas at Brussels,
confirmed this. The analyses of air taken from different parts of the world, at the
surface of the ocean and at different heights above the level of the sea, lead to the con-
clusion that air everywhere contains an equal amount of oxygen, or that if it docs vary
it does so within very inconsiderable limits.

As there is some basis (which will be mentioned shortly) for considering that the com-
position of the air at great altitudes is different from that at attainable heights — namely,
that it is richer in nitrogen — several fragmentary observations made at Munich and in
America gave reason for thinking that in the upward currents (that is in the region of
minimum barometric pressure or at the centres of meteorological cyclones) the air is
richer in oxygen than in the descending currents of air (in the regions of anticyclones
or of barometric maxima) ; but more carefully conducted observations showed this pro-
position to be incorrect. Improved methods for the analysis of air have shown that cer-
tain slight variations in the composition of air do actually occur, but in the first place
they depend on incidental local influences (on the passage of the air over mountains and
large surfaces of water, regions of forest and vegetation, and the like), and in the second!
place are limited by quantities which are scarcely distinguishable from possible errors in
the analyses.

The considerations which make one think that the atmosphere at great altitudes con-
tains less oxygen than at the surface of the earth are based more particularly on the law
of partial pressures (page 81). It obliges one to consider that the equilibrium of the
oxygen in the strata of the atmosphere is not dependent on the equilibrium of the nitro-

MTI;<H;KN AND .\n; 231

T)« lumlysia of air is effected by converfcing the oxygen into a non-
gaseous compound, so as to separate it from the air. The original
volume of the air is first measured, and then the volume of the remain-
ing nitrogen. The quantity of oxygen is calculated either from the
difference between these volumes or by the weight of the oxygen com-
pound formed. All the volumetric measurements have to be corrected
for pressure, temperature, and moisture (Chapters I. and II.). The
medium employed for converting the oxygen into a non-gaseous sub-
stance should enable its being taken up from the nitrogen to the very
end without evolving any gaseous substance. So, for instance,19 a mix-
ture of pyrogallol, C6H6O3, and a solution of a caustic alkali absorbs
oxygen with great ease at the ordinary temperature (the solution turns
black), but it is unsuited for accurate analysis because it requires an
aqueous solution of an alkali, and it alters the composition of the air
by acting on it as a solvent.20 However, for approximate determina-
tions this simple method gives entirely satisfactory results.

The determinations in a eudiometer (Chapter III.) give much more
exact results, if all the necessary corrections for changes of pressure,
temperature, and moisture be taken into account. This determination
is essentially carried on as follows : — A certain amount of air is intro-
duced into the eudiometer, and its volume is determined. Then about

gen, and that the variation in the densities of both gases with the height is determined
by the pressure of each gas separately. Details of the calculations and considerations
here involved are contained in my work On Barometric Levellings, 1876, p. 48.

On the basis of the law of partial pressure and of hypsometrical formulae, expressing
the laws of the variation of pressures at different altitudes, the conclusion may be deduced
that at the upper strata of the atmosphere the proportion of the nitrogen with respect
to the oxygen increases, but the increase will not exceed a fraction per cent., even at
altitudes of four and a half to six miles, the greatest height within the reach of men either
by climbing mountains or by means of balloons. This conclusion is confirmed by the
analyses of air collected by Welsh in England during his aeronautic ascents. The ques-
tion of the distribution of gases in the upper strata of the atmosphere is of particular
importance for understanding in what state the gaseous or vaporous masses occur which
are borne in space, and are one of the elementary forms of the heavenly bodies (accord-
ing to Laplace's and Kant's theory). I touch on this subject in speaking of the origin of
naphtha in my work On the Naphtha Industry, 1879.

19 The complete absorption of the oxygen may be attained by introducing moist phos-
phorus into a definite volume of air ; this is recognised by the fact of the phosphorus
becoming non-luminous in the dark. The amount of oxygen may be determined by
measuring the volume of nitrogen remaining. This method, however, cannot give accu-
rate results, owing to a portion of the air being dissolved in the water ; to the combination
of some nitrogen with oxygen ; to the necessity of introducing and withdrawing the
phosphorus, which cannot be accomplished without introducing bubbles of air ; and to the
numerous corrections of the volume (for moisture, temperature, and pressure), &c.

20 For rapid and approximate analyses (especially technical and hygienic), such a mix-
ture is very suitable for determining the amount of oxygen in mixtures of gases, from
which the substances absorbed by alkalis have first been removed. According to certain
observers, this mixture evolves a small quantity of carbonic oxide after absorbing oxygen.

232

PRINCIPLES OF CIIE3IISTKY

an equal volume of dry hydrogen is passed into the eudiometer, and the
volume again determined. The mixture is then exploded, as was
described in the determination of the composition of water. The
remaining volume of the gaseous mixture is again measured ; it will be
less than the second of the previously measured volumes. Out of three
volumes which have disappeared, one appertains to the oxygen and two
to the hydrogen, consequently one-third of the loss of volume indicates
the amount of oxygen held in the air.21

The most accurate method for the analysis of air, and one which is
accompanied by the least amount of error, consists in the direct weigh-
ing, as far as is possible, of the oxygen, nitrogen, water, and carbonic

FIG. 38. — Dumas and Bousingault's apparatus for the analysis of air bv weight. The globe B contains
10-15 litres. The air is first pumped out of it, and it lg weighed empty. The tube T connected
with it is filled with copper, and is weighed empty. It is heated in a charcoal furnace. \Vhen the
copper has become red-hot, the stop-cock r (near R) is slightly opened, and the air passes through
the vessels L, containing a solution of potash ; /, containing solutions and pieces of caustic
potash, which remove the carbonic anhydride from the air, and then through o and t, containing
sulphuric acid (which has been previously boiled to expel dissolved air) and pumice-stone, which
removes the moisture from the air. The pure air then gives up its oxygen to the copper in T.
When the air passes into T the stop-cock It of the globe B is opened, and it becomes rilled with
nitrogen. When the air ceases to flow in, the stop-cocks are closed, and the globe B and tube
T weighed. The nitrogen is then pumped out of the tube, and it is weighed again. The increase
in weight of the tube shows the amount of oxygen, and the difference of the second and
third weighings of the tuba, with the increase in "weight of the globe, gives the weight of the
nitrogen.

anhydride contained in air. For this purpose, the air is first passed
through an apparatus for retaining the moisture and carbonic an-
hydride (which will be considered presently), and is then led through

21 Details of eudiometrical analysis must, as was pointed out in Chap. III. note 82,
be looked for in works on analytical chemistry. The same must be remarked in reference
to the other analytical methods mentioned in this work. They are only described for the
sake of showing the diversity of the methods of chemical research.

NITROGEN AND All: '233

a tube, which is previously weighed, and contains shavings of metallic
copper. A long layer of such copper heated to redness absorbs all
the oxygen from the air, and leaves pure nitrogen, whose weight
must be determined. This is done by collecting it in a weighed and
exhausted globe, and the amount by weight of oxygen is shown by the
increase in weight of the tube with the copper after the experiment.

Air free from moisture and carbonic anhydride22 contains 23*15
parts of oxygen and 76*85 of nitrogen by weight,23 which, taking the
density of oxygen =16 and of nitrogen =14, gives the volumetric
composition of air as 2O84 volumes of oxygen and 79*16 of nitrogen.24

The possibility of the composition of air being altered by the mere
action of a solvent very clearly shows that the component parts of air
are in a state of mixture, in which any gases may occur ; they do not
in this case form a definite compound, although the composition of the
atmosphere does appear constant under ordinary conditions. The fact
that its composition varies under different conditions confirms the
truth of this conclusion, and therefore the constancy of the composition
of air must not be considered as in any way dependent on the nature
of the gases entering into its composition, but only as proceeding from
cosmic phenomena co-operating towards this constancy. It must be
admitted, therefore, that the processes evolving oxygen, and chiefly the
processes of the respiration of plants, are of equal force with those
processes which absorb oxygen over the entire surface of the earth.25

22 Air free from carbonic anhydride indicates after explosion the presence of a
small quantity of carbonic anhydride, as De Saussure remarked, and air free from moisture,
after being passed over red-hot copper oxide, seems invariably to contain a small
-quantity of water, as Boussingault has observed. These observations cause one to think
that air always contains a certain quantity of gaseous hydrocarbons, like marsh gas,
which, as we shall afterwards learn, is evolved from the earth, marshes, &c. Its amount,
however, does not exceed some hundredths of a per cent.

23 The analyses of air are accompanied by errors, and there are variations of composi-
tion attaining hundredths per cent. ; therefore one must limit oneself to the first places
in decimals in expressing the average normal composition of air.

24 The weight of a litre of hydrogen at 0° and 760 mm. pressure is 0'08958 gram,
therefore 20'8 litres of oxygen weigh 29'87 grams, and 79'2 litres of nitrogen 99'28 grams,
which gives the weight of a litre of air as T2914, instead of T293. This difference corre-
sponds with the possible errors of both the analysis of air and of the other data entering
into the calculation.

25 In Chapter III. note 4, an approximate calculation made for the determination of
the amount of oxygen in the entire atmosphere is evidently without solid foundation —
that is, it may be supposed that the composition of air varies from time to time when the
relation between vegetation and the processes absorbing oxygen changes ; but such a
supposition may be met by an argument of the following kind : the atmosphere of the
«arth has not, and should not have, a definite limit, and we have already seen (Chapter IV.
note 88) that there are observations confirming this, consequently ourVtmosphere should
vary in its component parts with the entire heavenly space. If the equilibrium now exist-
ing were destroyed, it would be rectified by means of the immense mass of rarefied air

234

PRINCIPLES OF CHEMISTRY

Air always contains more or less moisture 2G and carbonic anhydride,.
proceeding from the respiration of animals and the combustion of
carbon and carboniferous compounds. The latter shows the properties.

J?IU. 39.— Apparatus for the absorption and
washing of gases, known as Liebig's
bulbs. The gas enters m, presses on the
absorptive liquid, aud passes from m into
6, c, d, and e consecutively, and escapes
through/.

FIG. 49. — Geisler's potash bulbs. The gas enters-
at a, and passes through a solution of potash
in the lower bulbs, where the earl ionic anhy-
dride is absorbed, arid the gas escapes from 1>.
The lower bulbs are arranged in a triangle,
SD that the apparatus can stand without
support.

of an acid anhydride. In order to determine the amount of carbonic
.anhydride in air, substances are employed which absorb it — namely,
alkalies either in solution or solid. A solution of caustic potash, KHO,

is poured into light glass vessels, through
which the air is passed, and the amount
of carbonic anhydride is determined by the
increase in weight of the vessel. But it is
best to take a solid porous alkaline mass,
such as soda-lime.27 With a feeble current
of air a layer of soda-lime 20 c.m. in length
is sufficient to completely deprive the
air of the carbonic anhydride it con-
FIG. 4i.— Tube for the absorption of tains. A series of tubes containing calcium

carbonic acid. A wad of cotton

wool is placed in the bulb to prevent

the powder of soda-lime being which occurs in the heavenly space. If, for instance^

carried off by the gas. The tube the amount of oxygen were diminished, then it would

aSE"*!*11*1*1 Chl°ride °f be replenished at the expense of the oxygen pervad.

ing the space of the heavens.

* The amount of moisture contained in the air is considered in greater detail by
physics and meteorology, and the subject has been mentioned above, in Chapter I. note 1,
where the methods of absorbing moisture from gases were pointed out.

27 Soda-lime is prepared in the following manner : — Unslaked lime must be reduced to a
fine powder and mixed with a slightly wanned and very strong solution of caustic soda. The
mixing should be done in an iron dish, and they should be well stirred together until the lime
begins to slack. When the mass becomes hot, it boils, swells up, and solidifies, forming a
porous mass very rich in alkali and capable of absorbing carbonic anhydride. A lump of
caustic soda or potash presents a much smaller surface for absorption, and therefore
acts much less rapidly. It is necessary to place an apparatus for absorbing water after
the apparatus for absorbing the carbonic anhydride, because the alkali in absorbing the
latter evolves water.

NITROGEN AND All; 235

chloride for absorbing the moisture 28 is placed before the apparatus for
the absorption of the carbonic anhydride, and a measured mass of air
is caused to pass through the whole apparatus by means of an aspirator.
In this manner the determination of the moisture is combined with the
absorption of the carbonic acid. The arrangement shown in fig. 3&
is such a combination.

The amount of carbonic anhydride29 in free air is incomparably
more constant than the amount of moisture. The average amount of
carbonic anhydride in 100 volumes of dry air is approximately , ;J()- p.c. —
that is, 10000 volumes of air contain about 3 volumes of carbonic anhy-
dride, most frequently about 2 -95 volumes. As the specific gravity of
carbonic anhydride, referred to air =1'52, therefore 100 parts by weight
of air contain 0'045 part by weight of carbonic anhydride. This quantity
varies according to the time of year (more in winter), the altitude
above the level of the sea (less at high altitudes), on the proximity to-
forests and fields (less) or cities (greater), &c. But the variation is
small and rarely exceeds limits of 2^ to 4 ten-thousandths by volume.30
As there are many natural local influences which either increase the

^ It is evident that the calcium chloride employed for absorbing the water should be-
free from lime or other alkalis in order that it should not retain carbonic anhydride. Such
calcium chloride may be prepared in the following manner. An entirely neutral solution
of calcium chloride is prepared from lime and hydrochloric acid ; it is then evaporated first
over a water-bath and then over a sand-bath with great care. When the solution attains,
a certain strength a scum is formed, which solidifies on reaching the top. This scum is
collected, and will be found to be free from caustic alkalis. It is necessary in any case to
test it before use, as otherwise a large error may be introduced into the results, owing to
the presence of free alkali (lime). It is better still to pass carbonic anhydride through the
tube containing the calcium chloride for some time before the experiment, in order to
saturate any free alkali that may remain from the decomposition of a portion of the
calcium chloride by water, CaCl2 + 2H2O = CaOH2O + 2HCl.

29 Recourse is had to special methods when the determination regards only the carbonic
anhydride of the air. For instance, it is absorbed by an alkali which does not contain
carbonates (by a solution of baryta or caustic soda mixed with baryta), and then the
carbonic anhydride is expelled by an excess of an acid, and its amount determined by the
volume given off. During the last ten years the question as to the amount of carbonic
anhydride present in the air has been submitted to many voluminous and exact researches^
especially those of Reiset, Schloesing, Miintz, and Aubin, who showed that the amount of
carbonic anhydride is not subject to such variations as was at first supposed on the basia
of incomplete and insufficiently accurate determinations.

30 It is a different case in enclosed spaces in dwellings, cellars, wells, caves, and mines,,
where the renewal of air is hampered. Under these circumstances large quantities of
carbonic anhydride may accumulate. Even in cities, where there are many conditions for
the evolution of carbonic anhydride (respiration, decomposition, combustion), its amount
is greater than in free air, yet even in still weather the difference does not often exceed one
ten-thousandth (that is, rarely attains 4 instead of 2'4 vols. in 10000 vols. of air). Hundreds
of very careful comparative determinations made simultaneously around Paris and in
tlu- city itself, and constant daily determinations conducted at certain meteorological
stations (for instance, at Montsouris, near Paris), confirm this conclusion. On high
mountains and in deep valleys, as has been proved in the Pyrenees, the difference of

"236 PRINCIPLES OF CIIE3IISTKY

amount of carbonic anhydride in the air (respiration, combustion,
rotting, volcanic eruptions, ifec.), or diminish it (absorption by plants
and water), therefore the reason of the great constancy in the amount
of this gas in air must be looked for, in the first place, in the fact that
the wind mixes the air of various localities together, and, in the second
place, in the fact that the waters of the ocean, holding carbonic acid in
solution,31 form an immense reservoir for regulating the supply of this
gas in the atmosphere. As immediately the partial pressure of the
carbonic anhydride in the air decreases, the water evolves it, and when
the partial pressure increases, it absorbs it, nature consequently sup-
plies the conditions for a natural mobile state of equilibrium in this
instance, as in a number of others.32

Beyond nitrogen, oxygen, moisture, and carbonic acid, all other sub-
stances occurring in air are found in innnitesimally small quantities by
weight, and therefore the it-eight of a cubic measure of air depends, to a
sensible degree, on the above-named components alone. We have
already mentioned that at 0° and 760 mm. pressure the weight of a
cubic litre of air is 1-293 grams,33 the air being understood to be

the amount of carbonic anhydride is also very slight (at a height it is, however, less, as
would be expected).

51 In the sea as well as fresh water the carbonic acid occurs in two forms, directly
dissolved in the water, and combined with lime, as calcium bicarbonate (hard waters
sometimes contain very much carbonic acid in this form). If the tension of the carbonic-
anhydride in the first form varies with the temperature, and its amount with the partial
pressure, that in the form of acid salts is under the same conditions, because direct
experiments have shown a similar dependence in this case, although the quantitative
relations are different in the two cases.

52 In studying the phenomena of nature one inevitably arrives at the idea that the
universally reigning state of mobile equilibrium forms the chief reason of that harmonious
order which impresses all observers. It not unfrequently happens that we do not see the
causes regulating the order and harmony ; in the particular instance of carbonic anhy-
dride it is a striking circumstance that in the first instance a search was made for an
harmonious and strict uniformity, and in incidental (insufficiently accurate and fragmen-
tary) observations conditions were even found for concluding it to be absent. When, later,
the rule of this uniformity was confirmed, then the causes regulating such order were
also discovered. The researches of Schloesing were of this character. Deville's idea of
the dissociation of the acid carbonates of sea- water is suggested in them. In much else,
also, a right understanding can only be looked for in detailed investigation.

53 The difference of the weight of a litre of dry air (free from carbonic anhydride) at 0°
and 760 mm., at different longitudes and altitudes, depends on the fact that the force of
gravity varies under these conditions, and with it the pressure of the barometrical column
also varies. This is treated in detail in my works On tin- Elasticity of Gcuet SJad (>>/
Barometric Levellings.

In reality the weight is not measured in absolute units of weight (in pressure — refer
to works on mechanics and physics), but in relative units (grams, scale weights) whose
mass is one and the same, and therefore the variation of the weight of the weights itself
with the change of gravity must not be here taken into account, for the matter deals with
weights proportional to the masses, and with a change of locality the weight of the weights
varies as the weight of a given volume of air does.

UB 237

dry and free from carbonic anhydride. Taking the amount of the
latter as 0'03 per 100 volumes, wo obtain a greater weight — namely,
1-000156 times greater (consequently at 0° and 760 mm., instead of
l-L'9300 grains, the weight of a litre will then be 1 '29319 grams). The
weight of moist air in which the tension 34 of the aqueous vapour (par-
tial pressure) =fim\\.t and consequently the volume of vapour (its

density referred to air =0'62) =%- ~ f the whole pressure of the moist
air =760 mm.) is to the weight of an equal volume of dry air as
+ 0-62 , or as ™~~f, is to 1 ; that is, the weight of air

containing carbonic anhydride and moisture at 0° and 760 mm. pressure
is not 1-2930 grams, but this weight multiplied by 1-000156 and by

7fin ^ a Pressure (total) of air of H millimetres, a temperature

t, and elasticity of vapour f, the weight of a litre of air will be
(i.e., if at 0° and 760 mm. the weight of dry air = 1'293) equal to

x H~38/- For instance' if H = 73° mm" ' = 20°>

and f =. 10 mm. (the moisture is then slightly below 60 p.c.), the
weight of a litre of air = 1-1512 gram.35

The presence of ammonia, a compound of nitrogen and hydrogen,.
in the air, is indicated by the fact that all acids exposed to the air ab-
sorb ammonia from it after a time. De Saussure observed that aluminium
sulphate is converted by air into a double sulphate of ammonium and
aluminium, or the so-called ammonia alum. Quantitative determina-
tions have shown that the amount of ammonia 3n contained in air
varies at different periods. However, it may be accepted that 100 cubic

•"* The tension of the aqueous vapour in the air is determined by hygrometers and
other similar methods. It may also be determined by analysis (see Chapter I. note 1).

35 In determining the weight of small and heavy objects (crucibles, &c. in analysis, and
in determining the specific gravities of liquids, &c.) a correction may be introduced for
the loN.-i of trchjJit in the air of the room, by taking the weight of a litre of air displaced
as ra gram, and consequently 0'0012 gram for every cubic centimetre. But if gases or,
in general, large vessels are weighed, and the weighings have to be accurate, it is neces-
sary to take into account all the data for the determination of the density of the air
(t, H, and/), because sensitive balances can determine the possible variations of the
weight of air, as in the case of a litre the weight of air varies in centigrams, even at a
constant temperature, with variations of H and /. The following method was long ago
(1859) proposed and applied by me for this purpose. A large light and closed vessel is
taken, and its volume and weight in a vacuum are accurately determined, and verified from
time to time. On weighing it we obtain the weight in air of a given density, and by sub-
tracting this weight from its absolute weight and dividing by its volume we obtain the
density of the air.

56 Schloesing studied the equilibrium of the ammonia of the atmosphere and of the
rivers, seas, &c., and he showed that its amount is interchangeable between them. The

metre- i if ;ur do in>t contain les- than 1 ami m>t more than •"> milli-
gram- i it' ammonia. It is remarkable that mountain air contains more
ammonia than the air of \allevs. The air in t host1 place- where animal
-ubstance- underu'erin^ change are accumulated, and e.-pecially that of
-'able.-. _•'". iei allv contain- a much LiTeater quantity of t hi- Ljas. This is
• e • t-on of the peculiar pungent smell noticed in such place-. .More-
nmoiiia, a- we -hall learn in t he following chapter, combines with
acids. ;md should therefore be found in air as such combinations, as
air contains carbonic and nitric acids.

Tin- pre-ence of nitric acid in air is proved without doubt by tin-
fact ilia; rain-water contains a >omewhat considerable amount of
nitric acid, as we shall learn when speaking of this acid.

Furl her (as has been mentioned in Chapter I V.), air contains <>;<»<('.
and h\ dr< >_;eii peri ixide/ '

I!. --ides substances in a gaseous or vaporous state, ils there is alwavs
found a more or less considerable quantity of substances which are not
k m>\\ M iii a st ate of vapour. 'I hese substances are c-arried in the air as
<///x/. If ,-i linen surface, moistened with an acid, be placed in perfectly
pun- air. tlp-n the washings are found to contain sodium, calcium, iron,
and potas-ium.-1-1 Linen moistened with an alkali absorbs carbonic,
-ulphuric. phosphoric, and hydrochloric acids. l-'urther. the presence

• m,.,| i tl .-iiir. M/,,III- iiinl liy.li'.-rn p.-rnxiili' :it ill. i I • rapidlv dis-

s-Mii-i.li-r.-ilily. iinil.asw. li-.l.i./ ..... -isiiii-t

!• ] r. -i lit ill the ,iir HIM] its |nirit\. tiuit i~. tin- ;iinoiint of
in must he -mall. lire. iii-i , i <-i mtact \\ itli

i I In nr Iiy f\ i.l\ in-_' it li\ ;irtilicial iiicaii- in t In at nm-pluTi- : IW

; ,,,•,.•: ,.;,] pai ks tlii-.m-h the si'iil il.it in - |ti|..-- 1. .nlin- air

•• • : • ..lit tlii' ciini-

i • • ' tin il thn • , inu' i' . l''"i'

i./niii- 1 h.iu i-iiiuil r\ air. 'I'l I'uriu I In1 ilis-
I ! ,( i'r. animal life I'aiinut r\i-t n a ir i-ont aiiiin^r a

.1 i . ,m.| ali ..In. I. ( II. M. ,-. .... |, \Iiinl, h.uiiil to

NITROGEN AND A IK 239

•of organic substances in air has been proved by a similar experiment.
If a glass globe be filled with ice and placed in a room where are a
number of people, then the presence of organic substances, like albu-
minous substances, may be proved in the water which condenses on the
surface of the globe. It may be that the miasmas causing infection in
marshy localities, hospitals, and in epidemic illnesses proceed from the
presence of such substances in the air, as well as from the presence
of germs of lower organisms borne in the air as a minute dust.
Pasteur proved the presence of such germs in the air by the following
experiment : — He placed gun-cotton (pyroxylin), which has the appear-
ance of ordinary cotton, in a glass tube. Gun-cotton is soluble in a
mixture of ether and alcohol, forming the so-called collodion. A cur-
rent of air was passed through the tube for a long period of time, and
the gun-cotton was then dissolved in a mixture of ether and alcohol.
An insoluble residue was thus obtained which actually contained the
germs of organisms, as was shown by microscopical observations, and by
their capacity to develop into organisms (mould, &c.) under favourable
^conditions. The presence of these germs determines the property of
-air of bringing about the processes of rotting and fermentation — that
is, the fundamental alteration of organic substances, which is accom-
panied by an entire change in their properties. The appearance of
lower organisms, both vegetable and animal, is frequently to be
remarked in these processes. Thus, for instance, in the process of fer-
mentation, when, for example, wine is procured from the sweet juice
of grapes, a sediment separates out which is known under the name
•of lees, and contains peculiar yeast organisms. Germs are required
before these organisms can appear.40 They are borne in the air, and fall
into such substances from it. Finding themselves under favourable
conditions, the germs develop into organisms ; they are nourished at the
expense of the organic substance, and during growth change and destroy
it, and bring about corruption and rotting. This is why, for instance,
the juice of the grape when contained in the skin of the fruit, which
allows access of the air but is impenetrable to the germs, does not fer-
ment, does not alter so long as the skin remains intact. This is also
the reason why animal substances when kept from the access of air
may be preserved for a great length of time. Conserves for long sea

40 The idea of the spontaneous growth of organisms in a suitable medium, although
still upheld by many, has since the work of Pasteur and his followers (and to a certain
extent of his predecessors) been discarded, because it has been proved how, when, and
whence (from the air, water, &c.) the germs appear ; that fermentation as well as infec-
tious diseases cannot take place without them ; and mainly because it has been shown that
any change accompanied by the development of the organisms introduced may be brought
about at will by the introduction of the germs into a suitable medium.

240 PKINCIPLES OF CHEMISTRY

voyages are preserved in this way.41 Hence it is evident that however
infinitesimal the quantity the germs carried in the atmosphere may be,
still they have an immense significance in nature.4'2

Thus we see that air contains a great variety of substances. The
nitrogen, which is found in it in the largest quantity, has the least
influence on those processes which are accomplished by the action of air.
The oxygen, which is met with in a lesser quantity than the nitrogen*
on the contrary, takes a very important part in a number of reac-
tions ; it supports combustion and respiration, it brings about corruption
and every process of slow oxidation. The part played by the moisture
of air is known to everyone. The carbonic anhydride, which is met
with in still smaller quantities, has an immense significance in nature,
inasmuch as it serves for the nourishment of plants. The importance
of the ammonia and nitric acid is immense, because they are the sources
of the nitrogenous substances comprising an indispensable element
in all living organisms. And, lastly, the infinitesimal quantity of germs
evinces their significance in a number of processes. Thus it is not the
quantitative but the qualitative relations of the component parts of the
atmosphere which determine its importance in nature.43

Air, being a mixture of various substances, may suffer considerable
changes in consequence of incidental circumstances. It is particularly
important to remark that change in the composition of air which takes
place in dwellings and in various localities where human beings have to
remain during a lengthy period of time. The respiration of human
beings and animals alters the air.44 A similar deterioration of air is
produced by the influence of decomposing organic substances, and

41 In further confirmation of the fact that putrefaction and fermentation depend on
germs carried in the air, we may cite the circumstance that poisonous substances de-
stroying the life of organisms stop or hinder the appearance of the above processes.
Air which has been heated to redness or passed through sulphuric acid no longer contains
the germs of organisms, and loses the faculty of producing fermentation and putrefaction.

42 Their presence in the air is naturally due to the diffusion of germs into the atmo-
sphere, and owing to their microscopical dimensions they, as it were, hang in the air in
virtue of their large surfaces compared to their weight. In Paris the amount of dust
hanging in the air equals from f> (after rain) to 23 grams per 1000 c.m. of air.

45 we see similar cases everywhere. For example, the predominating mass of sand
and clay in the soil takes hardly any chemical part in the economy of the soil in respect
to the nourishment of plants. The plants by their roots search for substances which are
diffused in comparatively small quantities in the soil. If a large quantity of these
nourishing substances be taken, then the plants will not develop in the soil, just as animals
die in oxygen.

44 A man inbreathing burns about 10 grams of carbon per hour — that is, he produces
about 880 grams, or (as 1 c.m. of carbonic anhydride weighs about 2000 grains) almut
^ c.m. of carbonic anhydride. The air coming from the lungs contains 4 p.c. of carbonic
anhydride by volume. The exhaled air acts as a direct poison, owing to this gas and to
other impurities.

N AND AIR 241

especially of substances burning in it.1"' Hence it is necessary to have
regard to the purification of the air of dwellings. The renewal of air,
the replacing of respired by fresh air, is termed * ventilation,' 46 and the

45 For this reason candles, lamps, and gas change the composition of air almost in
the same way as respiration. In the burning of 1 kilogram of stearin candles, 50 cubic
metres of air are changed as by respiration— that is, 4 p.c. of carbonic acid will be
formed in this volume of air. The respiration of animals and exhalations from their skins,
and especially from the intestine and the excrements and the transformations taking place
in them, spoil the air to a still greater extent, because they introduce other volatile sub-
stances l>e>iile.^ carbonic anhydride into the air. At the same time that carbonic anhy-
dride is formed the amount of oxygen in the air decreases, and consequently the relative
amount of nitrogen increases, together with which there is noticed the appearance of mias-
mata which occur in but small quantity, but which are noticeable in passing from fresh
air into space full of such adulterated air. The researches of Schmidt and Leblanc and
others show that with 20'6 p.c. of oxygen (instead of 20'9 p.c.), when the diminution is due
to respiration, air already becomes noticeably heavy and unfit for respiration, and that the
heavy feeling experienced in such air increases with a lesser percentage of oxygen. It is
•difficult to remain for a few minutes in air containing 17'2 p.c. of oxygen. These obser-
vations were chiefly obtained by observations on the air of different mines, at different
depths below the surface. The air of theatres and buildings full of people also proves to
contain less oxygen ; it was once found that at the end of a theatrical representation the
air at the stalls contained 20'75 p.c. of oxygen, whilst the air at the upper part of the theatre
contained only 20'36 p.c. The amount of carbonic anhydride in the air may be taken
as a measure of its purity (Pettenkofer). When it reaches 1 p.c. it is very difficult for
human beings to remain long in such air, and it is necessary to set up a vigorous ventilation
for the removal of the adulterated air. In order to keep the air in dwellings in a uniformly
good state, it is necessary to introduce at least 10 cubic metres of fresh air per hour per
person. We saw that a man exhales about five-twelfths cubic metres of carbonic anhy-
dride per day. Accurate observations have shown that air containing one-tenth p.c. of
exhaled carbonic anhydride (and consequently also a corresponding amount of the other
substances evolved together with it) is not yet felt as spoilt ; and therefore the five-twelfth
cubic metres of carbonic anhydride should be diluted with 420 cubic metres of fresh air if
it be desired to keep not more than one-tenth p.c. (by volume) of carbonic anhydride
in the air. Hence a man requires 420 cubic metres of air per day, or 18 cubic metres per
hour. With the introduction of only 10 cubic metres of fresh air per person, the amount
of carbonic anhydride may reach one-fifth p.c., and the air will not then be of the
required freshness.

46 The ventilation of inhabited buildings is most necessary, and is even indispensable
in hospitals, schools, and similar buildings. In winter it is carried on by the so-called
calorifiers or stoves heating the air before it enters. The best kind of calorifiers in this
respect are those in which the fresh cold air is led through a series of channels heated by
the hot gases coming from a stove In ventilation, particularly during winter, care is taken
that the incoming air shall be moist, because in winter the amount of moisture in the
air is very small. Ventilation, besides introducing fresh air into a dwelling-place, must
also withdraw the air already spoilt by respiration and other causes — that is, it is neces-
sary to construct channels for the escape of the bad air, besides those for the introduction
of fresh air. In ordinary dwelling-places, where not many people are congregated, the
ventilation is conducted by natural means, in the heating by fires, through crevices,
windows, and various orifices in walls, doors, and windows. In mines, factories, and works
ventilation is of the greatest importance.

Animal vitality may still continue for a period of several minutes in air containing up
to 30 p.c. of carbonic anhydride, if the remaining 70 p.c. consist of ordinary air, but respi-
ration ceases after a certain time, and death may even ensue. The flame of a candle
is extinguished in an atmosphere containing from 5 to 6 p.c. of carbonic anhydride,

VOL. I. R

242 PRINCIPLES OF CHKMJSTRY

removal of impurities from the air is called ' disinfection.'47 The accu-
mulation of all kinds of impurities in the air of dwellings and cities i&
the reason why the air of mountains, forests, seas, and non-marshy
localities, covered with vegetation or snow, is distinguished for its fresh-
ness, and, in all respects, beneficial action.

but animal vitality can be sustained in it for a somewhat long time, although the
effect of such air is exceedingly painful to even the lower animals. There are mines in
which a lighted candle easily goes out from the excess of carbonic anhydride, but in which
the miners have to remain for a long time. The properties of air may be considerably
vitiated by the combustion of charcoal, wood, and similar substances in it, even when it
contains a comparatively small amount of carbonic anhydride. This doubtless depends on
the fact that certain gaseous substances are formed in the act of combustion (carbonic
oxide, acetylene, hydrocyanic acid, and others) which are positively injurious to breathe.
The action of charcoal fumes and smoke is based on this fact. The presence of 1 p.c. of
carbonic oxide is deadly even to cold-blooded animals. The air of explosive mines, where
explosions are made, is known to produce a state of insensibility resembling that produced
by charcoal fumes. Deep wells and vaults not .unfrequently contain similar substances,
and their atmosphere often causes suffocation. The atmospheres of such places cannot
be tested by lowering a lighted candle into it, as these poisonous gases would not extin-
guish the flame. This method only suffices to indicate the amount of carbonic anhydride.
If a candle keeps alight, it signifies that there is less than 5 p.c. of this gas. In doubtful
eases it is best to lower a dog or other animal into the air to be tested.

47 Different so-called disinfectants are capable of purifying the air, and of preventing
the injurious action of certain of its components by changing or destroying them. Dis-
infection is especially necessary in those places where a considerable amount of volatile
substances are evolved into the air, and where organic substances are decomposed ; for
instance, in hospitals, closets, &c. The numerous disinfectants are of the most varied
nature. They may be divided into the following chief categories : oxidising substances*
antiseptic substances, and absorbent substances. To the oxidising substances used for
disinfection belong chlorine, and various substances evolving it, because chlorine in the
presence of water oxidises the majority of organic substances. Further, to this class
belong the permanganates of the alkalis, as substances easily oxidising matters dissolved
in water ; these salts are not volatile like chlorine, and therefore act much more slowly,
and in a much more limited sphere. Antiseptic substances are those which convert
organic substances into such as are little prone to change, and prevent putrefaction and
fermentation. They most probably kill the germs of organisms occurring in miasmata.
The most important of these substances are creosote and phenol (carbolic acid), which
occur in tar, and which act in preserving smoked meat. Phenol is a substance little
soluble in water, volatile, oily, and having the characteristic smell of smoked objects. Its
action on animals in considerable quantities is injurious, but in small quantities, used in
the form of a weak solution, it prevents the change of animal matter. The smell of
privies, which depends on the change of excremental matter, may be easily removed by
means of chlorine or phenol. Salicylic acid, thymol, common tar, &c., are also substances
having the same property. They are used in special cases, but naturally not so generally.
Absorbent substances are of no less importance than the preceding two classes of disin-
fectants, inasmuch as they act regularly and are innocuous. They are such substances
as absorb the odoriferous gases and vapours emitted during putrefaction, which are
chiefly ammonia, sulphuretted hydrogen, and other volatile compounds. To this class
belong charcoal, certain salts of iron, gypsum, salts of magnesia, and such like substances,
as well as peat, mould, and clay. Their employment is profitable, not only for removing
the odour, but also in the radical destruction of miasmata. The questions both of disin-
fection and ventilation appertain to the most serious problems of common life and
hygiene. These questions are so vast that we are here able only to give a short outline
of their nature.

243

T

CHAPTER VI

THE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

IN the last chapter we saw that nitrogen does not immediately combine
with hydrogen, but that a mixture of these gases in the presence of
hydrochloric acid gas, HC1, forms ammonium chloride, NH4C1, on the
passage of a series of electric
sparks.1 In ammonium chlo-
ride, HC1 is combined with
NH:i, consequently N" with
H3 forms ammonia.2 Almost
all the nitrogenous sub-
stances of i)lants and ani-
mals evolve ammonia when
heated with an alkali. But
even without the presence
of an alkali the majority of

nitrogenous Substances, when FiG.J2.-The dry distillation of bones on a large scale.

decomposed or heated with a

limited supply of air, evolve

their nitrogen, if not entirely,

at all events partially, in

the form of ammonia. Thus,

when animal substances such

as skins, bones, flesh, hair, horns, ifcc., are heated without access

of air in iron retorts — or, as it is termed, are subjected to dry distil-

1 The ammonia in the air, water, and soil proceeds from the decomposition of the
nitrogenous substances of plants and animals, and also probably from the reduction of
nitrates. Ammonia is always formed in the rusting of iron. Its formation in this case
depends in all probability on the decomposition of water, and on the action of the hydro-
gen at the moment of its evolution on the nitric acid contained in the air (Cloez), or on
the formation of ammonium nitrite, which takes place under many circumstances. The
evolution of vapours of ammonia compounds is sometimes observed in the vicinity of
volcanoes.

2 If a silent discharge (as in the ozonisation of oxygen), or a series of electric
sparks (for instance, in a eudiometer), be passed through ammonia gas, it is decomposed

R2

The bones are heated in the vertical cylinders C (about
li metres high and 30 centimetres in diameter). The
products of distillation pass through the tubes T, into
the condenser B, and receiver F. When the distillation
is completed the trap H is opened, and the burnt bones
are loaded into trucks V. The roof M is then opened,
and a fresh quantity of bones charged into the cylinders.
The ammonia water is preserved, and goes to the pre-
paration of ammouiacal salts, as described in the follow-
ing drawing.

244 PRINCIPLES OF CHEMISTRY

lation, they decompose. A portion of the substances proceeding
from the decomposition remains in the retort and forms a carbonaceous
residue, whilst the other portion, in virtue of its volatility, escapes
through the tube leading from the retort. The vapours given off, on
cooling, form a liquid which separates into two layers ; the one, which
is oily, is composed of the so-called animal oils (oleum animate), the
other, an aqueous layer, contains a solution of ammonia salts. If this
solution be mixed with lime and heated, the lime takes up the elements
of carbonic acid from the ammonia salts, and ammonia is evolved as a
gas.3 In ancient times ammonia compounds were imported into Europe
from Egypt, where they were prepared from the soot obtained in the
employment of camels' dung as fuel in the locality of the temple of
Jupiter Ammon (in Lybia), and therefore the salt obtained was called
'sal-ammoniacale,' from which the name of ammonia is derived. Now
ammonia is exclusively obtained, on a large scale, either from the products
of the dry distillation of animal or vegetable refuse, from urine, or from
the ammoniacal liquors collected in the destructive distillation of coal

into nitrogen and hydrogen. This is a phenomenon of dissociation, as was explained in the
preceding chapter, p. 227. Therefore, a series of sparks do not totally decompose the
ammonia, but leave a certain portion undecomposed. One volume of nitrogen and three
volumes of hydrogen are obtained from two volumes of ammonia. The presence of free
ammonia — that is, ammonia not combined with acids — in a gas or aqueous solution may
be recognised by its characteristic smell. But many ammonia salts do not possess this
smell. However, on the addition of an alkali (for instance, caustic lime, potash, or soda),
they evolve ammonia gas, especially when heated. The presence of ammonia may be
made visible by introducing a substance moistened with strong hydrochloric acid into
its neighbourhood. A white cloud, or visible white vapour, then makes its appearance.
This depends on the fact that both ammonia and hydrochloric acid are volatile, and
on coming into contact with each other form solid sal-ammoniac, NH4C1, which forms a
cloud. This test is usually made by dipping a glass rod into hydrochloric acid, and*
holding it over the vessel from which the ammonia is evolved. With small amounts of
ammonia this test is, however, untrustworthy, as the white vapour is scarcely observable.
In this case it is best to take paper moistened with mercurous nitrate, HgNO3. This
paper turns black in the presence of ammonia, owing to the formation of a black com-
pound of ammonia with mercurous oxide. The smallest traces of ammonia, for instance,
in river water, may be discovered by means of the so-called Nessler's reagent, containing
a solution of mercuric chloride and potassium iodide, which forms a brown coloration
or precipitate with the smallest quantities of ammonia. Here it will be useful to give the
thermo-chemical data (in thousands of units of heat, according to Thomsen), or the
quantities of heat evolved in the formation of ammonia and its compounds in quantities
expressed by their formulae. Thus, for instance (N + H3) 26'7 indicates that 14 grams
of nitrogen in combining with 3 grams of hydrogen develop sufficient heat to raise the
temperature of 26'7 kilograms of water 1°. (NH5 + nH2O) 8'4 (heat of solution);
(NH3,nH20 + HCl,nH20)12-8; (N + H4 + Cl) 90'6 ; (NH3 + HC1) 41'9.

5 The same ammonia water is obtained, although in smaller quantities, in the
dry distillation of plants and of coal, which consists of the remains of fossil plants.
In all these cases the ammonia proceeds from the destruction of the complex nitrogenous
substances occurring in plants and animals. The ammonia salts employed in practice
are prepared by this method.

COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 245

for the preparation of coal gas. This ammoniacal liquor is placed in a
retort with lime and heated ; the ammonia is then evolved together
with steam.4 In practice, only a small amount of ammonia is used in
a free state — that is, in an aqueous solution ; the greater portion of it
is converted into different salts having technical uses, especially sal-
ammoniac, NH4C1, and ammonia sulphate, (NH4)2SO4. They are saline
substances which are formed because ammonia, NH3, combines with all
acids, HX, forming ammonia salts, NH4X. Sal-ammoniac, NH4C1, is

I'n,. -i:5.— Method of abstracting ammonia, on a large scale, from ammonia water obtained at gas
\\orks by the dry distillation of coal, or by the fermentation of urine, &c. This water is mixed
with lime and poured into the boiler C", and from thence into C' and C consecutively. The last
boiln- is heated directly over a furnace, and therefore no ammonia remains in solution after the
liquid liits been boiled io it. The liquid is therefore then thrown away. The ammonia vapour and
steiiiii pass from the boiler C, through the tube T, into the boiler C', and then into C", so that the
solution in C' becomes stronger than that in C, and still stronger in C". The boilers are furnished
with stirrers A, A', and A" to prevent the lime settling. From C" the ammonia and steam pass
through the tube T" into worm condensers surrounded with cold water, from thence into the
Wouite's bottle P, where the solution of ammonia is collected, and from whence the still uncon-
densed ammonia vapour is led into the flat vessel R, containing acid which retains traces of
ammoniii.

a compound of ammonia with hydrochloric acid. It is prepared by
passing the vapours of ammonia and water, evolved, as above described,
from ammoniacal liquor, into an aqueous solution of hydrochloric acid,
and on evaporating the solution sal-ammoniac is obtained in the form
of soluble crystals 5 resembling common salt in appearance and pro-

4 The technical methods for the preparation of ammonia water, and for the extraction
of ammonia from it, are to a certain extent explained in the figures accompanying the
text.

5 Usually these crystals are sublimed by heating them in crucibles or pots, when the
vapours of sal-ammoniac condense on the cold covers as a crust, in which form it cornea
into the market.

246 • PRINCIPLES OF CHEMISTRY

perties. Ammonia may be very easily prepared from this sal-ammoniac,
NH4C1, as from any other ammoniacal salt, by heating it with lime.
Calcium hydroxide, CaH2O2, as an alkali takes up the acid and sets
free the ammonia, forming calcium chloride, according to the equation
2NH4Cl + CaH2O2 = 2H2O + CaCl2 + 2NH3. In this reaction the
ammonia, as a gas, is evolved.6

It must be observed that all the complex nitrogenous substances of
plants, animals, and soils are decomposed when heated with an excess
of sulphuric acid, the whole of their nitrogen being converted into
ammonium sulphate, from which it may be liberated by treatment with
an excess of alkali. This reaction is so complete that it forms the
basis of Kjeldahl's method for estimating the amount of nitrogen in its
compounds.

Ammonia is a colourless gas, resembling those with which we are
already acquainted in its outward appearance but clearly distinguish-
able from any other gas by its very characteristic and strong smell. It
irritates the eyes, and it is positively impossible to inhale it. Animals
die in it. Its density, referred to hydrogen, is 8-5 ; hence it is lighter
than air. It belongs to the class of gases which are easily liquefied.7

6 On a small scale ammonia may be prepared in a glass flask by mixing equal parts
by weight of slacked lime and finely-powdered sal-ammoniac, the neck of the flask
being connected with an arrangement for drying the gas obtained. In this instance
neither calcium chloride nor sulphuric acid can be used for drying the gas, because
they absorb ammonia, and therefore solid caustic potash, which is capable of retaining
the water, is employed. The gas conducting tube leading from the desiccating apparatus
is introduced into a mercury bath, if dry gaseous ammonia be required, because water
cannot be employed in collecting ammonia gas. Ammonia was first obtained in this dry
state by Priestley, and its composition was investigated by Berthollet at the end of the
last century. Oxide of lead mixed with sal-ammoniac (Isambert) evolves ammonia
with still greater ease than lime. The cause and process of the decomposition is almost
the same, 2PbO -t- 2NH4C1 = Pb2OCl2 + H2O + 2NH3. Lead oxychloride is (probably
formed.

7 This is evident from the fact that its absolute boiling point lies about + 180° (Chap
II. Note 29). Consequently, it may be liquefied by pressure alone at the ordinary, and even
much higher, temperatures. The latent heat of evaporation of 17 parts by weight
of ammonia equals 4400 units of heat, and therefore liquid ammonia may be employed
for the production of cold. Strong aqueous solutions of ammonia, which in parting with
their ammonia act in a similar manner, are not unfrequently employed for this purpose^
Suppose a saturated solution of ammonia to be held in a closed vessel furnished with
a receiver. If the ammoniacal solution be heated, the ammonia, with a small quantity
of water, will pass off from the solution, and in accumulating in the apparatus will
produce a considerable pressure, and will therefore liquefy in the cooler portions of the
receiver. Hence liquid ammonia will be obtained in the receiver. The heating of the
vessel containing the aqueous solution of ammonia is then stopped. After having been
heated it contains only water, or a solution poor in ammonia. When once it begins to cool,
the ammonia vapours commence dissolving in it, the space becomes rarefied, and a rapid
vaporisation of the liquefied ammonia left in the receiver takes place. In evaporating in
the receiver it will cause the temperature in it to fall considerably, and will itself pass into

C().MI'()fNI)S (>!• MTIJOCKN AV1TII HYIHiOGKN AM> OXYGEN 247

Faraday employed the following method for liquefying ammonia.
Ammonia, when passed over dry silver chloride, AgCl, is absorbed by it
to a considerable extent, especially at low temperatures.8 The solid

the aqueous solution. In the end, the same ammoniacal solution as was originally taken
is re-obtained. Thus, in this case, on heating the vessel the pressure increases of itself,
and on cooling it diminishes, so that here heat directly replaces mechanical work. This
is the principle of the simplest forms of Carre's ice-making machines, shown in fig. 44.
C is a vessel made of boiler plates into which the saturated solution of ammonia is
j toured ; m is a tube conducting the ammonia vapour to the receiver A. All parts of
tlx1 apparatus should be hermetically joined together, and should be able to withstand a
pressure reaching ten atmospheres. The apparatus should be freed from air, which

-

=5*WeEST.jT.—

FIG. 44. — Carre's apparatus Described in text.

would otherwise hinder the liquefaction of the ammonia. The process is carried on as
follows : — The apparatus is first so inclined that any liquid remaining in A may flow
into C. The vessel C is then placed upon a stove F, and heated until the thermometer t
indicates a temperature of 130° C. During this time the ammonia has been expelled from
C, and has liquefied in A. In order to facilitate the liquefaction, the receiver A should
be immersed in a tank of water R (see the left-hand drawing in fig. 44). After about
half an hour, when it may be supposed that the ammonia has been expelled, the fire is
removed from under C, and this is now immersed in the tank of water R. The apparatus
is represented in this position in the right-hand drawing of fig. 44. Then the liquefied
ammonia evaporates, and passes over into the water in C. This causes the temperature
of A to fall considerably. The substance to be refrigerated is placed in a vessel G, in the
•cylindrical space inside the receiver A. The refrigeration is also kept on for about half
«,n hour, and with an apparatus of ordinary dimensions (containing about two litres
of ammonia solution), five kilograms of ice are produced by the consumption of one
kilogram of coal. In industrial works more complicatecktypes of Carre's machines are
employed.

8 Below 15° (according to Isambert), the compound AgCl,8NH3 is formed, and above
20° the compound 2AgCl,8NH5. The tension of the ammonia evolved from the latter

compound Au< 'l.:'N 1 1 .. thus obtained is introduced into ;( bent tube
itiu'. I'M. whose open end c i-tlirn fused up. The compound is then
slightly heated at it, and the ammonia comes <>\Y. OWJULC t" the easy
dissociation of tin1 compound. The other end of the tube is immersed
in a free/ini; mixture. The pressure of tlie --as

comiii" oil', comltiiied with the lo\v temperature

.

^\ at one end of the tube, causes the ammonia

c<^ ^"^ e\-olved to condense into a liquid, in \vliieli form

r;,, ;- •• i ,- it collects at the cold end of the tube. If the

heating be stopped, the silver chloride a^fain

absorbs t he ammonia. In this manner, one tube

" may serve for repeated experiments. Ammonia

may also be liquefied bv tlie ordinarv methods

that is. bv means of pumping dry ammonia -ras into a refrigerated
space. Jj<|uHifd ammonia is a colourless and very mobile liquid.'*
whose specific gravity at 0" is ()•(]:} (}•]. Andivett'). At the temperature
(about — 7" ) u'iven b\- a mixture of liquid carbonic anhydride and ether,
liquid ammonia crystallises, and in this form its odour is feeble, because
at so lo\v a temperature its vapour tension is very inconsiderable1. The
boil in-- point (at a pressure of TOO mm.) of li(|iiid ammonia is about — '\'2 \
Hence tliis temperature may be obtained at the ordinary pressure by
tlie e \'apo rat ion of li«|iielied ammonin.

Ammonia. eontaininL!1. as it doe>. much h\'dro--(Mi. is fti/Hiri/i »J
i'i,ni i»is/>f,/, ; it does not. however, burn regularly, and sometimes not
at all. in the ordinary atmospherie air. In ]>ui'e oxygen it burns with
a greenish -yellow llame,1" forminu' water, \\'hilst the nitrogen set tree

. , I ' •

pl'e- I . t ahi Hit -JII ; Cdlisei jllellt I V. at

the at l i|-< n re. v. hil-t at 1< >Wer telllpel

l.\ tl e . )hi , ctunptuiml. CMII^JI

.•" i • • i i. • \ In l,f nh-erved.

'' 'I In • " « if a n i moil ia ma v he acci miplishetl \\ ill milt an increase <•(' pressure

• ' ' • .; • a Mint i\e pi i We r FI .I1 eifjillfv ft -rill- a prnhlem which ha-, to II

extent In i .etlhv the Kr, |,cl, , M |-i 1 1 e, • r T. I .

The , • la a ill I,.- ell Ilia \ In etTei let] l.\ t he iliil ni phi t illlllll.

p, , ecket! heaker ,,j ;il»<lll I. Ill' lit I'. • Cllpacil \ . A -as < .lllrtilr: tllhe

• I ,,,:,' 1 n • I p] , . . . . i] ill 1 In • iitplei HI- -nlllt inll nf

! : tin i • i |e, cent platinum -pira 1 :- placet! in

(.lie 1 iea hi ; t • ! I |e pre-e|ice i i) t 1 ] e p 1 i t i 1 1 1 1 ! 1 1 i - i i X 1 1 I 1 ~et 1 a 1 1 1 1 1 1 1 1 r 11 . U 1 1 I I - 1

the plal • .1 m.-iv incamle cent. The iinnmiiiia i- then heatetl. ami

t: • • • • I eil I., th. . Illtit ''. The , •.;..•. II, a it hllhhle- ,,11 fr..]|| the illllllllllliil

int.. r.nit.ict v.ith lli. . -.en! plal inn. Tin i I'. .!!• .\\ i-i| I,;, a . ertain c. > ilimj efTect,

CO MPOUNDS OF NITROGEN WITH II V I > I :< H ; K N A N I > < >X YGEN 249

gives its oxygen compound — that is, oxide of nitrogen. The decompo-
sition of ammonia into hydrogen and nitrogen not only takes place at a
red heat and under the action of electric sparks, but also by means of
many oxidising substances ; for instance, by passing ammonia through
a tube containing red-hot copper oxide. The water thus formed may
be collected by substances absorbing it, and the quantity of nitrogen,
may be measured in a gaseous form, and thus the composition of
ammonia determined. In this manner it is very easy to prove that
ammonia contains 3 parts by weight of hydrogen to 14 parts by
weight of nitrogen ; and, by volume, 3 vols. of hydrogen and 1 vol. of
nitrogen form 2 vols. of ammonia. 1 1

Ammonia is capable of combining with a number of substances,
forming, like water, substances of various degrees of stability. It is
more soluble than any other known gas, both in water and in many
aqueous solutions. We have already seen, in the first chapter, that
one volume of water, at the ordinary temperature, dissolves about
700 vols. of ammonia gas. The great solubility of ammonia enables it
to be always kept ready for use in the form of an aqueous solution 12
which is commercially known as spirits of hartshorn. Ammonia water

owing to the combustion ceasing, but after a short interval this is, however, renewed, so
that one feeble explosion follows after another. During the period of oxidation without
explosion, white vapours of ammonium nitrite and red-brown vapours of oxides of nitrogen
make their appearance, while during the explosion there is complete combustion, and
consequently water and nitrogen are formed.

11 This may be verified by their densities. Nitrogen is 14 times denser than hydro-
gen, and ammonia is 8£ times. If 3 volumes of hydrogen with 1 volume of nitrogen gave
4 volumes of ammonia, then these 4 volumes would weigh 17 times more than 1 volume
of hydrogen ; consequently, 1 volume of ammonia would weigh 4 £ times heavier than the
same volume of hydrogen. But as these 4 volumes only give 2 volumes of ammonia,
therefore they weigh 8£ times heavier than hydrogen, which we find to be actually the
case.

12 Aqueous solutions of ammonia are lighter than water, and at 15°, taking water at
4° = 10000, their specific gravity, as dependent on p, or the percentage amount (by
weight) of ammonia, is given by the expressions = 9992 — 42'5p 4- 0'21p2 ; for instance, with
10 p.c. s = 9587. If the temperature be =t°, but not less than 10° or above 20°, then
the expression (15 — t) (l'5 + 0'14_p) must be added to the formula for the specific gravity.
Solutions containing above 24 p.c. have not been sufficiently investigated in respect to
the variation of their specific gravity. It is, however, easy to obtain more concentrated
solutions, and at 0° solutions approaching NH3,H.,O (48'6 p.c. NH3) in their composition,
and of sp. gr. 0'85, may be prepared. But such solutions give up the bulk of their
ammonia at the ordinary temperature, so that more than 24 p.c. NHS is rarely contained
in solution. Ammoniacal solutions containing a considerable amount of ammonia give
ice-like crystals at temperatures far below 0° (for instance, an 8 p.c. solution at —14°, the
strongest solutions at — 48°) which seem to contain ammonia. The whole of the ammonia
may be expelled from a solution by heating, even at a comparatively low temperature ;
therefore, in heating aqueous solutions containing ammonia a very strong solution of
ammonia is obtained in the distillate. Alcohol, ether, and many other liquids are also
capable of dissolving ammonia. Solutions of ammonia, when exposed to the atmosphere,

250

PRINCIPLES OF CHEMISTRY

is continually evolving ammoniacal vapour, and so has the characteristic
smell of ammonia itself. It is a very characteristic and important fact
that ammonia has an alkaline reaction, and colours litmus paper blue,
just like caustic potash or lime ; it is therefore sometimes called caustic
ammonia (volatile alkali). Acids may be saturated by ammonia water
or gas in exactly the same way as by any other alkali. In so doing,
ammonia combines directly with acids, and this forms the most essential
chemical reaction of this substance. If sulphuric, nitric, acetic, or any

give off a part of their ammonia, in accordance with the laws of the solution of gases in
liquids already considered by us. But the ammoniacal solutions at the same time
absorb carbonic anhydride from the air, and ammonium carbonate remains in the solu-
tion.

Solutions of ammonia are required both in the laboratory and in practice, and have
therefore to be frequently prepared. For this purpose the arrangement shown in fig. 46
is employed in the laboratory. In works the same arrangement is used, only on a larger
scale (with earthenware or metallic vessels). The gas is prepared in the retort, from

FIG. 46. — Apparatus for preparing solutions of ammonia.

whence it is led into the two-necked globe A, and then through a series of Woulfe's
bottles, B, C, D, E. The impurities spurting over collect in A, and the gas is dissolved
in B, but the solution soon becomes saturated, and a purer (washed) ammonia passes
over into the following vessels, in which only a pure solution is obtained. The bent
funnel tube in the retort preserves the apparatus from the possibility both of the pres-
sure of the gas evolved in it becoming too great (when the gas escapes through it into
the air), and also from the pressure incidentally falling too low (for instance, owing to a
cooling effect, or from the reaction stopping). If this takes place, the air passes into the
retort, otherwise the liquid from B would be drawn into A. The safety tubes in each
Woulfe's bottle, open at both ends, and immersed in the liquid, serve for the same purpose.
Without them, in case of an accidental stoppage in the evolution of so soluble a gas as
ammonia, the solution would be sucked from one vessel to another — for instance from E
into D, &c. In order to clearly see the necessity of the safety tubes in a gas apparatus,
it must be considered that the gaseous pressure in the interior of the arrangement must
exceed the atmospheric pressure by the height of the sum of the columns of liquid
through which the gas has to pass.

COMPOUNDS OF NITROGEN WITH II V I > I ; < >< ; K N .\ N 1 > n \ YUEN 25 1

other acid be brought into contact with ammonia it absorbs it, and in
so doing evolves a large amount of heat and forms a compound having
all the properties of a salt. Thus, for example, sulphuric acid, H0SO4,
in absorbing ammonia, forms (on evaporating the solution) two salts,
according to the relative quantities of ammonia and acid. One salt is
formed from NH3 + H2SO4, and consequently has the composition
NHSS04, and the other is formed from 2NH3 + H2SO4, and its composi-
tion is therefore N2H8SO4. The former has an acid reaction and the
latter a neutral reaction, and they are called respectively acid ammonium
sulphate (ammonium hydrogen sulphate), and normal ammonium sul-
phate, or simply ammonium sulphate. The same takes place in the
action of all other acids ; but certain of them are able to form normal
ammonium salts only, whilst others give both acid and normal ammonium
salts. This depends on the nature of the acid and not on the ammonia,
as we shall afterwards see. Ammonium salts are very similar in appear-
ance and in many of their properties to metallic salts ; for instance,
sodium chloride, or table salt, resembles sal-ammoniac, or ammonium
chloride, not only in its outward appearance but even in crystalline
form, in its property of giving precipitates with silver salts, in its solu-
bility in water, and in its evolving hydrochloric acid when heated with
sulphuric acid — in a word, a most perfect analogy is to be remarked
in an entire series of reactions. An analogy in composition is seen
if sal-ammoniac, NH4C1, be contrasted with table salt, NaCl ; and the
ammonium hydrogen sulphate, NH4HSO4, with the sodium hydrogen
sulphate, NaHSO4; or ammonium nitrate, NH4NO3, with sodium nitrate,
NaN03.13 It is seen, on comparing the above compounds, that the part
which sodium takes in the sodium salts is played in ammonium salts by
a group NH4, which is called ammonium. If table salt, as the product
obtained by the action of caustic soda or sodium hydroxide 011 hydro-

15 The analogy between the ammonium and sodium salts might seem to be destroyed
by the fact that the latter are formed from the alkali or oxide and an acid, with the sepa-
ration of water, whilst the ammonium salts are directly formed from ammonia and an
acid, without the separation of water ; but the analogy is restored if we compare soda to
ammonia water, and liken caustic soda to a compound of ammonia with water. Then the
very preparation of ammonia salts from such a hydrate of ammonia will completely re-
semble the preparation of sodium salts from soda. We may cite as an example the action
of hydrochloric acid on both substances.

NaHO + HC1 H20 + NaCl

Sodium hydroxide Hydrochloric acid Water Table salt.

NH4HO + HC1 = H20 + NH4C1

Ammonium hydroxide Hydrochloric acid Water . Sal-ammoniac.

Just as in soda the hydroxyl or aqueous radicle OH is replaced by chlorine, so it is in
ammonia hydrate.

252 PRINCIPLES OF CHEMISTRY

chloric acid, be called * sodium chloride,' then sal-ammoniac, as the
product obtained from caustic ammonia or ammonium hydroxide, is-
called ' ammonium chloride.'

The hypothesis that ammoniacal salts correspond with a complex
metal ammonium bears the name of the ammonium theory. It was
enunciated by the famous Swedish chemist Berzelius after the proposi-
tion made by Ampere. The analogy admitted between ammonium and
metals is probable, owing to the fact that mercury is able to form an
amalgam with ammonium similar to that which it forms with sodium or
many other metals. The only difference between ammonium amalgam
and sodium amalgam consists in the instability of the ammonium, which
easily decomposes into ammonia and hydrogen.14 Ammonium amalgam
may be prepared from sodium amalgam. If the latter be shaken up
with a strong solution of sal-ammoniac, the mercury swells up violently
And loses its mobility while preserving its metallic appearance. In so
doing, the mercury dissolves ammonium — that is, the sodium in the
mercury is replaced by the ammonium, and replaces it in the sal-
ammoniac, forming sodium chloride, NH4C1 + HgNa = NaCl + HgNH4.
Naturally, ammonium amalgam does not entirely prove the existence-
of ammonium itself in a separate state ; but it shows the possibility of
this substance existing, and, what is more important, its analogy with
the metals, because only metals dissolve in mercury, forming compounds,
termed * amalgams,' without altering its metallic form.15 Ammonium
amalgam crystallises in cubes, three times heavier than water ; it is-
only stable in the cold, and particularly at very low temperatures. It
begins to decompose at the ordinary temperature, evolving ammonia
and hydrogen in the proportion of two volumes of ammonia and one

14 Weyl, by working at considerable pressures, obtained the compound NH3K, and
then ammonium itself — by the action of sal-ammoniac on this substance— in the form of
a blue liquid, but his researches require confirmation. Ammonium amalgam was origi-
nally obtained in exactly the same way as sodium amalgam (Davy) ; namely, a piece of
sal-ammoniac was taken, and moistened with water (in order to render it a conductor
of electricity). A cavity was made in it, into which mercury was poured, and it
was laid on a sheet of platinum connected with the positive pole of a galvanic battery,
while the negative pole was put into connection with the mercury. On passing a current
the mercury increased considerably in volume, and became plastic, while preserving its
metallic appearance, just as would be the case were the sal-ammoniac replaced by a,
lump of a sodium salt or of many other metals. In the analogous decomposition of
common metallic salts, the metal contained in a given salt separates out at the negative
pole, immersed in mercury, by which the metal is dissolved. A similar phenomenon in
observed in the case of sal-ammoniac ; the elements of ammonium, NH4, in this case are
also collected in the mercury, and are retained by it for a certain time.

15 It would seem that hydrogen is also capable of forming an amalgam resembling the
amalgam of ammonium. If an amalgam of zinc be shaken up with an aqueous solution of
platinum chloride, without access of air, then a spongy mass is formed which easily
decomposes, with the evolution of hydrogen.

COMI'olNDS OF NITROGEN AVJTH HYDROGEN AND OXYGEN 253

volume of hydrogen, NH4 = NH3 + H. By the action of water,
ammonium amalgam gives hydrogen and ammonia water, just as
.sodium amalgam gives hydrogen and sodium hydroxide ; and therefore,
in accordance with the ammonium theory, ammonia water must be
looked on as containing ammonium hydroxide, NH,OH,1G just as an
aqueous solution of sodium hydroxide contains NaOH. The ammonium
hydroxide, like ammonium itself, is an unstable substance, which easily
dissociates, and which can only exist in a free state at low tempera-
tures.17 Ordinary solutions of ammonia must be looked on as the
products of the dissociation of this hydroxide, inasmuch as NH4OH
= NH3 + H2O. The liability to a greater or less decomposition is
proper, in the same degree, to substances containing NH3 as to
substances containing water.

All ammoniacal salts decompose at a red heat into ammonia and an
acid, which, on cooling in contact with each other, re-combine together.
If the acid be non-volatile, the ammoniacal salt, when heated, evolves
the ammonia, leaving the non-volatile acid behind ; if the acid be vola-
tile, then, on heating, both the acid and ammonia volatilise together,
and on cooling re-combine into the salt which originally served for the
formation of their vapours.18

Ammonia is not only capable of combining with acids, but also
Avith many salts, as was seen from its forming definite compounds,
AgCl,3NH3 and 2AgCl,3KH3, with silver chloride. So, also, am-
monia is absorbed by the chlorine, iodine, and bromine compounds
of many metals, and in so doing evolves heat. Certain of these com-

16 We saw above that the solubility of ammonia in water at low temperatures
attains to the molecular ratio NH3 + H2O, in which these substances are contained in
caustic ammonia, and perhaps it may be possible at exceedingly low temperatures to
obtain ammonium hydroxide, NH4HO, in a solid form. By regarding solutions as disso-
ciated definite compounds, we should see a confirmation of this view in the property
shown by ammonia of being extremely soluble in water, and in so doing of approaching
to the limit NH4HO.

17 In confirmation of the truth of this conclusion we may cite the remarkable fact
that there exist, in a free state and as comparatively stable compounds, a series of alka-
line hydroxides, NR4HO, which are perfectly analogous to ammonium hydroxide, and
present a striking resemblance to it and to sodium hydroxide, with the only difference
that the hydrogen in NH4HO is replaced by complex groups, E = CH3, C2H5, &c., for
instance N(CH3)4HO.

18 The fact that ammoniacal salts are decomposed when ignited, and not simply
sublimed, may be proved by a direct experiment with sal-ammoniac, NH4C1, which in a
state of vapour is decomposed into ammonia, NH3, and hydrochloric acid, HC1, as will
be explained in the following chapter. The readiness with which ammonium salts decom-
pose is seen from the fact that a solution of ammonium oxalate is decomposed with the
evolution of ammonia even at — 1°. Dilute solutions of ammonium salts, when boiled,
give aqueous vapour, having an alkaline reaction, owing to the presence of free ammonia
given off from the salt.

254 PRINCIPLES OF CHKMISTRY

pounds part with their ammonia even when left exposed to the air, but
others only do so at a red heat ; many give up their ammonia when
dissolved, whilst others dissolve without decomposition, and when
evaporated separate from their solutions unchanged. All these facts
only indicate that ammoriiacal, like aqueous, compounds dissociate with
greater or lesser facility.19 Certain metallic oxides also absorb ammonia
and are dissolved in ammonia water. Such are, for instance, the oxides
of zinc, nickel, copper, and many others ; the majority of such compounds
are unstable. The property of ammonia of combining with the oxides
of certain metals explains its action on certain metals.20 For this
reason, copper vessels are not suitable for holding liquids containing
ammonia. Iron is not acted on by such liquids.

The relation of ammonia and water to salts and other substances
becomes especially clear in the case when the salt is capable of combining
with both ammonia and water. Take, for example, copper sulphate,
CuSO4. As we saw in Chapter I., it gives with water blue crystals,
CuSO4,5H2O ; but it also absorbs ammonia in the same molecular
proportion, forming a blue substance, CuSO4,5NH3, and therefore the
ammonia combining with salts may be termed ammonia of crystallisation.

Such are the reactions of combination proper to ammonia. Let us
now turn our attention to the reactions of substitution proper to this
substance. If ammonia be passed through a heated tube containing
metallic potassium, then hydrogen is evolved, and a compound is
obtained containing ammonia in which one atom of hydrogen is re-
placed by an atom of potassium, NH2K (according to the equation
NH3 + K = NH2K + H). This body is termed potassamide. We shall
afterwards see that iodine and chlorine are also capable of directly
displacing hydrogen from ammonia, and of replacing it ; we shall also
see that in hydrocyanic acid, NCH, carbon has replaced hydrogen.
Hence the hydrogen of ammonia may be replaced in many ways by
different elements. If in so doing NH2 remains, the resultant sub-
stances are called amides, if only NH imides, and those in which the
whole of the hydrogen is displaced are termed nitrides. It may be
imagined that the albuminous substances— that is, the complex organic

19 Isambert studied the dissociation of ammoniacal compounds, as \\c have seen in
Note 8, and he showed that at low temperatures many salts are able to combine with a
still greater amount of ammonia, which proves an entire analogy with aqueous coin-
pounds ; and as in this case it is easy to isolate the definite compounds, and as the least
possible tension of ammonia is greater than that of water, therefore the ammonim al
compounds present a great and peculiar interest, both as a means for explaining the
nature of aqueous solutions, and as a confirmation of the conception of the formation of
definite compounds in them ; for these reasons we shall frequently turn to these com-
pounds in the further exposition of this work.

2° Chapter V. Note 2.

COMPOUNDS OF NITIUKJKN WITH HYDROGEN AND n.XYGEN 255

substances which we have already had occasion to mention — are the
amide compounds corresponding with saccharine substances. The most
important point to be remarked is that in the action of different
substances on ammonia it is the hydrogen that is substituted ; the
reactions proceed at the expense of the hydrogen and not of the
nitrogen, which remains in the resultant compound, so to say, un-
touched. The same is to observed in the action of various substances
on water. In the majority of cases the reactions of water consist in
the hydrogen being evolved, and in its being replaced by different
elements. The same takes place, as we have seen, in acids, in which
the hydrogen is easily displaced by metals. This chemical mobility of
hydrogen is distinctly connected with the great lightness of the atoms
of this element.

In chemical practice21 ammonia is often employed, not only for
saturating acids, but also for accomplishing reactions of double
decomposition with salts, and especially in separating insoluble basic
hydroxides from soluble salts. Let MHO stand for an insoluble basic
hydroxide, and HX for an acid. The salt formed by them will have a
composition MHO + XH - H20=MX. If aqueous ammonia, NH4OH,
be added to a solution of this salt, then the ammonia will change

n In practice, the applications of ammonia are very varied. The use of ammonia as a
stimulant, in the forms of the so-called ' smelling salts ' or of spirits of hartshorn, in cases
of faintness, &c., is known to everyone. The volatile carbonate of ammonium, or a
mixture of an ammonium salt with an alkali, is also employed for this purpose. Ammonia
also produces a well-known stimulating effect when rubbed on the skin, for which reason
it is sometimes employed for outward applications. Thus, for instance, the well-known
volatile salve is prepared from any liquid oil shaken up with a solution of ammonia. A
portion of the oil is thus transformed into a soapy substance. The solubility of greasy
substances in ammonia, which proceeds from the formation both of emulsions and soaps,
explains its use in extracting grease spots. It is also employed as an external application
for stings from insects, and for bites from poisonous snakes, and in general in medicine.
It is also remarkable that in cases of drunkenness a few drops of ammonia in water taken
internally rapidly renders a person sober. A large quantity of ammonia is used in
dyeing, either for the solution of certain dyes -for example, carmine — or for changing
the tints of others, or else for neutralising the action of acids. It is also employed in
the manufacture of artificial pearls. For this purpose the small scales of a peculiar
small fish are mixed with ammonia, and the liquid so obtained is blown into small hollow
glass beads, shaped like pearls.

In nature and the arts, however, the ammonium salts, not free ammonia, are most
frequently employed. In this form a portion of that nitrogen which is necessary for the
formation of albuminous substances is supplied to plants. Owing to this, a large
quantity of ammonium sulphate is now employed as a fertilising substance. But the same
part may be fulfilled by nitre, or by animal refuse, which in putrefying gives ammonia.
The nitre of the soil is formed from these ammonia-giving substances, because nitrogen
in combination with hydrogen is easily converted into oxygen compounds of nitrogen, as
we shall afterwards see. For this reason, if an ammoniacal (hydrogen) compound be
introduced into the soil in the spring, it will be converted into a nitrate (oxygen salt)
in the summer.

places \\-ith ilif metal M an<l thus form the insoluble basie hydroxide,
or. a- it i- -aid. ijive a precipitate. The mechanism of this double
< le.-oiii.p< >-it it 111 is a> f« ill« iws :

Thu-. for instance, if aqueous ammonia is added to a solution of a
-alt of aluminium. Ai, then hydrous alumina is separated out as a
colourless uvlat inous precipitate. '--

In order to ^ra^p the relation between ammonia and the oxygen
compounds of nitrogen it is necessary to reci i:_;ni.-e the general /<t/r of
fiiihxtitnt'ioii, applicable to all eases of substitution between elements,-'*
and therefore -howing what may be the cases of substitution between
ox\xfen and hvdrogen as component parts of water. The law of sub-
stitution may be deduced from mechanical principles if the molecule be
conceived as a system of elementary atoms occurring in a certain
eliemieal and mechanical equilibrium. \*>\' likening the molecule to a
,-vstem of bodies in a state of motion — for instance, to the sum total of
tin- sun. planets, and satellites, occurring in conditions of mobile equi-
librium— then we should expect the action of one part, in this system,
to be etjual and opposite to the other, according *:o Newton's third law
of mechanics. 1 fence, being i^iven a molecule of a compound, for
instance. !!.,(). N I I:t, .Nad, IK'l. Arc., then its every two parts must
in a chemical respect represent something alike in force and capacities,
and therefore </••/•// tiro part* into //•//!/•// <i mo/ccn^- <>f « compound ///"//
/,. ,l',,-'i'l'-<l di'i <-<ij,<thl< of ri'iiliH'luy eachothvr. In order that the api>li-
eat ion of i he la\\p should become clear, it is cvidenl that amoii'f com-

\ • ' • 'i lij i • liyl rates ttii'iii pt'ciuijir c

I pi in- iif an nia aili

• • • Mi' imi nf a t'n-sli iiiiant it y
l|. Mini .if tl... !M- l,

:h-,

• it fliLii-iiir. a- wi- -li.ill afteruanls mmv iull\ l.-iii

: , ,. n. III. -n ll." ivacti.m l,y \\hicli sudi an ^xrha !,-,• i, a.T, .ni| .1 i-hr,! pr,,-
nl.-'itin .All CI, \('l ; HCI, M. tliiit two Mil-planers. All siinl clili.riiH-,

litiil ..I .me i-li-iuriil. .1. li\ an. .tin r. A. ilm-s n..| aluay^ pro-

. r;. r.i ' i..-.-..i.ipli -linl l,\ lli.' jn-ti.-u nf III.' f n-.- .-li-iu.-iMs. Lilt tin- snl.sti-

. • • i -i -i ' i • ' ' • • 1 1 . "in' I'm- aiml IHT. t'.irniN t In- nni-1 i-iniiii t-a-r "f (t.xidat ion

! '1 tin' 1 i .'...; nl i-l it nt i"! i. I ha \ .• i n \ ic\\ t \\,- • nl ,-,) il lit imi

.i'-,'- • • • ill n t! "I ,, „;•,,. n i-i. i-iit. if n I'ru of

• :. c..Jii|i..iiii<l In- K-II..UII.

COMI'ofNDS ()!•' MTKor.KN WITH HYDROGEN AND <>XY<;KN 257

pounds the most stable should be chosen. We will, therefore, take
water as the most stable compound of hydrogen and oxygen,24 in order
to see the cases of substitution between hydrogen and oxygen ; but
for the sake of clearness we will start by taking the very stable mole-
cule of hydrochloric acid, HC1, as one which can be divided into H
and Cl only. According to the law of substitution, if these elements
are able to form a molecule, and a stable one, then they are able to
replace each other. And, indeed, we shall afterwards see that in a
number of instances a substitution between hydrogen and chlorine
conversely takes place. Given RH, then RC1 is possible, because
HC1 exists and is stable. The molecule of water, H20, may be
divided in two ways, because it contains 3 atoms : into H and (HO)
on the one hand, and into H2 and O on the other. Consequently, being
given RH, its substitution products will be R(HO) according to the
first form, and R2O according to the second ; being given RH2, its
corresponding substitution products will be RH(OH), R(OH)2, RO,
(RH).2O, £c. The group (OH) is the same hydroxyl or aqueous
radicle which we have already mentioned in the third chapter as a
component part of hydroxides and alkalis — for instance, Na(OH),
Ca(OH)2, Ac. It is evident, judging from HC1, that (OH) can be
substituted by Cl, because both are replaceable by H; and this is of
common occurrence in chemistry, because metallic chlorides — for in-
stance, NaCl and NH4C1 — correspond with hydroxides of the alkalis
Xa(OH) or NH4(OH). In hydrocarbons — for instance, C2H6 — the
hydrogen is replaceable by chlorine and by hydroxyl. Thus common
alcohol is C2H6, in which one atom of H is replaced by (OH) ; that
is, C2H5(OH). It is evident that the replacement of hydrogen by
hydroxyl essentially forms the phenomenon of oxidation, because RH
gives R(OH), or RHO. Hydrogen peroxide may in this sense be
regarded as water in which the hydrogen is replaced by hydroxyl ;
H(OH) gives (OH)2 or H2O2. For this reason chlorine, as we shall
afterwards see, exhibits in its reactions much analogy to hydrogen
peroxide, which may be termed free hydroxyl. The other form of
substitution — namely, that of O in the place of H2 — is also a common
chemical phenomenon. Thus common alcohol, C.2H6O, or C2H5(OH),
when oxidising in the air, gives, as every one knows, acetic acid,
C2H4O2, or C2H30(OH), in which H2 is replaced by O.

2* If hydrogen peroxide be taken as a starting point, then still higher forms of oxi-
dation than those corresponding with water should be looked for. They should possess
the properties of hydrogen peroxide, especially that of parting with their oxygen with
extreme ease (even by contact). Such compounds are known. Pernitric, persulphuric,
and similar acids present these properties, as we shall see in describing them.

VOL. I. S

I -: of thi< work \ve shall have occasion to ha\e

recoi;r>e io ihe law of sith.stit ution for explaining many chemical

] ill''!.' i HIS.

\Ve \\ill intv, apply ihe-e conceptions to ammoni;! or nin-o^eii
hvdride iii order to see it- relation to ihc oxygen conqiounds of
nitr'>_ ... i' i- e\'idenl t!i;it maiiv su o^taisccs mav pi'oceed trom
ammonia. Nil . or a1 iiieous ammonia , N I I (( ' ' I I ). from thcsulistitution
of tin i: hvdrou'en hv h vdroxvl, or of ||., hv OXVLTCII. A nd >uch i- the

( 1 ) ( MI-- atom of II in X I I ., i- sul»t it utcd l.y ( < > I I ). and N H .,(( )I I ) i
]>rodu<-t-d. Si;di ,-i su osta nci', still containing nutcli livdro^tMi. shoul
ha\i' ntany of the ]>ro[><Tt it-s of ammonia. li is known under tin
iiaiiif o] •/</••...' i/lii ni • i" ,' •' ' ;iiid. in f;ii-t. it i- cajiaolc. lil<c ammonia

'I i- i i i : i ul livtlroxyhuiiiiH' with liyili'nclilnric acid lu-,- thr c..i!i]ii,-it inn
Ml i « i -.-... :- ' ;• .. . ' , | ,,.,,,] 1,, ].,,_, i, i,,

i-:ili.-.l,.:!i\ ' •• . in A-hicli ,-IIM- tin- liy.lr...L'i'!i lilu-i •• ; t'l i • \^,>- liy.lim-iil. n- :«.•:•! l.y
• , • • . . • • i t r i i • : u • i i ] —

rll IK i

• ->• tin- trir Mi-id i- (lc<.. sidiscil. nni dircctlv into nit rniri'ii. Inn into

!:•;.-. sM.-: N'l :'.]) H('l Nll,nC] iiml in numy i.llu-r I-M-.-S. Ac.-.'i'.linuT M
I ' . ' ire ctf :',() pail^ nt' ct hyl nitrjtc. I'JId n. ami Jll i.urt ^ nf ;i

h\ IIP '-n -nlphiilc. the -i i!n! inn i< r\ a puiMti'd. ami a lar-jr aniMiint i >1 -ai-

: • . . - i-n mi ili.' !i\iln.\> 1-

: ' . : i • : • i!\.-.| in anli\ ilr.ai-

' •• : • ' . : - ' , fl ilc. Inch |.n !. - an\ ain-

ntii u. . \fti-r I-MIICI II!MII th.-

' . 'i : I • .• ,•• al li! i.'iU .

A I i : I ,-,,', I i, ill of

arid, Till Ill.-taiiri ! a KM -i,llll)l(' "t! UatiT like tin-

Ill. : i nt"d I'MI- atiMili, r, I : • . , , ; eete.l i luil liy

i lie alkali
• , eiivmn

•' ' : i".' ' : , , • . ::,,„,,,,!

ill i . . :;\ll-o

'.'II' II. . .. , ,,.!,, [,,,, |,v

• ' • I". I

i ! h,.v\ laniine iii .I.MI, , u si. hi

coMrnrxns OK NITROGKN AVJTII IIYDIH^JKN AND OXYGEN 259

of giving salts with acids ; for example, with hydrochloric acid,
NH3(OH)C1— which is a substance corresponding with sal-ammoniac,
in which one atom of hydrogen is replaced by hydroxyl. (2) The other
extreme case of substitution given by ammonium hydroxide, NH4(OH),
is \\lit-n the whole of the hydrogen of the ammonium is replaced by
oxygen ; and as ammonium contains 4 atoms of hydrogen, the highest
oxygen compound should be NO2(OH), or NHO3, as we find to be
really the case, because NHO;, is nitric acid, or the highest degree
of the oxidation of nitrogen.26 If instead of the two extreme aspects
of substitution we take an intermediate one, then we obtain one
of the intermediate oxygen compounds of nitrogen. For instance,
N(OH)3 is orthonitrous acid,27 with which corresponds nitrous acid,
NO(OH), or NHO2, equal to N(OH)3 - H2O, and nitrous anhydride
N2O3 = 2N(OH)3 - 3H2O. Thus nitrogen gives a series of oxygen

tion, like ammonia, precipitates basic hydrates, and it deoxidises the oxides of copper,
silver, and other metals. Hydroxylamine is obtained, in a great number of cases, for
instance, by the action of tin on dilute nitric acid, and also by the action of zinc on ethyl
nitrate and dilute hydrochloric acid, &c. The relation between hydroxylamine,
N 1 1 ,(OH), and nitrous acid, NO(OH), which is so clear in the sense of the law of substi-
tutions, becomes a reality in those cases when reducing agents act on salts of nitrous
acid. Thus Raschig (1888) proposed the following method for the preparation of the
hydroxylamine sulphate. A mixture of strong solutions of potassium nitrite, KNO2, and
hydroxide, KHO, in molecular proportions, is prepared and cooled. An excess of sul-
phurous anhydride is then passed into the mixture, and the solution boiled for a long
time. A mixture of the sulphates of potassium and hydroxylamine is thus obtained :
KNO2 + KHO + 2SO2 + 2H2O = NH2(OH),H2SO4 + K2SO4. The salts may be separated
from each other by crystallisation.

With respect to substances intermediate between NH3 and the oxides of nitrogen, we
must turn our attention to hyponitrous acid, NHO, and amidogen, which are mentioned
in Note 67.

-t; Nitric acid corresponds with the anhydride N2O5, which will afterwards be described,
but which must be regarded as the highest saline oxide of nitrogen, just as Na2O (and the
hydroxide NaHO) in the case of sodium, although sodium forms a peroxide possessing the
property of parting with its oxygen with the same ease as hydrogen peroxide, if not on
heating, at all events in reactions — for instance, with acids. So also nitric acid has its
corresponding peroxide, which may be called pernitric acid. Its composition is not well
known — probably NHO.j — so that its corresponding anhydride would be N2O7. It is
formed by the action of a silent discharge on a mixture of nitrogen and oxygen, so that
a portion of its oxygen is in a state similar to that in ozone. The instability of this sub-
stance (obtained by Hautefeuille, Chappuis, and Berthelot), which easily splits up with
the formation of nitric peroxide, and its resemblance to persulphuric acid, which we shall
afterwards describe, will permit our passing over the consideration of the little that is
further known concerning it.

27 Phosphorus, as we shall afterwards find, gives the hydride PH3, corresponding
with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous
arid, just as phosphoric acid is to nitric acid ; but phosphoric (or, better, orthophosphoric)
acid, PH3O.j, is able to lose water and give pyro- and meta-phosphoric acids. The latter
is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really
meta-nitric acid. So also nitrous acid, HNOo, is meta-nitrous (anhydrous) acid, and then
the ortho-acid is NH3O3 = N(OH)3.

s2

260 PRINCIPLES OF CHEMISTRY

compounds, which we will proceed to describe. Only let us first show
that the passage of ammonia into the oxygen compounds of nitrogen
up to nitric acid, as well as the converse preparation of ammonia
from nitric acid, are reactions which proceed directly and easily under
many circumstances. In nature the matter is complicated by a
number of influences and circumstances, but in the law the rela-
tions are presented in their simplest aspect. The bond between this
simplicity of laws and this complexity of phenomena forms the essence
of a scientific understanding of things.

It is easy to prove the possibility of the oxidation of ammonia into
nitric acid by passing a mixture of ammonia and air over heated
spongy platinum. This causes the oxidation of the ammonia, nitric
acid being formed, which partially combines with the excess of
ammonia.

The converse passage of nitric acid into ammonia is accomplished
by the action of hydrogen at the moment of its evolution.28 Thus
metallic aluminium, evolving hydrogen from caustic soda, is able to
completely convert nitric acid added to the mixture (really as a salt,
because the alkali gives a salt with the nitric acid) into ammonia,
NH03 + 8H=NH3 + 3H.2O.

The compounds of nitrogen with oxygen present an excellent
example of the law of multiple proportions, because they contain for 14
parts by weight of nitrogen 8, 16, 24, 32, and 40 parts, respectively, by
weight of oxygen. The composition of these compounds is as follows : —

N2O, nitrous oxide ; hydrate NHO.
N2O2, nitric oxide, NO.
N2O3, nitrous anhydride ; hydrate NHO2.
N2O4, peroxide of nitrogen, NO2.
N2O<5, nitric anhydride ; hydrate NH03.

Of these compounds,29 nitrous and nitric oxides, peroxide of nitrogen,
and nitric acid, NHO3, are characterised as being the most stable. The
lower oxides, when coining into contact with the higher, may give the
intermediate forms ; for instance, NO and NO2 form N2O3, and the
intermediate oxides may, in splitting up, give a higher and lower oxide.

28 The formation of ammonia is remarked in many cases of oxidation by means of
nitric acid. This substance is even formed in the action of nitric aci4 on tin, especially
if dilute acid be employed in the cold. A still more considerable amount of ammonia is
obtained if, in the action of nitric acid, there are conditions directly tending to the evolu-
tion of hydrogen, which then reduces the acid to ammonia ; for instance, in the action
of zinc on a mixture of nitric and sulphuric acids.

29 According to the determinations of Favre, Thomsen, and more especially of Berthelot,
on thermochemical data, it follows that in the formation of such quantities of the oxides
of nitrogen as express their formulae, if gaseou^nitrogen and oxygen be taken as the ,

COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 261

So N2O4 gives N2O3 and N2O3, or, in the presence of water, their
hydrates.

We have already seen that, under certain conditions, nitrogen
combines with oxygen, and we know that ammonia may be oxidised.
Tn these cases various oxidation products of nitrogen are formed, but
in the presence of water arid an excess of oxygen they always give
nitric acid. Nitric acid, as corresponding with the highest oxide, is
able, in deoxidising, to give the lower oxides, and for this reason we
will begin with it.

Nitric acid, NHO3, is likewise known as aqua fortis. In a free
state it is only met with in nature in small quantities, in the air and
rain-water after storms ; but even in the atmosphere nitric acid does
not long remain free, but combines with ammonia, traces of which are
always found in air. On falling on the soil and into running water,
Ac., the nitric acid everywhere comes into contact with bases (or their
carbonates), which easily act on it, and therefore it is converted into the
nitrates of these bases. Ammonia and other compounds of nitrogen,
if oxidised in the soil, are always in the presence of bases, and there-
fore also give salts of nitric acid, and not the free acid itself. Hence
nitric acid is always met with in the form of salts in nature. These
salts are called nitres. This name is derived from the Latin sal nitri.
The potassium salt KNO3 is common nitre, and the sodium salt NaNO3
Chili saltpetre, or cubic nitre. Nitres are formed when a nitrogenous
substance is slowly oxidised in the presence of an alkali by means
of the oxygen of the atmosphere. In nature there are very fre-
quent instances of such oxidation. For this reason, certain soils and

starting points, and if the compounds formed be also gaseous, the following amounts
of heat, expressed in thousands of heat units, are absorbed (hence a minus sign) : —

N20 N202 N,03 N204 N205

-21 -43 -22 -5 -1

-22 +21 +17 + 4

The difference is given in the lower line. For example, if N.,, or 28 grams of nitrogen,
combine with O — that is, with 16 grams of oxygen — then 21000 units of heat are absorbed,
or sufficient heat is assimilated to heat 21000 grams of water through 1°. Naturally,
direct observations are impossible in this case ; but if charcoal, phosphorus, or similar
substances, are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed
in both cases, then the difference (more heat will be evolved in burning in nitrous oxide)
gives the figures required. If, then, N.,O2, by combining with O.,, gives N.,,O4, then, as is
seen from the table, heat should be developed, namely 88000 units of heat, or NO + O =
19000 units of heat. The differences given in the table show that the maximum absorp-
tion of heat corresponds with nitric oxide, and that the higher oxides are formed from it
with evolution of heat. If liquid nitric acid, NHO3, were decomposed into N + O3 + H,
then 41000 heat units would be required ; that is, an evolution of heat takes place in its
formation from the gases. It should be observed that the formation of ammonia, NH5,
from the gases N + H3 evolves 12'2 thousand heat units.

rubhi>h heap- l'ir instance, lime rubbish (in tin- presence of a base

1 • ' . ' ;i . 1 1 a IIKIIV or less considerable a ii K MI n t of nit re. ( Mu-
ff the>e nitres tin- sodium nitrate is extracted from the earth in
lai'u'e quantities in ('hill, \\here it \\a- probably formed li\" the oxida-
tion < >f animal refuse. Thi- kind of nitre is employed in practice for
the manufacture of nitric acid and the other oxygen compounds of
nitro-en. Nitric arid is obtained from Chili *<i/tji>-frv by heating it
with > -a! i>]< a r'i<- nfiil. The hydrogen of the sulphuric acid replaces the
sodium in t lie n it re. 1 he sulphuric a eid then lorm> either an acid -alt .
Nal 1S< ».. or a norina] salt. Xa ,S( >,. while nitric acid is formed from the
nitre and i- volatilised. The decomposition is expressed 1 >v the equations :
(1) NaN< »,— II, S( ), = H N( ).,-u NallS* ),. if the arid salt he formed,
and (L() L'.\aN< ).. + J I,S< ),=:Na,S< ), • L' 1 1 N ( * . if the normal sodium
sulphate i> formed. \\'ith an excess of >ulphuric acid, and at a
moderate heat, and at the commencement of the reaction, the
decomposition proceeds according io the lirst etjuation : and <>n
furiher heating \\itli a >utlicient amount of nitre, according to the
second, because the acid .-alt NalIS(), itself acts like an acid (its
hydrogen bcin-- replaceable as in acids), according to the e<|iiation
N;i N < '., -f Nal IS( ) ,= Na,S( ) , + 1 1 N( ).,.

1 he sidpliurie acid, as it is said, here displaces the nitric acid from
it - compound with the base. This not unfre<[tiently gives rise to the
supposition that sulphuric1 acid has a particularly high degree of alhnity
or ener^v as compared with nitric acid, but. as we shall afterwards
see, the idea of a relathe allinity in acids and in bases is. in many
instances, untrustworthy; it should not be had recourse to so long
as ii be po»ible to explain a phenomenon without its introduc-
tion, inasmuch as the decree of atlinily cannot be measured. 1 he
aetion of the -ulphurie acid can be explained bv the fact that the
intrie aeii] formed is volatile. Nitric acid alone, ot all the sub-
stanee- ta]<in^r part in the reaction, i^ capable of being converted into
\apour ia< the temperature emplo\'ed). and it alone volatilises : the
remainmir sub-tances are not volatile, or. more -trictlv speaking, are
\eryslighily \datile. I f we stijtpose thai the sulphuric acid is able to
.-et 1 fee e\en onl\" a >mall <|iiantl(v of nitrie acid tl'om its salt, it IN
ullicieiil for explaining e\ cnttially the complete decomposit i(»n of the
nitre 1»\ the suljihuric acid, bei-au.se, onee the nitric acid is separated,
it i -. on heating, eon \erted into \apour, and pa--es trom the sphere
of action of the remaining substances: the free .sulphuric acid then
a 'ja i n et - free ;i } Ye- 1 1 -mall < | u a 1 1 1 1 1 v of 1 1 it fie acid, a nd so on until the
nitric acid i- completely di-pku-ed from the nitre. It is c\ ident that,
according to this explanation, it is necessary that the sulphuric acid

COMPOUNDS «>K MTKot.KN WITH II YIHIOCKN AND OXYGEN 263

be in excess (although but small) to the end of the reaction.
According to the equation expressing the reaction, it is required that
there should be 98 parts of sulphuric acid to 85 parts of sodium
nitrate ; but if these quantities be taken, the nitric acid is not entirely
displiu •('(! by the sulphuric acid. It is necessary that an excess of the
latter should be taken ; generally, 80 parts of nitre are taken to
98 parts of sulphuric acid, and therefore a portion of the sulphuric
acid remains in a free state to the end of the reaction. Thus, in
the reaction of sulphuric acid on nitre there is formed a n on- volatile
salt of sulphuric acid, which remains, together with an excess of this
acid, in the distilling apparatus, and nitric acid, which is converted

Fin. 17.— Method of preparing nitric acid on a large scale. A cast-iron retort, C, is fixed into the
furnace, and heated by the fire, B. The flame and products of combustion are at first lei along
the flue. M(in unler to heat the receivers), and afterwards into L. The retort is charged with Chili
.-altpctre and sulphuric acid, and the cover is luted on with clay and gypsum. A clay tube, a, is
fixed into the nock of the retort (in order to prevent the nitric acid from corroding the ca>t iron ),
and a bent glass tube, D, is luted on to it. This tube carries the vapours into a series of earthen-
ware receivers, K. Nitric acid mixed with sulphuric acid collects in the first. The purest nitric
acid is procured from the second, whilst that which condenses in the third receiver contains hydro-
chloric acid, and that in the fourth nitrous oxide. Water is poured into the last receiver in order
to condense the residual vapours.

into vapour, and may be condensed, because it is a liquid and volatile
substance. On a small scale, this reaction may be carried on in a glass
retort with a glass condenser. On a large scale, in chemical works, the
process is exactly similar, only iron retorts are employed for holding
the mixture of nitre and sulphuric acid, and earthenware three-necked
bottles are used instead of a condenser, :i° as shown in fig. 47.

50 It must be observed that sulphuric acid, at least when undiluted ((50° Baume*),
corrodes cast iron with difficulty, so that the acid may be heated in cast-iron boilers.

264 PRINCIPLES OF CHEMISTRY

Nitric acid so obtained always contains water. It is extremely
difficult to deprive it of all the admixed water without destroying a
portion of the acid itself and partially converting it into lower oxides,
because without the presence of an excess of water it is very unstable.
When rapidly distilled a portion is decomposed, and there are obtained
free oxygen and lower oxides of nitrogen, which, together with the
water, remain in solution with the nitric acid. Therefore it is neces-
sary to work with great care in order to obtain a pure hydrate of nitric
acid, HNO3, and especially to mix the nitric acid obtained from nitre,
as above described, with sulphuric acid, which takes up the water, and
to distil it at the lowest possible temperature — that is, by placing the
retort holding the mixture in a water or oil bath and carefully heating

Nevertheless, both sulphuric and nitric acids evince a certain action on cast iron, and
therefore the acid obtained will contain traces of iron. In practice sodium nitrate (Chili
saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to
take potassium nitrate, because it is purer and does not froth up so much as sodium
nitrate when heated with sulphuric acid. In the action of an excess of sulphuric acid on
nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of
nitrogen, which are dissolved in the nitric acid. A portion of the sulphuric acid itself is
also carried over as spray by the vapours of the nitric acid. Hence sulphuric acid occurs
as an impurity in commercial nitric acid. A certain amount of hydrochloric acid will
also be found to be present in it, because sodium chloride is generally found as an im-
purity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. Com-
mercial acid further contains a considerable excess of water above that necessary for the
formation of the hydrate, because water is first poured into the earthenware vessels
employed for condensing the nitric acid in order to facilitate its cooling and condensation.
Further, the acid of composition HNOj decomposes with great ease, with the evolution
of oxides of nitrogen. Thus the commercial acid contains a great number of impurities.
Generally its specific gravity is 1'33 (36° Baume), and it contains 53 p.c. of nitric acid.
The acid employed in medicine and in the laboratory contains one-third of nitric acid and
two-thirds of water, and its specific gravity is T2. The commercial acid is often purified
in the following manner : — Lead nitrate is first added to the acid because it forms non-
volatile and almost insoluble (precipitated) substances with the free sulphuric and
hydrochloric acids, and liberates nitric acid in so doing, according to the equations
Pb(NO3)o + 2HCl = PbCl2 + 2NHO5 and Pb(NOs).2 + H.,,SO4 = PbSO4 + 2NHO3. . Potas-
sium chromate is then added to the impure nitric acid, by which means oxygen is
liberated from the chromic acid, and this oxygen, at the moment of its evolution,
oxidises the lower oxides of nitrogen and converts them into nitric acid. A pure nitric
acid, containing no impurities other than water, may be then obtained by distilling
the acid, manipulated as above described, with care, and particularly if only the middle
portions of the distillate are collected. Such acid should give no precipitate, either
with a solution of barium chloride (a precipitate shows the presence of sulphuric acid)
or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric
acid), nor should it, after being diluted with water, give a coloration with starch con-
taining potassium iodide (a coloration shows the admixture of other oxides of nitrogen).
The oxides of nitrogen may be most easily removed from impure nitric acid by heat-
ing for a certain time with a small quantity of pure charcoal. By the action of nitric
acid on the charcoal carbonic anhydride is evolved, which carries off the NO, NOa,
and other volatile substances. On redistilling, pure acid is obtained. The oxides of
nitrogen occurring iu solution may also be removed by passing air through the nitric
acid.

OF NITI;O<;KN WITH HYI>RO<;FA AND OXYMK.V 265

it. The first portion of the nitric acid thus distilled boils at 86°, has a
specific gravity at 15° of 1*526, and solidifies at -50° ; it is very
unstable at higher temperatures. This is the normal hydrate, HNO3,
which corresponds with the salts, NMO3, of nitric acid. When diluted
with water nitric acid presents a higher boiling point, not only as
compared with that of the nitric acid itself, but also with that of water ;
so that, if very dilute nitric acid be distilled, the first portions passing
over will consist of almost pure water, until the boiling point in the
vapours reaches 121°. At this temperature a compound of nitric acid
with water, containing about 70 p.c. of nitric acid,31 distils over ; its
specific gravity at 15° = 1-521. If the solution contain less than
25 p.c. of water, then, the specific gravity of the solution being above
1*44, HNO3 evaporates off and fumes in the air, forming the above
hydrate, whose vapour tension is less than that of water. Such solu-
tions form fuming nitric acid. On distilling it gives monohydrated
acid,32 HNO3 ; it is a hydrate boiling at 121°, so that it is obtained
from both weak and strong solutions. Fuming nitric acid, under the
action not only of organic substances, but even of heat, loses a portion

31 Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling
point (see Chapter I. Note 60) for nitric acid was the compound 2HNO3,3H.2O, but Eoscoe
showed that its composition changes with a variation of the pressure and temperature
under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution
of constant boiling point contains 68'6 p.c., and at one-tenth atmosphere 66'8 p.c.
Judging from what has been said concerning solutions of hydrochloric acid, and from the
variation of specific gravity, I think that the comparatively large decrease of the
tensions of the vapours depends on the formation of a hydrate, NHO5,2H2O ( = 63'6 p.c.).
Such a hydrate may be expressed by N(HO)5, that is as NH4(HO) in which all the
equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will
then be the temperature of the decomposition of this hydrate.

Besides which, judging by the variation of the specific gravity (see my work cited in
Chapter I. Note 29), at least one more hydrate, NHOS,5H2O (41'2 p.c. HNO3), must be
acknowledged. Starting from water (p = 0) to this hydrate, the specific gravities of the
solutions at 15° is well expressed by s = 9992 + 57'4p + 0. 16p*, if water = 10000 at 4°.
For example, when p = 30 p.c., s = 11860. For more concentrated solutions, at least, the
above-mentioned hydrate, HNO3,2H.>O, must be taken, up to which the specific gravity
s = 9570 + 84-18p — 0'240p*; but perhaps (the results of observations of the specific gravity
of the solutions are not in sufficient agreement to make a decision) the hydrate
HNO5,8H.jO should be recognised, as is indicated by many nitrates (Al, Mg, Co., &c.),
which crystallise with this amount of water of crystallisation. From HNOs^H^O to
HNO3 the specific gravity of the solutions (at 15°) s = 10652 + W08p-0-I60p*. The
]>fiitahydrated hydrate is recognised by Berthelot on the basis of the thermo-chemical
data for solutions of nitric acid, because on approaching to this composition there is a
rapid change in the amount of heat evolved by mixing nitric acid with water. This
hydrate solidifies at about - 19°. One would think that a more detailed study of the
reactions of hydrated nitric acid would show the existence of change in the process and
rapidity of reaction in approaching these hydrates.

52 The normal hydrate HNO3, corresponding with the ordinary salts, may be termed
the monohydrated acid, because the anhydride N.,O5 with water forms this normal nitric-
acid. In thi> M-HSI; the hydrate HNO3,2HoO is the pentahydrated acid.

of its oxygen, forming lower oxides of nitrogen, which impart a r>-d-
lii'ini'ii i-iifn//,' in it ; -;;1 the pure arid is Colourless.

Nitric acid, as an tn-i<l /tt/ilrittt\ enters into i-eaetions of doul>le
der( imp' >-it ion \\ith bases, Italic hydrates (alkalis), and with salts. In
all the.-se ca.-cs a >alt of nitric aeid is obtained. An alkali and nitric
acid :gi\e water and a salt : so, also, a basu- oxide \\ith nitric acid
gives a salt and water; for instance, K 1 1< ) -f 1 1 N< );j= K N< ).,+ 1 1 ._,< >.
or with lime. CaO + i)llN(.):, = Ca(iN()3)., + lI2O. .Many of these salts
arc termed nitres/51 The composition of the ordinary salts of nitric-
acid may he expressed by the general formula M(N< ).,)„, where 31
indicates a metal replacing the hydrogen in one or several (K) equiva-
lents of nitric acid. We shall lind afterwards that the atoms M of
metals ai'e equivalent to one (^K, Na, ALC) atom of hvdrogen, or two
(I'a. .Mg. 1)U), or three (Al. In), or, in general, n atoms of hydrogen.
Th< .SV///N of 1/i/rif <iri<l are especially characterised by being all
xoluhli- In trutt-i-*' From the property common to all these salts

"'•" For technical and laboratory purposes recourse is frequently had to rat j n in /in/
nit fir iirii] — that is, the normal nitric acid, i I N( )-. containing lower oxides of nitrogen in
xJution. This acid is prepared hy decomposing nitre \vith half it- weight of strong sul-
phuric acid, or by distilling nitric acid \\ith an excess of sulphuric acid. The normal
nitric acid is iirst obtained, but it ]'artially decomposes, and gives the lower oxidation
products of nitrogen, which are dissolved by the nitric acid, to which they impart its
u>ual pale-brown or reddish colour. This acid fumes in the air. from which it attracts
moi-ture. forming a less volatile hydrate. If carbonic anhydride be pas-rd through the
red-brown fuming nitric acid tor a long period of time, especially if with the aid ot
n, i.iii rate heat, it expels all the lower oxido. and leaves a colourless acid free from these
oxide>. It i- necessary, in the preparation of the red acid, that the receivers should lie
kept quite cool, because it i- only \\hen cold that nitric acid is able to dissolve a large
proportion of the oxide- of nitrogen. The strong red fuming acid lias a specific gravity
r.'ii; at lit i . and ha-- a suffocating Mnell of the oxides of nitrogen. When the red acid is
mixed with water it turns green and then of a bluish colour, and \\ ith an excess of water
ultimateh becomes colourless. This i- owing to the fact that the oxides of nitrogen in
the pre-ence of water and nitric acid are changed, and give coloured solutions.

'I'hr act ion of red fumin nitric acid lor a mixture with sulphuric acid

Thus iron bee, ,1 ne-, CON e red uith a coating of oxides, and becomes insoluble in acids ; it
becomes, as I- -aid. passive. Tim- chromic acid land pota--mm dichromatel gives oxide
, ,i chromium in tin- red acid —that is. it is deoxidised. This i- owing to the presence of
the lower oxides of nit i-o-en. which are capable of being oxidised that i-. of passing into
„,.;,! hfe the lirjher oxides. Hut. generally, the acti ...... f fuming nitric acid, both

red and eoloiirle.- -. , . po v, ei'fu ! 1 \ oxidising.

• Ihdro-en i- not e\olved in the action of nitric acid (especially strong! on metals.
,.\en uith tho-e metal- which e \ol\e hydrogen under the action of other acid-. 'I'll is is
|iecaii-i the li \drogei i at t he i ..... no it ol i t s sepa ra t ion reduces t he nitric acid, with forma-

. ( 'ertain ba-ic alt o| nit ric acid. IK .u ever i for e\a mple. t he Ija^ic salt ol bi -mu tin.
are , iisolnble 111 wat ei\ v, 1 1 Ut. on the ot her i land, a 11 the normal -alt- are soluble, and

- . itll , ,eept loiial phenomenon a! ...... •_' acid-, becaii-e all the ordinar\ acid- form

COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 267

of entering into double decompositions, and owing to the volatility
of nitric acid, they, like cubic nitre, evolve nitric acid when heated
with sulphuric acid. They all, like nitric acid itself, are capable of
evolving oxygen when heated, and consequently of acting like oxidising
substances, and therefore, for instance, deflagrate with ignited carbon,
the carbon burning at the expense of the oxygen of the salt and forming
gaseous products of combustion.36

Nitric acid also enters into double decompositions with a number
of hydrocarbons not in any way possessing alkaline characters and not
reacting with other acids. Under these circumstances, the nitric acid
gives water and a new substance termed a nitro-compound. The
chemical character of the nitro-compound is the same as that of the
original substance ; for example, if an indifferent substance be taken,
then the nitro-coinpouiid obtained from it will also be indifferent ;
if an acid be taken, then an acid is obtained also. Benzene,
C6H6, for instance, acts according to the equation C6H6 + HNO3
=H2O + C6H5NO2. Nitrobenzene is produced. The substance taken,
CGHG, is a liquid hydrocarbon having a faint tarry smell, boiling at 80°,
and lighter than water ; by the action of nitric acid nitrobenzene is
obtained, which is a substance boiling at about 210°, heavier than
water, and having an almond-like odour ; it is employed in large
quantities for the preparation of aniline and aniline dyes.37 As
they contain both combustible elements (hydrogen and carbon), as

lead, &c., for hydrochloric acid the salts of silver, &c., are insoluble in water. How-
ever, the normal salts of acetic and certain other acids are all soluble.

111 Ammonium nitrate, NH4NO3, is easily obtained by adding a solution of am-
monia or of ammonium carbonate to nitric acid until it becomes neutral. On evapo-
rating this solution crystals of the salt are formed which contain no water of crystallisation.
It crystallises in prisms like those formed by common nitre, and has a refreshing taste ;
100 parts of water at t dissolve 54 + 0'61£ parts by weight of the salt. It is soluble in
alcohol, melts at 160°, and is decomposed at about 180°, forming water and nitrous oxide,
NH4NO3 = 2H.>O + N.>O. If ammonium nitrate be mixed with sulphuric acid, and the
mixture be heated at about the boiling point of water, then nitric acid is evolved, and
ammonium hydrogen sulphate remains in solution ; but if the mixture be heated rapidly
to 1()0°, then nitrous oxide is evolved. In the first case the sulphuric acid takes up
ammonia, and in the second place water. Ammonium nitrate is employed in practice for
the artificial production of cold, because in dissolving in water it lowers the temperature
very considerably. For this purpose it is best to take equal parts by weight of the salt
and water. The salt must first be reduced to a powder and then rapidly stirred up in
the water, when the temperature will fall from + 15° to — 10°, so that the water freezes.

Ammonium nitrate absorbs ammonia, with which it forms unstable compounds
resembling compounds containing water of crystallisation. At — 10° NH4NO3,2NH3 is
formed : it is a liquid of sp. gr. 1'50, which loses all its ammonia under the influence of heat.
At -f 28° NH4NO3,NH5 is formed : it is a solid which easily parts with its ammonia when
heated, especially in solution.

•~7 The action of nitric acid on cellulose, C6H10O5, is similar. This substance, which
forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in

268 PRINCIPLES OF CHEMISTRY

well as oxygen in unstable combination with nitrogen, in the form
of the radicle NO2 of nitric acid, the nitre-compounds, when ignited or

common writing-paper, and in flax, &c. ; under the action of nitric acid it forms water and
nitrocellulose, which, although it has the same appearance as the cotton originally taken,
differs from it entirely in properties. It explodes when struck, bursts into flame very
easily under the action of sparks, and acts like gunpowder, whence its name of pyroxy-
lin, or gun-cotton. The composition of gun-cotton is C6H7N3OU = C6H10O5 -f 8NHO3
— 3HoO. The proportion of the group NO2 in nitrocellulose may be decreased by limiting
the action of the nitric acid, and a compound is obtained which burns without explosion,
although it is capable of bursting into flame. This substance when dissolved in a mix-
ture of alcohol and ether is called collodion. The solution when poured on to any
surface loses all the ether and alcohol by evaporation, and leaves an amorphous mass in
the form of a transparent membrane insoluble in water. A solution of collodion is em-
ployed in medicine for covering wounds, and in wet-plate photography for giving on glass
an even coat of a substance into which the various reagents employed in the process are
introduced.

The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose,
and the other nitro-compounds, of burning with an explosion depends on the reasons
in virtue of which a mixture of nitre and charcoal deflagrates and explodes ; in both
cases the elements of the nitric acid occurring in the compound are decomposed,
the oxygen in burning unites with the carbon, and the nitrogen is set free ; thus a very
large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed
from the solid substances originally taken. These gases occupy an incomparably larger
volume than the original substance, and therefore produce a powerful pressure and
explosion. It is evident that in exploding with the development of heat (that is, in
decomposing, not with the absorption of energy, as is generally the case, but with the
evolution of energy) the nitro-compounds form stores of energy which are easily set free,
and that consequently their elements occur in a state of particularly energetic move-
ment, which is especially strong in the group NO2 ; this group is common to all nitro-
compounds, and all the oxygen compounds of nitrogen are unstable, easily decom-
posable, and (Note 29) absorb heat in their formation. On the other hand, the nitro-
compounds are instructive as an example and proof of the fact that the elements and
groups forming compounds are united in definite order in the molecules of a com-
pound. A blow, concussion, or rise of temperature is necessary to bring the com-
bustible elements C and H into the most intimate contact with NO2, and to distribute
the elements in a new order in new compounds.

As regards the composition of the nitro-compounds, it will be seen that the hydrogen
of a given substance is replaced by the complex group NO., of the nitric acid. The same
is observed in the passage of alkalis into nitrates, so that the reactions of substitution of
nitric acid — that is, the formation of salts and nitro-compounds — may be expressed in
the following manner. In these cases the hydrogen is replaced by the so-called radicle
of nitric acid NOo, as is evident from the following table : —

1 Caustic potash . . . KHO.

I Nitre K(NO2)O.

/Hydrate of lime . . CaH2O2.

1 Calcium nitrate . . . Ca(NO2)2O2.

(Glycerin C3H5H5O3.

I Nitroglycerin . . . C3H5(NO2)3O3.

(Phenol C6H5OH.

I Picric acid .... CeH2(NO2)3OH, &c.

The difference between the salts formed by nitric acid and the nitro-compounds con-
sists in the fact that nitric acid is very easily separated from the salts of nitric acid by

CCXMI'Ol'NDS OF NITROGEN WITH II YI»K< xiKN AND ()XV(iK.V 269

even struck, decompose with an explosion, owing to the pressure of the
vapours and gases formed — free nitrogen, carbonic anhydride, and
aqueous vapour. In the explosion of iiitro-compoimds much heat is
evolved, as in the combustion of gunpowder or detonating gas, and in
this case the force of explosion in a closed space is great, because from
a solid or liquid nitro-compound occupying a small space there proceed
vapours and gases whose elasticity is great not only from the small
space in which they are formed, but owing to the high temperature
corresponding to the combustion of the nitro-compound.38

The combustion of nitro-compounds, as well as that which nitrates
bring about (in gunpowder), originates in the weakness of the bond
which holds together the oxygen and nitrogen in nitric acid itself, as
well as in all the oxygen compounds of nitrogen. If the vapour of
nitric acid is passed through an even moderately heated glass tube, the
formation of dark-brown fumes of the lower oxides of nitrogen and the
separation of free oxygen may be observed — 2NH03=H2O + 2NO2 + O.
The decomposition is complete at a white heat — that is, nitrogen is
formed, 2NH03=H2O + N2 -f 05. Hence it is easily understood
that nitric acid may part with its oxygen to a number of substances
capable of being oxidised.39 It is consequently an oxidising agent.
Charcoal, as we have already seen, burns in nitric acid ; phosphorus,
sulphur, iodine, and the majority of metals also decompose nitric acid,

means of sulphuric acid (that is, by a method of double saline decomposition), whilst
nitric acid is not displaced by sulphuric acid from true nitro-compounds ; for instance,
nitrobenzene, CtfHs'NO.j. As nitro-compounds are formed exclusively from hydrocarbons,
they are described with them in organic chemistry.

The group NO2 of nitro-compounds in many cases (like all the oxidised compounds of
nitrogen) passes into the ammonia group or into the ammonia radicle NH2. It is evident
that this requires the action of reducing substances evolving hydrogen : RNO2 + 6H
= KNH5 + 2H.2O. Thus Zinin converted nitrobenzene, C6H5'NO.>, into aniline, C6H5'NHo,
by the action of hydrogen sulphide.

Admitting the existence of the group NO2, replacing hydrogen in various compounds,
then nitric acid may be considered as water in v/hich half the hydrogen is replaced by
the radicle of nitric acid. In this sense nitric acid is nitro-water, NCyOH, its anhydride
dinitro- water, (NO2)2O, and nitrous acid nitre-hydrogen, XOoH. In nitric acid the radicle
of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the
radicle of benzene.

It should here be remarked that the group NO3 may be recognised in the salts of
nitric acid, because the salts have the composition M^NOs),,, just as the metallic chlorides
have the composition MCI,,. But the group NOs does not form any other compounds
beyond the salts, and therefore it should be considered as hydroxyl, HO, in which H is
replaced by NO2.

38 The nitro-compounds play a very important part in mining and artillery. Detailed
accounts of them must be looked for in special works. The most important and histori-
ciil work in this connection is due to Berthelot, who elucidated much in connection with
explosive compounds by a series of both experimental and theoretical researches.

59 Nitric acid may be entirely decomposed by passing its vapour over highly incan-
descent copper, because the oxides of nitrogen first formed give up their oxygen to the

•^>nie MM heating anil oiliefs even at the ordinary" trmperat ure : the
suh>t:u ces taken are oxidised and t'ne nitric acid is deoxidised, yielding
compounds ci iii t aininu' less oxygen. < Milv a te\v metals, such as ufold
and platinum, do not act on nitric acid, hut the majority decompose it :
in so dojnu, an oxide • if the nr-tal is formed, which, if it has the
character ,,f ,-i base, acts on the remaining nitric acid ; therefore, with

'• . it- volume, it- \\i-i-iit and consequently its amount in a L'iveii

ic th'1 a :ii' 'ii n; of iixyur«'ii 1'V tin- increase iii
lit. l-'i ir nil ric acid t hi,- de

equal ii Hi 'JII N< )-, .".( 'n I I ,( ) -
N - ")( 'n( >. Tlii- reaction mn-t
1"' [.receded l.\ the formation
ol ci >)i|M'i- nit rate. ( 'in N( )- 1 .. he-
citn e , xide oi copper ton,,, this
salt u ith nil ric a< id. Tin, ,alt is
\ \ery nn-tal.le. and evolves oxv-

i. .* red heat. The C(,mplete iliTiilll-

|io it ion of nit ric ,irjd is al-o
ace, iinpli^lied li\ pa- -. i : rj a mix
hin i>l I - dro; , n and nitric acid
\ Mpi.ur - ! Im.u.L'li a red lioi tulic.
i-oii nil ro-.-n I.eiiiL' formed

dr. jell. SnilillMI ill«H de, ompo-es till- oxides

• ' i] ' Tlii method is sometimes n-..'d for

COMPOUNDS ()l' MT];«>(iI-:X WITH HVDRocKN AND OXYGEN 271

metals the result of the reaction is usually not an oxide of the metal,
but the corresponding salt of nitric acid, and, at the same time, one of
the lower oxides of nitrogen. The resulting salts of the metals are
soluble, and hence it is said that nitric acid dissolves nearly all metals.40
This case is termed the solution of metals by acids, although it is not a
case of simple solution, but a complex chemical change of the substances
taken. When treated with this acid, those metals whose oxides do not
combine with nitric acid yield the oxide itself, and not a salt ; for
example, tin acts in this manner on nitric acid, forming a hydrous
oxide, SnH2O3, which is obtained in the form of a white powder,
Sn + 4NH03=H,SnO3 + 4NO2 + H2O. Silver is able to take up still
more oxygen, and to convert a large portion of nitric acid into nitrous
anhydride, 4Ag + 6HNO3=4AgNO3 + N2O;, + 3H2O. Copper takes
up still more oxygen from nitric acid, converting it into nitric oxide,
a perfectly colourless gas ; and, by the action of zinc, nitric acid
is able to give up a still further quantity of nitrogen, forming nitrous
oxide, 4Zn + 10NHO3=4Zn(NO3).2 + N2O + 5H2O.41 Sometimes, and
especially with dilute solutions of nitric acid, the deoxidation pro-
ceeds as far as the formation of hydroxylamine and ammonia, and

40 The application of this acid for etching copper or steel in engraving is based on
this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which
nitric acid does not act, and then the ground is removed in certain parts with a needle, and
the whole is washed in nitric acid. The parts covered with the ground remain untouched,
whilst the uncovered portions are eaten into by the acid. Copper plates for etchings,
aquatints, &c., are prepared in this manner.

il The formation of such complex equations as the above often presents some diffi-
culty to the beginner. It should be observed that if the reacting and resultant sub-
stances be known, it is easy to form an equation for the reaction. Thus, if we wish to
form an equation expressing the reaction that nitric acid acting on zinc gives nitrous
oxide, NoO, and zinc nitrate, Zn(NO5).,, we must reason as follows : — Nitric acid contains
hydrogen, whilst the salt and nitrous oxide do not ; hence water is formed, and therefore
it is as though anhydrous nitric acid, N.>O5, were acting. For its conversion into nitrous
oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equi-
valents of zinc and to convert it into zinc oxide, ZnO. These four equivalents of zinc
oxide require for their conversion into the salt four more equivalents of nitric anhydride,
consequently five equivalents in all of the latter are required, or ten equivalents of nitric
acid. Consequently ten equivalents of nitric acid are necessary for four equivalents of
/inc in order to express the reaction in whole equivalents. It must not be forgotten,
however, that there are very few such reactions which can be entirely expressed by simple
equations. The majority of equations of reactions only express the chief and ultimate
products of reaction, and thus none of the three preceding equations express all that
in reality occurs in the action of metals on nitric acid. In no one of them is only one
oxide of nitrogen formed, but always several together or consecutively — one after the
other, according to the temperature and strength of the acid. And this is easily under-
stood. The resulting oxide is itself capable of influencing metals and of being deoxidised,
and in the presence of the nitric acid it may change the acid and be itself changed. The
equations given must be looked on as a systematic expression of the main aspects of re-
actions. Further, these reactions vary considerably with different temperatures and
varying strengths of acid.

272 PRINCIPLES OF CHEMISTRY

sometimes it leads to the formation of ammonia itself. The formation
of one or other nitrogenous substance from nitric acid is determined
not only by the nature of the reacting substances, but also by the
relative mass of water and nitric acid, and also by the temperature and
pressure, or the sum total of the conditions of reaction ; and as in a
given mixture these conditions even vary (the temperature and the
relative mass vary), therefore a mixture of different products of the
deoxidation of nitric acid is not unfrequently formed.

Thus the action of nitric acid on metals consists in their being
oxidised, whilst it is itself converted, according to the temperature,
concentration in which it is taken, and the nature of the metal, <fec.,
into either lower oxides, or even into ammonia.42 Many compounds
are oxidised by nitric acid like metals and other elements ; for instance,
lower oxides are converted into higher oxides. Thus, arsenious acid is
converted into arsenic acid, suboxide of iron into oxide, sulphurous
acid into sulphuric acid, the sulphides of the metals, M2S, into sulphates,
M.2SO4, &c. ; iii a word, nitric acid brings about oxidation, its oxygen
is taken up and transferred to many other substances. Certain sub-
stances are oxidised by strong nitric acid so rapidly and with so great
an evolution of heat that they deflagrate and burst into flame. Thus
turpentine, C10H16, bursts into flame when poured into fuming nitric
acid. In virtue of its oxidising property, nitric acid removes the
hydrogen from many substances. Thus it decomposes hydriodic acid,
separating the iodine and forming water ; and if fuming nitric acid be
poured into a flask containing gaseous hydriodic acid, then a rapid
reaction takes place, accompanied by flame and the separation of
violet vapours of iodine and brown fumes of oxides of nitrogen.43

42 It is observed that normal nitric acid oxidises many metals with much greater
difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by
dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid
or of the pure hydrate NHO-,. Nitric acid diluted with a large quantity of water does
not oxidise copper, but it oxidises tin ; dilute nitric acid also does not oxidise either silver
or mercury ; but, on the addition of nitrous acid, even dilute acid acts on the above metals.
This naturally depends on the smaller stability of nitrous acid, and on the fact that after
the commencement of the action the nitric acid is itself converted into nitrous acid, which
continues to act on the silver and mercury.

43 When nitric acid acts on many organic substances it often happens that not only
is hydrogen removed, but also oxygen is combined ; thus, for example, nitric acid con-
verts toluene, C7H8, into benzoic acid, C7H6O2. In certain cases, also, a portion of
the carbon contained in an organic substance burns at the expense of the oxygen of the
nitric acid. So, for instance, phthalic acid, C8H6O4, is obtained from naphthalene, C10H8.
Thus the action of nitric acid on the hydrocarbons is often most complex ; there takes
place (besides nitrification) the separation of carbon, the displacement of hydrogen,
and the combination of oxygen. There are few organic substances which can with-
stand the action of nitric acid. Hence nitric acid acts in a powerfully transforming
manner on a number of organic substances. It leaves a yellow stain on the skin, and in

COMPOUNDS ol' NITKO(JKN WITH IIVDIIoiiKN AND oXY<JK.\ 273

As nitric acid is very easily decomposed with the separation of
oxygen, it was for a long time supposed that it was not capable of
forming the corresponding nitric anhydride, N2O5 ; but first Deville,
and then Weber and others, discovered the methods of its formation.
Deville obtained nitric anhydride by decomposing silver nitrate by
chlorine under the influence of a moderate heat. Chlorine acts on the
al M »ve salt at a temperature of 95° (2AgNO3 + CL2 = 2 AgCl + N2O5 4- O),
and when once the reaction is started it continues by itself without
further heating. Brown fumes are given off, which are condensed in a
tube surrounded by a freezing-mixture. A portion condenses in this
tube and a portion remains in a gaseous state. The latter contains
free oxygen. A crystalline mass and a liquid substance are obtained in
the tube ; the liquid is poured off, and a current of dry carbonic acid
gas is passed through the apparatus in order to remove all traces of
volatile substances (liquid oxides of nitrogen) adhering to the crystals
of nitric anhydride. These form a voluminous mass of rhombic crystals
(density 1'64), which sometimes are of rather large size ; they melt at
about 30° and distil at about 47°. In distilling, a portion of the sub-
stance is decomposed. With water these crystals give nitric acid.
Nitric anhydride is also obtained by the action of phosphoric anhydride,
P2O5, on cold pure nitric acid (below 0°). During the very careful dis-
tillation of equal parts by weight of these two substances a portion
of the acid decomposes, giving a liquid compound, H2O,2N2Or>
=N2O5,2H]Sr03, whilst the greater part of the nitric acid gives the
anhydride according to the equation 2NHO3 + P2O3=2PHO3 + N2O5.
On heating, and sometimes even spontaneously with explosion, nitric
anhydride decomposes into nitric peroxide and oxygen, N2O5
=N2O4 + O.

Nitrogen peroxide, N2O4, and nitrogen dioxide, N02, express one
and the same composition, but they should be distinguished like ordinary
oxygen and ozone, although in this case their mutual conversion is
more easily accomplished, even by vaporisation ; also, O3 loses heat in
passing into O2, whilst N2O4 absorbs heat in' forming NO2.

Nitric acid in acting on tin and on many organic substances (for
example, starch) gives brown vapours, consisting of a mixture of N2O3
and NO2. A purer product is obtained by the decomposition of lead
nitrate by heat, Pb(NO3)2==2NO2 + O + PbO, when non-volatile lead

a large quantity causes a wound and entirely eats away the membranes of the body.
The membranes of plants are eaten into with the greatest ease by strong nitric acid in
just the same manner. One of the most durable blue vegetable dyes which is employed
in dyeing tissues is indigo, yet it is easily converted into a yellow substance by the
action of nitric acid, and small traces of free nitric acid may be recognised by this means.
VOL. I. T

274 PRINCIPLES OF CHEMISTRY

oxide, oxygen gas, and nitrogen peroxide are formed. The latter, in a
strongly cooled vessel, condenses into a brown liquid, which boils at
about 22°. The purest peroxide of nitrogen, solidifying at —9°, is
obtained when dry oxygen is mixed in a freezing-mixture with twice
its volume of dry nitric oxide, NO, when transparent prisms of nitrogen
peroxide are formed in the receiver ; they melt into a colourless liquid
at about — 10°. When the temperature of the receiver is above
— 9°, the crystals melt,44 and at 0° give a reddish-yellow liquid, like
that obtained in the decomposition of lead nitrate. The vapours of
nitrogen peroxide have a characteristic odour, and at the ordinary
temperature are of a dark-brown colour, but at lower temperatures the
colour of the vapour is much fainter. When heated, especially above
50°, the colour becomes a very dark brown, so that the vapours almost
lose their transparency.

The causes of these peculiarities of nitrogen peroxide were not
clearly understood until Deville and Trooste determined the density
and dissociation of the vapour of this substance at different temperatures,
and showed that the density varies. If the density be referred to that
of hydrogen at the same temperature and pressure, then it is found to
vary from 38 at the boiling point, or about 27°, to 23 at 135°, after
which the density remains constant up to those high temperatures at
which the oxides of nitrogen are decomposed. As, on the basis of the
laws enunciated in the following chapter, the density 23 corresponds
with the compound N02 (because the weight corresponding with this
molecular formula= 46, and the density referred to hydrogen as unity is
equal to half the molecular weight), therefore at temperatures above 1 35°
the existence of nitrogen dioxide only must be recognised. It is this
gas which is of a brown colour. At a lower temperature it forms
nitrogen peroxide, N2O4, whose molecular weight, and therefore density,
is twice that of the dioxide. This substance, which is isomeric with
nitrogen dioxide, as ozone is isomeric with oxygen, and has twice as
great a vapour density (46 referred to hydrogen), is formed in greater
quantity the lower the temperature, and crystallises at — 10°. The
reasons both of the variation of the colour of the gas (N.204 gives
colourless and transparent vapours, whilst those of NO2 are brown and
opaque) and the variation of the vapour density with the variation of

44 According to certain investigations, if a brown liquid is formed from the melted
crystals by heating above —9°, then they no longer solidify at —10°, probably because a
certain amount of N2O5 (and oxygen) is formed, and this substance remains liquid at
— 80°, or it may be that the passage from 2NO2 into N2O4 is not so easily accomplished
as the passage from N2O4 into 2NO2.

Liquid nitrogen peroxide (that is, a mixture of NO2 and N2O4) is employed in admix-
ture with hydrocarbons as an explosive.

COM P< ) I" N I )S o K NITROGEN WITH HYDROGEN AND QXYG K X 275

temperature are thus made quite clear, and as at the boiling point a
density 38 was obtained, therefore at that temperature the vapours
consist of a mixture of 79 parts by weight of N2O4 with 21 parts by
weight of ISTO2.45 It is evident that a decomposition here takes place
whose peculiarity consists in the fact that the product of decomposition,
NO2, is polymerised (i.e. becomes denser, combines with itself) at a
lower temperature ; that is, the reaction,

N2O4=NO

is a reversible reaction, and consequently the whole phenomenon repre-
sents a dissociation in a homogeneous gaseous medium, where the
original substance, N2O4, and the resultant, NO2, are both gases. The
measure of dissociation will be expressed if we find the proportion of the
quantity of the substance decomposed to the whole amount of the sub-
stance. At the boiling point, therefore, the measure of the decomposi-
tion of nitrogen peroxide will be 21/(79 + 21)=O21, or 21 p.c. ; at
135° it=l, and at 10° it =0— that is, the N2O4 is not then de-
composable. Consequently here the limits of dissociation are — 10° and
135° at the atmospheric pressure.46 Within the limits of these tem-
peratures the vapours of nitrogen peroxide have not a constant density,
and above and below these limits definite substances exist. Thus
above 135° N2O4 has ceased to exist and NO2 alone remains. It is

45 Because if x equal the amount by weight of N.2O4, its volume will = x/46, and the
amount of NO3 will =100 — x, and consequently its volume will =(100 — x), 23. But the
mixture, having a density 38, will weigh 100, consequently its volume will =100/38.
Hence Z/46 + (100 -*)/23 = 100/86, or x = 79'0.

46 The phenomena and laws of dissociation, considered by us in only separate and
particular instances, are discussed in detail in works on theoretical chemistry. Besides,
certain points in the doctrine of chemical equilibria are still subject to some doubt owing
to the recent date at which the exact study of this subject commenced. Nevertheless,
in respect to nitrogen peroxide, as an historically important example of dissociation in a
homogeneous gaseous medium, we will cite the results of the careful investigations
(1885-1886) of E. and L. Natanson, who determined the densities under variations of
temperature and pressure. The measure of dissociation, expressed as above (it may also
be expressed otherwise — for example, by the ratio of the substance decomposed to that
unaltered), proves to increase at all temperatures as the pressure diminishes, which
would be expected for a homogeneous gaseous medium, as a decreasing pressure aids
the formation of the lightest product of dissociation (that having the least density or
largest volume). Thus, in Natansons' experiments the measure of dissociation at 0° in-
creases from 10 p.c. to 30 p.c., with a decrease of pressure of from 251 to 38 mm. ; at 49'7° it
increases from 49 p.c. to 93 p.c., with a fall of pressure of from 498 to 27 mm., and at 100° it
increases from 89'2 p.c. to 99'7 p.c., with a fall of pressure of from 732'5 to 11'7 mm. At
180° and 150° the decomposition is complete — that is, NO2 only remains at the low pres-
sures (less than the atmospheric) at which the Natansons made their determinations ;
but it is probable that at considerable pressure (of several atmospheres) molecules of
N2O4 would still be formed, and it would be exceedingly interesting to trace the pheno-
mena under the conditions of both very considerable pressures and of relatively large
volumes.

T 2

276 PRINCIPLES OF CHEMISTRY

evident that at the ordinary temperature there is a partially dis-
sociated system or mixture of nitrogen peroxide, N2O4, and nitrogen
dioxide, NO2. In the brown liquid boiling at 22° probably a portion
of the N2O, has already passed into NO2, and it is only the colourless
liquid and crystalline substance at — 10° that can be considered as pure
nitrogen peroxide.47

The above explains the action of nitrogen peroxide on water at low
temperatures. N2O4 then acts on water like a mixture of the anhy-
drides of nitrous and nitric acids. The first, X2O3, may be looked on
as water in which the two atoms of hydrogen are replaced by the radicle
NO, while in the second the hydrogen is replaced by the radicle NO2,
proper to nitric acid ; and in nitrogen peroxide one atom of the
hydrogen of water is replaced by NO and the other by NO.,, as is seen
from the formulae —

H)0. N0j0. N02)0. NO )0.

Hj°> N0i°> N02)°> N02,r°>

or H20 ; N2O:J ; N2O5 ; N2O4.

In fact, nitrogen peroxide at low temperatures gives with water (ice)
both nitric, HNO3, and nitrous, HNO2, acids. The latter, as we shall
afterwards see, splits up into water and the anhydride, N2O3. If, how-
ever, warm water act on nitrogen peroxide, only nitric acid and oxide
of nitrogen are formed : 3NO2 + H2O=NO + 2NHO3.

Although NO.; is not decomposed into N and O even at 500°,
still in many cases it acts as an oxidising agent. Thus, for instance,
it oxidises mercury, converting it into mercurous nitrate, 2NO2 +
Hg=HgNO3-|-NOj it being itself deoxidised into nitric oxide, into

47 The fact that the presence of a portion of dioxide, NO.,,, must be acknowledged in
liquid nitrogen peroxide, N2O4, at temperatures of from 0° to 22°, is not only of great
significance for the theory which regards solutions as liquid systems of equilibrium, consist-
ing of combined and decomposed substances, but it also shows the nature of solutions of
gaseous substances, because the NO2 must be regarded as a gas dissolved in the volatile
liquid N2O4.

Liquid nitrogen peroxide is said by Geuther to boil at 22°-26°, and to have a sp. gr.
at 0° = 1-494 and at 15° = 1'474. It is evident that, in the liquid as in the gaseous state,
the variation of density with the temperature depends not only on physical, but also on
chemical changes, as the amount of N2O4 decreases and the amount of NO.2 increases with
the temperature, and they (as polymeric substances) should have different densities, as we
find, for instance, in the hydrocarbons C5H10 and C10H.,o.

It may not be superfluous to here mention that the measurement of the specific heat
of a mixture of the vapours of N2O4 and NO2 enabled Berthelot to determine that the
transformation of 2NO2 into N2O4 is accompanied by the evolution of about 18000 units
of heat, and as the reaction proceeds with equal facility in either direction, it will be
exothermal in the one direction and endothermal in the other ; and this clearly demon-
strates the possibility of reactions of both aspects proceeding in either direction, although,
as a rule, reactions evolving heat proceed with greater ease.

OF NITH<X;KN WITH HYDROGEN AND OXYGEN 277

which nitrogen dioxide in many other instances passes, and from which
it is easily formed.48

Xifrinix anhydride, N2O3, corresponds49 with nitrous acid, NHO2,
and with the latter corresponds a series of salts, the nitrites — for ex-
ample, the sodium salt NaNO2, the potassium salt KNO.2, the ammonium
salt (NH ,)X( ),, :i" the silver salt AgNO2,M <kc. Neither the anhydride
nor the hydrate of the acid is known in a perfectly pure state. The
anhydride has only been obtained as a very unstable substance, and has
not yet been investigated with proper fulness ; and when efforts are
made to obtain the acid NHO2 from its salts, it always gives water and
the anhydride, whilst the latter, as an intermediate oxide, easily splits
up into NO + NO2. But the salts of nitrous acid are distinguished for
their great stability. Potassium nitrate, KNO3, may be converted into
potassium nitrite by depriving it of a portion of its oxygen ; for
instance, by fusing it (at a not too great heat) with metals, such as
lead, KNO3 + Pb=KNO2 + PbO. The resultant salt is soluble in
water, whilst the oxide of lead is insoluble. With sulphuric and other
acids the solution of potassium nitrite 52 immediately evolves a brown
gas, nitrous anhydride : 2KN02 + H2SO4=K2SO4 + N2O3 + H2O. The
same gas (N2O3) is obtained by passing nitric oxide at 0° through
liquid peroxide of nitrogen,53 or by heating starch with nitric acid of sp.

48 Nitric acid of sp. gr. 1'51 in dissolving nitrogen peroxide becomes brown, whilst
nitric acid of sp. gr. T32 is coloured greenish blue, and acid of sp. gr. below 1'15 remains
colourless on absorbing nitrogen peroxide.

49 Nitrogen peroxide as a mixed substance has no corresponding independent salts.

50 Ammonium nitrite may be easily obtained in solution by a similar method of double
decomposition (for instance, of the barium salt with ammonium sulphate) to the other
salts of nitrous acid, but it decomposes with great ease when evaporated, with the evo-
lution of gaseous nitrogen, as has been already mentioned (Chap. V.). If the solution,
however, be evaporated at the ordinary temperature under the receiver of an air-pump,
a solid saline mass is obtained, which is easily decomposed when heated. The dry salt
even decomposes with an explosion when struck, or when heated to about 70° — NH4NO2 =
iiHoO + N2. It is also formed by the action of aqueous ammonia on a mixture of nitric
oxide and oxygen, or by the action of ozone on ammonia, and in many other instances.

••' Silver nitrite, AgNO2, is obtained as a very slightly soluble substance, as a preci-
pitate, 011 mixing solutions of silver nitrate, AgNO5, and potassium nitrite, KNO2. It
is soluble in a large volume of water, and this is taken advantage of to free it from
silver oxide, which is also present in the precipitate, owing to the fact that potassium
nitrite always contains a certain amount of oxide, which with water gives the hydroxide,
forming oxide of silver with silver nitrate. The solution of silver nitrite gives, by double
decomposition with metallic chlorides (for instance, barium chloride), insoluble silver
chloride and the nitrite of the metal taken (for instance, barium nitrite, Ba(NOo)._>).

•'- Probably potassium nitrite, KNO2, when strongly heated, especially with metallic
oxides, evolves N and O, and gives potassium oxide, K2O, because nitre is liable to such
a decomposition, but it has, as yet, been but little investigated.

53 It is evident that the reaction NoO5 = NO2 + NO is reversible, and that it resembles
the conversion of N2O4 into NO2, but as yet this reaction has not been thoroughly

278 .% PRINCIPLES OF CHEMISTRY

gr. 1'3. At a very low temperature it condenses into a blue liquid boiling
below 0°,54 but then partially decomposing into NO + NO2. Nitrous an-
hydride evinces a remarkable capacity for oxidising. Ignited bodies burn
in it, nitric acid absorbs it, and then acquires the property of acting on
silver and other metals, even when diluted. Potassium iodide is
oxidised by this gas just as it is by ozone (and by peroxide of hydrogen,
chromic and other acids, but not by dilute nitric acid nor by sulphuric
acid), with the separation of iodine. This iodine may be recognised
(see Ozone, Chap. IV.) by its turning starch blue. The smallest traces
of nitrites may be easily discovered by this method. If, for example,
starch and potassium iodide are added to a solution of potassium
nitrite (there will be no change, there being no free nitrous acid), and
then sulphuric acid be added, then the nitrous acid (or its anhydride)
immediately set free evolves iodine, which communicates a blue colour
to the starch. Nitric acid does not act in this manner, but in the
presence of zinc the coloration takes place, which proves the formation
of nitrous acid in the deoxidation of nitric acid.55 Nitrous acid (or
even a mixture of HNO2-l-NO) acts directly on ammonia, forming
nitrogen and water, HNO2 + NH3=N2 + 2H20.56

As nitrous anhydride easily splits up into NO2 + NO, so with warm
water it, like NO2, gives nitric acid and nitric oxide, according to the
equation 3N2O3 + H2O=4NO + 2NHO3.

Being in a lower degree of oxidation than nitric acid, nitrous acid

studied. The brown colour of the vapours of nitrous anhydride probably depends on
the presence of NO.2.

If nitrogen peroxide be cooled to — 20°, and half its weight of water be added to it drop
by drop, then the peroxide is decomposed, as we have already said, into nitrous and nitric
acids ; the former does not then remain as a hydrate, but straightway passes into the
anhydride, and, therefore, if the resultant liquid be slightly warmed vapours of nitrous
anhydride, N2O5, are evolved, and condense into a blue liquid, as Fritzsche showed.
This method of preparing nitrous anhydride evidently gives the purest product.

54 According to Thorpe, N2O3 boils at +18°. According to Geuther, at +3'5°, and its
sp. gr. at 0° = 1-449.

55 In its oxidising action nitrous anhydride gives nitric oxide, N<jO5 = 2NO + O. Thus
its analogy to ozone becomes still closer, because in ozone it is only one-third of the
oxygen that acts in oxidising ; from O-, there is obtained O, which acts as an oxidiser, and
common oxygen O3. In a physical aspect the affinity between N.2O3 and O3 is expressed
by both substances being of a blue colour when in the liquid state.

56 This reaction is taken advantage of for converting the amides, NHoR (where R is
an element or a complex group) into hydroxides, RHO. In this case NPL>R + NHOo forms
2N + H.,O + RHO ; NHo is replaced by HO, the radicle of ammonia by the radicle of water.
This reaction is employed for transforming many nitrogenous organic substances having
the properties of amides into their corresponding hydroxides. Thus aniline, CgHj'NH.^,
which is obtained from nitrobenzene, C6H5'NO.,, (Note 37), is converted by nitrous anhy-
dride into phenol, C6H5'OH, which occurs in the creosote extracted from coal tar. Thus
the H of the benzene is successively replaced by NCX>, NH.,, and HO — a method which is
suitable for other cases also.

COMPOUNDS OF NITROCKX WITH HYDROGEN AND OXVCKN 279

and its anhydride are oxidised in solutions by many oxidising
substances —for example, by potassium permanganate — into nitric acid. '"
Nitric oxide, NO. — This permanent gas 58 (that is, unliquefiable by
pressure without the aid of cold) may be obtained from all the above-
described compounds of nitrogen with oxygen. The deoxidation of
nitric acid by metals is the usual method employed for its preparation.
Dilute nitric acid (sp. gr. 1*18, but not stronger, as then N203 and
NO2 are produced) is poured into a flask containing metallic copper.59
The reaction commences at the ordinary temperature. Mercury and
silver also give nitric oxide with nitric acid. In these reactions with
metals one portion of the nitric acid is employed in the oxidation of the
metal, whilst the other, and by far the greater, portion combines with the
metallic oxide so obtained, with formation of the nitrate corresponding
with the metal taken. The first action of the copper on the nitric acid
is thus expressed by the equation

u O + 2NO.

The second reaction consists in the formation of copper nitrate —
+ 3CuO=3H2O + 3Cu(NO3)2.

Nitric oxide is a colourless gas which is only slightly soluble in
water (^ of a volume at the ordinary temperature). Reactions of
double decomposition in which nitric oxide readily takes part are not
known — that is to say, it is an indifferent, not a saline, oxide. Like the
other oxides of nitrogen, it is decomposed into its elements at a red heat.
The most characteristic property of nitric oxide consists in its capacity
for directly and easily combining with oxygen (owing to the evolution
of heat in the combination). With oxygen it forms nitrous anhydride

57 The action of a solution of potassium permanganate, KMnO4, on nitrous acid in
the presence of sulphuric acid is determined by the fact that the higher oxide of man-
ganese, MnoO7, contained in the permanganate is converted into the lower oxide MnO,
which as a base forms manganese sulphate, MnSO4, and the oxygen serves for the oxida-
tion of the N2O3 into N.,O3, or its hydrate. As the solution of the permanganate is of &
red colour, whilst that of manganese sulphate is almost colourless, this reaction is clearly
seen, and may be employed for the recognition and determination of nitrous acid and
its salts.

58 The absolute boiling point =98° (see Chap. II. Note 29).

59 Kammerer proposed preparing nitric oxide, NO, by pouring a solution of
sodium nitrate over copper shavings, and adding sulphuric acid drop by drop. The
oxidation of ferrous salts by nitric acid also gives NO. One part of strong hydrochloric
acid is taken and iron is dissolved in it (FeCl.2), and then an equal quantity of hydro-
chloric acid and nitre is added to the solution. On heating, nitric oxide is evolved. When
nitric oxide is prepared by either of the above methods, the apparatus first becomes full
of brown fumes of nitrogen peroxide, formed by the oxygen of the air and the nitric
oxide, and therefore the pure gas can only be collected after it has displaced all the air
in the apparatus, and when the latter becomes full of colourless gas.

280 PRINCIPLES OF CHEMISTRY

and nitrogen peroxide, 2NO + O=N2O3, 2NO + O2=2NO.2. If nitric
oxide is mixed with oxygen and immediately shaken up with caustic
potash, it is almost entirely converted into potassium nitrite, whilst
after a certain time, when the formation of nitric peroxide has already
commenced, a mixture of potassium nitrite and nitrate is obtained. If
oxygen is passed into a bell jar tilled with nitric oxide, then brown
fumes of nitrous anhydride and nitric peroxide are formed, which in the
presence of water give, as we already know, nitric acid and nitric
oxide, so that in the presence of an excess of water and oxygen the
whole of the nitric oxide is easily and directly converted into nitric
acid. This reaction of the re-formation of nitric acid from nitric oxide,
air, and water, 2NO + H2O + O3 = 2HNO3, is frequently made use of in
practice. The experiment showing the conversion of nitric oxide into
nitric acid is very striking and instructive. As the intermixture of the
oxygen with the oxide of nitrogen proceeds, the nitric acid formed dis-
solves in water, and if an excess of oxygen has not been added the
whole of the gas (nitric oxide), being converted into HNO3, is
absorbed, and the water entirely fills the bell jar previously containing
the gas.60 It is evident that nitric oxide 61 in combining with oxygen

40 This transformation of the permanent gases nitric oxide and oxygen into liquid
nitric acid in the presence of water, and with the evolution of heat, presents a most
striking instance of liquefaction produced by the action of chemical forces. They per-
form with ease the work which physical (cooling) and mechanical (pressure) forces do
with difficulty. In this the motion, which is so clearly the property of the gaseous mole-
cules, is extinguished. In other cases of chemical action its appearance arises from
latent energy — that is, in all probability, from the movement of the atoms in the molecules.

61 Nitric oxide is capable of entering into many characteristic combinations ;
it is absorbed by the solutions of many acids (for instance, tartaric, acetic, phos-
phoric, sulphuric), and also by the solutions of many salts, especially those formed by
suboxide of iron (for instance, ferrous sulphate). In this case a brown compound
is formed which is exceedingly unstable, like all the analogous compounds of
nitric oxide. The amount of nitric oxide combined in this manner is in atomic pro-
portion with the amount of the substance taken ; thus ferrous sulphate, FeSO4,
absorbs it in the proportion of NO to 2FeSO4. Ammonia is obtained by the action of
a caustic alkali on the resultant compound, because the oxygen of the nitric oxide and
water are transferred to the ferrous oxide, forming ferric oxide, whilst the nitrogen
combines with the hydrogen of the water. According to the investigations of Gay
(1885), the compound is formed with the evolution of a large quantity of heat, and is
easily dissociated, like a solution of ammonia in water. This subject must be regarded
as not sufficiently studied. On passing nitric oxide through nitric acid, nitrogen per-
oxide and nitrous anhydride are formed, whose solutions in the nitric acid are, as we
have already mentioned, of various colours. It is evident that oxidising substances (for
example, potassium permanganate, KMnO4, Note 57) are able to convert it into nitric-
acid. If the presence of a radicle NO.,,, composed like nitrogen peroxide, must be recog-
nised in the compounds of nitric acid, then a radicle NO, having the composition of
nitric oxide, may be admitted in the compounds of nitrous acid. The compounds in
which the radicle NO is recognised are called nitroso-compoiinds. The compounds are
described in Prof. Bunge's work (Kief, 1868).

COMPOUNDS OF NJTK'ocl-N WITH BYDROGEN AND OXYGEN 281

has n strong tendency to give only the higher types of nitrogen com-
pounds, which we see in nitric acid, HNO;5 or NO2(OH), in nitric an-
hydride, N2O:s or (NO2)._,O, and in ammonium chloride, NH4C1. If X
stand for an atom of hydrogen, or its equivalents, chlorine, hydroxyl, Arc.,
and if O, which is, according to the law of substitution, equivalent to
H2, be indicated by X2, then the three above-named compounds of
nitrogen should be considered as compounds of the type or form NX5.
For example, in nitric acid X5=O2 + (OH), where O2=X4, and
OH=X ; whilst nitric oxide is a compound of the form NX2. Hence
this lower form, as is true of lower forms in general, strives by combina-
tion to attain to the higher forms proper to the compounds of a given
element. NX2 passes consecutively into NX3- — namely, into N2O3 and
NHO2, NX4 (for instance NO2) and NX5.

As the decomposition of nitric oxide begins at temperatures above
600°, many substances burn in it ; for instance, ignited phosphorus con-
tinues to burn in nitric oxide, but sulphur and charcoal are extinguished
in it. This is due to the fact that the heat evolved in the combustion of
these two substances is insufficient for the entire decomposition of the
nitric oxide, whilst the heat developed by burning phosphorus suffices
to produce this decomposition. That this is the true explanation of
the behaviour of nitric oxide in these cases is proved by the fact that
charcoal when very strongly ignited will burn in the gas.62

The compounds of nitrogen with oxygen which we have so far con-
sidered may all be prepared from nitric oxide, and may themselves be
converted into it. Thus nitric oxide stands in intimate connection
with them.'53 The passage of nitric oxide into the higher degrees of

62 A mixture of nitric oxide and hydrogen is inflammable. If a mixture of both
gases be passed over spongy platinum, the nitrogen and hydrogen even combine, forming
ammonia. A mixture of nitric oxide with many combustible vapours and gases is very in-
flammable. A very characteristic flame is obtained in burning a mixture of nitric oxide
and the vapour of the combustible carbon bisulphide, CS.,. The latter substance is very
volatile, so that it is sufficient to pass the nitric oxide through a layer of the carbon bisul-
phide (for instance, in a Woulfe's bottle) in order that the gas escaping should contain a
considerable amount of the vapours of this substance. This mixture continues to burn
when set light to, and the flame emits a large quantity of the so-called ultra-violet rays,
which are capable of bringing about chemical combinations and decompositions, and
therefore the flame may be employed in photography in the absence of sufficient day-
light (magnesium and electric light have the same property). A mixture of nitric
<>xi<lt> with many gases (for instance, ammonia) explodes in a eudiometer.

63 The oxides of nitrogen do not proceed directly from oxygen and nitrogen by contact
alone, naturally because their formation is accompanied by the absorption of a large
quantity of heat, namely (see Note 29), about 21500 heat units are absorbed when 16 parts
of oxygen and 14 parts of nitrogen combine, consequently the decomposition of nitric oxide
into oxygen and nitrogen is accompanied by the evolution of this amount of heat; and
therefore with nitric oxide, as with all explosive substances and mixtures, the reaction
once started is able to proceed by itself. In fact, Berthelot remarked the decomposition
of nitric oxide in the explosion of fulminate of mercury. This decomposition does not take

282 PRINCIPLES OF CHEMISTRY

oxidation and the converse reaction is employed in practice as a
means for transferring the oxygen of the air to substances capable of
being oxidised. Having nitric oxide, it may easily be converted, with
the aid of the oxygen of the atmosphere and water, into nitric acid,
nitrous anhydride, and nitric peroxide, and by their means employed to-
oxidise other substances. In this oxidising action nitric oxide is again
formed, and it may again be converted into nitric acid, and so on with-
out end, if only there be oxygen and water. Hence the fact, which
at first appears to be a paradox, that by means of a small quantity of
nitric oxide in the presence of oxygen and water it is possible to oxidise
an indefinitely large quantity of substances which cannot be directly
oxidised either by the action of the atmospheric oxygen or by the
action of nitric oxide itself. The sulphurous anhydride, SO2, which
is obtained in the combustion of sulphur and in roasting many metallic
sulphides in the air, is an example of this kind. In practice this
gas is obtained by burning sulphur or iron pyrites, the latter being
thereby converted into oxide of iron and sulphurous anhydride. In
contact with the oxygen of the atmosphere this gas does not pass into-
the higher degree of oxidation sulphuric anhydride, SO3, and if it does
form sulphuric acid with water and the oxygen of the atmosphere,
SO2 +H2O + O=H2SO4, it does so very slowly. With nitric acid (and
especially with nitrous acid, but not with nitrogen peroxide) and water,
sulphurous anhydride, on the contrary, very easily forms sulphuric acid,
and especially so when slightly heated (about 40°), the nitric acid (or,
better still, nitrous acid) being converted into nitric oxide —

3SO2 + 2NHO3 -f 2H2O=2H2S04 + 2

The presence of water is absolutely indispensable here, otherwise
sulphuric anhydride is formed, which combines with the oxides of
nitrogen (nitrous anhydride), forming a crystalline substance contain-
ing oxides of nitrogen (chamber crystals, which will be described in the
chapter on sulphur). Water destroys this compound, forming sulphuric
acid and separating the oxides of nitrogen. The water must be taken
in a greater quantity than that required for the formation of the hydrate
H2SO4, because the latter absorbs oxides of nitrogen. With an excess of
water, however, solution does not take place. If, in the above reaction,
only water, sulphurous anhydride, and nitric or nitrous acid be taken in

place spontaneously ; substances even burn with difficulty in nitric oxide, probably because
a certain portion of the nitric oxide in decomposing gives oxygen, which combine* with
another portion of nitric oxide and forms nitric peroxide, a somewhat more stable com-
pound of nitrogen and oxygen. The further combinations of nitric oxide with oxygen all pro-
ceed with the evolution of heat, and take place spontaneously by contact with air alone.
From these examples it is seen how the use of thermochemical data is limited by facts.

COMPOUNDS ol' MTI{()(iKN WITH HYDROGEN AM) OXYGEN 28B

a definite quantity, then a definite quantity of sulphuric acid and nitric
oxide will be formed, according to the preceding equation ; but there
the reaction ends, the excess of sulphurous anhydride, if there be any,
will remain unchanged. But if we add air and water, then the nitric
oxide will unite with the oxygen to form nitrogen peroxide, and the
latter with water to form nitric and nitrous acids, which again give
sulphuric acid from a fresh quantity of sulphurous anhydride. Nitric
oxide is again formed, which is able to start the oxidation afresh if
there be sufficient air. Thus it is possible with a definite quantity of
nitric oxide to convert an indefinitely large quantity of sulphurous
anhydride into sulphuric acid, water and oxygen only being required.64
This may be easily demonstrated by an experiment on a small scale, if
a certain quantity of nitric oxide be first introduced into a flask, and
sulphurous anhydride, steam, and oxygen be then continually passed in.
Thus the above-described reaction may be expressed in the following
manner :

if we consider only the original substances and those finally formed.
Thus a definite quantity of nitric oxide may serve for the conversion of
an indefinite quantity of 'sulphurous anhydride, oxygen, and water into
sulphuric acid. In reality, however, there is a limit to this, because a
portion of the resulting oxides of nitrogen are dissolved by the sulphuric
acid, so that in employing even pure oxygen the amount of free (undis-
solved) or active nitric oxide decreases little by little. If air, and not
pure oxygen, be employed for the oxidation, as it is necessary to do in
practice, then it is necessary to remove the nitrogen of the air and to
introduce a fresh quantity of air. A certain quantity of nitric oxide
will pass away with this nitrogen, and will in this way be lost.65

64 The instance of the action of a small quantity of NO in inciting a definite
chemical reaction between large masses (SOo + O + H.>O = H.2SO4) is very instructive,
because the particulars relating to it have been studied, and show that intermediate
forms of reaction may be discovered in the so-called contact or catalytic phenomena.
The essence of the matter here is that A ( = SO.>) reacts upon B ( = O and H2O) in the pre-
sence of C, because it gives BC, a substance which forms AB with A, and again liberates
C. Consequently C is a medium, a transferring substance, without which the matter does
not proceed of its own accord. Many similar phenomena may be found in other depart-
ments of life. Thus the merchant is an indispensable medium between the producer and
the consumer ; thus experiment is a medium between the phenomena of nature and the
cognisant faculties; thus Umi:uiige, forms, and laws are media which are as necessary
for the consolidation i.f social intercourse as nitric oxide for the relations between sul-
phurous anhydride and oxygen and water.

65 If the sulphurous anhydride be prepared by roasting iron pyrites, FeS.,>, then
each equivalent of pyrites (equivalent of iron 56, of sulphur 82, of pyrites 120) requires
.six equivalents of oxygen (that is 96 parts) for the conversion of its sulphur into sul-

284

PRINCIPLES OF CHEMISTRY

The preceding series of changes serve as the basis of the manu-
facture of sulphuric acid or so-called chamber acid. This acid is
prepared on a very large scale in chemical works because it is the
cheapest acid whose action can be applied in a great number of cases.
It is thus used in immense quantities.

The process is carried on in a series of chambers (or in one divided
by partitions as in fig. 50, which shows the beginning and end of a
chamber) constructed of sheet lead. These chambers are placed one

FIG. 50. — Section of sulphuric acid chambers, the first and last chambers only bein^ represented.
The tower to the left is called the Glover's tower, and that on the right the Gay-Lusaac'B rower.
Less than -^ of the natural size.

after the other, and communicate with one another by tubes or special
orifices so placed that the inlet tubes are in the upper portion of the
chamber, and the outlet in the lower and opposite end. The current of

phuric acid (for forming 2H2SO4 with water), besides l£ equivalents (24 parts) for con-
verting the iron into oxide, Fe2O3 ; hence the combustion of the pyrites for the formation
of sulphuric acid and ferric oxide requires the introduction of an equal weight of oxygen
(120 parts of oxygen to 120 parts of pyrites), or five times its weight of air, whilst four
parts by weight of nitrogen will remain inactive, and in the removal of the exhausted
air will carry off the remaining nitric oxide. If not all, at least a large portion of the
nitric oxide may be collected by passing the escaping air, still containing some oxygen,
through substances which absorb oxides of nitrogen. Sulphuric acid itself may be
employed for this purpose if it be taken as the hydrate H2SO4, or containing only a
small amount of water, because such sulphuric acid dissolves the oxides of nitrogen,

COMPOUNDS OF N IT I : < '< ; K N WITH HYDKOGEN AND < > X YGEN 285

steam and gases necessary for the preparation of the sulphuric acid
passes through these chambers and tubes. The acid as it is formed falls
to the bottom of the chambers, or runs down their walls, and flows from
one chamber to another (from the last to the first, to permit of which
the partitions do not reach to the bottom of the chambers), and there-
fore the floor and walls of the chambers should be made of a material
on which the sulphuric acid will not act. Among the ordinary metals
lead is the only one suitable. The other metals, such as iron, zinc, or
copper, are corroded by the acid ; glass and earthenware are not acted
011, but would not withstand the changes of temperature which occur in
the chambers, and would be difficult to join closely ; whilst wood and
similar materials are destroyed by the acid.

For the formation of the sulphuric acid it is necessary to introduce
sulphurous anhydride, steam, air, and nitric acid, or some oxide of
nitrogen, into the chambers. The sulphurous anhydride is produced by
burning sulphur or iron pyrites. This is carried on in the furnace with
four hearths to the left of the drawing. Air is led into the chambers
and furnace through orifices in the furnace doors. The current of air
and oxygen is regulated by opening or closing these orifices to a greater
or less extent. The ingoing draught in the chambers is brought about
by the fact that heated gases and vapours pass into the chambers whose
temperature is further raised by the reaction itself, and also by the
remaining nitrogen being continually withdrawn from the outlet (above
the tower K) by a tall chimney situated near the chambers. Nitric
acid is prepared from a mixture of sulphuric acid and Chili saltpetre, in
the same furnaces in which the sulphurous anhydride is evolved (or in
special furnaces). Not more than 8 parts of nitre are taken to 100 parts
of sulphur burnt. On leaving the furnace the vapours of nitric acid
and oxides of nitrogen mixed with air and sulphurous anhydride first
pass along the horizontal tubes T into the receiver B B, which is partially
cooled by water flowing in on the right-hand side and running out on
the left by o, in order to reduce the temperature of the gases entering
the chamber. The gases then pass up a tower filled with coke, and
shown to the left of the drawing. In this tower are placed lumps of

They may be easily expelled from this solution by heating or by dilution with water, as
they are only slightly soluble in aqueous sulphuric acid. Besides which, sulphurous
anhydride acts on such sulphuric acid, being oxidised at the expense of the nitrous anhy-
dride, and forming nitric oxide from it, which again enters into the cycle of action.
Therefore the sulphuric acid which has absorbed the oxides of nitrogen escaping from
the chambers in the tower K (see fig. 50) is led back into the first chamber, where it
comes into contact with sulphurous anhydride, by which means the oxides of nitrogen
are reintroduced into the reaction which proceeds in the chambers. This is the use of
the towers (Gay-Lussac's and Glover's) which are erected at either end of the chambers.

coke (partially distilled coal), over which trickle.- sulphuric acid from
t IK- reservoir M. This arid In-. absorbed in the cud town1 K t lie oxides of
nit i-o -•. MI e -raping tVom t lie chamber. This end lower is also Tilled with
voki . over whirh a stream of strong sulphuric acid trickles from the
reservoir \i. The acid sjireads over the coke, and, owinv; to the lar^e
surface oil'ered 1 »y the coke, absorbs tlie greater part of the oxides of
nitrogen escaping from the chambers. The sulphuric acid in passing
down the tower becomes saturated with the oxides of nitrogen, and
(lows out at A into a special receiver (in the drawing situated bv tlie
side i if the t'u rnaces ). froii i which it is forced up the tube.-. I, //'bv steam
pressure into the reservoir M, situated above the first tower. The Leases
passing i hroii u'li this tower from the furnace on coming into contact
with tlie sulphuric acid take up the oxides of nitrogen contained in it.
and these are thus returned to the chamber ami ai^-iin participate in the
reaction. The sulphuric acid left after their extraction Hows into the
chambers. I hus, on leaving the first coke tower the sulphurous anhy-
dride, air. and vapours of nitric acid and ot the oxides of nitrogen pass
through the upper tube /// into the chamber. J I ere they come into
contaci with steam introduced bv lead tubes into various parts of the
chamber. The reaction takes place in the presence of water, the sul-
phuric acid falls to the bottom of the chamber, and the same process
lakes place in the following chambers until the whole of the sulphurous
anhvdride is consumed. A somewhat greater proportion of air than is
strictly necessary is passed in, in order that no sulphurous anhvdride
should be left unaltered for want of sufficient oxygen. The presence of
an excess of o.xvovn is shown bv the colour of the ibises escaping from
the la.-,1 chamber (into I)). If they be (.if a pale colour it indicate.-- an
i iis.u thciencv ot air (and the presence of sulphurous anhvdride), as other-
\\ise peroxide ot nitrogen would be formed. A very dark colour shows
an excels < it air. which is also disadvantageous, because it increases the
inevitable loss ol nitric oxide by increasing the mass of escaping
Leases.''1'

1 l'.\ ili •• •• •. - a- iniicli a ~ -J.M KM iini kiliiLfraiiis dt' I'li.-iniliiT in-ill, cnm-.iinin^ uliuut
f,(i |..c. n] I In li\ ili-.iii- I i ,S( ), ;ii ii I ill ii nit. Id |i«-r ( -i -nt . of \Viitrr, ni.i \ In- 1 1 i.i nu tart urnl
i . • < | '!,i n: ' 1 1 r.iHin ,-ul ,,,- i uci r,.-, en | MI- it \ ' \\ ii In nit -l.i|i|M-'- -I. Tli is process
ii.i In i i ; In -i n -I i ,i ili-jl'1'1' "1 pi-l't'i-rt inli t ll;il ii- iiillrll :i - .".I 111 parts M| 1
r< "111 i I miii I n!i p:irt- nl -nl pi i iir, \s hiUl t lit' I ln'iin-l i<-;i] iinn

...".: |. . , ! Th,. anil part U itll it- r\rc-> "I Uiitrr <Hi hciil III-.

!,M| . a, I M ,•]- . II., \\c\cr. : ii.' ami >•< ml a 1 1 1 i 1 1 •_• alnuil 7.', per cent, of
;, c,i i i',.,11111^ . aln M.|\ IH-JIII to arl mi I In- le.-nl u hen he.ite.l. ami 1 herefore

|, ,T I . ,i in (, nr arl ..•!< - ,ni -ulphurie iirnl. The ai|ll, • arid (.Ml I'.allliir)

n ,i , ,| , haiiilier ami. The ami eolieeiil rali-il to HO

COMPOUNDS OF MTi:«xiKN WITH 11 YI>i;n<;KN AND OXYGEN 287

t,.i'i<li\ X.2O,f'7 is similar to water in its volumetric composi-
tion. Two volumes of nitrous oxide are formed from two volumes of
nitrogen and one volume of oxygen, which may be shown by the common

vitriol acid is also used. In Kngland alone more than 1000 million kilograms of
chamber acid arc produced by this method. The formation of sulphuric acid by the
action of nitric acid was discovered by Drebbel, and the first lead chamber was erected
by Roebuck, in Scotland, in the middle of the last century. The essence of the process
was ably brought to light at the beginning of this century, when many improvements
were introduced into practice.

67 Hyponitrous acid, corresponding with nitrous oxide (as its anhydride), is not
known in a pure state, but its salts are known. They are prepared by the reduction of
nitrous land consequently of nitric) salts by sodium amalgam. If this amalgam be
added to a cold solution of an alkaline nitrite until the evolution of gas ceases, and
the excess of alkali saturated with acetic acid, an insoluble yellow precipitate of silver
hyponiti ite. X AgO, will be obtained on adding a solution of silver nitrate. The hyponi-
trite is insoluble in cold acetic acid, and decomposes when heated, with the evolution of
nitrous oxide. If rapidly heated it decomposes with an explosion. It is dissolved
unchanged by feeble mineral acids, whilst the stronger acids (for example, sulphuric
•and hydrochloric acids) decompose it, with the evolution of nitrogen, nitric and nitrous
acids remaining in solution. Among the other salts of hyponitrous acid, HNO, the salts
of lead, copper, and mercury are insoluble in water. This is almost all that is at
present known (according to the researches of Divers) concerning this compound, which
in its composition and reactions presents a certain analogy to hypochlorous acid.
There is even reason to think that the composition of silver hyponitrite, AgNO, is more
complex than was at first supposed. As a substance which has not been sufficiently
fully invest iuated, hyponitrous acid must be ranked with those compounds which yet
present much that is doubtful. It is evident from the very method of its formation that
it belongs to the class of compounds which are intermediate between the oxygen and
hydrogen compounds of nitrogen. If its composition be N HO, then perhaps it =NH3,
in which two equivalents of hydrogen are replaced by oxygen (see p. 258). A substance
of this composition, containing the hydrogen combined with the nitrogen, should most
probably be isomeric, and not identical, with the true hydrate of nitrous oxide, because
in the latter the hydrogen would be in the form of hydroxyl. Among such insufficiently
investigated compounds — which, however, are of great interest — we must rank amidogen^
or lujdrazhic. X\>H4, which was prepared by Curtius (1887) by means of ethyl diazo-
acetate, or triazoacetic acid. Curtius and Jay (1889) showed that triazoacetic acid,
CHNo.COOH (the formula should be tripled), when heated with water or a mineral
acid gives (quantitatively) oxalic acid and amidogen (hydrazine), CHN2.COOH +
2ELO = C.2O..,(OH)o + N.,H4 — i.e. (empirically), the oxygen of the water replaces the nitro-
gen of the azoacetic acid. The amidogen is thus obtained in the form of a salt.
With acids amidogen forms very stable salts of the two types, N2H4HX, and N2H4H.^X,,
as, for example, with HC1, HUSO4, &c. These salts are easily crystallised ; in acid solu-
tions they act as powerful reducing agents, evolving nitrogen ; when ignited they are
decomposed into ammoniacal salts, nitrogen, and hydrogen; with nitrites they evolve
nitrogen. The sulphate, N.,H4,H.2SO4, is sparingly soluble in cold water (8 parts in 100
of water), but is very soluble in hot water ; its specific gravity is 1'378, it fuses at 254°,
with decomposition. The hydrochloride, NoH4,2HCl, crystallises in octahedra, is very
soluble in water, but not in alcohol ; it fuses at 198°, evolving hydrogen chloride, and
forming the salt N2H4HC1 ; when rapidly heated it decomposes with an explosion ; with
platinie chloride it immediately evolves nitrogen, forming platinous chloride. By the
action of the alkalis the salts N2H,,2HX give hydrate of amidogen, N2H4,H2O,
which is a fuming liquid, boiling at 119°, almost without odour, and whose aqueous
solution corrodes glass and india-rubber, has an alkaline taste and poisonous properties.
The reducing capacities of the hydrate are clearly seen from the fact that it reduces the

288 PRINCIPLES OF CHEMISTRY

method of the analysis of the oxides of nitrogen (by passing them over
red-hot copper or sodium). In contradistinction to the other oxides of
nitrogen, it is not directly oxidised by oxygen, but it may be obtained
from the higher oxides of nitrogen by the action of certain deoxidising
substances ; thus, for example, a mixture of two volumes of nitric
oxide and one volume of sulphurous anhydride if left in contact with
water and spongy platinum is converted into sulphuric acid and
nitrous oxide, 1>NO + SO, + H,O=H,SO4 + N2O. Nitric acid, also,
under the action of certain metals —for instance, of zinc fi8— gives nitrous
oxide, although in this case mixed with nitric oxide. The usual method
of preparing nitrous oxide consists in the decomposition of ammonium
nitrate by the aid of heat, because in this case only water and nitrous
oxide are formed, NH4N03 = 2H2O + N2O (a mixture of NH4C1 and
KNO3 is sometimes taken). The decomposition G(J proceeds very easily
in an apparatus like that used for the preparation of ammonia or
oxygen — that is, in a retort or flask with a gas-conducting tube. The
decomposition must, however, be carried on carefully, as otherwise
nitrogen is formed from the decomposition of the nitrous oxide.70

Nitrous oxide is not a permanent gas (absolute boiling point + 36°),
it is easily liquefied by the action of cold under a high pressure ; at
15° it may be liquefied by a pressure of about 40 atmospheres. This
gas is usually liquefied by means of the force pump71 shown in fig. 51.

metals platinum and silver from their solutions. With mercuric oxide it explodes. It
reacts directly with the aldehydes HO, forming N2R2 and water ; for example, with lien/-
aldehyde it gives the very stable insoluble benzalazine (CgHsCHN).?, of a yellow colour.
Further research should explain the relation of these very interesting salts to amidogen
(N2H4) itself, which has not yet been isolated. Amidogen must be regarded as a substance
which stands to ammonia in the same relation as hydrogen peroxide stands to water.
Water, H(OH), gives, according to the law of substitution, as was clearly to be expected.
(OH)(OH) — that is, peroxide of hydrogen is the free radicle of water (hydroxyli. So also
ammonia, H(NH2), forms hydrazine, (NH2)(NH2) — that is, the free radicle of ammonia,
NH2, or amidogen. In the case of phosphorus a similar substance, us we shall after-
wards see, has long been known under the name of liquid phosphuretted hydrogen, P ,H j.

68 It is remarkable that electro-deposited copper shavings give nitrous oxide with a
10 p.c. solution of nitric acid, whilst ordinary copper gives nitric oxide. It is here
evident that the physical and mechanical structure of the substance effects the course of
the reaction — that is to say, it is a case of contact.

69 This decomposition is accompanied by the evolution of about 25000 calories per
molecular quantity NH^NOs, and therefore takes place with ease, and sometimes with
an explosion.

70 In order to remove any nitric oxide that might be present, the gas obtained is
passed through a solution of ferrous sulphate. As nitrous oxide is very soluble in cold
water (at 0° 100 volumes of water dissolve 130 volumes of N2O, at 20°, 67 volumes), it
must be collected over warm water. The nitrous oxide is much more soluble than nitric-
oxide, which is in agreement with the fact that the nitrous oxide is much more easily
liquefied than the nitric oxide.

71 Faraday obtained liquid nitrous oxide by the same method as liquid ammonia,

COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 289

As it is liquefied with comparative ease, and as the cold produced by its
vaporisation is very considerable,72 it (as also liquid carbonic anhy-
dride) is often employed in investigations requiring a low temperature.

iJ''io. 51. — Natterer's apparatus for the preparation of liquid nitrous oxide and carbonic anhydride.
The gas first passes through the vessel V, for drying, and then into the pump (a section of the
upper part of the apparatus is given on the left). The piston t of the force pump is moved by the
crank E and fly-wheel turned by hand. The gas is pumped into the iron chamber A, where it is
liquefied. The valve S allows the gas to enter A, but not to escape from it. The cliamber and
pump are cooled by the jacket B, filled with ice. When the gaa is liquefied the vessel A is uu-
scri'\ved from the pump, and the liquid may be poured from it by inverting it and unscrewing the
valve v, and the liquid then runs out of the tube x.

by heating dry ammonium nitrate in a closed bent tube, one arm of which was immersed
in a freezing mixture. In this case two layers of liquid are obtained at the cooled end,
a lower layer of water and an upper layer of nitrous oxide. This experiment should be
conducted with great care, as the pressure of the nitrous oxide in a liquid state is con-
siderable, namely (according to Regnault), at +10° = 45 atmospheres, at 0° = 86 atmo-
spheres, at -10° = 29 atmospheres, and at -20° = 28 atmospheres. It boils at -92°,
and the pressure is then therefore = 1 atmosphere.

72 Liquid nitrous oxide, in vaporising at the same pressure as liquid carbonic
-anhydride, gives rise to_almost equal or even slightly lower temperatures. Thus at a
VOL. I. U

290 PRINCIPLES OF CHEMISTRY

Nitrous oxide forms a very mobile, colourless liquid, which acts on the
skin, and which is incapable in a cold state of oxidising either
metallic potassium, phosphorus, or carbon ; its specific gravity is
slightly less than that of water (O94). When evaporated under the
receiver of an air-pump, the temperature falls to — 100°, and the
liquid solidifies into a snow-like mass, and partially forms transparent
crystals. Both these substances are solid nitrous oxide. Mercury
is immediately solidified in contact with evaporating liquid nitrous
oxide.73

When introduced into the respiratory organs (and consequently
into the blood also) nitrous oxide produces a peculiar kind of drunken-
ness accompanied by spasmodic movements, and hence this gas, dis-
covered by Priestley in 1776, received the name of * laughing gas.' On a
prolonged respiration it produces a state of insensibility (it is an
anaesthetic like chloroform), and it is therefore employed in dental and
surgical operations.

Nitrous oxide is easily decomposed into nitrogen and oxygen by the
action of heat, or a series of electric sparks ; and this explains why a
number of substances which cannot burn in nitric oxide do so with
great ease in nitrous oxide. In fact, when nitric oxide gives some
oxygen on decomposition, this oxygen immediately unites with a fresh
portion of the gas to form nitric peroxide, whilst nitrous oxide does
not possess this capacity for further combination with oxygen.74 A
mixture of nitrous oxide with hydrogen explodes like detonating
gas, gaseous nitrogen being formed, N2O + H2=H2O + N2. The
volume of the remaining nitrogen is equal to the original volume of
nitrous oxide, and is equal to the volume of hydrogen entering into

pressure of 25mm. carbonic anhydride gives a temperature as low as —115°, and nitrous
oxide of —125° (Dewar). The similarity of these properties and even of the absolute
boiling point (CO2 + 32°, N2O + 86°) is all the more remarkable because these gases have
the same molecular weight = 44 (Chap. IV. Note 10, and Chap. VII.).

73 A very characteristic experiment of simultaneous combustion and of intense.
cold maybe conducted by means of liquid nitrous oxide; if liquid nitrons oxide be
poured into a test tube containing some mercury, then the mercury will solidify, and if
a piece of red-hot charcoal be thrown upon the surface of the nitrous oxide it will con-
tinue to burn very brilliantly, giving rise to a high temperature.

74 In the following chapter we shall consider the volumetric composition of the
oxides of nitrogen. It explains the difference between nitric and nitrous oxide.
Nitrous oxide is formed with a diminution of volumes (contraction), nitric oxide without
contraction, its volume being equal to the sum of the volumes of nitrogen and oxygen of
which it is composed. By oxidation, if it could be directly accomplished, two volumes of
nitrous oxide and one volume of oxygen would not give three but four volumes of nitric
oxide. These facts must be taken into consideration in comparing the calorific equiva-
lents of formation, the capacity for supporting combustion, and other properties of nitrous
and nitric oxides, N2O and NO.

COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 291

combination with the oxygen ; hence in this reaction equal volumes of
nitrogen and hydrogen replace each other. Nitrous oxide is also very
easily decomposed by red-hot metals ; and sulphur, phosphorus, and
charcoal burn in it, although not so brilliantly as in oxygen. An equal
quantity of a substance in burning in nitrous oxide evolves more heat
than in burning in oxygen ; which most clearly shows that in the
formation of nitrous oxide by the combination of nitrogen with oxygen
there was not an evolution but an absorption of heat, there being no
other source for the excess of heat in the combustion of substances in
nitrous oxide. If a given volume of nitrous oxide be decomposed by
a metal — for instance, sodium — then there remains, after cooling and
total decomposition, an exactly equal volume of nitrogen to that of
the nitrous oxide taken ; consequently, the oxygen is, so to say, distri-
buted between the atoms of nitrogen without producing an increase
in the volume of the nitrogen.

u 2

1 1 YI>I:O< ;i-;\ combines with oxv^'en in the proportion of t\vo volumes to
one. The composition l»y volume of nitrous oxide is exaetlv similar
it is composed of two volumes of nitrogen and one volume of oxv^en.
P.v decomposing ammonia liv tlie action of ;in electric spark it is easv
to prove tli;it it contains one volume of nitrogen to three volumes of
hvdroLi'en. So, similarly, it is found, whenever ;t compound is decom-
posed and tlie volumes of the ibises proceeding from it are measured,
that tlie volumes of the ibises or vapours entering into combination
are in a verv simple proportion one to another \\"ith water, nitrous
oxide, tv<-.. this may be proved by direct observation, bui in themajoritv
• if cases, and especially with substances which, although volatile that
is, capable of passing into a gaseous (or vaporous) state are liquid at
the ordinarv temperature, such a direct method of observation pre-
sents manv difficulties. Hut then, if the densities of the vapours and
erases lit- known, the same simplicitv in their ratio is shown bv calcula-
tion. The \olumeof a substance is proportional to its weight and
inverselv proport ional to its densitv. and therefore hy dividing the
amount liv weight of each substance entering into the com]>osition of
a compound li\- its densitv in the gaseous or \aporous state we shall
olitain factors \\hich will be in the same proportion as the volumes of
the substances entering into the composition of the compound.' So.

i If the v.> i-ht In' indii-iitci] l,y I'. tli.> density l>\ I >. iinil Ilir VohniH' hy \', tlini

P KV.
I)

,,-'-,- , dill; nil t hi' \ -ti'lll nf Illr cv |,]T i<.|| ; I'. I ). mid V. If I >

• . mi Irii'iil \ h in ( liiip. I. Nnli' '.' . Ilii- cnhic niciisurc «\'

' • f he I .iki'll .1 :i Unit nl' Viilllllir t lu'li l\ I . I'.llt \\ ll;lli'\ iT it
mm ..!-.• f.iki'ii. In MM , .-I ... | ,t .T. :i 1 1 1 r- .1 r 'I mil t, tin- hunk. (In-

MOLECULES AND ATOMS 293

for example, water contains eight parts by weight of oxygen to one
part by weight of hydrogen, and their densities are 16 and 1, conse-
quently their volumes (or the above-mentioned factors) 1 and ^, and
therefore it is seen without direct experiment that water contains two
volumes of hydrogen for every one volume of oxygen. So also, know-
ing that nitric oxide contains fourteen parts of nitrogen and sixteen
parts of oxygen, and knowing that the specific gravities of both these
gases are fourteen and sixteen, we find that the volumes in which nitro-
gen and oxygen combine for the formation of nitric oxide are in the
proportion of 1:1. Consequently, nitric oxide is a combination of equal
volumes of nitrogen and oxygen. We will cite another example. In
the last chapter we saw that the density of NO2 only becomes constant
and equal to twenty -three (referred to hydrogen) above 135°, and there-
fore a method of direct observation of the volumetric composition of
this substance would be very difficult at so high a temperature. But
it may be easily calculated. N02, as is seen from its formula and
analysis, contains thirty-two parts by weight of oxygen to fourteen
parts by weight of nitrogen, forming forty- six parts by weight of NO2,
and knowing the densities of these gases we find that one volume of
nitrogen with two volumes of oxygen gives two volumes of nitrogen
peroxide. Therefore, knowing the amounts by weight of the substances
participating in a reaction or forming a given substance, and knowing
the density of the gas or vapour,2 the volumetric relations of the sub-

the expression of chemical composition, then the weight of an atom is taken as unity.
The density of gases, D, is also taken in reference to the density of hydrogen, and the
volume V in metrical units (cubic centimetres or cubic metres, &c.), if it be a matter of
absolute magnitudes of volumes, and if it be a matter of chemical transformations— that
is, of relative volumes — then the volume of an atom of hydrogen, or of one part by weight
of hydrogen, is taken as unity, and all volumes are expressed according to these units.

2 As the volumetric relations of vapours and gases, next to the relations of substances
by weight, form the most important province of chemical learning and the most important
means for the attainment of chemical conclusions, and inasmuch as these volumetric
relations are determined by the densities of gases and vapours, therefore the methods
of determining the densities of vapours (and also of gases) form important means
in chemical research. These methods are described in detail in works on physics and
physical and analytical chemistry, and therefore we here only touch on the general
principles of the subject.

If we know the weight p and volume v, occupied by the vapour of a given substance
at a temperature t and pressure h, then its density may be directly obtained by dividing
p by the weight of a volume v of hydrogen (if the density be expressed according to
hydrogen, see Chap. II. Note 28) at t and h. Hence, the methods of determining the
density of vapours and gases are based on the determination of p, v, t, and h. The two
last data (the temperature t and pressure h) are given by the thermometer and barometer
and the heights of mercury or other liquid confining the gas, and therefore do not
require further explanation. It need only be remarked that : (1) In the case of easily-
volatile liquids there is no difficulty in procuring a bath with'a constant temperature,
but that it is, nevertheless, best (especially considering the inaccuracy of thermo-

294 PRINCIPLES OF CHEMISTRY

stances acting in a reaction or entering into the composition of a com-
pound, may be also determined.

meters) to have a medium of absolutely constant temperature, and thereforeTto take
either a bath in which some substance is melting — such as melting ice at 0° or crystals j^of
sodium acetate, melting at + 56° — or, as is more generally practised, to place the^vessel
containing the substance to be experimented with in the vapour of a liquid boiling at a
definite temperature, and, knowing the pressure under which it is boiling, to determine
the temperature of the vapour. For this purpose the boiling points of water at different
pressures are given in Chap. I. Note 11, and the boiling points of certain easily procurable
liquids at various pressures are given in Chap. II. Note 27. (2) With respect to tempera-
tures above 300° (below which mercurial thermometers may be conveniently employed),
they are most simply obtained constant (to give time for the weight and volume of a
substance being observed in a given space, and to allow that space to attain the calcu-
lated temperature t) by means of substances boiling at a high temperature. Thus, for
instance, at the ordinary atmospheric pressure the temperature t of the vapour of
sulphur is about 448°, of phosphorus pentasulphide 518°, of tin chloride 606°, of cad-
mium 770°, of zinc 930° (according to Violle and others), or 1040° (according to Deville),
&c. (3) The indications of the hydrogen thermometer must be considered as the most
exact (but hydrogen diffuses through ignited platinum, and therefore nitrogen is then
employed). (4) The temperature of the vapours used as the bath should in every case
be several degrees higher than the boiling point of the liquid whose density is to be
determined, in order that no portion should remain in a liquid state. But even in this
case, as is seen from the example of nitric peroxide (Chap. VI.), the vapour density
does not always remain constant with a change of t, as it should were the law of the
expansion of gases and vapours absolutely exact (Chap. II. Note 26). If variations of
a chemical and physical nature similar to that which we saw in nitric peroxide take
place in the vapours, the main interest is centred in constant densities, which do not
vary with t, and therefore the possible effect of t on the density must always be kept
in mind in having recourse to this means of investigation. (5) Usually, for the sake of
convenience of observation, the vapour density is determined at the atmospheric pres-
sure which is read on the barometer ; but in the case of substances which are volati-
lised with difficulty, and also of substances which decompose, or, in general, vary at
temperatures near their boiling points, it is best or indispensable to conduct the deter-
mination at low pressures, whilst for substances which decompose at low pressures the
observations have to be conducted under a more or less considerably increased pressure.
(6) In many cases it is convenient to determine the vapour density of a substance in
admixture with other gases, and consequently under the partial pressure, which may be
found, knowing the volume of the mixture and that of the intermixed gas (see Chap. I.
Note 1). This method is especially important for substances which are easily decom-
posable, because, judging by the phenomena of dissociation, a substance is able to remain
unchanged in the atmosphere of one of its products of decomposition. Thus, Wurtz
determined the density of phosphoric chloride, PClj, in admixture with the vapour of
phosphorous chloride, PCls. (7) It is evident, from the example of nitric peroxide, that a
change of pressure may alter the density and aid decomposition, and therefore identical
results are sometimes attained (if the density be variable) by raising t and lowering // ;
.but if the density does not vary under these variable conditions (at least, not to an
observable degree exceeding the limits of experimental error), then this conx/anf density
indicates the gaseous and invariable state of a substance. The laws hereafter laid down
refer only to such vapour densities. But the majority of volatile substances show such
a constant density at a certain distance from their boiling points up to the starting point
of decomposition. Thus, the density of aqueous vapour does not vary for t between
the ordinary temperature and 1000° (there are no trustworthy determinations beyond
this) and for pressures varying from fractions of an atmosphere up to several atmo-
spheres. If, however, the density does vary considerably with a variation of h and t,

MOLECULES AND ATOMS

295

Such an investigation (either direct, or by calculation from the densi-
ties and composition) of every chemical reaction, resulting in the forma-

the fact may servo as a guiding string for the investigation of the chemical changes
which are undergone by the substance in a state of vapour, or, at least, as an indi-
cation of a deviation from the laws of Boyle, Mariotte, and Gay-Lussac (for the expansion
of gases from t). In certain cases the separation of one form of deviation from the other
may be explained by special hypotheses.

With respect to the means of determining p and v, with a view to finding the vapour
density, we may distinguish three chief methods: (a) by weight, by ascertaining the

FIG. 52. — Apparatus for determining the vapour density
by Dumas' method. A small quantity of the liquid
whose vapour density is to be determined is placed in
the glass globe, and heated in a water or oil bath to a
temperature above the boiling point of the liquid.
When all the liquid has been converted into vapour
and has displaced all the air from the globe, the latter is
; sealed up and weighed. The capacity of the globe is
then measured, and in this manner the volume occu-
pied by a known weight of vapour at a known tem-
perature is determined.

FIG. 53.— Deville and Troost's apparatus for
determining the vapour densities, accord-
in g to Dumas' method, of substances
which boil at high temperatures. A porce-
lain globe containing the substance whose
vapour density is to be determined is
heated in the vapour of mercury (350°),
sulphur (410°), cadmium (850°), or zinc
(1040°). The globe is sealed up in an
oxyhydrogen flame.

weight of a definite volume of vapour ; (6) by volume, by measuring the volume occupied
by the vapour of a definite weight of a [substance ; and (c) by displacement. The last-
mentioned is essentially volumetric, because a known weight of a substance is taken,
and the volume of the air displaced by the vapour at a given t and h is determined.

The method by weight (a) is the most trustworthy and historically important. Dumas'
method is typical. An ordinary spherical glass or porcelain vessel, like those shown
respectively in figs. 52 and 53, is taken, and an excess of the substance to be experimented
upon is introduced into it. The vessel is heated to a temperature t higher than the boil-
ing point of the liquid ; this gives a vapour which displaces the air, and fills the sphe-
rical space. When the air and vapour cease escaping from the sphere, it is fused
up or closed by some means; and when cool, the weight of the vapour remaining
in the sphere is determined (either by direct weighing of the vessel witk the vapour
and introducing the necessary corrections for the weight of the air and of the vapour
itself ; or the weight of the volatilised substance is determined by chemical methods),
and the volume of the vapour at t and the barometric pressure h is then calculated.

ti.m ofdetinite chemical compounds, shows that the volumes of tlic react-
ing substances in a -aseous ,,r vaporous state are either e«|iml or arc. in

^;j

Jt

"-- "

'•\

—-ASBESTOS

MOLECULES AND ATOMS 297

simple multiple proportion.3 This forms thejirst law of those discovered
by Gii ;i- 1, tixxnc. This law may be formulated as follows : The amounts of
nnfistances entering into chemical reaction, occupy under similar physical
conditions, in a gaseous or vaporous state, equal or multiple volumes.
This law not only refers to elements, but also to compounds entering
into mutual chemical combination ; thus, for example, one volume of
ai 1 11 uonia gas combines with one volume of hydrogen chloride. For in
the formation of sal-ammoniac, NH4C1, there enters into reaction 17 parts
by weight of ammonia, NH.,, which is 8^ times denser than hydrogen,
and 36 -5 parts by weight of hydrogen chloride, whose vapour density is
18| times that of hydrogen, as has been proved by direct experiment.
By dividing the weights by the respective densities we find that the
volume of ammonia, NH3, is equal to two, and so also the volume of
hydrogen chloride. Hence the volumes of the compounds which here
combine together are equal to each other. By taking into considera-
tion that the law of Gay-Lussac holds good, not only for elements, but
also for compounds, it should be expressed as follows : Substances
interact with one another in commensurable volumes of their vapours.*

space b is heated to a constant temperature t (by the surrounding vapours of a liquid of
constant boiling point), and the air (or other gas enclosed in this space) is allowed to
attain this temperature, and when it has done so a glass bulb containing a weighed quan-
tity of the liquid to be experimented with is dropped into the space. The liquid is imme-
diately converted into vapour," and displaces the air into the graduated cylinder e. The
amount of this air is calculated from its volume, and consequently the volume at t, and,
therefore, also the volume occupied by the vapour, is found. The general arrangement
of the apparatus is given in fig. 55.

5 Vapours and gases, as was explained in the second chapter, are subject to the same
laws, which are, however, only approximate. It is evident that for the deduction of the
laws which will presently be enunciated it is only possible to take into consideration a
perfect gaseous state (removed from the liquid state) and 'chemical invariability in
which the vapour density is constant — that is, the volume of a given gas or vapour
varies like a volume of hydrogen, air, or other gas, with the pressure and temperature.

It is necessary to make this statement in order fliat it may be clearly seen that the
laws of gaseous volumes, presently to be described, are in the most intimate connec-
tion with the laws of the variations of volumes with pressure and temperature. And
as these latter laws (Chap. II.) are not infallible but only approximately exact, the same,
therefore, applies to the laws presently to be described. And as it is possible to find more
exact laws (a second approximation) for the variation of v with p and t (for example,
Van der Waals' formula, Chap. II. Note 38), so also a more exact expression of the
relation between the composition and the density of vapours and gases is also possible.
But to prevent any doubt arising at the very beginning as to the breadth and general
application of the laws of volumes, it will be sufficient to mention that the density of
such gases as oxygen, nitrogen, and carbonic anhydride is already known to remain
constant (within the limits of experimental error) between the ordinary temperature
and a white heat ; whilst, judging from what is said in my work on the ' Tension
of Gases' (vol. i. p. 9), it may be said that, as regards pressure, the density remains
very constant even when the deviations from Mariotte's law are very considerable. How-
ever, in this respect the number of data is yet too small for forming an exact conclusion.

4 We must recollect that this law is only ^proximate, like Boyle and Mariotte's law,

Tli.' law of o iiul lining volumes and the l;t\\ «i multiple proportions
\\eiv discoveied independently of each other the one in 1-Yanee by
( lay-Lr.ssae, the oilier in Kmjand by 1 )altoii. alinosi simultaneously.
In tin- ianmiau'e of the atomic hypothesis ii mav lie -aid that atomic
quantities of elements occupy eijiial o!' multiple Volumes.

The tirM la\v of (!ay-Lussae expresses the relation l>et \veen the
volumes ' >t the component parts ot a compound ; let u- now consider
tin- relation existing between the volumes of the eomponent parts and
of the c impounds which proceed trom them. This ma\" sometimes be
determined by direct observation. Thus the volume occupied by \vatei1,
formed by 1 \vo volumes of hydrogen and one \olume of oxvgen, ma}' be
determined bv the aid of the apparat us shown in tie;. ~i(\. 'Jne long
gla-s tube is closed at the top and open ai the bottom, which is
immersed in a cylinder containing mercury. 1 he cio>cd end i>
fui'iii-hed \\ ith \\'ires like a eudiometer (p. H»> ). The t u'oe is idled with
inercurv. and then a certain \olume of detonating u'as is introduced.
Thi> ga> is obtained from the decomposition of water, and therefore in
every three volumes contains two volumes of hydrogen and one volume
of oxygen. The t ube is surrounded by a second and wider glass tube, and
the vapour of a substance boiling above \(^> that is. whose boiling point
is higher than that of water is passed through the annular space be-
tween them. Amvl alcohol, whose boiling point is \-\\l , may betaken
for this purpose. The amyi alcohol is boiled in the vessel to the right
hand and its vapour passed bet ween the walls of the t \\ o tubes. In the
case of amvl alcohol, the otiter glass t ube would be connected with a
Condenser to present the escape into the air of the unpleasant-smelling
vapour. The detonating gas is thtis heated up to a temperature of 1 .'»!' .
\\ hi'ii its \olume becomes constant it is measured, the height of the
column of mercury in the tube above the le\el of the mercury in the
cylinder being noted. Let this volume equal r; it will therefore con-
tain ; '• of oxygen and r! r of h\-drogen. The current of \apour is
then stopped, t he open end of the tube tilled \\ith merrur\', and then
closed, and the gas exploded ; water is formed, which condenses into a

liquid. 'I IH • \oluine oeell pled by the \ a poll I' ot the Water fol'llU'd lias

no\\ to bi' det ermined. l-'or this purpose the \apoiir of the amvl
aleohol is a -a in passed between the 1 llbes, and the \\ hole of t he \\ at er
foi'ined is converted iiito \ajiour at the --,(\\\c lempei'al lire as that at
\\hii-h t he detonat ing gas \\as measured, and the cylinder of mercury
beinu' raised until the column of mercury in the t ube stands at t he same
hcj._dii abo\-e 1 he surface of the luercui'V in the cylinder as it did lie fore

tin- explosion. it is found that the volume of the water formed is e«ju;il to
two-thin Is i- — that is. it is equal to the volume of the hydrogen contained
in the \vattT. ( 'on sequent 1 v, the \ olumet ric com posit ion of water should
IK- expressed in the following terms: Two volumes of hydrogen
combine with one volume of oxygen to form t wo volumes of aqueous
vapour. For substances which are gaseous at the ordinarv tempera-
ture. this direct method of observation is sometimes very easilv con-
ducted : for instance, with ammonia, nitric, and nitrous oxides. Thus
to determine the composition by volume of nitrous oxide, the above-
described apparatus may be employed. Nitrous oxide is introduced
into the tube, and having measured its volume a series of electric
spark- are passed through the gas, and it is found that two volumes of
nitrous oxide give three volumes of gases namely, two volumes of
nitrogen and one volume of oxygen. Consequently, the composition of
nitrous oxide is similar to that of water : two volumes of nitrogen and
one volume of oxygen give two volumes of nitrous oxide. J>y decom-
posing ammonia it is found to be composed in such a manner that two
volumes give one volume of nitrogen and three volumes of hydrogen ;
;d-o. two volumes of nitric oxide are formed by the union of one volume
of oxygen with one volume of nitrogen. The same mav naturally be
proved bv calculation from the vapour densities, as was described above ;
hence the composition by volume of vapours may be determined in the
simplest manner, if their density, composition, and the densities of the
component parts are known. Thus, according to Thomsen, the density
of N<) referred to hvdrogen— 15, and it contains 14 parts of nitrogen,
and 1«; parts of oxygen in l\0 parts : hence the volumetric composition
is 1 volume of nitrogen and 1 volume of oxygen in '2 volumes of
nit ric oxide.

Comparisons of various results made bv the aid of direct observa-
tion.- or calculation, an example of which has just been cited, led (Jay-
LUS-U.C to the conclusion that flu' rnlnitu' (>/'</ coinnonii'l in <> i/nmoim or

,n, -I, (,/' //if <-,,in i>»n< at j«i ft* <>/' irfiii'fi if ixftirntvil (and consequently
tin- -inn of the \olumes of t he eleli MM 1 1 s of \\hicll it is for! ..... 1). This
the „

M<>LK<TLES AND ATOMS 301

When a compound is formed from two or more components, there
is or is not a contraction ; the volume of the acting substances is in
this case either equal to or greater than the volume of the resultant
compound. The reverse is naturally observed in the case of decom-
positions, when from one substance there are produced several of
simpler nature. Therefore in the future we shall term combination
a reaction in which a contraction is observed — that is, a diminution in
the volume of the component bodies in a state of vapour or gas ; and
we shall term decomposition a reaction in which an expansion is pro-
duced ; while those reactions in which the volumes in a gaseous or
vaporous state remain constant (the volumes being naturally com-
pared at the same temperature and pressure) we shall term reactions
of substitution or of replacement or of double decomposition. Thus
the transition of oxygen into ozone is a reaction of combination, the
formation of nitrous oxide from oxygen and nitrogen will also be a
combination, the formation of nitric oxide from the same will be
a reaction of substitution, the action of oxygen on nitric oxide a
combination, and so on.

The degree of contraction proceeding in the formation of chemical
compounds not unfrequently leads to the possibility of distinguishing
the degree of change which takes place in the chemical character of
the components when combined. In those cases in which a contrac-
tion takes place, the properties of the resultant compound are very
different from the properties of the substances of which it is composed.
Hence ammonia bears no resemblance in its physical or chemical
properties to the elements from which it is derived ; a contraction takes
place in a state of vapour, and consequently approach ment of parts —
the distance between the atoms is diminished, and from gaseous sub-
stances there is formed a liquid substance, or at least one which is
easily liquefied. For this reason nitrous oxide formed by the conden-
sation of two permanent gases is a substance which is somewhat
easily converted into a liquid ; again nitric acid, which is formed
from elements which are permanent gases, is a liquid, whilst, on the
contrary, nitric oxide, which is formed without contraction and is de-
composed without expansion, remains a gas which is as difficult to
liquefy as nitrogen and oxygen. In order to obtain a still more com-

and B. A substance AB is produced by their combination. It may, according to the
law of multiple proportion, combine not only with substances C, D, &c., but also with A
and with B. In this new combination the volume of AB, combining with the volume of
A, should be in simple multiple proportion with the volume of A ; hence the volume of
the compound AB is in simple proportion to the volume of its component parts. There-
fore, only one law of volumes need be accepted. We shall afterwards see that the third
law of volumes may also embrace in itself the two first laws.

302 PRINCIPLES OF CHEMISTRY

plete idea of the dependence of the properties of a compound on the
properties of the component substances, it is further necessary to know
the quantity of heat which is developed in the formation of the com-
pound. If this quantity be large — as, for example in the formation of
water — then the amount of energy in the resultant compound wilt be
considerably less than the energy of the elements entering into its com-
position ; whilst, 011 the contrary, if the amount of heat evolved in the
formation of a compound be small, or if there even be an absorption
of heat, as in the formation of nitrous oxide, then the energy of the
elements is not destroyed or is only altered to a slight extent : hence,
notwithstanding the contraction (compression) involved in its forma-
tion, nitrous oxide supports combustion.

The preceding laws were deduced from purely experimental and
empirical data, and as such evoke further consequences, as the law of
multiple proportions evoked the atomic theory and the law of equiva-
lents (Chapter IV.). In point of view of the atomic conception of the
constitution of substances, the question naturally arises as to what
then are the relative volumes proper to those physically-indivisible
molecules which chemically react on each other and consist of the
atoms of elements. The simplest possible hypothesis in this respect
would be that the volumes of the molecules of substances are equal ; or,
what is the same thing, to suppose that equal volumes of vapours and
gases contain an equal number of molecules. This proposition was
first enunciated by the Italian savant, Avogadro, in 1810, It was
also admitted by the French physico-mathematician, Ampere (1815),
for the sake of simplifying all kinds of physico-mathematical concep-
tions respecting gases. But Avogadro and Ampere's propositions were
not generally received in science until Gerhardt in the forties had
applied them to the generalisation of chemical reactions, and had
demonstrated by aid of a series of phenomena that the reactions of
substances are actually accomplished with the greatest simplicity, and
before all, that such reactions take place between those quantities of
substances which occupy equal volumes, and until he had stated the
hypothesis in an exact manner and deduced the consequences that
necessarily flow from it. Following Gerhardt, Clausius, in the fifties,
placed the hypothesis of the equality of the number of molecules in
equal volumes of gases and vapours on the basis of the kinetic theory
of gases. Since then the hypothesis of Avogadro and Gerhardt has
become the basis of contemporary physical, mechanical, and chemical
conceptions ; the consequences arising from it have often been subject
to doubt, but in the end have been verified by the most diverse methods,
and now, when all efforts to refute those consequences have proved

MOLECULES AND ATOMS 303

fruitless, the hypothesis must be considered as verified,6 and the law of
Avogadro-Gcrhardt must be spoken of as fundamental, and as of great
importance for the comprehension of the phenomena of nature. The
law may now bo formulated in two aspects. In the first place, from a
physical aspect : equal volumes of gases (or vapours) at equal tempera-
tures and pressures contain an equal number of molecules — or of such
quantities of matter which are neither mechanically nor physically
divisible — previous to chemical change. In the second place, from a
chemical aspect, the same law may be expressed thus : the quantities of
,sv fixfances entering into chemical reactions occupy, in a state of vapour,
equal volumes. For our purpose, the chemical aspect is the most im-
portant, and therefore, before developing the law and its consequences,
we will stay to consider the chemical phenomena from which the law is
deduced or by which it may be explained.

When two isolated substances interact with each other directly and
easily — as, for instance, an alkali on an acid— then it is found that the
reaction is accomplished between quantities which in a gaseous state
(at equal temperatures and pressures) occupy equal volumes. Thus,
ammonia, NH3, reacts directly with hydrochloric acid, HC1, forming
sal-ammoniac, NH4C1, and in this case the NH3, or 17 parts by weight
of ammonia, occupy the same volume as the 36*5 parts by weight of
hydrochloric acid.7 Ethylene, C2H4, combines with chlorine, C12, in
only one proportion, forming ethylene dichloride, C2H4C12, and this
combination proceeds directly and with great facility ; the reacting
quantities occupying equal volumes. Chlorine reacts with hydrogen in
only one proportion, forming hydrochloric acid, HC1, and in this case

8 It must not be forgotten that Newton's law of gravity, or of the unity of the forces
causing a body to fall to the earth, and the planets to be attracted to and revolve round the
sun, was first a hypothesis, but it became a trustworthy, perfect theory, and acquired
the qualities of a fundamental law owing to the concord between its consequences
and reality. All laws, all theories of natural phenomena, are first hypotheses. Some
are rapidly established by their exact consequences, which agree with facts ; others only
take root by slow degrees.

7 This is not only seen from the above calculations, but maybe proved by experiment.
A glass tube, divided in the middle by a stop-cock, is taken, and one portion filled with
(//•// hydrogen chloride (the dryness of the gases is strictly necessary, because ammonia
and hydrogen chloride are both very soluble in water, and hence a small trace of water
may contain a large amount of these gases in solution) and the other with dry ammonia,
under the atmospheric pressure. One orifice (for instance, of that portion which contains
the ammonia) is firmly closed, and the other is immersed under mercury, and the cock is
then opened. Solid sal-ammoniac is formed, but if the volume of one gas be greater
than that of the other, some of the first gas will remain. By immersing the tube in the
mercury in order that the internal pressure shall equal the atmospheric pressure, it may
easily be shown that the volume of the remaining gas is equal to the difference between
the volumes of the two portions of the tube, and that the remaining gas is that whose
volume was the greater.

304 PRINCIPLES OF CHEMISTRY

equal volumes interact with each other. If an equality of volumes is
observed in cases of combination, it should be all the more frequently
encountered in cases of decomposition taking place in substances which
split up into two others. Indeed, acetic acid breaks up into marsh
gas, CH4, and carbonic anhydride, CO2, and in the proportions in which
they are formed from acetic acid they occupy equal volumes. Also
from phthalic acid, C8H6O4, there may be obtained benzoic acid,
C7H6O2, and carbonic anhydride, CO2, and as all the elements of
phthalic acid enter into the composition of these substances, therefore,
although they cannot re-form it by their direct action on each other (the
reaction is not reversible), still they form the direct products of its
decomposition, and they occupy equal volumes. But benzoic acid,
O7H6O2, is also composed of benzene, C6H6, and carbonic anhydride,
CO2, which also occupy equal volumes.8 There is an immense number
of similar examples among those organic substances to whose study
Gerhardt consecrated his whole life and work, and he did not allow such
facts as these to escape his attention. Still more frequently in the
phenomena of substitution, when two substances act on one another,
and two are produced without a change of volumes — that is, when the
first definition given (p. 4) ^corresponds with that given on p. 301 — it
is found that the two substances acting on each other occupy equal
volumes as well as each of the two resultant substances. Thus,
in general, reactions of substitution take place between volatile acids,
HX, and volatile alcohols, R(OH), with the formation of ethereal salts,
RX, and water, H(OH), and the volume of the vapour of the reacting
quantities, HX, R(OH), and RX, is the same as that for water
H(OH), whose weight, corresponding with the formula, 18, occupies 2
volumes, if 1 part by weight of hydrogen occupy 1 volume and the
density of aqueous vapour referred to hydrogen is 9. Such general
examples, of which there are many,9 show that the reaction of
equal volumes forms a chemical phenomenon, of frequent occur-
rence, indicating the necessity of acknowledging the law of Avogadro-
Gerhardt.

<* Let us demonstrate this by figures. From 122 grams of benzoic acid there is
obtained (a) 78 grams of benzene, whose density referred to hydrogen = 89, hence the
relative volume = 2, and (6) 44 grams of carbonic anhydride, whose density = 22, and
hence the volume = 2. It is the same in other cases.

9 A large number of such generalised reactions, showing reaction by equal
volumes, occur in the case of the hydrocarbon derivatives, because many of these com-
pounds are volatile. The reactions of alkalis on acids, or anhydrides on water, Ax-.,
which are so frequent between mineral substances, present but few such examples,
because many of these substances are not volatile and their vapour densities are
unknown. But essentially, the same is seen in these cases also ; for instance, sulphuric
.acid, H2SO4, breaks up into the anhydride, SO3,and water, H2O, which exhibit an equality

.MOLECULES AND ATO.MS 305

But the question arises : What is the relation of volumes if the
ivac-tion <»f two substances takes place in different proportions, accord-
ing to the law of multiple proportions ? A definite answer can only be
given in cases which have been very thoroughly studied. Thus chlorine,
in acting on marsh gas, CH4, forms four compounds, CH3C1, CH2CL>,
CHC13, and CC14, and it may be established by direct experiment that
the substance CH3C1 (methylic chloride) precedes the remainder, and
that the latter proceed from it by the further direct action of chlorine.
And this substance, CH3C1, is formed by the reaction of equal volumes
of marsh gas, CH4, and chlorine, C12, according to the equation CH4 +
C1.,=CH3C1 + HC1, A great number of similar cases are met with
amongst organic — that is, carbon — compounds. Gerhardt was led to
the discovery of his law by having investigated many such reactions,
and from observing that in them the reaction of equal volumes precedes
all others.

But if nitrogen or hydrogen give several compounds with oxygen,
the above proposed question cannot be answered with complete clear-
ness, because the consecutive formation of one degree of combination
after the other cannot be so strictly defined. It may be supposed, but
neither affirmed with assurance nor confirmed by experiment, that nitro-
gen and oxygen first give nitric oxide, NO, and only afterwards the brown
vapours N2O3 and NO2. Such a sequence in the combination of nitro-
gen with oxygen can only be supposed on the basis of the fact that NO
forms N2O3 and NO2 directly with oxygen. If it be admitted that NO
(and not N2O or N02) be first formed, then this instance would also
confirm the law of Avogadro-Gerhardt, because nitric oxide contains
equal volumes of nitrogen and oxygen. So, also, it may be admitted
that, in the combination of hydrogen with oxygen, hydrogen peroxide
is first formed (equal volumes of hydrogen and oxygen), which is de-
composed by the heat evolved into water and oxygen, all the more as
this explains the presence of traces of hydrogen peroxide (Chap. IV.)
in almost all cases of the combustion or oxidation of hydrogenous sub-
stances ; for it cannot be supposed that water is first formed and then
the peroxide of hydrogen, because up to now such a reaction has not
been accomplished, whilst the formation of H20 from H2O2 is very
easily reproduced.10

of volumes. Let us take another example where three substances combine in equal
volumes : carbonic anhydride, COo, ammonia, NH3, and water, H3O (the volumes of all are
equal to 2), form acid ammonium carbonate, (NH4)HCO3.

10 This opinion as to the primary origin of hydrogen peroxide and of the forma-
tion of water by means of its decomposition, which was always held by me (in the first
editions of this work), has in latter days become more generally accepted, thanks more
especially to the work of Traube. Probably, it explains most simply the necessity for

VOL. I. X

306 PRINCIPLES OF CHEMISTRY

Thus a whole series of phenomena show that the chemical reaction
of substances actually takes place, as a rule, between equal volumes,
but this does not remove the possibility of the frequent reaction of un-
equal volumes, although, in this case, it is often possible to discover a
preceding reaction between equal volumes.11

the presence of traces of water in many reactions, as, for instance, in the explosion of
carbonic oxide with oxygen, and perhaps the very theory of the explosion of detonating
gas and of the combustion of hydrogen gain in clearness and truth if we take into con-
sideration the preliminary formation of hydrogen peroxide and its decomposition. We
may here point out the fact that Ettingen (at Dorpat, 1888) observed the existence of
currents and waves in the explosion of detonating gas by taking photographs, which
showed the periods of combustion and the waves of explosion, which should be taken into
consideration in the theory of this subject. As the formation of H.2O2 from O2 and H2
corresponds with a less amount of heat than the formation of water from H2 and O, it
may be that the temperature of the flame of detonating gas depends on the pre-formation
of hydrogen peroxide.

11 The possibility of reactions between unequal volumes, notwithstanding the general
application of the law of Avogadro-Gerhardt, may, in addition to what has been said
above, depend on the fact that the participating substances, at the moment of reaction,
undergo a preliminary modification, decomposition, isomeric (polymeric) transformation,.
&c. Thus, if NO2 seems to proceed from NoOj, if O2 is formed from Og, and the converse,
then it cannot be denied that the production of molecules containing only one atom is
also possible — for instance, of oxygen — as also of higher polymeric forms — as the
molecule N from N2, or H3 from H.2. In this manner it is naturally possible, by means of a
series of hypotheses, to explain the cases of the formation of ammonia, NH5 from 3 vols,
of hydrogen and 1 vol. of nitrogen. But it must be observed that perhaps our
information in similar instances is, as yet, far from being complete. If the existence of
hydrazine or diamide N2H4 (Chap. VI. Note 6) be verified, then, perhaps, an amide NoH.>
will be discovered [this substance is now stated to'have been obtained] in which 2 vols.
of hydrogen are combined with 2 vols. of nitrogen — that is, the reaction is accom-
plished between equal volumes. If it be shown that diamide gives nitrogen and
ammonia (3N2H4 = N2 + 4NH3) under the action of sparks, heat, or the silent discharger
<fec., then it will be possible to admit that it is formed before ammonia. And, perhaps,
the still less stable amide N2H2, which may also decompose with the formation of
ammonia, is produced before the amide N2H4.

I mention this to show that the fact of there being apparent exceptions to the law of re-
actions between equal volumes does not prove the impossibility of their being included under
the law on further study of the subject. Having admitted a certain law or hypothesis, then
consequences must be deduced from it, and if by their means a clearness and harmony
is arrived at — and all the more, if by their means that which could not otherwise be
known can be recognised — then the consequences verify the hypothesis. This was the case
with the law now under discussion. The simplicity alone of the deduction of the weight
proper to the atoms of the elements, or the fact alone that having admitted the law it
follows, as will afterwards be shown, that the vis viva of the molecules of all gases
is a constant quantity, is alone a sufficient reason for retaining the hypothesis
if not for believing in it as a fact beyond doubt. And as by the acceptance
of the law it became possible to foretell even the properties and atomic weights of
elements which had not yet been discovered, and as these predictions afterwards proved
to be in agreement with the actual facts, it is evident that the law of Avogadro-Gerhardt
penetrates deeply into the nature of the chemical relation of substances. This being
granted, it is possible at the present time to expound and deduce the truth under
consideration in many ways, and in every case it, like all that is highest in science (for
example, the law of the indestructibility of matter, the law of the conservation of energy..

MOLECULES AND ATOMS 307

The law of Avogadro-Gerhardt may also be easily expressed in an
algebraical form. If the weight of a molecule, or of that quantity of a
substance which enters into chemical reaction, and occupies in a state
of vapour, according to the law, a volume equal to the molecules of

other bodies, be indicated by the letters M,, M2 or, in

general, M, and if the letters D,, D2 , or, in general, D,

stand for the density or weight of a given volume of the gases or
vapours of the corresponding substances under certain definite con-
ditions of temperature and pressure, then the law requires that

M1=M, -M = C

D! D2" "D

where C is a certain constant. This expression directly shows that the

volumes corresponding with the weights M l , M2 M, are equal

to a certain constant, because the volume is proportional to the weight
and inversely proportional to the density. The magnitude of C is
naturally conditioned by and dependent on the units taken for the
expression of the weights of the molecules and the densities. The
weights of the molecules (equal to the sum of the atomic weights of
the elements forming a given substance) are usually expressed by
taking the weight of an atom of hydrogen as unity, and hydrogen is
now also chosen as the unit for the expression of the densities of gases
and vapours ; it is therefore only needful to find the magnitude of the
constant for any one compound, as it will be the same for all others.
Let us take water. Its reacting mass is expressed (conditionally)
by the formula or molecule H.2O, for which M=18, if H=l, as
we already know from the composition of water. Its vapour density,
or D, compared to hydrogen =9, and consequently for water 0=2, and

therefore and in general for the molecules of all substance ^. = 2.

Consequently, the weight of a molecule is equal to twice its vapour
density expressed in relation to hydrogen, and conversely the density of
a gas is equal to half the molecular weight referred to hydrogen.

The truth of this may be seen from an infinitely large number of

and the law of gravity, &c.), proves to be not an empirical deduction from direct obser-
vation and experiment, not a direct result of analysis, but a creation, a penetration of an
inquiring mind, which is guided and directed only by experiment and observation — a
synthesis to which the exact sciences are capable to an equal extent with the highest forms
of art. Without such a synthetical process of reasoning, science would only be a
collection of the results of arduous labour, and would not be distinguished by that force
with which it is really endowed when once it succeeds in attaining a synthesis, or con-
cordance of outward form with the inner nature of things, without departing from the
analysis of individual parts ; in short, when it discovers by means of outward forms,
which are apparent to the sense of touch, to observation, and to the mind, the internal
signification of things— discovering simplicity in complexity and uniformity in
diversity.

x 2

308 PRINCIPLES OF CHEMISTRY

observed vapour densities by comparing them with the results obtained
by calculation. As an illustration, we may point out that for ammonia,
NH3, the weight of the molecule or quantity of the reacting sub-
stance, as well as the composition and weight corresponding with the
formula, is expressed by the figures 14 + 3 = 17. Consequently, M = 17.
Hence, according to the law, D = 8'5. And this result is also obtained
by experiment. The density, according to the formula and experiment,
of nitrous oxide, N20, is 22, of nitric acid 15, and of nitric peroxide 23.
In the case of nitrous anhydride, N.2O3, as a substance which dissociates
into NO + N02, the density should vary between 38 (so long as the
N2O3 remains unchanged), and 19 (when NO + NO2 is obtained).
There are no figures of constant density for H.,O2, NHO3, N2O4, and
many similar compounds which are either wholly or partially decom-
posed in passing into vapour. Salts and similar substances either have
no vapour density because they do not pass into vapour (for instance,
potassium nitrate, KNO3) without decomposition, or if they pass
into vapour without decomposing, their vapour density is observed
with difficulty only at very high temperatures. The practical de-
termination of the vapour density at these high temperatures (for
example, for sodium chloride, ferrous chloride, staniious chloride, etc.),
requires special methods which have been w7orked out by Sainte-Claire,
Deville, Crafts, Nilson and Pettersson, Meyer, Scott, and others.
Having overcome the difficulties of experiment, it is found that the
law of . Avogadro-Gerhardt holds good for such salts as potassium
iodide, beryllium chloride, aluminium chloride, ferrous chloride, <tc. —
that is, the density obtained by experiment proves to be equal to half
the molecular weight — naturally within the limits of experimental
error or of possible deviation from the law.

Gerhardt deduced his law from a great number of examples of
volatile carbon compounds. We shall become acquainted wdth certain of
them in the following chapters ; their entire study, from the complexity
of the subject, and from long-established custom, forms the subject
of a special branch of chemistry, termed 'organic ' chemistry. With all
these substances the observed and calculated densities are very similar.

When the consequences of a law are verified by a great number of
observations, it should be considered as confirmed by experiment. But
this does not exclude the possibility of apparent deviations. They may

evidently be of two kinds : the fraction - may be found to be either

greater or less than 2 — that is, the calculated density may be either
greater or less than the observed density. When the difference between
the results of experiment and calculation fall within the possible errors -

MOLECULES AND ATOMS 309

of experiment (for example, equal to hundredths of the density), or
within a possible error owing to the laws of gases having an only
approximate significance (as is seen from the deviations, for instance,
from the law of Boyle and Mariotte), then the fraction proves but

slightly different from 2 (between 1-9 and 2-2), and such cases
as these may be classed among those which ought to be expected
from the nature of the subject. It is a different matter if the quotient

of 1- be several times, and in general a multiple, greater or less than

•J. The application of the law must then be explained or it must be
laid aside, because the laws of nature admit of no exceptions. We will

therefore take two such cases, and first one in which the quotient 4j

i* i/reatcr than 2, or the density obtained by experiment is less than is
in accordance with the Itur.

It must be admitted as a consequence of the law of Avogadro-
Gerhardt, that there is a decomposition in those cases where the volume
of the vapour corresponding with the weight of the amount of a
substance entering into reaction is greater than the volume of two
parts by weight of hydrogen. Suppose the density of the vapour of
water to be determined at a temperature above that at which it is
decomposed, then, if not all, at all events a large portion of the water
will be decomposed into hydrogen and oxygen. The density of such a
mixture of gases, or of detonating gas, will be less than that of aqueous
vapour ; it will be equal to 6 (compared with hydrogen), because
1 volume of oxygen weighs 16, and 2 volumes of hydrogen 2 ;
and, consequently, 3 volumes of detonating gas weigh 18, and 1
volume 6, whilst the density of aqueous vapour =9. Hence, if the
density of aqueous vapour be determined after its decomposition, the
quotient - would be found to be 3 and not 2. This phenomenon

might be considered as a deviation from Gerhardt's law, but this would
not be correct, because it may be shown by means of diffusion through
porous substances, as described in Chap. II., that water is decomposed
at such high temperatures. In the case of water itself there can
naturally be no doubt, because its vapour density agrees with the law
at all temperatures at which it has been determined.12 But there are

12 As the density of aqueous vapour remains constant within the limits of experi-
mental accuracy, even at 1000°, when dissociation has certainly commenced, it must be
admitted that only a very small amount of water is decomposed at these temperatures.
If even 10 p.c. of water were decomposed, then the density would be 8'57 and the
quotient M/D = 2'1, but at the high temperatures here concerned the error of experiment
is not greater than the difference of this quantity. And probably at 1000° the dissocia-

310 PRINCIPLES OF CHEMISTRY

many substances which decompose with great ease directly they are
volatilised, and which only exist as solids or liquids, but not in a state
of vapour. There are, for example, many salts of this kind, besides all
definite solutions having a constant boiling point, all the compounds of
ammonia — for example, all ammonium salts — &c. Their vapour
densities, determined by Bineau, Deville, and others, show that they
do not agree with Gerhardt's law. Thus, the vapour density of
sal-ammoniac, NH4G1, is nearly 14 (compared with hydrogen),
whilst its molecular weight is riot less than 53-5, whence the
vapour density should be nearly 27 according to the law. The
molecule of sal-ammoniac cannot be less than NH4C1, because it is
formed from the molecules NH3 and HC1, and contains single atoms of
nitrogen and chlorine, and therefore it cannot be divided ; it, further,
never enters into reactions with the molecules of other substances (for
instance, potassium hydroxide or nitric acid) in quantities of less than
53'5 parts by weight, &c. The calculated density (27) is here double

the observed density (about 14), hence — =4 and not 2. For this

reason the vapour density of sal-ammoniac for a long time served as
an argument for doubting the truth of the law. But it proved other-
wise, after the matter had been fully studied. The low density
depends on the decomposition of sal-ammoniac, on volatilising, into
ammonia and hydrogen chloride. The observed density is not that of
sal-ammoniac, but of a mixture of NH3 and HC1, which should be
nearly 14, because the density of NH3=8'5 and of HC^IS'S, and
therefore the density of their mixture (in equal volumes) should be
13'3.13 The actual decomposition of the vapours of sal-ammoniac was
demonstrated by Pebal and Than by the same method as the decom-
position of water, by passing the vapour of sal-ammoniac through a
porous substance. The experiment demonstrating the decomposition
during volatilisation of sal-ammoniac may be made very easily, and is
a very instructive point in the history of the law of Avogadro-Gerhardt,
because without its aid it would never have been imagined that sal-
ammoniac decomposed in volatilising, as this decomposition bears all
the signs of simple sublimation ; consequently, the knowledge of the
decomposition itself was forestalled by the law. The whole force

tion is far from being equal to 10 p.c. Therefore, the variation in the vapour denaifij
of water cannot be the means of ascertaining the amount of its dissociation.

13 This explanation of the vapour density of sal-ammoniac, sulphuric acid, and
similar substances which decompose in being distilled was the most natural to resort to
as soon as the application of the law of Avogadro-Gerhardt to chemical relations
was begun; it was, for instance, given in my work on Specific volumes, 1850,
p. 99. The formula, M/D = 2, which later was applied by many other investigators, had
already been applied in that work.

MOLECULES AND ATOMS 311

and visible use of the discovery of the laws of nature lies in, and is
expressed by, the fact that they enable the unknown to be foretold, and
the unobserved to be foreseen. The arrangement of the experiment is
based on the following reasoning.14 According to the law and to
experiment the density of ammonia, NH3, is 8J, and of hydrochloric
acid, HC1, 18|. if the density of hydrogen=l. Consequently, in a
mixture of NH3 and HC1, the ammonia will penetrate much more
rapidly through a porous mass, or a fine orifice, than the heavier hydro-
chloric acid, just as in a former experiment the hydrogen penetrated
more rapidly than oxygen. Therefore, if the vapour of sal-ammoniac
comes into contact with a porous mass, the ammonia will pass through
it in greater quantities than the hydrochloric acid, and this excess of
ammonia may be discovered by means of moist red litmus paper, which
should be turned blue. If the vapour of sal-ammoniac were not
decomposed, it would pass through the porous mass as a whole, and the
colour of the litmus paper would not be altered, because sal-ammoniac-
is a neutral salt. Thus, by testing with litmus the substance passing
through the porous mass, it may be decided whether the sal-ammoniac-
is decomposed or not when passing into vapour. Sal-ammoniac
volatilises at so moderate a temperature that the experiment may be
conducted in a glass tube heated by means of a lamp, an asbestos
plug being placed in the central portion of a glass tube.15 The asbestos
forms a porous mass, which is not changed at a high temperature. A
piece of dry sal-ammoniac is placed at one side of the asbestos plug,
.and is heated by a Bunsen's burner. The vapours formed are driven
by a current of air forced from a gasometer or bag through two tubes
containing pieces of moist litmus paper, one blue and one red paper in
each. If the sal- ammoniac be heated, then the ammonia appears on the
opposite side of the asbestos plug, and the corresponding litmus turns
blue. And as an excess of hydrochloric acid remains on the side where
the sal-ammoniac is heated, therefore it turns the litmus at that end
red. This proves that the sal-ammoniac, when converted into vapour,
splits up into ammonia and hydrochloric acid, and at the same time
gives an instance of the possibility of correctly conjecturing a fact on
the basis of the law of Avogadro-Gerhardt.

So also the fact of a decomposition may be proved in the other

M

instances where -^ proved greater than 2, and hence the apparent

14 The beginner must remember that an experiment and the mode in which it is
carried out must be determined by the principle or fact which it is intended to illustrate,
and not vice versa as some suppose. The idea which determines the necessity of an
experiment is the chief consideration.

15 It is important that the tubes, asbestos, and sal-ammoniac should be dry, as other-
wise the moisture retains the ammonia and hydrogen chloride.

312 PRINCIPLES OF CHEMISTRY

deviations appear in reality as an excellent proof of the general applica-
tion and significance of the law of Avogadro-Gerhardt.

In those cases where the quotient vy proves to be less than 2, or

the observed density greater than that calculated by a multiple number
of times, the matter is evidently more simple, and the fact observed
only indicates that the weight of the molecule is as many times greater,
as that taken as the quotient obtained is less than 2. So, for instance,
in the case of ethylene, whose composition is expressed by CH2, the
density was found by experiment? to be 14, and in the case of amy-
lene, whose composition is also CH2, the density proved to be 35, and
consequently the quotient for ethylene=l, and for amylene=?. If
the molecular weight of ethylene be taken, not as 14, as might be
imagined from its composition, but as twice as great— namely, as 28 —
and for amylene as five times greater — that is as 70 — then the molecular
composition of the first will be C2H4, and of the second CAH10, and for

both of them ^j- will be equal to 2. This application of the law,

which at *first sight may appear perfectly arbitrary, is nevertheless
strictly correct, because the amount of ethylene which reacts — for
example, with sulphuric and other acids — is not equal to 14, but to 28
parts by weight. Thus with H2SO4, Br2, or HI, &c., ethylene com-
bines in a quantity C2H4, and amylene in a quantity CftH10, and not
CH2. On the other hand, ethylene is a gas which liquefies with diffi-
culty (absolute boiling point= + 10°), whilst amylene is a liquid boiling
at 35° (absolute boiling point= + 192°), and by admitting the greater
density of the molecules of amylene (M=70) its difference from the
lighter molecules of ethylene (M = 28) becomes clear. Thus, the

smaller quotient ^y is an indication of polymerisation, as the larger

quotient is of decomposition. The difference between the densities of
oxygen and ozone (Chap. IV.) is a case in point.

On turning to the elements, it is found in certain cases, especially
with metals — for instance, mercury, zinc, and cadmium — that that
weight of the atoms which must be acknowledged in their compounds
(of which mention will be afterwards made) appears to be also the
molecular weight. Thus, the atomic weight of mercury must be taken
as=200, but the vapour density=100, and the quotient=2. Con-
sequently, the molecule of mercury contains one atom, Hg. It is the same
with sodium, cadmium, and zinc. This is the simplest possible molecule,
which necessarily is only possible in the case of elements, as the mole-
cule of a compound must contain at least two atoms. However, the

MOLECULES AND A.TOM8 313

molecules of many of the elements prove to be complex — for instance,
the weight of an atom of oxygeii=16, and its density=16, so that its
molecule must contain two atoms O2, which might already be concluded
by comparing its density with that of ozone, whose molecule contains
O;j (Chap. IV.). So also, the molecule of hydrogen equals H2, of
chlorine C12, of nitrogen N2, <tc. If chlorine react with hydrogen, the
volume remains unaltered after the formation of hydrochloric acid,
H2 + Cl2=HCl-fHCl. It is a case of substitution between the one
and the other, and, therefore, the volumes remain constant. There are
elements whose molecules are much more complex — for instance, sulphur,
S6 — although by heating the density is reduced to a third, and S2 is
formed. Judging from the vapour density of phosphorus (D=62)
the molecule contains four atoms P4. Hence, many elements, when
polymerised, appear in molecules which are more complex than
the simplest possible. In carbon, as we shall afterwards find, a
very complex molecule must be admitted, as otherwise its nonvola-
tility and other properties cannot be understood. And if compounds
are decomposed by a more of less powerful heat, and if polymeric
substances are depolymerised (that is, the weight of the molecule
diminishes) by a rise of temperature as N2O4 passes into NO2,
or ozone, O3, into ordinary oxygen, O2, then we might expect to
find the splitting- up of the complex molecules of elements into
the simplest molecule containing a single atom only — that is to
say, if O2 be obtained from O3, then the formation of O might also
be looked for. The likelihood of such a proposition is indicated
by the vapour of iodine. Its normal density =127 (Dumas, Deville,
arid others), which corresponds with the molecule I2. At tempera-
tures above 800° (up to which the density remains almost con-
stant), this density distinctly decreases, as is seen from the verified
results obtained by Victor Meyer, Crafts, and Troost. At the ordinary
pressure and 1000° it is about 100, at 1250° about 80, at 1400° about
75, and it apparently strives to reduce itself to one-half — that is, to 63.
Under a reduced pressure this splitting- up, or depolymerisation, of
iodine vapour actually reaches a density 1G of 66, as Crafts demon-
strated by reducing the pressure to 100 mm. and raising the temperature
to 1500°. From this it may be concluded that at high temperatures and
low pressures the molecule I2 gradually passes into molecule I containing
one atom like mercury, and that something similar occurs with other ele-
ments at a considerable rise of temperature, which tends to bring about

16 Just as we saw (Chap. VI. Note 46) an increase of the dissociation of N2O4 and the
formation of a large proportion of NO2, with a decrease of pressure. The splitting-up of
Io into I -f I is a similar dissociation.

IU4 I'KINCIPLKS OF CHKM1STKY

•In- disunion of compounds and the decomposition of complex mole-
cules.17

1 V-idrs tin's,- cases of apparent discrepancv from the la\v of Avogadro-
(lerhardt there is yet a third, which is the last, and one which is very
instruct ive. In the investigation of separate substances they have to
be isolated in the purest possible form, and their chemical and physical
properties, and among them the vapour density, then determined.
If it he normal --that is, if l') = ]\I '1 — it is a pi oof of the purity of the
substance and of its freedom from all impurities. If it be abnormal
that is, if 1> be not equal to M '2 then for those who do not believe
in the law it appears as a new argument against it and nothing more :
but to those who have already grasped the important significance of
the law it heroines clear that there is some error in the observation,
or thai the density was determined under conditions in which the
vapour dues not follow the laws of Hoyle or Gay-Lussac, or else that
the substance has not been sulliciently purified, and contains impurities,
Arc. The law of Avogadro-Gerhardt then appeal's as a convincing
evidence of the necessity of a fresh and more exact research. And as
yet the causes of error have always been found. There are not a few
examples in point in the recent history of chennMrv. We will cite
one instance. In the case of pvrosulphurvl chloride, S.,O;)G1.,, ]\I — '2 1 •">,
and consequently I> should — 107 '•">, instead of which < >girr and others

• A It In >iiLdi at first there appeared to IK- a similar phenomenon in t lie case of chlorine,
I: wa- afterwards proved that if there is a decrease of density it is onh a small one. In
tin- ca-e of bromine it is not much ;_rreatcr. and i- far from lieim: equal to that for iodine.
A- \ve in general very often involuntarily confuse chemical processes with physical.
it may be that, a physical process ot change in the coef'ticieiit of expansion with a change
df temperature and molecular weight participates, if not \\hollv. at all e\ents partiiilly,
in tli,. matter ,,f the decrease of the density of chlorine, bromine, and iodine. Thus. I
ha ve remarked > ( 'ompte-, 1 1 end n-. Is7»'>i that the coef'ticieiit of expansion of ^ases incri-Rses
uitli theii- molecular weight, and iChap. II. Note'Ji'n the results i,t direct cxjierimeiit
^li,,w the coefh'eient of expansion of hydrobromic acid iM si i t , , 1 ,,- tniti.'jsc. instead of
()-IMi:;|-,7 for hydn>-en iM =lii. Therefore, in the case of the \ap.Mir of iodine | M 'jr.J'i a
N, r\ l.ii-e coeflicient of expansion is to l.e expected, and from this cause alone the
,1,.,,-itv would fall. As (he molecule ,,f chlorine C] , i:, li-hter i -71 ithau that of bromine
, • ir.in. v.hic], i li-hter than that of indim- ( =1».",.| i, therefore the order in which the
decnmpo-ability of the \a]iours of the^-e haloids is nlisc-rvcd i'orr«->poinls \\ ilh the ex]iected

;1-. o-iiiii. the,, :.t IIIIMI it - den-iiy would already be 1111,. Therefore it may be that, the
,li-- ocial 1011 of io( line i. only an ap]iarent phenomenon. However, on t he other hand,
the hea\ y \ aponr- of mercury iM •Jim. I> lu(l) scarcely decrea-e in den-it v at a t em-
I , , -,,t in -e i ,| I .Mm .[) '.is. according to Victor Me\eri; but it mii-1 not lie forgotten that
,!,,. molecule nf inercur\ contain- only one at . mi. u h i 1 -t that of iodine contains two, which
v.,,,,l(l !•_• nil's much. (.ine.Co,,-^ »i thi- kind uhich are difficult )o decide by experimental
m, ,,,, ',. pecially the accurate determination of /i mii-t remain l<>icj without bein^ defi-
nitelv explained byrea on of the dif'ticultv. and -omet imes impo--.ibility, of separating
I,;-, .i,-;,] from chemical chan-,-.

MOLECULES .\.\-l) ATOMS 315

obtained 53*8 — that is, a density half as great ; and further, Ogier
(1882) demonstrated clearly that the substance is not dissociated by
distillation into SO3 and S02C12, or any other two products, and thus
the abnormal density of S2O5C12 remained unexplained until D. P.
Konovaloff (1885) showed that the previous investigators were working
with a mixture (containing SO3HCl),and that pyrosulphuryl chloride has
a normal density of approximately 107. Had not the law of Avogadro-
Gerliardt served as a guide, the impure liquid would have still passed
as pure ; all the more as the determination of the amount of chlorine
could not aid in the discovery of the impurity. Thus, by following a
true law of nature we are aided in the attainment of true deductions.

All cases which have been studied confirm the law of Avogadro-
Oerhardt, and as by it a deduction is obtained, from the deter-
mination of the vapour density (a purely physical property), as to the
size of the molecule or quantity of a substance entering into chemical
reaction, therefore, this law links together the two provinces of learn-
ing— physics and chemistry — in the most powerful manner. Besides
which, the law of Avogadro-Gerhardt places the conceptions of mole-
•cules and atoms on a firm foundation, which was previously wanting.
Although since the days of Dalton it had become evident that it was
necessary to admit the existence of the atom (the chemical individual
indivisible by chemical or other forces) of elements, and the groups of
.atoms or molecules of compounds indivisible by mechanical and physical
forces ; still the relative magnitude of the molecule and atom was not
defined with sufficient clearness. So, for instance, the atomic weight of
oxygen might be taken as 8 or 16, or any multiple of these numbers,
and nothing indicated a means for the acceptation of one or another of
these magnitudes ; 18 whilst as regards the weight of the molecules of
elements and compounds there was no trustworthy conception whatever.

18 And so it was in the fifties. Some took O = 8, others O = 16. Water in the first
case would be HO and hydrogen peroxide HOo, and in the second case, as is now gene-
rally accepted, water H.2O and hydrogen peroxide H.^O2 or HO. Discussion and confu-
sion were reigning. In 1860 the chemists of the whole world met at Carlsriihe for the
purpose of arriving at some agreement and uniformity of opinion. I was present at this
Congress, and well remember how great was the difference of opinion, and how a condi-
tional agreement was defended with the greatest acumen by the ranks of science, and with
what warmth the followers of Gerhardt, at whose head stood the Italian professor,
Canizzaro, followed up the consequences of the law of Avogadro. In the reign of
.scientific freedom (without which science would make no progress, and would remain
petrified as in the middle ages) and with the simultaneous necessity of scientific conser-
vatism (without which the roots of past study could give no fruit) a conditional agree-
ment was not arrived at, and ought not to have been, but instead of it truth, in the form
of the law of Avogadro-Gerhardt, received by means of the Congress a wider develop-
ment, and soon afterwards conquered all minds. Then the new so-called Gerhardt
Atomic weights established themselves, and in the seventies they had already become
.generally used.

316 PKlXni'LKS OF CHK-MISTKY

With the establishment of the law the conception of the molecule
was fully denned, and from it the conception of the magnitude of the
atom of elements.

The particle or chemical particle or molecule must le consi'l' /•"/ O8
the quantity of a substance entering into chemical rwrfiuii //•//// <tth»-r
molecules, and tuhich in a state of vapour occupies the same voIn/)» at
two parts by weight of hydrogen.

The molecular weight (which has been indicated by M) of a sub-
stance is determined by its composition, transformations, and vapour
density.

The molecule is not divisible by the mechanical and physical
changes of substances, but in chemical reaction it is either altered in
its properties, or quantity, or structure, or in the nature of the move-
ment of its parts.

An agglomeration of molecules, which are alike in all chemical
respects, makes up the masses of definite homogeneous substances in all
states.19

Molecules consist of atoms in a certain state of distribution and
movement, just as the solar system20 is made up of inseparable parts
(the sun, planets, satellites, comets, &c.). The greater the number of

19 A volume of gas, a drop of a liquid, or the smallest crystal, presents an agglomeration
of a number of molecules, in a state of movement, continuously repeated (like the stars of
the milky way), distributing themselves in order or forming their new systems. If the aggre-
gation of all kinds of heterogeneous molecules be possible in a gaseous state, where the
molecules are considerably removed from each other, then in a liquid state, where the
molecules are already close together, such an aggregation becomes possible only in that
aspect of mutual reaction between the molecules which appears in their chemical attrac-
tion, and especially in the faculty of heterogeneous molecules for combining together.
Solutions and other so-called indefinite chemical compounds should be regarded in this
light. According to the representation evolved in this work we should regard them as
containing both the compounds of the heterogeneous molecules themselves and the pro-
ducts of their decomposition, as in peroxide of nitrogen NoO4 and NOo. And we must
consider that those molecules A, which at a given moment are combined with B in AB,
will in the following moment become free in order to again enter into a combined form.
The instances of chemical equilibria proper to dissociated systems cannot be regarded in
any other sense.

20 This strengthens the fundamental idea of the unity and harmony of the type of
creation which forms one of those ideas which impress themselves on man in all ages,
and give rise to a hope of arriving in time, by means of a lengthy labour of discoveries,
observations, experiments, laws, hypotheses, and theories, at a comprehension of the
internal and invisible structure of concrete substances with the same degree of clearness
and exactitude as has been attained in the visible structure of the heavenly bodies. It
is not many years ago since the law of Avogadro-Gerhardt took root in science. It is
within the memory of many living scientific men. It is not surprising, therefore, that as
yet little progress has been made in the province of molecular mechanics ; but the theory
of gases alone, which is intimately connected with the conception of molecules, show-- l<y
its success that the time is approaching when our knowledge of the internal structure of
matter will grow rapidly.

MOLECULES AND ATOMS 317

atoms in a molecule, the more complex is the resultant substance. The
equilibrium between the dissimilar atoms may then be more or less
>tablo, and may for this reason give more or less stable substances.
Physical and mechanical transformations alter the velocity of the
movement and the distances between the individual molecules, or of the
atoms in the molecules, or of their sum total, but they do not alter the
original equilibrium of the system ; whilst chemical changes, on the
other hand, alter the molecules themselves, that is, the velocity of
movement, the relative distribution, and the quality and quantity of
the atoms in the molecules.

Atoms are the smallest quantities or indivisible chemical masses of
the elements forming the molecules of elements and compounds.

Atoms have weight, the sum of their weights forms the weight of
the molecule, and the sum of the weights of the molecules forms the
weight of masses, and is the cause of gravity, and of all the phenomena
which depend on the mass of a substance.

The elements are characterised, not only by their independent exist-
ence, their incapacity of being converted into each other, <kc., but also
by the weight of their atoms.

Chemical and physical properties depend on the weight, composi-
tion, and properties of the molecules forming a substance, and on the
weight and properties of the atoms forming the molecules.

This is the substance of those conceptions of molecular mechanics
which lie at the basis of all contemporary physical and chemical
constructions since the establishment of the law of Avogadro-Gerhardt.
The fecundity of the principles enunciated is encountered at every step
in the entire sum of the particular cases forming the present store of
chemical data. We will here cite a few examples of the application of
the law.

As the weight of an atom must be understood as the minimum
quantity of an element entering into the composition of all the mole-
cules formed by it, therefore, in order to find the weight of an atom of
oxygen, let .us take the molecules of those of its compounds which have
been already described, together with the molecules of certain of those
carbon compounds which will be described in the following chapter : —

Molecular Amount of Molecular Amount of

Weight. Oxygen. Weight. Oxygen.

H2O 18 16 HN03 63 48

N2O 44 16 CO 28 16

NO 30 16 CO2 44 32

KO9 46 32

318 PRINCIPLES OF CHEMISTRY

The number of substances taken might be considerably increased,,
but the result would be the same — that is, the molecules of the com-
pounds of oxygen would never be found to contain less than 16 parts
by weight of this element, but always n!6, where n is a whole number.
The molecular weights of the above compounds are found either directly
from the density of their vapour or gas, or from their reactions. Thus,
the vapour density of nitric acid (as a substance which easily decom-
poses above its boiling point) cannot be accurately determined, but the
fact of its containing one part by weight of hydrogen, and all its pro-
perties and reactions, indicate the above molecular composition and no
other. In this manner it is very easy to find the atomic weight of all
the elements, knowing the molecular weight and composition of their
compounds. It may, for instance, be easily proved that less than nl2
parts of carbon never enters into the molecules of carbon compounds,
and therefore C must be taken as 12, and not as 6 which was the case be-
fore Gerhardt. In similar manner the atomic weights now accepted for
the elements oxygen, nitrogen, carbon, chlorine, sulphur, etc., were found
and indubitably established, and they are even now termed the Gerhardt
atomic weights. As regards the metals, many of which do not give a
single volatile compound, we shall afterwards see that there are also
methods by which their atomic weights may be established, but never-
theless the law of Avogadro-Gerhardt is here also ultimately re-
sorted to, in order to remove any doubt which may be encountered.
Thus, for instance, although much that was known concerning the
compounds of beryllium necessitated its atomic weight being taken as
Be=9 — that is, the oxide as BeO and the chloride BeCl2 — still certain
analogies gave reason for considering its atomic weight to be Be=13'5r
in which case its oxide would be expressed by the composition Be2O3,
and the chloride BeCl3.21 It was then found that the vapour density
of beryllium chloride was approximately 40, when it became quite clear
that its molecular weight was 80, and as this satisfies the formula
BeCl2 but does not suit the formula BeCl3, it therefore became neces-
sary to regard the atomic weight of Be as 9 and not as 13*.

21 If Be = 9, and beryllium chloride be BeCL>, then for every 9 parts of beryllium
there are 71 parts of chlorine, and the molecular weight of BeCl3 = 80 ; hence the vapour
density should be 40 or w40. If Be = 18'5, and beryllium chloride be BeCl3, then to 13'5
of beryllium there are 106*5 of chlorine ; hence the molecular weight would be 120, and
the vapour density (50 or nGO. The composition is evidently the same in both cases,
because 9 : 71 ;: 13'5 : 106'5. Thus, if the symbol of an element designate different
atomic weights, what seem to be very different formulae may equally well express both
the percentage composition of compounds, and those properties which are required by
the laws of multiple proportions and equivalents. The chemists of former days
accurately expressed the composition of substances, and accurately applied Dalton's laws,
by taking H = l, O = 8, C = 6, Si = 14, &c. The Gerhardt equivalents are also satisfied by

MOLECULF.s AND ATOMS 319

With the establishment of a true conception of molecules and atoms,
chemical formula* became direct expressions, not only of composi-
tion,22 but also of molecular weight or vapour density, and consequently

because O = ltf, C = 12, Si = 28, &c., are multiples of them. The choice of
one or the other multiple quantity for the atomic weight is impossible without a firm and
concrete conception of the molecule and atom, and this is only obtained as a consequence
of the law of Avogadro-Gerhardt, and therefore the contemporary atomic weights are
the results of this law.

- In order to calculate the percentage amounts of the elements contained in a given
compound from its formula, it is necessary to use a very simple proportion sum. Thus,
for example, to find the percentage amount of hydrogen in hydrochloric acid we reason
as follows: — HC1 shows that hydrochloric acid contains 35'5 of chlorine and 1 part of
hydrogen. Hence, in :'>C>-r> parts of hydrochloric acid there is 1 part by weight of
hydrogen, consequently 100 parts by weight of hydrochloric acid will contain as many
more units of hydrogen as 100 is greater than 36' 5 ; therefore, the proportion is as

1 Of)

follows— # : 1 : : 100 : 36'5 or x = = 2*739. Therefore 100 parts of hydrochloric acid con-
36'5

tain 2'739 parts of hydrogen. In general, when it is required to transfer a formula into
its percentage composition, we must replace the symbols by their corresponding atomic
weights and find their sum, and knowing the amount by weight of a given element in it, it
is easy by proportion to find the amount of this element in 100 or any other quantity of
parts by weight. If, on the contrary, it be required to find the formula from a given
percentage composition, we must proceed as follows : Divide the percentage amount of
each element entering into the composition of a substance by its atomic weight, and
compare the figures thus obtained together — they should be in simple multiple proportion
to each other. Thus, for instance, from the percentage composition of hydrogen peroxide,.
5'88 of hydrogen and 94'12 of oxygen, it is easy to find its formula ; it is only necessary to
divide the amount of hydrogen by unity and the amount of oxygen by 16. The numbers
5'88 and 5'88 are thus obtained, which are in the ratio of 1 : 1, which means that in
hydrogen peroxide there is one atom of hydrogen to one atom of oxygen.

The following is a proof of the above practical rule : — That to find the ratio of the
number of atoms from the percentage composition, it is necessary to divide the per-
centage amounts by the atomic weights of the corresponding substances, and to find
the ratio which these numbers bear to each other. Let us suppose that two radicles
(simple or compound), whose symbols and combining weights are A and B, combine
together, forming a compound composed of x atoms of A and y atoms of B. The
formula of the substance will be AxBy. From this formula we know that our compound
contains xA parts by weight of the first element, and yB of the second. In 100 parts of

our compound there will be (by proportion) — f °'a? . of the first element, and 100^B ..,

xA + yB xA + yB

of the second. Let us divide these quantities, expressing the percentage amounts by the

corresponding combining weights ; we then obtain - — — . for the first element and

xA + yB

y for the second element. And these numbers are in the ratio x : y — that is, in

the ratio of the number of atoms of both substances.

It may be further observed that even the very language or nomenclature of
chemistry acquires a particular clearness and conciseness by means of the conception of
molecules, because then the names of substances may directly indicate their composition.
Thus the term ' carbon dioxide ' tells more about and expresses CO^ better than carbonic
acid gas, or even carbonic anhydride. Such nomenclature is already employed by many. But
expressing the composition without an indication or even hint as to the properties, would
be neglecting the advantageous sides of the present nomenclature, Sulphur dioxide,
SOo, expresses the same as barium dioxide, BaO^, but sulphurous anhydride indicates
the acid properties of SO.2. Probably in time one harmonious chemical language will
succeed in embracing both advantages,

320 PRINCIPLES OF CHEMISTRY

of a series of fundamental chemical and physical data, inasmuch as a
number of the properties of substances are dependent on their vapour
density, or molecular weight and composition. Therefore, the vapour

density D = — . For instance, the formula of ethyl ether is C4H10O,

hence it corresponds with the molecular weight 74, and the vapour
density of 37, which is the fact. Therefore, the density of vapours and
gases ceased to be an empirical magnitude obtained by experiment
only, and it acquired a rational meaning. It is only then needful to
remember that 2 grams of hydrogen, or the molecular weight of this
primary gas in grams, occupies, at 0° and 760 mm. pressure, a volume
of 22'3 litres (or 22300 cubic centimetres), in order to directly reduce
the weights of cubical measures of gases and vapours from their
formulae, because the molecular weights of all other vapours at 0° and
760 mm. occupy the same volume, 22*3 litres. Thus, for example, in the
case of carbonic anhydride, CO2, the molecular weight M=44, hence 44
grams of carbonic anhydride at 0° and 760 mm. occupy a volume of
22 '3 litres — consequently, a litre weighs 1*97 grams. By combining the
laws of gases — Gay-Lussac's, Mariotte's, and Avogadro-Gerhardt's — we
obtain 23 a general formula for gases

where s is the weight in grams of a cubic centimetre of a vapour or gas
.at a temperature t and pressure p (expressed in centimetres of mer-
cury) if the molecular weight of the gas— M. Thus, for instance, at
100° and 760 millimetres pressure (i.e., at the atmospheric pressure)
the weight of a cubic centimetre of the vapour of ether (M = 74) is
equal to s=0-0024.24

As the molecules of many elements (hydrogen, oxygen, nitrogen,

23 This formula (which is given in my work on 'The Tension of (! uses,' and in a
somewhat modified form in the ' Comptes Rendus,' Feb. 1876) is deduced in the following
manner. According to the law of Avogadro-Gerhardt, M = 2D for all pises, where M i-
the molecular weight and D the density referred to hydrogen. But they equal the weight
S0 of a cubic centimetre of a gas in grams at 0° and 76 c.m. pressure, divided by
0-0000896, for this is the weight in grams of a cubic centimetre of hydrogen. But the
weight s of a cubic centimetre of a gas at a temperature t and under a pressure /;
.(in centimetres) is equal to spjlft (l + at). Therefore, s0 — s.76 (l + at)p ; hence
D = 76./S (l + a£)/0.0000896j9, whence M = 152s (l+ai)/0*000089dp, which gives the above
•expression, because I/a = 278. m/v, where tn is the weight and v the volume of a
vapour, may be taken instead of s.

24 The above formula may be applied in order to ascertain directly the molecular
weight for a given vapour density, as s = the weight of vapour m, divided by the volume
v, and consequently by experiment, M = 6,255 m (278 + t)/pv. Therefore, instead of the
formula (see Chap. II. Note 38), pv = ~R('273 + t), where R varies with the mass and
nature of a gas, we may apply the formula ^>u = 6,255(?/i/M) (278 + £), and taking a
.weight of a gas m equal to its molecular weight, pv — 6,255 (278 + t) for all gases.

MOLECULES AND ATOMS 321

chlorine, bi online, sulphur — at least at high temperatures) are of uni-
form composition, therefore, the formula; of the compounds formed by
them, directly indicates the composition by volume. So, for example,
thr formula HNO3 directly shows that in the decomposition of nitric
acid there is obtained 1 vol. of hydrogen, 1 vol. of nitrogen, and 3 vols.
of oxygen.

And as a great number of mechanical, physical, and chemical
properties are directly dependent on the elementary and volumetric
composition, and on the vapour density ; so the accepted system of
atoms and molecules gives the possibility of simplifying a number of
most complex relations. For instance, it may be easily demonstrated
that, the vis viva of the molecules of all vapours and gases is alike. For
it is proved by mechanics that the vis viva of a moving mass=^ wi?2,
where in is the mass and v the velocity. For a molecule m=M, or the
molecular weight, and the velocity of the movement of gaseous
molecules=a constant which we will designate by C, divided by the
square root of the density of the gas25=C/D^, and as D=M/2,
therefore, the vis viva of molecules =C2 — that is, a constant for all
molecules. Q.E.D.26 The specific heat of gases (as we shall afterwards
.see), and many other of their properties, are determined by their
density, and consequently by their molecular weight. Gases and
vapours in passing into a liquid state evolve the so-called latent heat,
which also proves to be in connection with the molecular weight. The
observed latent heats ofj carbon bisulphide, CS2=90, of ether.

25 Chap. I. Note 34.

16 The velocity of the transmission of sound through gases and vapours closely
bears on this. It = v' Kpg/D (1 + at) where K is the ratio between the two specific
heats (it is approximately T4 for gases containing 2 atoms in a molecule), p the pressure
of the gas expressed by weight (that is, the pressure expressed by the height of a column
of mercury multiplied by the density of mercury), g the acceleration of gravity, D the
weight of a cubic measure of the gas, o = 0'00367, and t the temperature. Hence, if K
be known, and as D can be found from the composition of a gas, we can calculate the
velocity of the transmission of sound in that gas. Or if this velocity be known, we can
find K. The relative velocities of sound in two gases can be determined with peculiar
ease (Kundt).

If a horizontal glass tube (about 1 metre long and closed at both ends) be full of a
gas, and be firmly fixed in the middle, then it is easy to bring the tube and gas into a
stair of vibration, by rubbing it from centre to end with a damp cloth. The vibration of
the gas is easily rendered visible, if the interior of the tube be dusted with lycopodium
tlir yellow powder-dust or spores of the lycopodium plant is of ten employed in medicine),
In -fore- the gas is introduced and the tube fused up. The fine lycopodium powder forms
itself into figures, whose number depends on the velocity of sound in the gas. If there
be 10 figures, then the velocity of sound in the gas is ten times slower than in glass. It
is evident that this is an easy method of comparing the velocity of sound in gases. It
lias been demonstrated by experiment that the velocity of sound in oxygen is four times
less than in hydrogen, and the square roots of the densities and molecular weights of
hydrogen and oxygen stand in this ratio.
VOL. I.

UNIVRSITY

r,H.11O. = tU. of ben/eno, C,,I I , ..= 1< »!». of alcohol. ( '.,1 1 ti< ». = -ji in. ot
chloroform, (1IK11;,. = (>7, Are. These figures shoxvthe amount of heat
expended in converting one part by weight of the above sub-tanee-
into vapour. A LjTeat uniformity is ol»ser\ ed if the measure- of heat

be reftTlvd to the weights of the molecules. For carbon bisulphide
the forjiiida ( '>., expre-ses a \\ciuht 7<>, hence the latent heat of eva-
poration referred to t he molecular quantitv, C'S.,= 7<> X (.IO = <»S4< >, for
et her ='.M ").")('), Tor beii/em^cOOL', f«.r alcohol = '.IL'MI ). f , ,r chloroform
= SUU7. for water — HiiM I. vvc. Consequently, for molecular <jiiant it ies.
the latent heat varies comparatively lit tie, from 7<»nu to lunui) heat
units, whilst for single parts by weight it is for water ten time- greater
than for chloroform and many others.-7

\Ve will cite vet one iiioi'e exa in] ile. showi'iii; tlie direct dependence
of tin- properties of a substance on the molecular \\ciuhi. If one
molecular part bv weight of the variou> chloride- foi- in-tance. of
-odium, calcium, barium. vVc. be dissolved in "Ji»ii molecular parts bv
weight of wattjr ('for instance, in 3(iOO ^rams) then it is found that
the greater the molecular weight of the >alt di-sol\ cd. the Create]- is
the -pecitic ^rax'itN" of the resultant solution. -s Inu- :

llo\\c\er. all ]))•()] iert ie> of sub-tance- do not depend on the
lecular weight alone.2'1 Xot oidv chemical, but also many physical.

MI jilipli.-d In lh«. lati-iit lii'sit nl' cvajioratii'ii. il.ul :ill i-M-nt-. lirin-'s to li^'lit n

' . . ill'.---. \\ lliri, , ,| 1|.-|'\\ i-i' I'llK I ' 1 '. ' •-• ' ' ! '''• I'' '-lilt of iili-cl'.

M. ...•i-ii l,i r i|iian1 it ics «\ li.|iii.l- n]>|ir,ir lo c\j>fjnl alnm^l npial amount- <>!' In -a I

•' iilioM1. a const a nl quantity.

i . t lifir Spcrifii ( , i ,, \ it v." I -s7. p. |-j:,.

1 \m-.u-j ' In- mo ' nti-noil h\ i.j tlir a|i]ilic:itioii . i.l ll n. • ct ion ;ii-i -in- i'roin

- ! . .. o| A: ..-_-, i. in. < .. rlur.lt. an- llio-f -m. -ra 1 1 -a I \< >\, i-otii-i-rninjj ililutr- solutions
. l.i-i-n mi-ntioiii-il in tin- Cliap. I. i.Noti • I1.'. IH. r.n. .M . It i- ln-t iii'cc-sar\
In ' - hiii'l.iiin-ntal .-\|,.-|-inicnt-. 'rraul.r. I't.-if'l'.-r. an.l inoi-.-i'-pi-cially. !>!• N'rirs.
iilii- -lioui-il lli.it cci-tain |..-riall\ jn-i-pari-d lilni- i ].r.-ci|.ilati'il nu-niliranr < nl
.1 - ul.-l i I - nip I.-. lli.,-.< v, hidi an- l'..riin-<l IVoin lln- -.ill- "I coi>i>i-r ;nnl
in- |ir.,|,crt y of allos\in^ tin- |M-,I->' of \\ati-r\\liilst
i in it. an. I it wa- thiTflor.- i .. il.lc l.\ (hi- in. -an- to

HOLECUUBS AND ATOMS 323

properties are determined by the composition of the molecules, and by the
properties of the elements forming them. Thus the density of solids

determine the osmotic pressure, as in the experiment described in Chap. I. Note 19.
De Vries found in vegetable cells a convenient means of determining such (isotonic)
solutions, which produce indentical osmotic pressures. A thin slice of a coloured plant
tis^in — f,,r instance, of Tradescantia discolor — is moistened under the microscope with
the solution to be tested. If its osmotic pressure be equal, or less than that of the
liquid contained in the cells, no visible change occurs ; but if the liquid taken be endued
with a greater osmotic pressure than the cellular sap, then the water will pass from
the cell, and the coloured matter of the cell will shrink away from the envelope, and
this process is easily observed under the microscope. Knowing, then, the osmotic
pressure for any one substance — for instance, for sugar — with different strengths of solu-
tions, it is possible to find the osmotic pressure of all other substances investigated,
because it is shown by direct experiment that the osmotic pressure increases in pro-
portion to the strength of the solution. Thus having fixed on any one substance — for
instance, sugar — and on one of its solutions, we may see the substance of the results
which have been attained.

If (as on p. 64) a one per cent, solution of sugar be taken, then according to the
experiments made by Pfeiffer (1877) its osmotic pressure = 53 '5 centimetres at 14°.
According to the formula of sugar, C^H^On, its molecular weight M = 342, and as the
weight of a cubic centimetre of a one per cent, solution of sugar — 1'003 gram, therefore
the weight of sugar in a cubic centimetre of the solution, or s in the preceding formula
(p. 320 : 6255 s (278 + J) = Mp), is equal to O'OIOOS gram, and therefore, according to this
formula (as M = 342 and t = 14), p = 52'6 centimetres. This shows that if the sugar were,
instead of being in solution, in a state of vapour, then in following the law of Avogadro-
Gerhardt it would produce a pressure equal to the osmotic pressure. This deduction
(whose sense is at present not clear) forms the substance of Van't Hoff's doctrine
(Chap. I. Note 19), when i = I (Chap. I. Note 49).

Consequently, the molecular weight determines the osmotic pressure (and together
with it the vapour tension and temperature of freezing, according to Note 49, Chap. I.),
and therefore the molecular weight itself may be determined by the osmotic pressure as
well as by the vapour density.

But so simple a relation only exists for dilute solutions of substances like sugar,
which do not conduct an electric current, for which *' = !. For salts and acids which
conduct a current, this factor varies up to » = 4 (Chap. I. Note 49). Arrhenius explains
this phenomenon by supposing (partially after Hittorf and Clausius) that a portion of
such substances in solutions, and especially in dilute solutions, occurs in a state of dis-
sociation, and owing to this the number of molecules is multiplied (Chap. I. Note 45).
As the conceptions of this order have been as yet but very little developed, and as in
regarding solutions from this point of view, which is warmly supported by Ostwald, the
water or solvent in general, which certainly plays an important part in solutions, and
especially dilute ones, is entirely lost sight of, I consider it premature at present to ex-
plain the theory of Arrhenius, but think that it contains the seeds for further develop-
ment and for its being merged into a fuller theory of solutions.

For the matter now under our consideration we need not see in the fact of the
variability of i any hindrance to employing (a) the determination of the osmotic pressure,
(b) the freezing point of a solvent (the so-called Raoult's method), and (c) the variation
of the vapour density as means for finding the molecular weight of a substance in solu-
tion, not only in the ordinary cases when * = 1, but even in those cases where i~> 1.
These methods have already proved useful for solving the question of the molecular
weight in many particular instances among the hydrocarbons, and some portion of their
application to inorganic compounds will be mentioned in the further course of this work.

It may not be superfluous to remark that the osmotic pressure in the cells of organisms
attains several atmospheres, and probably forms one of the causes determining the

Y 2

324 PRINCIPLES OF CHEMISTRY

and liquids (as will afterwards be shown) is chiefly determined by the
weights of the atoms of the elements entering into their composition,
inasmuch as heavy (free) elements and compounds are only met with
among substances containing elements with large atomic weights, sucli
as gold, platinum, and uranium. And these elements themselves, in a
free state, are the heaviest of all elements. Substances containing such
light elements as hydrogen, carbon, oxygen, and nitrogen (like many
organic substances) never have a high specific gravity ; in the majority
of cases it scarcely exceeds that of water. The density generally
decreases with the augmentation of the amount of hydrogen, as the
lightest element, and a substance is often obtained lighter than water.
The refractive power of substances also entirely depends on the com-
position and the properties of the component elements.30 The history

individual functions of the cells. The further development of the questions touching
on this subject should in this manner not only aid the perfecting of the theory of solu-
tions but also the further progress of physiological science.

50 With respect to the optical refractive power of substances, it must first be observed
that the coefficient of refraction is determined by two methods : (a) either all the data
. are referred to one definite ray — for instance, to the Frauenhofer (sodium) line D of the
solar spectrum — that is, to a ray of definite wave length, and often to that red ray (of the
hydrogen spectrum) whose wave length is 656 million parts of a millimetre ; (b) or
Cauchy's formula is used, showing the relation between the coefficient of refraction and

T>

dispersion to the wave length n = A + - , where A and B are two constants vary-
ing for every substance but constant for all rays of the spectrum, and a is the wave length
of that ray whose coefficient of refraction is n. In the latter method the investigation
usually concerns the magnitudes of A, which are independent of dispersion. We shall
afterwards cite the data, investigated by the first method, by which Gladstone, Landolt,
and others established the conception of the refraction equivalent.

The coefficient of refraction n for a given substance decreases, as has long been
known, with the density of a substance D, so that the magnitude (n — l) -=-D = c is almost
constant for a given ray (having a definite wave length) and for a given substance. This
constant is called the refractive energy, and its product with the atomic or molecular
weight of a substance the refraction equivalent. The coefficient of refraction of oxygen is
1-00021, of hydrogen, 1*00014, their densities (referred to water) are 0'00143 and 0'00009,
and their atomic weights, O = 16, H = l ; hence their refraction equivalents are 3 and 1-5.
Water contains H2O, consequently the sum of the equivalents of refraction is (2 x 1'5) +
3 = 6. But as the coefficient of refraction of water = 1-331, therefore its refraction equi-
valent = 5"958, or nearly 6. The comparison shows that, approximately, the sum of the
refraction equivalents of the atoms forming compounds (or mixtures) is equal to the re-
fraction equivalent of the compound. According to the researches of Gladstone, Landolt,
Hagen, Briihl and others, the refraction equivalent of the elements are — H = l'8,Li = 8'8,
B = 4'0, C = 5'0, N = 4-l (in its highest state of oxidation, 5'3), 0 = 8*0, F = T4, Na = 4'8,
Mg = 7-0, Al = 8-4, Si = 6'8, P - 18'8, S = 16'0, Cl = 9'9, K = 8*1, Ca = 10'4, Mn - 12'2, Fe = 12'0,
(in the salts of its higher oxides 20'1), Co = 10'8, Cu = ll'6, Zn = 10'2, As = 15'4, Bi = l.V:',,
Ag = 15'7, Cd = 13-6, 1 = 24-5, Pt = 26'0, Hg = 20'2, Pb = 24'8, &c. The refraction equi-
valents of many elements could only be calculated from the solutions of their compounds.
The composition of a solution being known it is possible to calculate the refraction
equivalent of one of its component parts, those for all its other components being known.
The results are founded on the acceptance of a law which cannot be strictly appl i<-« 1.
Nevertheless the conception of the refraction equivalents gives an easy means for directly,

CTLES AND ATOMS 325

of chemistry presents a striking example in point —Newton fore-
saw from the high refractive index of the diamond that it would
contain a combustible substance since so many combustible oils have
a high refractive power.

As regards purely chemical relations, especially the understanding
of reactions and the structure of substances, the fecundity of the law
of Avogadro-Gerhardt is evinced at every step of the contemporary
path of chemistry. Starting from the laws and conceptions of
Lavoisier the chemistry of our .time is entirely founded on the laws
of Dalton and Avogadro-Gerhardt, on the doctrine of Berthollet
respecting the equilibria brought about in chemical actions, and on the
conceptions of dissociation introduced into the science by Saint-Claire
Deville.

although only approximately, obtaining the coefficient of refraction from the chemical com-
position of a substance. For instaiice, the composition of carbon bisulphide, CS2 = 76,
and from its density, 1'27, we find its coefficient of refraction to be 1'618 (because the
refraction equivalent = 5 + 2 x 16 = 87), which is very near the actual figure. It is evident
that in the above representation compounds are looked on as simple mixtures of atoms,
and the physical properties of a compound as referable to the properties present in
the elementary atoms forming it. If this representation of the presence of simple
atoms in compounds did not exist, then the effort to combine by a few figures a whole
mass of data relating to the coefficient of refraction of different substances would hardly

IT i- necessary to dearly distinguish between the two closclv-allied
Term-. charcoal ;nid carbon. Charcoal is well known to e\ erybodv,
although it is no easy matter to obtain it in a chemically pure state.
Pure charcoal i- a simple, insoluble, infu-ible. combustible substance
produced by heating organic matter, and lias the familiar aspect of a
Mark mass, devoid of any crystalline structure, and completely in-
- diible. Charcoal is a sub-tance possessing a certain combination of
]>hv.sical and chemical propert ies. This suh>tance. \\-hilst in a state of
ignition, combines directly \\itli oxygen: in organic substances it is
found in combination \vjtli hydrogen, <i.\\'j4'eii, ';it ro^'en. and sulphur.
Hut in all these combination- there is IH> real charcoal, as in the same
sense there is no ice in steam. AVliat is found in such combinations is
termed 'carbon - that is, an element common 10 charcoal, to t)io>e sub-
Mances \\liidi can be formed b\- it. and also to tho-e substances from
v.hicli it can be obtained. Carbon mav take the toriu of charcoal,
but appeal's also a- diamond and as graphite. It is true that no
other element has siidi a limitation in its terminolou'V. '*. \ygen was
alwavs called 'o\\'^en, \\hether it \vas in a f ree gase()iis state, or oxygen
in the tor m ot o/i >i ie. or o\ \'^en in \\ a t er. ( >r in nit ric acid or in carbonic
anhvdride. llut here there is a certain confu-n»n. In water it is
evident thai there is no oxvgen in ;. ga.-eous form, such as can be
obtained in a free -tale, no oxvgen in the form of o/one, but a sub
stance \\hidi i- capable of producing both oxygen, o/one. and \valer.
As an element oxvLren possesses a kno\\n chemical indi\ iduality, and
;ni influence on the properties < >\ tho-e combinations into which it
enters. llvdroifen gas is a substance which read- with dillicultv. but
h \ 'dro'_fi •! i as an elemeni represents in its combinations an easily dis-
placeable component jiart. ('arbon may be considered as an atom of
carbon matter, and charcnal as a collection of -uch atoms forming u
whole substance, as molecules in the mass of the substance. The
accepted atomic Weight of carbon i- I 1', because that is the least

CARBON AND THE II YDROCA !{!',< ).\s 327

tju.-intity of carbon which enters into combination in molecules of its
compounds ; but the weight of the molecules of charcoal is probably
very great. This weight remains unknown, because charcoal is capable
of but few direct reactions, and those only at a high temperature (when
the weight of its molecules probably changes, as when ozone changes into
oxygen), when it does not turn into vapour. Carbon exists in nature,
both in a free and combined state, in most varied forms and aspects.
Carbon in a free state is found in at least three different forms, as
charcoal, graphite, and the diamond. In a combined state it enters
into the composition of what are called organic substances — a
multitude of substances which are found in all plants and animals. It
exists as carbonic anhydride both in air and in water, and in the soil
and crust of the earth as salts of carbonic acid and as organic remains
The variety of the substances of which the structure of plants and
animals is built up is familiar to all. Wax, oil, turpentine, and tar,
cotton and albumin, the tissue of plants and the muscular fibre of
animals, vinegar and starch, are all vegetable and animal matters, and
all carbon compounds.1 The sphere of carbon compounds is so vast

1 Wood is the non- vital part of ligneous plants ; the vital part of ordinary trees is
situated between the bark and the lignin. Every year a layer of lignin is deposited
on this part by the juices which are absorbed by the roots and worked up by the leaves ;
for this reason the age of trees may be determined by the number of lignin layers depo-
sited. The following year the juices travel over a new layer, and in this way the layer
already deposited serves only as a support for the vital parts of the tree. A living tree
may be regarded as an agglomeration of many plants living on one support. The woody
matter consists principally of fibrous tissue on to which the lignin or so-called incrust-
ing matter has been deposited. The tissue has the composition C6H10O5, the substance
deposited on it contains more carbon and hydrogen and less oxygen. This matter is
saturated with moisture when the wood is in a fresh state. Fresh birch wood contains
about 31 p.c. of water, lime wood 47 p.c., oak 35 p.c., pine and fir about 37 p.c. When
dried in the air the wood loses a considerable quantity of water and not more than 19 p.c.
remains. By artificial means this loss of water may be increased. If water be driven
into the pores of wood it becomes heavier than water, as the ligniu of which it is com-
posed has a density of about 1-6. One cubic centimetre of birch wood does not weigh
more than 0'901 grams, fir 0'894, lime tree 0'817, poplar 0'765 when in a fresh state ; when
in a dry state birch weighs 0'622, pine 0'550, fir 0'355, lime 0'430, guaiacum T342,
ebony 1*226. It is not out of place to remark here that on one deciatin (2*7 acres)
of woodland the yearly growth averages an amount of 3000 kilograms, or 180 poods of
\vood, but rarely reaches as much as 5000. The average chemical composition of wood dried
in air may be expressed as follows :— Hygroscopic water 15 p.c., carbon 42 p.c., hydrogen
5 p.c., oxygen and nitrogen 37 p.c., ash 1 p.c. Wood parts with its hygroscopic water at
150°, and decomposes at about 800°, giving a brown, brittle, so-called red charcoal ; above
350° black charcoal is produced. From the above-mentioned average composition of
wood it is evident that the hydrogen may be accepted as being in about the quantity to
combine with the oxygen, as the hydrogen contained in the wood requires for its combus-
tion about forty parts by weight of oxygen. Therefore all that burns of the wood is the
carbon which it contains, 100 parts of wood only giving out as much heat as forty parts
of charcoal. Charcoal gives out much more useful heat than wood because the water con-
tained in the wood, or formed by the combination of its oxygen and hydrogen, has to be

828 PRINCIPLES OF CHEMISTRY

that it forms a separate branch of chemistry, which treats of the com-
pounds of carbon, part of which are met with in plants and animals.

evaporated during its combustion. The above-mentioned composition of wood shows
that it would be far more profitable to use charcoal for heating purposes than wood, if it
were possible to obtain it in such quantities as correspond with its percentage ratio— that
is, forty parts per 100 parts of wood. Generally, however, the quantity produced is far
less, not more than 30 p.c., because part of the carbon is given off as gas, tar, &c. If
wood has to be transported great distances, or if it is necessary to obtain a very high
temperature by burning it, then even as little as 25 p.c. of charcoal from 100 parts of
wood may be advantageous. Charcoal (wood) develops on burning 8000 heat unitsr
whilst wood dried in air does not develop more than 2800 units of heat ; therefore
seven parts of charcoal give as much heat as twenty parts of wood ; but twenty parts of
wood are only capable of yielding five parts of charcoal. This datum may be used for com-
paring the heating power of wood and charcoal. As regards the temperature of combus-
tion, it is far higher with charcoal than with wood, because twenty parts of burning wood
give, besides the carbonic anhydride which is also formed with charcoal, eleven parts of
water, the evaporation of which requires a considerable amount of heat.

The composition of the growing parts of plants, the leaves, young branches, shoots, <tc.T
differs from the composition of the wood in that these vital parts contain a considerable
quantity of sap which contains much nitrogenous matter (in the wood itself there is very
little), mineral salts, and a large amount of water. Taking, for example, the- composition
of clover and pasture hay in the green and dry state. In 100 parts of green clover there
is about 80 p.c. of water and 20 p.c. of dry matter, in which there are about 3'5 parts of
nitrogenous matter, about 9'5 parts of soluble and about 5 parts of insoluble
non-nitrogenous matter, and about 2 p.c. of ash. In dry clover or clover-hay there is
about 15 p.c. of water, 13 p.c. of nitrogenous matter, and 7 p.c. of ash. This com-
position of grassy substances shows that they are capable of forming the same sort of char-
coal as wood itself. It also shows the difference of nutritive properties existing between
wood and the substances mentioned. These latter serve as food for animals, because
they contain those substances which are capable of being dissolved (entering into the
blood) and forming the body of animals ; such substances are proteids, starch, &c. Let
us remark here that with a good harvest an acre of land gives in the form of grass as
much organic substance as it yields in the form of wood.

One hundred parts of dry wood are capable of giving, by means of dry distillation,
besides 25 p.c. of charcoal and 10 p.c. or more of tar, 40 p.c. of watery liquid, containing
acetic acid and wood spirit, and about 25 p.c. of gases, which may be used for heating or
lighting purposes, because they do not differ from ordinary illuminating gas, which caji
indeed be obtained from wood. As wood-charcoal and tar are costly products, in some
cases the dry distillation of wood is carried on principally for producing them. For this
purpose those kinds of woods are particularly advantageous which contain resinous sub-
stances, especially coniferous trees, such as fir, pine, &c. ; birch, oak, and ash give much
less tar, but on the other hand they yield more watery liquid. The latter is used
for the manufacture of wood spirit, CH4O, and acetic acid, C2H4Oo. In such cases
the dry distillation is carried on in stills. Stills are nothing more than hori-
zontal or vertical cylindrical retorts, made of boiler plate, heated with fuel and
having apertures at the top and sometimes also at the bottom for the exit of the light and
heavy products of distillation. The dry distillation of wood in stoves is carried on in two
ways, either by burning a portion of the wood inside the stove in order to submit the
remainder to dry distillation by means of the heat obtained in this manner, or by placing
the wood in a stove the thin sides of which are surrounded with a flue leading from the
fuel, placed in a space below.

The first means does not give such a large amount of liquid products of the dry
distillation as the latter. In the latter process there is generally an outlet below for

CARBON AM> THE HYDROCARBONS

329

This branch of chemistry is known under the name of organic
chemistry — that is, the chemistry of carbon compounds, or, more
strictly, of the hydrocarbons and their derivatives.

If any one of these organic compounds be strongly heated without
access of air — or, better still, in a vacuum — it decomposes with more or
less facility. When organic substances are heated in air, it is well
known that they burn ; but if the supply of air be insufficient, or the
temperature be too low for combustion, and if the first volatile pro-
ducts of transformation of the organic matter are subjected to con-
densation (for example, if the door of a stove be opened), an imperfect
combustion takes place, and smoke, with charcoal or soot, is formed.2

emptying out the charcoal at tlie close of the operation. For the dry distillation of 100
parts of wood from forty to twenty parts of fuel are used.

There are many steps between the method of burning wood in stacks (Note 4), and
that of burning it in an enclosed space — namely, those in which the burning of the char-

FIG. 57.— Apparatus for the dry distillation of wood. The retort a containing the wood is heated by
the flues c e. The steam and volatile products of distillation pass along the tube g through the
condenser ni, wln-iv they are condensed. The form, distribution, and dimensions of the apparatus

vary.

coal is accompanied by a certain production of tar. This is effected by means of trenches
dug in the earth and having sloping bottoms, by which contrivance the tar separated by
the charring of the wood flows into special receivers. This process is much used in
the north of Eussia.

In the north of Russia wood is so plentiful and cheap that this locality is admirably
fitted to become the centre of a general trade in the products of the dry distil-
lationof wood. Coal (Note 6), sea-weed, turf, animal substances (Chap. VI.), &c., are
also submitted to the process of dry distillation.

2 The result of imperfect combustion is not only the loss of a part of the fuel and the
production of smoke, which in some respects is inconvenient and injurious to health, but
also a low flame temperature, which means that a less amount of heat is transmitted to
the object heated. Imperfect combustion is not only always accompanied by the forma-
tion of soot or unburnt particles of charcoal, but also by that of carbonic oxide, CO, in the

PRINCIPLES OF CHEMISTRY

The nature of the phenomenon, and the products arising from it, are
the same as those produced by heating alone, as that part which is in a
state of combustion serves to heat the remainder of the fuel. The decom-
position which takes place on heating a compound composed of carbon,
hydrogen, and oxygen is as follows : — A part of the hydrogen is sepa-
rated in a gaseous state, another part in combination with oxygen,
and a third part separates in combination with carbon, and sometimes in
combination with carbon and oxygen in the form of gaseous or volatile
products, or, as they are also called, the products of dry distillation.
If the vapours of these products are passed through a highly-heated
tube, then they are changed again in a .similar manner, and finally
resolve themselves into hydrogen and charcoal. All these various pro-
ducts of decomposition contain a smaller amount of carbon than the
primary organic matter ; part of the carbon separates in combination
with hydrogen and oxygen, but part of it remains in a free state, form-
ing charcoal.3 It remains in that space where the decomposition takes

-moke (Chapter IX.) which burns, emitting much heat. In works and factories where
lari:c quantities of fuel are consumed, many appliances are adopted to ensure perfect com-
bustion, and to combat against such a ruinous practice as the imperfect combustion of
fuel. The most effective and radical means consists in employing the combustible gases
i regenerative and water gases), because by their aid perfect combustion can be easily
realised without a loss of heat-producing power and tl'i> highest temperature can
be reached. When solid fuel is used (such as coal, wood, and turf), imperfect combustion
is most liable to occur when the furnace doors are opened for the introduction of fresh
fuel. The step furnace may often prove a remedy for this defect. In the ordinary
furnace fresh fuel is placed on the burning fuel, and the products of dry distillation of
the fresh fuel have to burn at the expense of the oxygen remaining uncombined with
the burnt fuel. Imperfect combustion is observed in this case also from the fact that
the dry distillation and evaporation of the water of the fresh fuel lying on the top of that
burnt, lowers the temperature of the flame, because part of the heat becomes latent.
On this account a large, amount of smoke (imperfect combustion) is observed when a fresh
quantity of fuel is introduced into the. furnace'. This may be obviated by constructing
the fill-mice (or managing the stoking] in such a way that the products of distillation pass
through the red-hot charcoal lemaining from the burnt fuel. It is only necessary in
order to ensure this to allow a. sufficient quantity of air for perfect combustion. All this
may lie easily attained by the use of step fire-liars. The fuel is shovelled into a funnel
and falls on to the fire-bars, which are disposed in the form of a staircase. h burning
charcoal is below, and therefore, the flame formed by the fresh fuel is heated by the con-
tact of the red-hot, burning charcoal. An air supply through the lire grate, and its equal
distribution on the fire-bars (otherwise the air will blow through the empty space and
lower the temperature i, a proper proportion between the supply of air and the chimney
draught, and a perfect admixture of air with the flame (without an undue excess of air),
•ire the means by which we can strive against the imperfect combustion of such kinds of
fuel as wood, peat, and ordinary (smoky) coal. Coke, steam coal, anthracite, burn with-
out smoke, because they do not contain hydrogenous substances which furnish the pro-
ducts, of drv distillation, but imperfect combustion may occur with them also; then the
smoke contains carbonic oxide.

•"' The various kinds of coal used in practice and found in nature are the products of
the transmutation of the remains of organic matter. There is no organic substance
which contains in itself sufficient oxvgen not onlv to combine with hydrogen in order to

('AUIJON AND THi; II YI>li< H'A I!H< >NS 331

place in the shape of a black, infusible, non-volatile charcoal familiar
to all. The earthy matter and all non-volatile substances (ash) form-
ing a part of the organic matter, remain behind with the charcoal. The
tar-like substances, which require a high temperature in order to de-
compose them, also remain mixed with charcoal. If a volatile organic
substance, such as a gaseous compound containing oxygen and hydro-
gen, be taken, the carbon separates on passing the vapour through a

form water, but also to unite with the carbon to form carbonic anhydride. The
greater part of vegetable tissue consists of the cellulose, C6H10O5 ; from its composi-
tion it is i-vidt'iit that the oxygen is insufficient to transform the hydrogen into water and
the carbon into carbonic anhydride, because for this purpose it would require 17
• •quivalents and it only contains 5. This reasoning also refers to all the remaining
organic substances. Under the action of air, organic substances are capable of oxidising
to such an extent that all the carbon and all the hydrogen they contain will be trans-
formed into carbonic anhydride and water. The refuse of plants and animals are subjected
to such a change whether they slowly decompose and putrefy, or rapidly burn, with direct
access to air. But if the supply of air be limited, then, in virtue of the above-stated
reasoning, there can be no complete transformation into water and carbonic anhydride,
and therefore, if organic matter decomposes under these conditions, charcoal must
remain, as it is a non-volatile substance. All organic substances are unstable, they do
not resist heat, and in time easily change at ordinary temperatures, particularly if water
be present. Therefore it is easy to understand that charcoal may be obtained in many
cases through the transformation of substances entering into the composition of
•organisms, but that it is never found in a pure state.

The transformation of organic matter is not, however, so simple as would appear
from the preceding statements ; that is to say, water and carbonic anhydride are not the
•only products separated from organic substances. Carbon, hydrogen, and oxygen are
capable of giving a multitude of compounds ; some of these are volatile compounds,
gaseous, soluble in water — they are carried off from organic matter, undergoing change
without access of air. Others, on the contrary, are non-volatile, rich in carbon, constant
under the influence of heat and other agents. The latter remain in admixture with
charcoal where the decomposition takes place; such, for example, are tarry sub-
stances. The quantity of those bodies which are found mixed with the charcoal is very
varied, and depends on the energy and duration of the decomposing influence. For
instance, when wood is first acted on by heat, the moisture separates ; it then turns
brown, but still contains a large amount of oxygen and hydrogen. If the action be
further continued the quantity of these latter elements diminishes, and the proportion of
carbon in the residue increases, although a part of it is carried off in the shape of the
volatile products of decomposition. The greater the heat, the less the quantity of char-
< oal obtained, and the less the amount of hydrogen and oxygen contained in the remain-
ing charcoal. The annexed table shows, according to the data of Violette, those changes
to which wood is subjected at various temperatures when submitted to dry distillation by
nu'ansof superheated steam: —

Residue

Temperature from 100 parts In 100 parts of the residual charcoal

of alder wood

C H 0 andN A>h

150° lOO'O 47'5 <)•! 46'8 O'l

850° 29'7 76-6 4'1 18'4 0'6

1082° 18'7 Hl-i) 2-8 14-1 1'6

l :,oo

17-8 95-0 0-7 :!-s T7

332 PRINCIPLES OF CHEMISTRY

tube heated to a high temperature. Organic substances, when burning
with an insufficient supply of air, give off soot — that is, charcoal — pro-
ceeding from carbon compounds in a state of vapour, the hydrogen
of which has, by combustion, been converted into water ; so, for instance,
turpentine, naphthalene, and other hydrocarbons which are with diffi-
culty decomposed by heat, easily yield carbon in the form of soot during
combustion. Chlorine and other substances, which, like oxygen, are
capable of taking up hydrogen, and also substances which are capable
of taking up water, can also separate carbon from (or char) most organic
substances.

When organic substances are burning in an insufficient supply of
air, they both separate and leave charcoal. Part of it remains behind
as a residue, as, for instance, after burning wood in a stove some remains
in the form of charcoal. Wood charcoal is prepared in large quantities
in a similar manner — that is, by the partial combustion of wood.4 In
nature a like process of carbonisation of vegetable refuse takes place in
its transformation under water, as shown by the marshy vegetation
which forms peat.5 In this manner, doubtless, the enormous masses

4 The object of producing charcoal from wood has been explained in Footnote 1.
Wood charcoal is obtained in so-called stacks by partially burning the wood, or by
means of dry distillation— that is, by heating wood in an enclosed space (in retorts,
Note 1) — without the access of air. It is principally manufactured for metallur-
gical processes, especially for smelting and forging iron — that is, for use in smithies.
The preparation of charcoal in stacks has one advantage, and that is that it may be
done on any spot in the forest. But in this way all the products of dry distillation
are lost. For charcoal burning, a pile or stack is generally built, in which the logs are
closely placed, either horizontally, vertically, or inclined, forming a stack of from six to
fifty feet in diameter and even larger. Under the stack there are several horizontal air
passages, and an opening in the middle to let out the smoke. The surface of the stack is
covered with sods to a considerable thickness, especially the upper part, in order to hinder
the free passage of air and to concentrate the heat inside. When the stack is kindled, the
pile begins to settle down by degrees, and it is then necessary to look after the turf
casing and keep it in repair. As the combustion spreads throughout the whole pile, the
temperature rises and real dry distillation commences. It is then necessary to stop the
air holes, in order as much as possible to prevent unnecessary combustion. The nature
of the process is, that part of the fuel burns and develops the heat required for sub-
jecting the remainder to dry distillation. The charring of the stack lasts about a fort-
night, and is brought to a termination by scattering earth over the incandescent charcoal
to stop the combustion. The charring is stopped when the products of dry distillation,
which are emitted, no longer burn with a brilliant flame, but the pale blue flame of
carbonic oxide appears. Dry wood in stacks yields about one-fourth of its weight of
charcoal.

5 When dead vegetable matter undergoes transformation in air, in the presence of
moisture, there remains a substance much richer in carbon — namely, humus, black earth
or mould. 100 parts of humus in a dry state contain about 70 p.c. of carbon. The roots,
leaves, and stems of plants which wither and fall off form a soil rich in humus.
The non-vital vegetable substances (ligneous tissue) first form brown matter (ulmic
compounds), and then black matter (humic substances), which are both insoluble
in water ; after this a brown acid is produced, which is soluble in water (apocrenic acid),.

N AND THE IIVDIJOCAKIJONS 333

of coal were formed,6 which, following the example set by England, are
now utilised everywhere as the principal material for heating steam

uncl lastly a colourless acid also soluble in water (crenic acid). Alkali dissolves a part
of the original brown and black substances, forming solutions of a brown tint (ulmic
and humic ai-idsi, which sometimes communicate their colour to springs and rivers. The
composition of the humus in the soil is generally in connection with its fertility; firstly,
because putrefying plants develop carbonic anhydride and ammonia, and yield the saline
substances which are necessary to vegetation; secondly, because humus is capable of
attracting the moisture of the air and of absorbing water (twice its weight) and in
this way keeps the soil in a damp condition, which is indispensable for nourishment ;
thirdly, humus renders the soil porous, and, fourthly, renders it more capable of absorb-
ing the heat of the sun's rays. On this account black earth is often most remarkable for
its fertility. One object of manuring is to increase the quantity of humus in the soil,
and any easily changeable vegetable or any animal matter (composts) may be used. The
boundless tracts of black earth soil in Eussia are capable of bestowing countless wealth
on the country.

The origin and extent of black earth soil is treated in detail in Professor Dokou-
^haeff's works.

If those substances which produce humus undergo decomposition under water, less
carbonic anhydride is formed, a quantity of marsh gas. CH4, is evolved, and the
solid residue forms an acid humus, found in great quantities in marshy places, and
called peat. Peat especially abounds in the lowlands of Holland, North Germany,
Ireland, and Bavaria. In Russia it is likewise found in large quantities, especially in the
North-west districts. The old hard forms of peat resemble in composition and properties
brown coal ; the newest formations, as yet unhardened by pressure, form very porous
masses which retain traces of the vegetable matter from which they have been formed.
Dried (and sometimes pressed) peat is used as fuel. The composition of peat varies
considerably with the locality in which it is found. When dried in air it does not contain
less than 15 p.c. of water and 8 p.c. of ash ; the remainder consists of 45 p.c. of carbon,
4 p.c. of hydrogen, 1 p.c. of nitrogen, and 28 p.c. of oxygen. Its heating power is about
equivalent to that of wood. The brown earthy varieties of coal were probably formed
from peat. In other cases they have a marked woody structure, and are then known as
lignites. The composition of the brown sorts of coal resembles in a marked degree that
of peat — namely, in a dried state brown coal contains on an average 60 p.c. of carbon, 5
p.c. of hydrogen, 26 p.c. of oxygen and nitrogen, and 9 p.c. of ash. In Russia brown coal is
met with in many districts near Moscow, in the governments of Toula and Tver and the
neighbourhood ; it is very generally used as fuel, particularly when found in thick
-Muis. The brown coals generally burn with a flame like wood and peat, and are
iikin to them in heating power, which is two or three times less than that of the best
coal.

6 Grass and wood, the vegetation of primaeval seas and similar refuse of all geological
periods, must have been, in many cases, subjected to the same changes they now
undergo — that is, under water they formed peat and lignites. Such substances, pre-
served for a long time underground, subjected to the action of water, compressed
by the new strata formed above them, transformed by the separation of their
more volatile component parts (peat and lignites, even in a finished state, still
continue to evolve nitrogen, carbonic anhydride, and marsh gases), form coal. Coal is a
dense homogeneous mass, dark brown or black, with an oily or glassy lustre, or, more
rarely, dull, without any evident vegetable structure ; this distinguishes it in appearance
from the majority of lignites. The density of coal (not counting the admixture of
pyrites, &c.) varies from 1'25 (dry bituminous coal) to 1'6 (anthracite, flameless), and
CM u reaches 1'9 in the very dense variety of coal found in the Olonetzky government
(termed thungite), which, according to the investigations of Professor Inostrantzeff, may
be regarded as the extreme member of the various forms of coal.

In order to explain the formation of coal from vegetable matter, Caigniard de la Tour

334 PRINCIPLES OF CHEMISTRY

boilers, and in general for all purposes of heating and burning.7 Russia

enclosed pieces of dried wood in a tube and heated them to the boiling point of mercury,
when the wood was changed into a semi-liquid black mass from which a substance
exceedingly like coal separated. In this manner some kinds of wood formed coal, which,
on being heated, left caking coke, others non-caking; precisely us we iind with the
natural varieties of coal. Violette repeated these experiments with wood dried
at 150°, and showed that when wood is decomposed in this way a gas. a watery liquid,
and a residue are formed. The latter, at a temperature of 200°, has the properties of wood
charcoal incompletely burnt ; at 300° and higher a homogeneous mass like coal is formed,
which at 340° is dense and without cavities. At 400° the residue resembles anthracite.
In nature probably the decomposition was effected by heat alone in rare cases ;
more generally it was effected by means of water and heat, but in either case the result
ought to be almost the same.

The average composition of coal compiled from many analyses, disregarding the
ash, is as follows : 84 parts of carbon, 5 parts of hydrogen, 1 part of nitrogen, 8 parts of
oxygen, 2 of sulphur. The medium quantity of ash is 5 p.c., but there are coals which
contain a large quantity, and naturally they are not so advantageous for use as fuel. As
regards the amount of water, coal in this respect is much more profitable than lignite or
peat, as it does not usually contain more than 10 p.c. of water. The anthracites form
a remarkable variety of coals, and are indeed sometimes placed in a separate category ;
they do not give any volatile products, or but a very small amount, as they contain but
little hydrogen compared to oxygen. In the average composition of coal we saw that for
5 parts of hydrogen there were 8 parts of oxygen ; therefore 4 parts by weight of the
hydrogen are capable of forming hydrocarbons, because 1 part of hydrogen is necessary
in order to form water with the 8 parts of oxygen. These 4 parts by weight of hydrogen
can convert 48 parts of carbon into the form of benzene or similar volatile products,
because 1 part of hydrogen by weight in these substances combines with 12 parts of
carbon. The anthracites differ essentially from this ; neglecting the ash, their average
composition is as follows : 94 parts of carbon, 3 of hydrogen, and 3 of oxygen and
nitrogen. According to the analyses of A. A. Voskresensky, the Grousheffsky anthracite
(Don district) contains: C = 93'8, H = 1'7, ash = l'5. Therefore the anthracites contain
but little hydrogen capable of combining with the carbon to form hydrocarbons, which
burn with a flame. Anthracites are the most ancient forms of coal. The newest and
least tranformed, which resemble some of the brown varieties, are the dry coals (Griiner's
first group). They burn with a flame like wood, and leave a coke having the appearance
of lumps of coal, half their component parts being absorbed by the flame (they contain
much hydrogen and oxygen). The remaining varieties of coal (2nd group, gas coal ;
3rd, smithy coal ; 4th, coking ; and 5th, anthracite, according to Griiner) in all respects
form connecting links between the dry coals and the anthracites. These coals burn with
a very smoky flame, and on being heated leave coke, which bears the same relation to
coal as charcoal does to wood. The quantity and quality of coke varies considerably
with the different sorts of coal from which it is formed. In practice coals are most often
distinguished by the properties and quantity of the coke which they give. In this par-
ticular the so-called bituminous coals are especially valuable, as even small coal (baked i
of this kind gives by dry distillation large spongy masses of coke. If large pieces of
these kinds of coal are subjected to dry distillation, they, as it were, melt, flow together.
and form caking masses of coke. The best coking coals give (55 p.c. of dense caking
coke. Such coal is very valuable for metallurgical purposes (see Note 8). Besides coker
the dry distillation of coal produces gas (see further, illuminating gas), coal-tar (which
gives benzene, carbolic acid, naphthalene, tar for artificial asphalt, &c.), and also a
watery alkaline liquid (with wood and lignites the liquid is acid from acetic acid) which
contains ammonium carbonate (see Note 6).

7 In England in 1850 the output of coal was already 48 million tons, and in latter
years (1884-1888) it rose to about 160 millions. Besides this the other countries con-

< '.\RHo.\ AND TIIK HYDROCARBONS 385

possesses many very rich coalfields, amongst which the Don district is
most worthy of remark. *

During the imperfect combustion of volatile substances containing
carbon and hydrogen, the hydrogen and part of the carbon first burn,
and the remainder of the carbon forms soot. If oil of turpentine,

tribute 230 millions — Russia about 3^ millions, the United States of America come next
to England with an output of 75 million tons, then Germany 60 millions ; France
produces but little (20 millions), and takes about 5 million tons from England. Besides
household purposes, coal is chiefly used as fuel for steam-engines. As every horse-power
( = 75 kilogrammetrea per second) of a steam-engine expends on the average more than
2."i kilograms in 24 hours, or in a year (counting stoppages) not less than 5 tons per
horse-power, and there are not less than 40 million horse-power at work in the world,
the consumption of coal for motive-power is at least equal to half the whole production.
For this reason coal serves as a standard for industrial development. About 15 p.c. of
(•(nil is used for the manufacture of cast iron, wrought iron, steel, and articles made of
them.

8 The principal workable coal beds of Eussia are : The Don basin (115 million poods
per annum, 62 poods = l ton), the Polish basin (Dombrovo and others 110 million poods
per annum), the Toula and Riazan beds of the Moscow basin (up to 25 million poods),
the Ural basin (10 million poods), the Caucasian (Kliboul, near Kutais), the Khirjhis
steppes, the smithy coal basin (Gov. of Tomsk), the Sahaline, &c. The Polish and Mos-
cow basins do not give any coking coals. The presence of every variety of coal (from the
dry coal near Lisichanena on the Donetz to the anthracites of the entire south-east basin),
the great abundance of excellent metallurgical coal (coking, see Note 6) in the western
part of the basin, its vast extent (as much as 25,000 sq. versts), the proximity of the
scams to the surface (the shafts are now from 20 to 100 fathoms deep, and in England
and Belgium as deep as 500 fathoms), the fertility of the soil (black earth), the proximity
of the sea (about 100 versts from the Sea of Azoff) and of the rivers Donetz, Don, and
Dnieper, the most abundant seams of excellent iron ore (Korsan Mogila, Krivoy rog,
Soulin, &c., &c.), copper ore, mercury ore (near Nikitooka, in the Bakhmouth district of
the Ekaterinoslav Gov.), and other ores, the richest probably in the whole world, the
beds of rock-salt (near the stations of the Stoupka and Brianzovka), the excellent clay of
all kinds (china, fire-clay), gypsum, slate, sandstone, and other wealth of the Don coal
Ixisin, give complete assurance of the fact that with the growth of industrial activity in
Russia this bountiful land of the Cossacks and New Russia will become the centre of the
most extensive productive enterprise, not only for the requirements of Russia alone, but
of the whole world, because in no other place can be found such a concentration of
favourable conditions. The growth of enterprise and knowledge, together with the
extinction of the forests, which compels Russia to foster the production of coal, will help
to bring about this desired result. The forest wealth of North Russia and the naphtha
treasures of Caucasus can only contribute to its advancement, and have not the power
to check that influence which the Don coal basin should have on the industrial state of
Russia. England with a whole fleet of merchant vessels exports annually about 25
million tons of coal, the price of which is higher than on the Donetz (where a
pood of worked coal costs less than 5 copecks on the average), where anthracites and
semi-anthracites (like Cardiff or steam coal, which burns without smoke) and
coking and metallurgical coals are able both in quantity and quality to satisfy the most
fastidious requirements of the industry already existing and rapidly increasing every-
where. In 1850 the world's consumption and production of coal was only 5000 million
poods, and now the quantity has risen to 25000 million poods. The coal mines of
England and Belgium are approaching to a state of exhaustion, whilst in those of the
Don basin, only at a depth of 100 fathoms 1200000 million poods of coal lies waiting,
to be worked.

336 PRINCIPLES OF CHEMISTRY

or naphthalene or other hydrocarbons, be burned in the open air, soot is
formed in large quantities, because these substances contain a large
amount of carbon. Tar, pitch, and similar substances for this reason
burn with a smoky flame. Thus, soot is finely-divided charcoal,
separated during the imperfect combustion of the vapours and gases
of carbonaceous substances rich in carbon. Specially-prepared soot
(lampblack) is very largely used as a black paint, and a large quantity
goes for the manufacture of printers' ink. The quantity of organic
matter remaining undecomposed in the charcoal depends on the
temperature to which it has been submitted. Charcoal, prepared at the
lowest temperature, still contains a considerable quantity of hydrogen
and oxygen — even as much as 4 p.c. of hydrogen and 20 p.c. of
oxygen. Such charcoal still preserves the structure of the substance
from which it was obtained. Ordinary charcoal, for instance, in which
the layers of the tree are yet visible, is of this kind. On submitting it
to further heating, a fresh quantity of hydrogen with carbon and
oxygen (in the form of gases or volatile matter) may be separated, and
the purest charcoal will be obtained on submitting it to the greatest
heat.9 If it be required to prepare pure charcoal from soot, it is
necessary first to wash it with alcohol and ether, in order to get rid of
the soluble tarry products, and then submit it to a powerful heat to
drive off the impurities containing hydrogen and oxygen. Charcoal,
however, when completely purified does not change in appearance.
Everybody knows that charcoal is a black amorphous substance,
without any signs whatever of crystallisation (probably a colloid). Its
porosity,10 bad conducting power for heat, capability of absorbing the

9 As it is difficult to separate from the charcoal the admixture of ash — that is, the
earthy matter contained in the vegetable substance used for producing charcoal — in order
to obtain it in its purest condition, it is necessary to use such organic substances as do
not contain any ash, for example, completely refined or purified crystallised sugar,
crystallised tartaric acid, &c.

10 The cavities in charcoal are the passages through which those volatile products
formed at the same time as the charcoal have passed. The degree of porosity of
charcoal varies considerably, and has a technical significance, in different kinds of
charcoal. The most porous charcoal is very light; a cubic metre of wood charcoal
weighs about 200 kilograms. Many of the properties of charcoal which depend exclu-
sively on its porosity are shared by many other porous substances, and vary with
the density of the charcoal and depend on the way it was prepared. The power which
charcoal has of absorbing gases, liquids, and many substances in solution, has refe-
rence to this. The densest kind of charcoal is formed by the action of great heat on
sugar. The lustrous gray dense charcoal formed in gas retorts is also of this character.
This dense charcoal collects on the internal walls of the retorts subjected to great heat,
and is produced by the vapours and gases separated from the heated coal in the retorts.
In virtue of its density, such charcoal becomes a good conductor of the galvanic current
and approaches graphite. It is principally used in galvanic batteries. Coke, or the
•charcoal remaining from the imperfect combustion of coal and tarry substances, is also but

CARBON AM) THE II YI>K< •( 'AJUV 337

luminous rays (clue to its blackness and opacity), and many other quali-
ties are familiar from everyday experience.11 The specific gravity of
charcoal varies from T4 to 1-9, and that it floats on water is due to the
air contained in its pores. If charcoal be reduced to a powder and
moistened with spirit, it immediately sinks in water. It is infusible in
the furnace and even at the temperature of the oxyhydrogen flame. In
the heat generated by means of a strong galvanic current, charcoal
only softens, but does not completely melt, and on cooling it is found
to have undergone a complete change both in properties and appearance,
and is more or less transformed into graphite. The physical stability

slightly porous, brilliant, does not soil or mark paper, is dense, almost devoid of the faculty
of retaining liquids and solids, and does not absorb gases. The light sorts of charcoal
produced from charred wood, on the other hand, show this absorptive power in a most
marked degree. This property is particularly developed in that very fine and friable
charcoal prepared by heating animal substances, such as hides and bones. The absorp-
tive power of charcoal with reference to gases, is similar to the condensation of gases in
spongy platinum. Here evidently there is a phenomenon of the adherence of gases to a
•solid, precisely as liquids have the property of adhering to various solids. One volume
of charcoal will absorb the following volumes of gases (charcoal is capable of absorb-
ing an immense amount of chlorine, almost equal its own weight) : —

Saussure

Favre

Heat emitted

Boxwood Charcoal.

Cocoanut Charcoal.

IMT -_'ram of gas.

NH- HO

172 vols.

41)4 units.

CO., 85

97

158

X.,0 40

99 „

169 „

HC1 85

ir,r, „

274 „

The quantity of gas absorbed by the charcoal increases with the pressure, and is
approximately proportional to it. The quantity of heat given out by the absorption
nearly approaches that set free on dissolving, or passing into a liquid condition.

Charcoal absorbs not only gases, but a number of other substances. For instance,
alcohol, which contains disagreeably smelling fusel oil, on being mixed with charcoal or
filtered though it, loses the bulk of the fusel oil. The practice of filtering substances
through charcoal in order to get rid of foreign matters is often applied in chemical and
manufacturing processes. Oils, spirits, various extracts, and vegetable and other solu-
tions containing an admixture of colouring or odorous matters, and also water, are
filtered through charcoal in order to purify them. The bleaching power of charcoal
may be tested by using various coloured solutions — such as aniline dyes, litmus, &c.
Charcoal, \\liicli has absorbed one substance to saturation, is still capable of absorbing
certain other substances. The more porous charcoal is, the greater its surface, and this
is the reason why animal charcoal, produced in a very finely-divided state, especially by
h.-iiting bones, makes the best sort for the purposes of absorption. Bone charcoal is
used 'in large quantities in sugar works for filtering syrups, and all saccharine solutions,
in order to purify them, not only from colouring and odorous matter, but also from lime.
\vhich is mixed with the syrups in order to render them less unstable during boiling>
The absorption of lime by animal charcoal depends, in all probability, in a great degree
on the mineral component parts of bone charcoal.

11 Charcoal is a very bad conductor of heat, and therefore forms an excellent insulator
or packing to prevent the transmission of heat. A charcoal lining is often used in
crucibles for heating many substances, as it does not melt and resists a far greater heat
than many other substances.

VOL. I. Z

338 PRINCIPLES OF CHEMISTRY

of charcoal is, without doubt, allied to its chemical stability. It is
evidently a substance devoid of energy, as charcoal is insoluble in all
known liquids, and at an ordinary temperature does not combine n-'iflt
anything ; it is an inactive substance, like nitrogen.12 But these
properties of charcoal change with a rise of temperature ; thus, in contra-
distinction from nitrogen, charcoal, at a high temperature, combines
directly with oxygen. This is well known, as charcoal burns in air.
Indeed, not only does oxygen combine icith charcoal at a red lt<'«t,
but sulphur, hydrogen, and also iron and some other metals do so
at a very high temperature— that is, when the molecules of the char-
coal have reached a state of great instability— whilst at ordinary tem-
peratures neither oxygen, sulphur, nor metals act on charcoal in any
way. When burning in oxygen, charcoal forms carbonic anhydride,
C02, whilst in the vapours of sulphur carbon bisulphide, CS2, is
formed, and wrought iron, when acted on by carbon, becomes cast iron.
At the great heat obtained by passing the galvanic current through
carbon electrodes, the charcoal combines with hydrogen, forming
acetylene, C2H2. Charcoal does not combine directly with nitrogen,
but in the presence of metals and alkaline oxides, nitrogen is absorbed,
forming a metallic cyanide, as, for instance, potassium cyanide, KCN.
From these few direct combinations which charcoal is capable of
entering into, may be derived those numerous carbonaceous compounds
which enter into the composition of plants and animals, and which are
obtained artificially. Certain substances containing oxygen give up a
part of it to carbon at a relatively low temperature. For instance,
nitric acid when boiled with charcoal gives carbonic anhydride and
nitric peroxide. Sulphuric acid is reduced to sulphurous anhydride
when heated with carbon. When heated to redness charcoal ab-
sorbs oxygen from a large number of the oxides. Even such oxides
as those of sodium and potassium, when heated to redness, yield their
oxygen to charcoal, although they do not part with it to hydrogen.
Only a few of the oxides, like silica (oxide of silicon), and lime (calcium
oxide), resist the reducing action of charcoal. Charcoal is capable of
changing its physical condition without undergoing any alteration in
its essential chemical properties — that is, it passes into isomeric or

12 The unalterability of charcoal under the action of atmospheric influences, which
produce changes in the majority of stony and metallic substances, is often made use of
in practice. For example, charcoal is frequently strewn in boundary ditches. Tin-
surface of wood is often charred to render it durable in those places where the soil is
damp, and wood itself would soon rot. The chambers (or in some works towers i through
which acids pass (for example, sulphuric and hydrochloric) in order to bring them into
contact with gases or liquids, are filled with charcoal or coke, because, at ordinary tc in
peratures, it resists the action of even the most energetic acids.

CABB03 AND THE HYDROCARBONS 339

(t//t,fr(rj>i>' /ni'iiia. The two particular forms in which carbon appears
arc t lie tlinHinnil and f/r<i/i/i iff. The identity of the composition of these
with charcoal is proved by burning an equal quantity of all three
separately in oxygen (at a great heat), when each gives the same
quantity of carbonic anhydride -namely, 12 parts of charcoal, diamond,
or graphite in a pure state, yield on burning 44 parts by weight of
carbonic anhydride. The physical properties present a marked con-
trast ; the densest sorts of charcoal have a density of only 1-9, whilst
the density of graphite is about 2-3, and that of the diamond 3'5. A
great many other properties depend on the density, for instance, com
bustibility. The lighter charcoal is, the more easily it burns ; graphite
burns with considerable difficulty even in oxygen, and the diamond
burns only in oxygen and at a very high temperature. On burning,
charcoal, the diamond, and graphite develop different quantities of heat.
One part by weight of wood charcoal converted by burning into
carbonic anhydride develops 8080 heat units :. dense charcoal
separated in gas retorts develops 8050 heat units ; natural graphite,
7*00 heat units ; and the diamond, 7770. The greater the density
the less the heat evolved by the combustion of the carbon.13

By means of intense heat charcoal may be transformed into
graphite. If a charcoal rod 4mm. in diameter and 5mm. long be enclosed
in an exhausted receiver and the current from 600 Bunsen's elements,
placed in parallel series of 100, be passed through it, the charcoal
becomes strongly incandescent, partially volatilises, and is deposited in
the form of graphite. If sugar be placed in a charcoal crucible and
a powerful galvanic current passed through it, it is baked into a mass
similar to graphite. If charcoal be mixed with wrought iron and
heated, cast iron is formed, which absorbs as much as five per cent, of
charcoal. If molten cast iron be suddenly chilled, the carbon remains
in combination with the iron, forming so called white cast iron ; but if
the cooling proceeds slowly the greater part of the carbon separates
in the form of graphite, and if such cast iron (so called grey cast
iron) be dissolved in acid, the carbon remains in the form of graphite.
Graphite is met with in nature, sometimes in the form of large
compact masses, sometimes it permeates rocky formations like the
schists or slates, and in fact is met with in those places which, in all

'" When subjected to pressure, charcoal loses heat, hence the densest form stands to
the less druse as a solid to a liquid, or as a compound to an element. From this the
conclusion mi>jht be drawn, that the molecules of graphite are more complex than those
of charcoal, and those of the diamond still more so. The specific heat shows the same
tiling, because, as we shall see on further exposition, the complexity of a molecule leads
to a diminution of the specific heat. At ordinary temperatures, the specific heat of
charcoal is 0"24, graphite 0*20, the diamond 0'147.

z 2

340 PRINCIPLES OF CHEMISTRY

probability, were submitted to the action of subterranean heat.14 The
graphite in cast iron, and sometimes also natural graphite, occasionally
appears in a crystalline form in the shape of six-sided plates, but more
often it occurs as a compact amorphous mass having the characteristic
properties of the familiar black-lead pencil.15

The diamond is a crystalline and transparent form of carbon. It
crystallises in octahedra, dodecahedra, cubes, and other forms of the
regular system.11' The efforts which have been made to produce
diamonds artificially, although they have not been absolutely fruitless,
have not as yet led to the production of large-sized crystals, because
those means by which crystals are generally formed are inapplicable
to carbon. Indeed, carbon in all its forms being insoluble and infu-
sible does not pass into a liquid condition by means of which crystalli-

14 There are places where anthracite gradually changes into graphite, as the
strata sink. I myself had the opportunity of observing this gradual transformation in
the valley of Aosta, near Mont Blanc, not far from Courmayeur, near the warm mineral
waters.

"l5 Pencils are made of graphite worked up into a homogeneous mass by disintegiu-
ting, powdering, and cleansing it from earthy impurities ; the best kinds are made of
completely homogeneous graphite sawn up into the requisite sticks. Graphite is found
in many places. In Russia the so-called Aliberoffsky graphite is particularly renowned ;
it is found in the Altai mountains near the Chinese frontier ; in many places in Finland
and likewise on the banks of the Little Tungouska, Sidoroff also found a considerable
quantity of graphite.

Graphite, like most forms of charcoal, still contains a certain quantity of hydrogen,
oxygen, and ash, so that in its natural state it does not contain more than (J« p.o. of
carbon.

In practice graphite is purified simply by washing it when in a finely-ground state,
by which means the bulk of the earthy matter may be separated. The following prr,
cess, proposed by Brodie, consists in mixing the powdered graphite with TTT part of its
weight of potassium chlorate. The mixture is then heated with twice its weight of strong
sulphuric acid until no more odoriferous gases are emitted ; on cooling, the mixture is
thrown into water and washed; the graphite is then dried and heated to a red heat;
after this it considerably shrinks in volume, and turns into a very fine powder, which is
then washed. When mixed with clay, graphite is used for making crucibles and pots
for melting metals. By acting on graphite several times with a mixture of potassium
chlorate and nitric acid heated up to 60°, Brodie transformed it into a yellow insoluble
acid substance which he called graphitic acid, CUH4O5. The diamond remains unchanged
when subjected to this treatment, whilst amorphous charcoal completely oxidises.
Availing himself of this possibility of distinguishing graphite from the diamond or amor-
phous charcoal, Berthelot showed that when compounds of carbon and hydrogen are
decomposed by heat, amorphous charcoal is mainly formed, whilst when compounds of
carbon with chlorine, sulphur, and boron are decomposed, graphite is principally
deposited.

16 Diamonds are sometimes found in the shape of small balls, and in that case it is
impossible to cut them, because directly the surface is ground or broken they fall into
minute pieces. Sometimes minute diamond crystals form a dense mass like sugar, ami
this is generally reduced to diamond powder and used for grinding. Some known
varieties of the diamond are almost opaque and of a black colour. Such diamonds a re
as hard as the ordinary ones, and are used for polishing diamonds and other precious
stones, and also for rock boring and tunnelling.

N ANJJ TIIK JI YI>K< K'AKIK >NS 341

sition could take place. Diamonds have several times been success-
fully produced in the shape of minute microscopic crystals having the
appearance of a black powder, but when viewed under the microscope
they appeared transparent, and possessed that hardness which is the
peculiar characteristic of the diamond. This diamond powder is de-
posited on the negative electrode, when a weak galvanic current is
passed through liquid chloride of carbon.17

Judging from the fact that carbon forms a number of gaseous (car-
bonic oxide, carbonic anhydride, methane, ethylene, acetylene, tfec.) and
volatile (such are, for instance, many hydrocarbons and their most simple
derivatives) substances, and also because the atomic weight of carbon,
C=12, approaches that of nitrogen, N = 14, and that of oxygen, O = 16,
a iid the compounds CO (carbonic oxide) and N2C2 (cyanogen) are gases,
it must be argued that if carbon formed the molecule C2 like N2 and O2,
it would be a gas. And as through polymerism, or the mutual combina-
tion of molecules (as O2 passes into O3 or NO2 into N2O4) the tempera-
tures of ebullition and fusion rise (which is particularly clearly proved
with the hydrocarbons of the CnH2,, series), it ought to be considered
that the molecules of charcoal, graphite, and the diamond are very com-
plex, seeing that they are not soluble, not volatile, and not fusible.
The aptitude which the atoms of carbon show for combining together
and forming complex molecules appears in all carbon compounds.
Among the volatile compounds of carbon many are well known, the
molecules of which contain C5 . . : C-10 . . . C20 . . . C30, &c., in
general C,,, where n may be very large, and in none of the other ele-
ments is this faculty of complexity so developed as in carbon.18 Up

17 Hannay, in 1H80, obtained diamonds by heating a mixture of heavy liquid
hydrocarbons (paraffin oils) with magnesium in a thick iron tube. This investigation
was, however, not repeated. Diamonds are found in a particular dense rock, known by
tin- name of itacolumnite, and are dug out of the debris produced by the destruction
of the itacolumnite by water. When the debris is washed the diamonds remain behind ;
they are principally found in Brazil, in the provinces of Rio and Baliia, and at the Cape
of Good Hope. The debris gives the black or amorphous diamond, carbonado, and th^
ordinary colourless or yellow translucent diamond. As the diamond possesses a very
marked cleavage, the first operation consists in splitting it, and then roughly and finely
polishing it with diamond powder. It is very remarkable that Professors P. A. Latchinoff
and Erofeeff found (1887) diamond powder in a meteoric stone which fell in the govern-
ment of Penza, in the district of Krasnoslobodsk, near the settlement of Novo Urei
(Sept. 1(». lSHr>i. Up to that time charcoal and graphite (a special variety, cliftonite) had
been found in meteorites and the diamond only conjectured to be therein. The Novo
Urei meteorite was composed of siliceous matter and metallic iron (with nickel), like
many other meteorites.

18 The existence of a molecule S6 is known, and it must be held that this accounts
for the formation of hydrogen persulphide H.,S5. Phosphorus appears in the molecule
P4 and gives P4H._>. When expounding the data on specific heat we shall have occasion
to return to the question of the complexity of the carbon molecule.

342 PRINCIPLES OF CHEMISTRY

to the present time there are no grounds for determining the degree
of polymerism cf the charcoal, graphite, or diamond molecules, and it
can only be supposed that they contain C,, where ?t is a large quantity.
Therefore charcoal and those complex noii- volatile organic substances
which represent the gradual transitions to charcoal 19 and which form
the principal masses of the organisms, contain a store or accumulation
of internal power in the form of the energy binding the atoms into
complex molecules. When charcoal or complex compounds of carbon
burn, the energy of the carbon and oxygen is turned into heat, and
this fact is taken advantage of at every turn for the generation of heat
from fuel.20

No other two elements are capable of combining together in
such variety as carbon and hydrogen. The hydrocarbons of the
C,,H2m series in many cases widely differ from each other, although
they have some properties in common. All hydrocarbons, whether
gaseous, liquid, or solid, are combustible substances sparingly soluble
or insoluble in water. The liquefied gaseous hydrocarbons, also those
which are liquid at ordinary temperatures, and those solid hydrocarbons
which have been liquefied by fusion, have the appearance and property
of oily liquids, more or less viscid or fluid.21 The solid hydrocarbons

19 The hydrocarbons, poor in hydrogen (far from the limit) and containing many
atoms of carbon, like chrysene and carbopetrocene, itc., C,,Ho(,,_W) are solids, and less
fusible as n and in increase. They present a marked approach to the properties of the
diamond. And in proportion to the diminution of the water in the carbohydrates
C/jHoTOOwi — for example, in the liumic compounds iXote ~>) — the transition of complex
organic substances to charcoal is very evident. That residue resembling charcoal and
graphite whk;h is obtained by the separation (by means of copper sulphate and sodium
chloride) of iron from white cast-iron containing carbon, chemically combined with the
iron, also seems, especially after the researches of G. A. Zaboudsky, to be a complex
substance containing C^H^O^. The endeavours which have been directed towards
determining the measure of complexity of the molecules of charcoal, graphite, and the
diamond will probably at some period lead to the solution of this problem, and will most
likely prove that the various forms of charcoal, graphite, and the diamond contain mole-
cules of different and very considerable complexity. The constancy of the grouping of
benzene, CCH6, and the wide diffusion and facility of formation of the carbohydrates.
containing C6 (for example, cellulose, CCH10O3, glucose, C0H12O<3),give reason for thinking
that the group C6 is the first and simplest of those possible to free carbon, and it may
be hoped that some time or other it may be possible to get charcoal in this group. 1 Vr-
haps in the diamond there may be found such a relation between the atoms as in tin-
benzene group, and in charcoal such as in the carbohydrates.

20 When charcoal burns, the complex molecule C,, is resolved into the simple mole-
cules ?iCO.>, and therefore part of the heat — probably no small amount — is expended in
the destruction of the complex molecule Ctt. Perhaps by burning the most complex
substances, which are the poorest as regards hydrogen, it maybe possible to form an
idea of the work required to split up C;i into separate atoms.

21 The viscosity, or the degree of mobility, of liquids is determined by their internal
friction. It is estimated by passing the liquids through narrow [capillary) tubes, the
mobile liquids passing through with greater facility and speed than the. viscid ones. The

< \KHON AND THE HYDROCARBONS ;343

more or less resemble wax in their properties, although ordinary oils
and wax generally contain oxygen in addition to carbon and hydrogen,
but in relatively small proportion. There are also a few hydrocarbons
which have the appearance of tar — as, for instance, metacinnamene and
gutta-percha. Those liquid hydrocarbons which boil at a high tempera-
ture are like oils, and those which have a low boiling point resemble
rthrr, whilst the gaseous hydrocarbons in many of their properties are
akin to hydrogen. All this tends to show that, in hydrocarbons phy-
sically considered, the properties of solid 11011- volatile charcoal are
strongly modified and hidden, whilst those of the hydrogen predominate.
All hydrocarbons are neutral substances (neither basic nor acid), but
under certain conditions they enter into peculiar reactions. It has
been seen in those hydrogen compounds which have been already con-
sidered (water, nitric acid, ammonia) that the hydrogen in almost all
•cases enters into reaction, being displaced by metals. The hydrogen of
the hydrocarbons, it may be said, has no metallic character — that is to
say, it is not directly 22 displaced by metals, even by such as sodium and

viscosity varies with the temperature and nature of the liquids, and, in the case of solu-
tions, changes with the amount of the substance dissolved, but is not proportional to it.
S,. thai, for example, with alcohol at 20° the viscosity will be 69, and for a 50 p.c. solu-
tion 1C>(>, the viscosity of water being taken as 100. The volume of the liquid which passes
through by experiment (Poiseuille) and theory (Stokes) is proportional to the tune,
the pressure, and the fourth power of the diameter of the (capillary) tube, and inversely
proportional to the length of the tube ; this renders it possible to form comparative esti-
mates of the coefficients of internal friction and viscosity.

As the complexity of the molecules of hydrocarbons and their derivatives increases
by the addition of carbon (or CH2), so does the degree of viscosity also rise. The exten-
eries ot 'investigations referring to this subject still await the necessary generalisation.
That connection which (already partly observed) ought to exist between the viscosity
•and the other physical and chemical properties, forces us to conclude that the magnitude
of internal friction plays an important part in molecular mechanics. In investigating
organic con-pounds and solutions, similar researches ought to stand foremost. Many
observations have already been made, but not much has yet been done with them; the
bare tacts and some mechanical data exist, but their relation to molecular mechanics has
not been cleared up in the requisite degree. It has already been seen from existing data
that the viscosity at the temperature of the absolute boiling point becomes as small as
in ga

-' In a number of hydrocarbons and their derivatives such a substitution of metals
for the hydrogen maybe arrived at by indirect means. The property shown by acetylene,
(11,. and its analogues of forming metallic derivatives is in this respect particularly
characteristic. Judging from the fact that carbon is an acid element (that is, gives an
ac id anhydride with oxygen), but comparatively slightly acid (because carbonic acid is
not. at all an energetic acid; compounds of chlorine and carbon, even CC14, are not
decomposed by water as is the case with phosphorus chloride, and even silicic chloride
or boric chloride, although they correspond with acids of but little energy), one might
expect to find in the hydrogen of hydrocarbons this faculty for being substituted by
metals. The metallic compounds which correspond with hydrocarbons are known under
the name of organo-metallic compounds. Such, for instance, is zinc ethyl, Zn(C
which corresponds with ethyl hydride or ethane, C.^H0, where two atoms of hydrogen
have been exchanged for one of zinc.

3-14 PKINCJPLKS OF CHE.MISTKY

potassium. ( >n the application of more or less heat, all hydrocarbons
decompose, 2:i forming charcoal and hvdr< »gen. The majority of hydro-
carbons do not combine \vith the oxygen of the air or oxidise at ordi-
nary temperatures, lint under the action of nitric acid and many other
oxidising substances most of them are subject to oxidation, in which
either a jiortion of the hydrogen and carbon :.•- separated, or the oxygen
outers into coin bin at ion. or else the elements of hydrogen peroxide enter
into combination with the hydrocarbon.-1 When heated in air. hydro-
carbons luirii. and, according to the amount of carbon they contain,
their combustion is attended with a separation of soot — that is. linely-
dividod charcoal — which imparts great brilliancy to the Hamo, and on
this account many of them are used for the purposes of illumination —
as. tor instance, kerosene, coal u'as, oil of turpentine. As hydrocarbons
contain reducing elements (that is. those capable of combining with oxy-
gen) they often act as reducing agents — as. for instance, when heated
\vith oxide of copper, they burn, forming carbonic anhydride and water,
and leayo metallic copper. According to Gerhardt. all hydrocarbons
contain an even number of hydrogen atoms. Therefore, the general
formula for all hydrocarbons is ('„ II., whore n and /// are Nvholr-
ntimbers. This fact is known as tin- /////• <>/' >•/•> ,, mmifti't'n. Ht^nce, tluv
simjile>t possible hydrocarbons ought to be: C.'Il.,, ('H,, dl,, . . .
C.J1,, (,'.,11,, C.JI*,, (,<.JIS . . . but all of them are not in existence, as
they are subject to certain limitations.

Some of the hydrocarbons are capable of combination, whilst others-
do not show that power. Those "which contain less hydrogen belong to
the former category, and those which, for a given quantity of carbon,
contain the maximum amount of hydrogen belong to the latter. The
composition of tho>e last mentioned is expressed by the general formula
C,,!!.,,, ,. Thoe >o -railed ant n rated />>/</rnr<tr/><>>i* are incapable
of combination.-' •! ud^inn' from this, the hydrocarbons ( 'H (, (\, H s.

when ni^iilurat.-il li\ .Irncarl.mi-, sir,, slmki-n willi M

form '.-Ivr..!- — i.,r i-xiiini.!.-, C.H, yii-lils (Ml,;<),.

1 M\ iii'ticlr 1,11 lln- Mil.j.-r-t iippcarcd iii tin' jnunial (if tlir Si. I'rdT-liiir- Acadcinv
of Science- in Isr,l. Cp i,, |li;,t time, .-iltlinii-li Mian\ a(l(liti\c c'Hiuliinations witli livdro
1-iirl.un-, MIL I tln-ir fli-ri\.iti\e- U.T,. knowii. tlie\ lia.l iml l.eeii ^enei-aliserl, and were even

c()nlinnall\ i|ii«ited a- .,,-,-- ,,t -id.-i it ut i. ,n. Tim-, tin- c. .ml.in.it i. f etliylene, ( '.,H ,,

with ehlni-ine. ('!,. w,i- i. ('ten i-e-ardi-il a> .1 fdriiiiitiiin d tin- pnidncts of the >nl^t it ut ion
r.f C.H-CI iiinl HCI i iip|»o-,ed were held to-Hher a- fh.' Water of crystullisn

tidii i- in -all . I. • eariici th.ui ll,,. [*:>!. J,,u n/nl t,f' Hi, I', t , ,,//>/•// .] r,i, i, -,,,,/ 1 I
• . . ..iii|».iiinU. In general, accordin- to the law of limits..

CAKI'.OX AND THE JIYI>K< )( 'A KP,< >N> 345-

C3H,0, &c. . . do not exist. Those containing the maximum amount of
hydrogen will be represented by CH4 (n=l, 2w + 2 = 4), C2H6 (n='2)f
C3H8 (rc=3), C4H10, lire. This may be termed the law of limits.
Placing this in juxtaposition with the law of even numbers, it is easy
to perceive that the possible hydrocarbons can be ranged in series, the
terms of which may be expressed by the general formuhe C,(H2,(+2,
C,,H2,,, C,,H2,,_,, Arc. . . Those hydrocarbons which belong to any one-
of the series expressible by a general formula are said to be homologous
with one another. Thus, the hydrocarbons CH4, C2HG, C3H8, C4H10,
£c. . , are members of the limiting (saturated) homologous series
C,(H.2,(+.2. That is, the difference between the members of the series is
GEL.26 ISTotonly the composition, but also the properties, of the mem-
bers of a series tend to classification in one group. For instance, the
members of the series C,,H2w+2 are not capable of forming additive
compounds, wrhilst those of the series C,(H.2,( are capable of combining
with chlorine, sulphuric anhydride, tire. ; and the members of the
C,(H2;,_G group, belonging to the coal tar series, are acted on by nitric-
acid, and have other properties in common. The physical properties,
of the members of a given homologous series vary in such a manner, the
boiling point generally rises and the internal friction increases as n in-
creases 2" — that is, with an increase in the atomic weight ; the specific-
gravity also successively changes as n becomes greater.28

an unsaturated hydrocarbon, or its derivative, on combining with rX2, gives a substance
which is saturated or else approaching the limit. The investigations of Frankland with
many organo-metallic compounds clearly showed the limit in the case of metallic com-
pounds, which we shall constantly refer to later on.

-tt The conception of homology has been applied by Gerhardt to all organic com-
pounds in his classical work, ' Traite de Chimie Organique,' finished in 1855 (4 vols.)>
where he divided all organic compounds into fatty and aromatic, which is in principle
still adhered to at the present time, although the latter are more often called benzene
derivatives, on account of the fact that Kekule, in his beautiful investigations on the
structure of aromatic compounds, showed the link which unites them all with the
' nucleus ' benzene, C0H0.

27 This is always true for hydrocarbons, but for derivatives of the lower homo-
logues it is sometimes different ; for instance, in the series of saturated alcohols.
0,,^,,,+! (OH), when n = 0, we obtain water, H(OH), which boils at 100°, and whose specific
gravity at 150 = 0".)'.M)-2 ; when w = l, wood spirit CH5 (OH), which boils at 66°, and at 15°
ha- a specific gravity = (V7964 ; when w = 2, ordinary alcohol C2H.-,(OH), boiling at 78°,
specific gravity at 15° = 0'7986, and with further increase of CH.2 the specific gravity in-
creases. For the glycols CnH.2« (OH)2 the phenomenon of a similar kind is still more
striking ; at first the temperature of the boiling pointand the density increase, and then
for higher (more complex) members of the series diminish. The reason for this pheno-
menon, it is evident, must be sought for in the influence and properties of water, and
that strong affinity which, acting between hydrogen and oxygen, determines many of
the exceptional properties of water (Chap. I.).

28 As, for example, in the saturated series of hydrocarbons C,,Hon+.o, the lowest
member (« = 0) must be taken as hydrogen H.,,, a gas which (t.c. below —190°) is
liquefied with great difficulty, and when in a liquid state has doubtless a very small

346 PRINCIPLES OF CHEMISTRY

Many of the hydrocarbons which are met with in nature are the
products of organisms, and do not belong to the mineral kingdom. A
still greater number are produced artificially. These are formed by what
is termed the combination of residues. For instance, if a mixture of
the vapours of hydrogen sulphide and carbon bisulphide be passed
through a tube in which copper is heated, this latter absorbs the sul-
phur from both the compounds, and the liberated carbon and hydrogen
•combine to form a hydrocarbon, methane. If a mixture of bromo-
benzene, C6H5Br, and ethyl bromide, C.2H5Br, be heated with metallic
sodium, the sodium combines with the bromine of both compounds,
forming sodium bromide. From the first combination the group CGH-,
remains, and from the second C2H5. Having an odd number of hydrogen
atoms, they in virtue of the law of even numbers cannot exist alone,
therefore they combine together forming the compound C6HVC2H5 or
C8H , o (ethylbenzene). Hydrocarbons are also produced by the breaking
up of more complex organic or hydrocarbon compounds, especially
by heating — that is, by dry distillation. For instance, gum- benzoin
contains an acid called benzoic acid, C7HGO2, the vapours of which,
when passed through a heated tube, split up into carbonic anhydride,
CO2, and benzene, C6H6. Carbon and hydrogen only unite directly in
one degree of combination —namely, to form acetylene, having the com-
position C2H2, which, as compared with other hydrocarbons, exhibits
a very great constancy at a high temperature.211.

density. Where n = l, 2, 8, the hydrocarbons CH4, CoHc, C,-;Hg are gases, more and more
readily liquefiable. The temperature of the absolute boiling point for CH4= — 1003, and
for ethane C.,,H6, and in the higher members it rises. The hydrocarbon C,H10, already
liquefies about 0°. C5HK> boils at from + 9° (Livoff) to 37°, C0HU from 58° to 78°, Arc.
The specific gravities in a liquid state at 15° are : —

0-63 0-66 0'70 0'75 U'«5

29 If, at the ordinary temperature (assuming therefore that the water formed will be
in a liquid state), a gram molecule (26 grams) of acetylene, C2H.>, be burnt, 310 thousand
calories will be emitted (Thomsen), and as 12 grams of charcoal produce 97 thousand
calories, and 2 grams of hydrogen 69 thousand calories, therefore, if the hydrogen and
carbon of the acetylene were burnt there would be only 2x97 <>9, or 263 thousand
calories produced. It is evident, then, that acetylene in its formation absorbs :ilo -it',:;,
or 47 thousand calories — that is, the reaction of charcoal and hydrogen is endotherinal
-and the product is in this respect a substance resembling nitrous oxide, hydrogen
peroxide, &c. Such reasoning is, however, open to certain defects of one kind or another,
especially when a reaction, which can really only take place at a high temperature, is
supposed to be effected at ordinary temperatures or vice versa. Calculations cannot
be made for high temperatures, on account of insufficient evidence as to specific
heat.

For considerations referring to the combustion of carbon compounds, we will first
enumerate the quantity of heat separated by the combustion of definite eheinical earhon
compounds, and then a few figures touching the kinds of fuel applied in a practical
way.

CAKI50X AND TllK 11 V !>!{()( 'A \i\}( >.\S 347

There is one known substance among the saturated hydrocarbons
composed of 1 atom <>t' carbon and 4 atoms of hydrogen ; this is a com-

I. For molecular quantities in perfect combustion, the following amounts of heat
.iven out (when gaseous carbonic anhydride and liquid water are formed i. according

to the data of Thomson: (1) for gaseous C ,,11.,,,+., : 52'8 + 158'H/t thousand calories;
T C,,H.,,( : 17-7 + 158'l« thousand calories; (3) according to Stohniann (1888) for
liquid satuated alcohols, C,,H.,,,, +._>() : 1T8 + 156'3/i, and as the latent heat of evaporation
= about 8-2-rO'6», in a gaseous state, 20'0+156'9«; (4) for monobasic saturated liquid
acids, C((HX,,,O.., : 95'3 + 154'3/<. and as their latent heat of evaporation is about 5'0 + 1'2/i,
in a gaseous form, about (.)()-f 155/i ; (5) for solid saturated bibasic acids, C/iH.^j -2O4 :
25:>-s l.VJT.n, if they are expressed as C,,H., )(C.,,HoOj, then 5r4 + 152'6w; (6) for ben-
zene audits liquid homologues (still according to Stohmann) C,,Ho,, — 6 : 158'6 + 156'3«,
and in a gaseous form about 155 + 157«; (7) for the gaseous homologues of acetylene,
C,,H._,,,-o, (according to Thomsen), 5 + 157/t. It is evident from the preceding figures
that the group CH._,, or the substitution of CH5 for H, on burning gives out from 152 to
If.!) thousand calories. This is less than that given out by C + H.2, which is 97 + 69 or
166 thousand ; the reason of this difference (it would be still greater if charcoal were
gaseous) is the amount of heat separated during the formation of CH.v The heat of com-
bustion of the following solids (determined by Stohmann) is expressed, not in terms of
the molecular weight of the substance, but per unit of weight : C6H1UO5, cellulose, 4146 ;
starch, 4123; dextrose, C6HK,OG, 3692; cane sugar, CuH.,.>On, 3866; naphthalene, C10H8,
9621; urea, CN.JtjO, 2465 ; white of eggs, 5579 ; dry rye bread, 4421; wheaten bread,
4302 ; tallow, 98(55 ; butter, 9192 ; linseed oil, 9323.

II. The number of units of heat given out during the complete combustion and
cooling of the following ordinary kinds of fuel in their usual state of dryness and purity
are : — (1) for wood charcoal, anthracite, semi-anthracite, tarry coal and coke, from 7200
toS-200; (2) for dry, long naming coals, and the best brown coals, from 6200 to 6800;
(3) for perfectly dry wood, 3500 ; hardly dry, 2500 ; (4) perfectly dry peat, best kind,
4500 ; compressed and dried, 3000 ; (5) petroleum refuse and similar liquid hydro-
carbons, about 11000 ; (6) illuminating gas of the ordinary composition (about 45 vols.
H, 40 vols. CH4, 5 vols. CO, and 5 vols. N), about 12000 ; (7) generator gas (see next
Chap.), containing 2 vols. carbonic anhydride, 30 vols. carbonic oxide, and 68 vols.
nitrogen for one part b/j iveight of the whole carbon burnt, 5300, and for one part
by weight of the gas, 910, units of heat; and (8) water gas (see next Chap.) containing
4 vols. carbonic anhydride, 8 vols. N.,,, 24 vok. carbonic oxide, and 46 vols. H2, for one
part by weight of the carbon consumed in the generator 10900, and for one part by
weight of the gas, 3600, units of heat. In these figures, as in all calorimetric observa-
tions, the water produced by the combustion of the fuel is supposed to be liquid. As
regards the temperature reached by the fuel, it is important to remark that for solid
fuel it is indispensable to admit (to ensure complete combustion) twice the amount of
air required, but liquid, pulverised fuel, and especially gaseous fuel, does not require an

98 of air ; therefore, a kilogram of charcoal, giving 8000 units of heat, requires about
24 kilograms of air (3 kilograms of air per 1000 calories) and a kilogram of generator gas
requires only 0'77 kilogram of air (0'85 kilo, of air per 1000 calories), 1 kilogram of water
gas about 4'5 of air (1'25 kilos, of air per 3000 calories).

III. As one of the most important and extensive applications of combustible matter
is the formation of steam and conversion of it into work, we will take as an example the
calculation for a boiler burning prr hour ( = 3600 seconds) 100 kilograms of anthracite,
giving out 8000 units of heat, and containing 90 p.c. C, 3 p.c. H, 3 p.c. O, and 4 p.c. of
a-li and nitrogen. For the combustion of 100 kilos, of such coal, giving 800 thousand
units of heat, about 2400 kilos, (counting double the amount) of air will be required,
therefore in the smoke there will be 2500 kilos., and as the specific heat of the products
of combustion is about 0'25, so, taking the temperature of the smoke passing from
underneath the boiler, < = 200° (higher than the temperature of the air), the smoke will

carry off 125 thousand units of heat, and 187-5 units of heat per second will be communi-

348 PRINCIPLES OF CHEMISTRY

pound containing the highest percentage of hydrogen (CH4 contains,
25 per cent, of hydrogen), and at the same time it is the only
hydrocarbon whose molecule contains a single atom of carbon This
saturated hydrocarbon, CH,, is called mnrxh-f/nx or methane. If vege-
table or animal refuse suffers decomposition in a space where the air
•has not free access, or no access at all, then the decomposition is
accompanied with the formation of marsh gas, and this either at an
ordinary temperature, or at a comparatively much higher one. On this
account plants, when decomposing under water in marshes, give out
this gas. It is well known that if the mud in bogs be stirred up, the act
is accompanied with the evolution of a large quantity of gas bubbles ;
these may, although slowly, also separate of their own accord. The
gas which is evolved consists principally of marsh-gas.30 If wood, coal,
or many other vegetable or animal substances are decomposed by the
action of heat without access of air — that is, are subjected to dry dis-
tillation— they, in addition to many other gaseous products of decom-
position (carbonic anhydride, hydrogen, and various other substances),
evolve a great deal of methane. Generally, the gas which is used for
lighting purposes is obtained by this means, and, therefore, always
contains marsh gas, mixed with hydrogen and other vapours and gases,
although it is subsequently purified from many of them.31 As the

cated to the boiler, and to the surrounding air. Subtracting 27'5 units of heat for
radiation, we have 160 units of heat per second communicated to the water. If the
pressure in the boiler be 5 atmospheres, £ = 152° (page 54), and the temperature of the
feed water 12°, then (the heat of warming and evaporation of 640 units of heat, see
page 53) 0'25 kilo, of steam at 150° per second (900 kilos, per hour) will be produced.
If this steam does its work without waste of any kind, and be condensed in the con-
denser at 50°, then, according to the second law of the mechanical theory of heat, not
more in any case than 160 (150 — 50) •*- (150 + 273) or 37-H units of heat, will be converted
into work, which (according to the first law of the mechanical theory of heat) will equal
424 x 87'8 or 15927 kilogrammetres of work per second, or 212 horse-power (one horse
power =75 kilogrammetres per second).

But as at least 35 p.c. (in the best machines) is lost in inevitable loss of heat and
through friction, only about 140 horse-power can be obtained with the above-mentioned
quantity of fuel in a steam-engine of excellent construction. Generally, 1 kilo, of coal
per horse-power per hour is consumed in the best steam-engines (with expansion and
condensation). Only in gas engines at present does the work of the fuel approach the
theoretical limit.

30 It is easy to collect the gas which is evolved in marshy places if a glass bottle be
inverted in the water and a funnel put into it (in water) ; if the mud of the bottom be
now agitated, the bubbles which rise may be easily caught in the funnel.

31 Illuminating gas is generally prepared by heating gas coal (see Note 6) in oval
cylindrical horizontal cast-iron or clay retorts. Several such retorts BB (fig. ~>>~
disposed in the furnace A, and heated together. When the retorts are heated to a
red heat, lumps of coal are thrown into them, and they are then closed with a closely-
fitting cover. The illustration shows the furnace, with five retorts. Coke (see Note
1, dry distillation) remains in the retorts, and the volatile products in the form of vapours
and gases travel along the pipes d, rising from each retort. These pipes branch above
the stove, and are hermetically sealed into the receiver/ (hydraulic main) placed above

350 PRINCIPLES OF CHEMISTRY

decomposition of the organic matter, which forms coal, is still going on
underground, the evolution of large quantities of marsh gas frequently

the furnace. Those products of the dry distillation which most easily pass from the
gaseous into the liquid and solid states collect in the hydraulic main. From the hydraulic
main the vapours and gases travel along the pipe g and the series of vertical pipes j (which
are sometimes cooled by water trickling over the surface), where the vapours and gases
cool from the contact of the colder surface, and a fresh quantity of vapour condenses. The
condensed liquids pass from the pipes g andy and into the troughs H. These troughs
always contain liquid at a constant level (the excess flowing away), so that the gas cannot
escape, and thus forms, as it is termed, a hydraulic joint. In the state in which it leaves
the condensers, the gas consists principally of the following vapours and gases: (1) vapour
of water, (2) ammonium carbonate, (3) liquid hydrocarbons, (4) hydrogen sulphide, H.>S,
(5) carbonic anhydride, C(X, (6) carbonic oxide, CO, (7) sulphurous anhydride, SO.,, but a
great part of the illuminating gas consists of (8) hydrogen, (9) marsh gas, (10) olefiant
gas, C.,H4, and other gaseous hydrocarbons. The hydrocarbons (3, 9, and 10), the hydrogen,
and carbonic oxide are capable of combustion, and are useful component parts, but the
carbonic anhydride, the hydrogen sulphide, and sulphurous anhydride, as well as the
vapours of ammonium carbonate, form an injurious admixture, because they do not burn
(CO.,, SO.,)) and lower the temperature and brilliancy of the flame, or else, although they
are capable of burning, they give out during combustion sulphurous anhydride (for
example, H.jS, CS._>, and others), which has a disagreeable smell, is injurious when
inhaled, and spoils many surrounding objects. In order to separate the injurious pro-
ducts, the gas is washed with water, a cylinder (not shown in the illustration) filled with
coke continually moistened with water serving for this purpose. The water coming into
contact with the gas dissolves the ammonium carbonate ; hydrogen sulphide, carbonic
anhydride, and sulphurous anhydride, being only partly soluble in water, have to be got
rid of by a special means. For this purpose the gas is passed through moist lime or
another alkaline liquid, as the above-mentioned gases have acid properties and are
therefore retained by the alkali. In the case of lime, calcium carbonate, sulphite and
sulphide, all solid substances, are formed. It is necessary to renew the purifying
material, as its absorbing power ceases. A mixture of lime and sulphate of iron,
FeSO4, acts still better, because the latter, with lime, Ca(HO)._>, forms ferrous hydroxide,
Fe(HO)..,, and gypsum, CaSOt. The suboxide (partly turning into oxide) of iron absorbs
H.,S, forming FeS and HoO, and the gypsum retains the remainder of the ammonia,
the excess of lime absorbing carbonic anhydride and sulphuric anhydride. [In English
works a native hydrated ferric hydroxide is used for removing hydrogen sulphide.]
This purification of the gas takes place in the apparatus L, where the gas passes through
netting m, covered with sawdust mixed with lime and sulphate of iron. It is necessary
to remark that in the manufacture of gas it is indispensable to draw off the vapours from
the retorts, so that they should not remain there long (otherwise the hydrocarbons-
would in a considerable degree be resolved into charcoal and hydrogen), and also to avoid
a great pressure of gas in the apparatus, otherwise a quantity of gas would escape at
all cracks such as must inevitably exist in such a complicated arrangement. For
this purpose there are special pumps (exhausters) so regulated that they only pump off
the quantity of gas formed (the pump is not shown in the illustration). The purified
gas passes through the pipe n into the gasometer ("gasholder) P, a dome made of iron
plate. The edges of the dome dip into water poured into a ring-shaped channel g, in which
the sides of the dome rise and fall. The gas is collected in this holder, and distributed
to its destination by pipes communicating with the pipe o, issuing from the dome. The
pressure of the dome on the gas enables it, on issuing from a long pipe, to penetrate
through the small aperture of the burner. 100 kilograms of coal give about 20 to 80 cubic
metres of gas, having a density from four to nine times greater than that of hydrogen.
A cubic metre (1000 litres) of hydrogen weighs about 87 grams ; therefore 100 kilograms
of coal give about 18 kilograms of gas, or about one-sixth of its weight. Illuminating gas is

CABBON AND THI-: IIVDKOC.MMK.XS 351

occurs in coal-mines.3'2 When mixed with air it forms an explosive-
mixture, which forms one of the great dangers of coal mining, as
subterranean work has always to be carried on by lamp-light. This
danger is, however, overcome by the use of Humphry Davy's safety

generally lighter tliiin marsh gas, as it contains a considerable amount of hydrogen, and is
only heavier than marsh gas when it contains much of the heavier hydrocarbons. Thus
olefiant gas, CoHj, is fourteen times, and the vapours of benzene thirty-nine times, heavier
than hydrogen, and illuminating gas sometimes contains 15 p.c. of its volume of them.
The brilliancy of the flame of the gas increases with the quantity of olefiant gas and
similar heavy hydrocarbons, as it then contains more carbon for a given volume and a
greater number of carbon particles are separated. Gas usually contains from 35 to 60 p.c.
of its volume of marsh gas, from 30 to 50 p.c. of hydrogen, from 3 to 5 p.c. of carbonic
oxide, from i> to 10 p.c. heavy hydrocarbons, and from 3 to 10 p.c. of nitrogen. Wood
gives almost the same sort of gas as coal, and almost the same quantity, but the wood
gas contains a great deal of carbonic anhydride, and the vapours of water and tarn-
liquids, but on the other hand there is almost complete absence of sulphur compounds.
Tar, oils, naphtha, and such like materials furnish a large quantity of good illuminating
gas. An ordinary burner of 8 to 12 candle power burns 5 to 6 cubic feet of coal gas per
hour, but only 1 cubic foot of naphtha gas. One pood (36 Ibs. Eng.) of naphtha gives
."DO cubic feet of gas — that is, one kilogram of naphtha produces about one cubic metre
of gas. The formation of combustible gas by heating coal
was discovered in the beginning of the last century, but
only put into practice towards the end by Le-Bon in France
and Murdoch in England. In England, Murdoch, together
with the renowned Watt, built the first gas works in 1805.
It is worthy of remark that 1 candle-power per hour en-
tails a consumption of not less than 7 grams of coal gas, or 1 *>
grams of naphtha gas. whilst .lamps consume but 4 grams of
kerosene per candle-power, and ordinary-sized candles burn
6-9 grams of stearin, wax, or paraffin, or 13 grams of
tallow per hour. In practice illuminating gas is not only
used for lighting (electricity and kerosene are cheaper in
i;u--ia >. but also as the motive-power for gas engines (see
p. 172), which consume about half a cubic metre per horse-
power per hour ; gas is also used in laboratories for heating
purposes. When it is necessary to concentrate the heat,
either the ordinary blowpipe (fig. 59) is applied, placing
the end in the flame and blowing through the mouthpiece ;
or. in other forms, gas is passed through the blowpipe;
when a large, hot, smokeless flame is required for heating
crucibles or glass-blowing, a foot-blower is used. High
temperatures, which are often required for laboratory and
manufacturing purposes, are most easily attained by the
use of gaseous fuel (illuminating gas, generator gas, and
water gas, which will be treated of in the following chap-
ter) because complete combustion may be effected without
an excess of air. It is evident that for obtaining high tem-
peratures means must be taken to diminish the loss of heat

bv radiation. Fr ;-, 59 -Blowpipe. Air is

* ... r.,, . . , . Mown in at the trumpet -

Which is set free in coal mines contains a shaped month pi txv. ami
good deal of nitrogen, some carbonic anhydride, and a large escapes in fine stream from
quantity of marsh gas. The best means of avoiding «„ tile SrSy of tteltS
explosion consists in efficient ventilation. It is best to tuht'-
light coal mines with electric lamps.

1amp/u Sir Humphry I );i\ y ob-er\ ed that on introducing a piece ot' wire
u'au/.e into a tlame, it absorbs so much heat that combust ion docs not
proceed beyond it (the unburiit gase- which pass through it mav be
kindled on tin- other side). In accordance with this, the ilanie of the
l>avv lamp is surrounded with a thick glass (as shown in t he drawing),
and lias no comnitinicat ion whatever with the explosive* mixture except
through a wire ^au/.e which prevent- it igniting the mixtui'e of the
marsh-gas i-suin^ from the coal with air. In -ome place.-, particularly
in those districts \vhere petroleum is found — as, for instance, near l>aku,
where a temple of the Indian tire-worshippers \vas built, and in
Ponnsvl vania, and other places — marsh-gas in abundance issues from
the earth, and it is used, like coal gas. for the purpose- of lighting and
\varming.:u Fairly pure marsh gas ;<:> may be obtained by heating a
mixture of an acetate with an alkali. Acetic acid, < '., 1 1 ,( >.,. on being
heated is decomposed into marsh gas and carbonic anhydride,
C,H ,( ), = CH, + CO,.

An alkali for instance. Nall() gives with acetic acid a salt,
(.".,! l;.Na(J.,, and on decomposition \\'ith excels of alkali, this retains
carbonic anhydride, and forms a carbonate, Xa ,( '< ).,. and marsh gas
is ft >rnied.

C2H;iNaO,-f NalK )=Na._,CO;i + CH ,

Marsh o-as i- difficult to liijtiefv ; it is .almost insoluble in water,
and is without taste or smell. The most important point in connection
with it- chemical reaction.- is that it does not combine directlv with

•" Tin' Davy lamp is used fur li-htin- mill and
.iilu-i- niiii.'s where cninlui-.til.lr ..MS is luund. 'I'hr
wii-k i.f tin- lamp is I-J:I-I.^IM| in a thick -la —
cylin.l.T. which is tinnly ln-l<l in a metallic hol.l.-r.
( )\ el' ! lii- a metallic c\ limler and the u ire 'jaii/.e are
[ihiceil. The |n-i)duct- <>l ci utihu-t i< in jia— thnni^li
the ;jMu/e. and the air enters thniiiuh the -pace
I iet ween tin1 ry HlldiT ;ll)i| the \\ 1 1 i • - i u . c. TM ell -lire
greater >at'et\ the lamp caninit lie i ipmcd \\ it limit.

. \ 1 1 1 1 ;j II i - 1 1 1 1 1 L' the 1 1 a 1 II i • .

"' In I'enn-ylvauia il.e\«,inl the A ll.-jha n y inoini-
tain-i many of the -hall-, -mils I'm1 petroleum only
emitted LM-. lui! many useful application- tor it,
\\ere found, and il was ci.ndiicteil in metallic pij»es
to work-, hundred- of mile- di-tant , principally for
i ne! a I 1 1 1 I'L1 ica 1 | 'H I pi ise-.

• '' Ilie plll'e-t L'a- 1- prepared h\ I] 1 i X i 1 1 '_' the

i 1 1 11 id ii list a nee called /.i nc methyl, /in < 'H-ij, wi til
\\-.\\i r. \\ hen t he ftillnw ill'' read i,.n (icclll's :

CAKIJON AND TIIK HYDROCARBONS 353

anything, whilst the other hydrocarbons which contain less hydrogen
than expressed by the formula CMH3/l+2 are capable of combining with
hydrogen, chlorine, certain acids. Arc.

Marsh gas is the only hydrocarbon which contains one atom of
carbon in its molecule,36 and stands, therefore, to that element in the
same relation as water does to oxygen or ammonia to nitrogen. The mul-
titude of hydrocarbons which carbon has the power of forming must be
derived from this unique and simplest hydrocarbon. By arranging the
molecules (and consequently equal volumes) in the following order —

HH ; OH, ; NH3 ; CH$ ;

HC1 ; SH2- ; PH3 : SiH4 ;

it is evident that hydrogen and chlorine are univalent, oxygen and
sulphur bivalent, nitrogen and phosphorus trivalent, carbon and sili-
con quadrivalent. And if the law of substitution gives a very simple
explanation of the formation of hydrogen peroxide as a compound con-
taining two aqueous residues (OH) (OH), then on the basis of this
law all hydrocarbons ought to be derived from methane, CH4, as being
the simplest hydrocarbon. The increase in complexity of a molecule
of methane is brought about by the faculty of mutual combination
which exists in the atoms of carbon, and, as a consequence of the most
detailed study of the subject, much of that which might be foreseen and
conjectured from the law of substitution has been actually brought
about in such manner as might have been predicted. Although this sub-
ject, on account of its magnitude, appertains, as has been already stated,
to the sphere of organic chemistry, yet it has been touched on here in
order to show, although only in part, the best investigated example of the
application of the law of substitution. According to this law, a mole-
cule of methane, CH4, is capable of undergoing substitution in the four
following ways : — (1) Methyl substitution, when the radicle equivalent
to hydrogen CH3, called methyl, replaces the hydrogen. In CH4 this
radicle is combined with H, and therefore can replace it, as (OH) re-
places H because with it it gives water ; (2) methylene substitution,
or the exchange between H2 and CH2 (this radicle is called methylene)
is founded on a similar division of the molecule CH4 into two equiva-
lent parts, H.2 and CH2 ; (3) acetylene substitution, or the exchange
between CH on the one hand and H3 on the other ; and (4) carbon
substitution — that is, the substitution of H4 by an atom of carbon C,
which is founded on the law of substitution similarly to the methyl
substitution. These four cases of substitution render it possible to
understand the principal relations of the hydrocarbons. For instance,

36 Methylene, CHo, does not exist. When attempts are made to obtain it (for
example, by removing X.> from CEUXo), C2H4 or C3H6 are produced — that is to say, it
undergoes polymerisation .

VOL. I. A A

354 PRINCIPLES OF CHEMISTRY

the law of even numbers is seen from the fact that in all the cases of
substitution mentioned the hydrogen atoms increase or decrease by
an even number ; but, as in CH2 they are likewise even, it follows that
no matter how many substitutions they produce there will always be
an even number of hydrogen atoms obtained. Indeed, when H is re-
placed by CH3 there is an increase of CH2 ; when H2 is replaced by
CH2 there is no increase of hydrogen ; in the acetylene substitution
CEI replaces H3, therefore there is an increase of C and a decrease of
H2 ; in the carbon substitution there is a decrease of H4. In a similar
way the law of limit may be deduced as a corollary of the law of
substitution. In fact, the largest possible quantity of hydrogen
introduced corresponds with the methyl substitution, but it leads
to the introduction of CH2 ; therefore, no matter how many times this
exchange may be effected with CH4, and, denoting the number of times
by (w-1), the result will always be CH4(w-l)(CH2), or C,,H2n+2
as the highest hydrogen compound. TJnsaturated hydrocarbons, con-
taining less hydrogen, are evidently only formed when the increase of
the new molecule derived from methane proceeds from one of the other
forms of substitution. When the methyl substitution alone takes place
with methane, CH4, it is evident that the saturated hydrocarbon formed
is C2H6 or (CH3)(CH3).37 This is called ethane. Directly, by means
of the methylene substitution alone, ethylene, C2H4, or (CH2) (CH2) may
be obtained from CH4, and by the acetylene substitution, C2H2 or
(CH)(CH), or acetylene, both the latter being unsaturated hydro-
carbons. Thus we have all the possible hydrocarbons with two atoms
of carbon in the molecule, C2H6, ethane, C2H 15 ethylene, arid C2H2,
acetylene. But with them, according to the law of substitution, the
same forms of substitution may be repeated — that is, the methyl,

37 Although the methods of formation and the reactions connected with hydrocarbons
are not described in this work, because they are dealt with in organic chemistry, yet, in
order to clearly show the mechanism of those transformations by which the carbon
atoms are built up in the molecules of the carbon compounds, we here give a general
example of reactions of this kind. From marsh gas, CH4, on the one hand, the substi-
tution of chlorine or iodine, CH5C1, CH3I, for the hydrogen may be effected, and, on the
other hand, such metals as sodium may be substituted for the hydrogen, CH3Na. Such
and other similar products of substitution are exceedingly characteristic of hydrocarbons,
and serve as a means of obtaining other more complex substances from given carbon com-
pounds. If we place the two above-named products of substitution of marsh gas i metallic
and haloid) in mutual contact, the metal combines with the halogen, forming a very stable
compound — namely, common salt or sodium chloride and the carbon-groups which were
in combination with them separate in mutual combination, as shown by (lie equation :

CH5C1 + CH3Na = NaCl + C2H6.

This is the most simple example of the formation of a complex hydrocarbon from these
* radicles. The cause of the reaction must be sought for in the property which chlorine
and sodium have of entering into mutual combination.

CARBON AND THE HYDROCARBONS 355

inethylene, acetylene, and even carbon substitutions (because C2H6 will
still contain hydrogen when C replaces H4) — and therefore further sub-
stitutions will serve as a source for the production of a set of saturated
and unsaturated hydrocarbons, all containing more and more carbon
in the molecule and, in the case of the acetylene substitution and
carbon substitution, containing less and less hydrogen. Thus by
IH> <uis of the law of substitution we can foresee not only the limit
C,,H2,l+2, but an unlimited number of unsaturated hydrocarbons, C,(H2)1,
CrtH.,n_2 .... CHH.2(,,_m), where m varies from 0 to n - I,38 and
where n increases indefinitely. From these facts not only does the
existence of a multitude of polymeric hydrocarbons, differing in mole-
cular weight, become evident, but it is also seen that there is a possi-
bility of cases of isomerism with the same molecular weight. Already
in the first unsaturated series CwH2n that polymerism so common to
hydrocarbon compounds is apparent, because all the terms of this
series C2H4, C3Hr,, C4H8 .... C30H60 .... have one and the
same composition CH2, but different molecular weights, as has been
already explained in Chapter VII. (page 312). The differences in the
vapour density, boiling points, and melting points, of the quanti-
ties entering into reactions,39 and the means of preparation40 also so
clearly tally with the doctrine of polymerism, that this example will
always be the clearest and most conclusive for the conception of
polymerism and molecular weight. Such a case is also met with among
•other hydrocarbons. Thus benzene, C6H6, and cinnamene, C8H8,
Correspond with the composition of acetylene or to a compound of
the composition CH.41 The first boils at 81°, the second at 144° ;

38 When m = n — 1, we have the series C,,H._). The lowest member is acetylene, C.±H.<>.
Amongst the hydrocarbons with C-, there ought to be two of the formula CsHo, one
with the structure CH,CCH, and the other CCH.,,C. But one of these is unknown. For
C4H>, CHCCCH is known the diacetylene of v. Baeyer, an explosive, easily destructible,
gaseous substance. Nothing more is known of the series C,,Ho.

39 For instance, ethylene, C2H4, combines with Br2, HI, H2SO4, as a whole molecule,
as also does amylene, C5H10, and, in general, CnH2rt — that is, the reacting masses are
l>n>i>orti<>n;il to the molecular weight or n, or to the density of the vapours (page 307).

40 For instance, ethylene is obtained by removing the water from ethyl alcohol,
•C.,,H5(OH), and amylene, C5H10, from amyl alcohol, Cr,Hu(OH), or in general CltH.yi from

41 Acetylene and its polymerides have an empirical composition CH, ethylene and
its homolognes (and polymerides) CH2, ethane CH3, methane CH4. This series presents
a good example of the law of multiple proportions, but such diverse proportions are met
with between the number of atoms of the carbon and hydrogen in the hydrocarbons
already known that the accuracy of Dalton's law might be doubted. Thus the difference
in weight between the substances C30H02 and CsoHgQ is so slight that it is within the in-
evitable errors of analysis, but their reactions and properties are so distinct that they
can be distinguished beyond a doubt. Without Dalton's law, chemistry could not have
brrn brought to its present condition, but it cannot alone express all those shades which
-are quite clearly understood and predicted by the law of Avogadro-Gerhardt.

A A 2

356 PRINCIPLES OF CHEMISTRY

the specific gravity of the first is 0*899 ; that of the second, O925, at
0° — that is, here also the boiling point rises with the increase of
molecular weight, and so also, as might be expected, does the density.

Cases of isomerism in the exact sense of the word — that is, when,
with an identity of composition and of molecular weight, the pro-
perties of the substances are different — are very numerous among
the hydrocarbons and their derivatives. Such cases are particularly
important for the comprehension of molecular structure, and they
also, like the polymerides, may be predicted from the above-mentioned
conceptions, expressing the principles of the structure, of the carbon
compounds 42 based on the law of substitution. According to it, for
example, it is evident that there can be no isomerism in the cases of
the saturated hydrocarbons C2H6 and C3HS, because the former is
CH.j, where methyl has taken the place of H, and as all parts of
the hydrogen atoms of methane ought to be supposed to have the
same relation to carbon, it is all the same which of them be subjected
to the methyl substitution — the resulting product will only be ethane,
CH3CH3 ; 43 and therefore the same argument applies in the case of
propane, CH3CH2CH3, where one compound only can be imagined. It

42 The conception of the structure of hydrocarbon compounds — that is, the expression
of those unions and correlations which their atoms have in the molecules — was for a long
time limited to the representation that organic substances contained complex radicles
(for instance, ethyl C2H5, methyl CH3, phenyl C6H5, &c.) ; then about the year 1840 the
phenomena of substitution and the correspondence of the products of substitution with
the primary bodies (nuclei and types) were observed, but it was not until about the year
I860 and later when, on the one hand, the teaching of Gerhardt about molecules was
spreading, and, on the other hand, the materials had accumulated for discussing the
transformations of the simplest hydrocarbon compounds, that conjectures began to
appear as to the mutual connection of the atoms of carbon in the molecules of the com-
plex hydrocarbon compounds. Then Kekule and A. M. Butleroff began to express the
connection between the separate atoms of carbon, regarding it as a quadrivalent element.
Although in their methods of expression and in some of their views they differ from each
other, and also from the way in which the subject is treated in this work, yet the essence
of the matter — namely, the comprehension of the causes of isomerism and of the union
between the separate atoms of carbon — remains the same. In addition to this, starting
from the year 1870, there appears a tendency, which from year to year increases, to dis-
cover the actual spacial distribution of the atoms in the molecules. Thanks to the
endeavours of Le Bel (1874), Van't Hoff (1874), and Wislicenus (1887) in observing cases
of isomerism— such as the effect of different isomerides on the direction of the rota-
tion of the plane of polarisation of light — this tendency promises much for chemical
mechanics, but the details of the still imperfect knowledge in relation to this matter
must be sought for in special works devoted to organic chemistry.

43 Direct experiment shows that however CH3X is prepared (where X = for instance
Cl, <fec.) it is always one and the same. If, for instance, in CX4, X is gradually replaced
by hydrogen until CH3X is produced, or in CH4 the hydrogen by various means is
replaced by X, or else, for instance, if CH-.X be obtained by the decomposition of more
complex compounds, the same product is always obtained.

This was shown in the year 1800, or there about, by many methods, and is the funda-
mental conception of the structure of hydr.ocarbon compounds. If the atoms of hydro-

CARBON AND THE HYDROCARBON- 357

is to be expected, however, that there should be two butanes, C4H10,
and this is actually the case. In one methyl may be considered
as replacing the hydrogen of one of the methyls, CH3CH2CH2CH3 ;
and in the other CH3 may be considered as substituted for H in

PITT

CH.2, and there it will consist of CH3CH QJJS- It may be also re-
garded as methane, in which three of hydrogen are exchanged for
three of methyl. On going further in the series it is evident that the
number of possible isomerides will be still greater. But we have
limited ourselves to the simplest examples, showing the possibility and
actual existence of isomerides. C2H4 and CH2CH2 are, it is evident,
identical, but there ought to be, and are, two hydrocarbons of the
composition C3H6, propylene and trimethylene ; the first is ethylene
CH2CH2, in which one part of hydrogen is exchanged for methyl
CH2CHCH3, and trimethylene is ethane, CH3CH3, only with the
exchange of methylene for two hydrogen atoms from two methyl

pTT

groups — that is, pjj2CH2, t4 where the methyl introduced is united

to both the atoms of carbon existing in CH3CH3. It is evident
that the cause of isomerism here is, on the one hand, the difference of
the amount of hydrogen in union with the particular atoms of carbons,
and, on the other, the different connection between the separate atoms
of carbon. In the first case they may be said to be chained together
(more generally to form ' an open chain '), and in the second case, to be
locked up (to form a ' closed chain ' or ' ring '). Here, also, it is easily
understood that on increasing the quantity of carbon atoms the num-
ber of conjectured and existing isomerides will greatly increase. If,
also, in addition to the substitution of one of the radicles of methane
for hydrogen a further exchange of part of the hydrogen for some of
the other groups of elements X, Y .... occurs, the quantity of
possible isomerides still further increases in a considerable degree.
For instance, there are even two possible isomerides for the derivatives
of ethane, C2H6 : if two parts of the hydrogen be exchanged for X2, one

gen in methyl were not absolutely identical in value and position (as, for instance, in
CH3CH2CH3, or CH3CH2X), then there would be as many different forms of CH3X
as there were diversities in the atoms of hydrogen in CH4. The scope of this work does
not permit of a more detailed account of this matter. It is given in works on organic
chemistry.

41 The union of carbon atoms in closed chains or rings was first suggested by Kekule
as an explanation of the structure and isomerism of the derivatives of benzene, C^Hg,
forming aromatic compounds (Note 26). It is now incontestable, although the ques-
tion of isomerism between the benzene derivatives cannot be considered as finally
solved, that this closed connection exists, and gives a peculiar character to these com-
pounds.

• >r>S PRINCIPLES OF CHEMISTRY

will have tin- ethvlene structure, CI I .,XC! I.,X. and the other an ethyl-
iilene structure, ( '1 !.,( 'I f X., ; such are, tor instance, ethylene chloride.
CM, C1CH ._,<.'], and ethylidene chloride, CH ,CIK'I,. And as in the

place of the lirst part of hydrogen not only metals may he substi-
tuted. bin (']. r>r. I.(<>H) the water radicle. ( X 1 1.,) the ammonia radi-
ele. ( X< )., ) the ! adicle of nitric acid, iVc.. so al-o in exchange for two
parts of hvdro^en < '. XII, S, <\"c.. ma\' he suhst ituted, whence it will l>c
understood that the <|iiantity of isomeri< le> is somet imcs very <;reat.
It is impossible here to describe how the isomerides are distinguished
from cadi other, in what reactions they occur, how and when one
changes int'i anotlior, Arc. : for this, taken together with the descrip-
tion of the hydrocarbons already known, and their derivatives, forms a
most extended and most thoroughly investigated branch of chemistry,
called tii'i/iiin<' '.'1/i-i/nxt ni . launched with a ma>s of closely-observed
phenomena and strictly-deduced i^eneralisations, tin'-- branch of
chemistry has been treated separately for the reason that in it the
hydrocarbon iri'oups are subjected to transformations which are not
met with in such <|iiantitv in dealing with anv of the other elements
or their hydrogen comjiounds. It was important for us to show that
notwithstanding the uTeat variety of the hydrocarbons and their
products, '•' they are all of them governed b\'the law of substitution,
and referi'iiiL;- our readers for detailed information to works on organic
chemistry, we will limit ourselves to a short ex posit ion of the proper! ies
of the two simplest unsaturated hydrocarbons : ethvleiie, CI !.,( 'I I . and
acetylene. ( 'I I (' 1 1 . and ashort acquaintance wit h ]>et roleum as t he nat ura 1
source of a ma-s of hydrocarbons. l-J/i //I' m . «r u/r/fmif </(>*, C..\\., is

; II.M.. ••.,,-!, -MI .|.-i-:vii:iv.'>. ill Tin- .-ili-oii. .K 'I'lu-M- :in- liy.li'i-c:irl...n- in wliii-h

liV.ir., _• ii ,..--..-|.. MI .-..I !..r l>;j<ii;'.r;/l |<)H'. Tl iinpl.^t < ,f :ll(--,r i - ni.-t li\ l-nlc. ,||< .1.

i! ,,l,1.i in.-. I l)y III.- -Irv <li-till:il:..n i-t \\... •!. Tin- n.imii.ui spirits
.,1. ('!!,( HI . ;in<l -lu-..l. < M , M|| .. ,.t,n-».-p..ii(l \\illi .•tluin«>.

l. ( 'I I ,( I I I'll , < III '. ;ll 1-1 i-c.p!'Mp\ 1 ulcollMl. ( I I .,( 'I I ( )II 'C1I-. pl'll-

| . 1 L:I\, -,.n,!.( '-I I •()! I - \\hicli. \\itli -t.-i-.rir :ui«l (illii-r ariils.

. .-..i-n-pnii.! \\iili pi-..|.:i -.-•. ( ' II . All ;il,-..li.,U ;M-.-,-itpalil.-..1 |,,nii-
I: \\i:l! :i.-iiU. ju-l ;i I'-.i-in Mi-.liiuirys.-ill'.. -'1- AI.l.-li\,li-s

- . :: : l.ir in-htiir. . ... •-• : .. M . • i , \ . I , -. (Ml ;( i. i-..rn-.| •!> \\.'lli .-thy!

!.- ; t.. n-,inl m-j.-niic a.-iiU M- h\ .li'. iciirh. ,11-, in which liy.ln ';_r<-n

,! nil !M,\U I ( I 1 i . . i . s\ I i I In- , • \ pi ;1 i 1 1 . -. I ill tlir l'<ill..\\ 11)'.: cllJI) I.T.

Tli.-iv .-in . nnii'l,.'.- -I inl.-nii.Tliiil.' .-, m i p. ,11 1 1< I : I'. i|- .-\niiipl.-. tin- n Iil,-li\ <lr-a Iri ill. ils.
nl. -MI,,.; .i<-;, | ,,r lr..ir., '. .-••''! . .Vr. Tlm^tlii liy.ln. \\ :i.-i.l-> ;ir.- liy.lr.M-;ii-lt..n^ ill \vliirh

;II-!.,)M .•. !,!<•!,<• IK- .1 .•..n-.'-pMiul \Mtll (Ml,, ii I l<l 1 1,. . 111.- r.H i-t it nil.. II ( ' ,11 , ( > I I < I ( '< > , I 1 I.
1 ! l.i ; i'.- «• pi.,. 111. 1 u. ,ul.| I h.- 1 1. .]..], I .j|l , iu h.-l-t- II ; irpliir.-il l.\ < 'I. 111'. I . I III- II ill-n-

CARBON AND THE HYDROCAHlJoNS 359

the lowest known member of the unsaturatecl hydrocarbon series of the
composition C,,H.);r As in composition it is equal to two molecules of
marsh gas deprived of two molecules of hydrogen, it is evident that it
illicit be, and actually can, together with hydrogen, be produced,
although but in small quantities, by heating marsh gas. On being
heated, however, olefiant gas is decomposed into charcoal and marsh
gas : C.,H4 = CH4-|-C; and therefore in those cases where marsh gas
is produced by heating, olefiant gas, hydrogen, and charcoal will also be
formed, although only in small quantities. The lower the temperature at
which complex organic substances are heated, the greater the quantity
of olefiant gas found in the gases given off ; at a white heat it is en-
tirely decomposed into charcoal and marsh gas. If coal, wood, and
more particularly petroleum, tars and fatty substances are subjected to
dry distillation, they give off illuminating gas, which contains more or
less olefiant gas.

Olefiant gas, almost free from other gases,46 may be obtained
from ordinary alcohol (if possible, free from water), if it be mixed with
five parts of strong sulphuric acid and the mixture heated to slightly
above 100°. Under these conditions, the sulphuric acid removes the
elements of water from the alcohol, C2H5(OH), arid gives olefiant gas,
C.2HGO=H.2O + C.,H,. The greater molecular weight of olefiant gas
compared with marsh gas points out that it may be comparatively
easily converted into a liquid by means of pressure or great cold ;
this may be effected, for example, by the evaporation of liquid nitrous
oxide. Its absolute boiling-point is +10°, it boils at —103° (1 atmo-
sphere), liquefies at 0°, at a pressure of 43 atmospheres, and solidifies
at —160°. Ethylene is colourless, has a slight ethereal smell, is slightly
soluble in water, and somewhat more soluble in alcohol and in ether
(in five volumes of spirit and six volumes of ether).47

Like other unsaturated hydrocarbons, olefiant gas readily enters

4(5 Ethylene bromide, C.^H4Br.,, when gently heated in alcoholic solution with finely
divided zinc, yields pure ethylene, the zinc merely taking up the bromine (Sabaneyeff).

47 Ethylene decomposes somewhat easily under the influence of the electric spark,
or a high temperature. In this case the volume of the gas formed may remain the
same when olefiant gas is decomposed into carbon and marsh gas, or may increase to
double its volume when hydrogen and carbon are formed, CoH4 = CH.1 + C = 2C + 2H;>.
A mixture of olefiant gas and oxygen is highly explosive; two volumes of this gas require
six volumes of oxygen for its perfect combustion. The eight volumes thus taken then
resolve themselves into a mixture of eight volumes of the products of combustion, water and
carbon i i- anhydride, C..jH4 + 3O., - 2CO._, + 2H..O. On cooling, after the explosion, diminution
of volume occurs because the water becomes liquid. For two volumes of the olefiant gas
taken, the diminution will be equal to four volumes, and the same for marsh gas. The
quantity of carbonic anhydride formed by both gases is not the same. Two volumes of
marsh gas give only two volumes of carbonic anhydride, and two volumes of ethylene give
four volumes of carbonic anhydride (2CO..,).

360 PRINCIPLES OF CHEMISTRY

into combination with certain substances, such as chlorine, bromine,
iodine, fuming sulphuric acid, or sulphuric anhydride. If olefiant gas
be sealed up with a small quantity of sulphuric acid in a glass vessel,
and be subjected to continued shaking (as, for instance, by suspending
it from a moving part of a machine), the prolonged contact and repeated
mixing causes the olefiant gas, little by little, to combine with the
sulphuric acid, forming C2H4H.2SO4. If, after this absorption, the
sulphuric acid be diluted with water and distilled, alcohol separates,
which is produced in this case by the olefiant gas combining with
the elements of water, C2H4 + H2O = C2H6O. In this reaction (Ber-
thelot) we see an excellent example of the fact that if a given sub-
stance, like olefiant gas, is produced by the decomposition of another,
then in the reverse way this substance, entering into combination, is
capable of forming the original substance — in our example, alcohol.
Therefore the reaction, CH3CH2(OH)=H(OH) +CH2CH,, ought
to be classed as a reversible reaction. In combination with various
molecules, X2, ethylene gives saturated compounds, C2H4X2 — that is,
either CH2XCH2X, or CH3CHX2, which correspond with ethane,
CH3CH3, or C2H~.48

Acetylene, C2H2 = CHCH, is a gas ; it was first prepared by
Berthelot (1857). It has a very pungent smell, is characterised
by its great constancy under the action of heat, and is the only
product of the direct combination of carbon with hydrogen. This
combination takes place under the action of great heat, such as is
produced by a very strong electric current between carbon points. A
brilliant arc (voltaic) is then formed between the carbon electrodes,
which contains particles of carbon passing from one pole to the other.
If the carbons be surrounded with an atmosphere of hydrogen, the
carbon in part combines with the hydrogen, forming C2H2. Acetylene
may be formed from olefiant gas if two atoms of hydrogen be taken
from it. This may be effected in the following way : the olefiant gas is first
made to combine with bromine, giving C2H4Br2 ; from this the hydro-
bromic acid is taken by means of an alcoholic solution of caustic potash,
leaving the volatile product C2H3Br ; and from this yet another part
of hydrobromic acid is withdrawn by passing it through anhydrous
alcohol in which metallic sodium has been dissolved, or by heating it
with a strong alcoholic solution of caustic potash. Under these cir-

48 The homologues of ethylene, C H2n, are also capable of direct combination with
halogens, &c., but with various degrees of facility. The composition of these homologues
can be expressed thus : (CH^CH^);/ (CH)2Cr where the sum of x + z is always an even
number, and the sum of x + z + r is equal to half the sum of '6x + z, whence z 4- 2r = x ; by
this means the possible isomerides are determined. For example, for butylenes,
(CH3)2(CH)2, (CH3)2(CH2)C, (CH2)(CH2)2CH, and (CH2)4 are possible.

CABBOH ANJ) TIIK HVDKOCAIM'.ONS :;C,1

cuinstances (Berthelot, Sawitsch, Miasnikofi) the alkali takes up the
hydrobromic acid from C^H.^^Br, forming C,(H2n_2.

Acetylene is also produced in all those cases where organic sub-
stances are decomposed by the action of a high temperature — for
example, by dry distillation. On this account a certain quantity is
always found in coal gas, and gives to it, at all events in part, its
peculiar smell, but the quantity of acetylene in coal gas is very small.
If the vapour of alcohol be passed through a heated tube a certain
quantity of acetylene is formed. It is also produced by the imperfect
combustion of olefiaiit arid marsh gas — for example, if the flame of
coal gas has not free access to air.49 The inner part of every flame
contains gases in imperfect combustion, and in them some amount
of acetylene.

Acetylene, being further removed than ethylene from the limit
C,,H2,,+2 of hydrocarbon compounds, has a still greater faculty of combi-
nation than is shown by olen'ant gas, and therefore can be more readily
separated from any mixture containing it. Actually, acetylene not only
combines with one and two molecules of I2, HI, H2SO4, C12, Br2,
*kc. . . . (many other unsaturated hydrocarbons combine Avith them) but
also with cuprous chloride, CuCl, forming a red precipitate. If a gaseous
mixture containing acetylene be passed through a solution of cuprous
chloride (or AgNO3) and ammonia water, the other gases do not com-
bine, but the acetylene gives a red precipitate (or gray with silver),
which on being struck with a hammer decomposes with an explosion.
This red precipitate gives off acetylene under the action of acids.
In this manner pure acetylene may be produced. Acetylene and its
homologues also readily react with corrosive sublimate, HgCl2 (Kou-
cheroff, Favorsky). Acetylene burns with a very brilliant flame, which
is accounted for by the comparatively large amount of carbon it
contains.50

The formation and existence in nature of large masses of petroleum
or a mixture of liquid hydrocarbons, principally of the series C/iH2+2
and C,(H2n is in most respects remarkable.51 In some mountainous dis-

49 This is easily accomplished with those gas lamps which are used in laboratories
and mentioned in the Introduction, page 11. In these lamps the gas is first mixed with
4vir in a long tube, above which it is kindled. But if it be lighted inside the pipe it does
not fully burn, but forms acetylene, on account of the cooling effect of the walls of the
metallic tube ; this is observed by the smell, and may be shown by passing the
issuing gas (by aid of an aspirator) into an ammoniacal solution of cuprous chloride.

60 Amongst the homologues of acetylene, C,,H.2W-2, the lowest is C5H4 ; allylene,
CH3CCH, and allene, CH2CCH2, are known, but the closed structure, CH2(CH)2, is
unknown.

51 The saturated hydrocarbons predominate in American petroleum, especially in
its more volatile parts ; in Baku naphtha the hydrocarbons of the composition CrtH2H form

362 PEINCIPLES OF CHEMISTKY

tricts — as, for instance, by the slopes of the Caucasian chain, on inclines
in a parallel direction to the chain— an oily liquid issues from the earth
together with salt water and hot gases (methane and others) ; it has a
tarry smell, dark brown colour, and is lighter than water. This liquid
is called naphtha or rock oil (petroleum), and is obtained in large quan-
tities by making wells and deep bore-holes in those places where traces
of naphtha are observed, the naphtha being sometimes thrown up from
the wells in fountains of considerable height.52 The evolution of
naphtha is always accompanied by salt water and marsh gas. Naphtha
has from ancient times been worked in Russia in the Apsheron penin-
sula near Baku, and is also now worked in Burmah (India), in Galicia
near the Carpathians, and in America, especially in Pennsylvania and
Canada. Naphtha does not consist of one definite hydrocarbon, but of
a mixture of many, and its density, external appearance, and other
qualities vary with the amount of the different hydrocarbons of which
it is composed. The light kinds of naphtha have a specific gravity about
0'8 and the heavy kinds up to 0*98. The former are very mobile liquids,
and are more volatile ; the latter contain less of the volatile hydro-
carbons and are less mobile. When the light kinds of naphtha are dis-
tilled the boiling point in the vapours constantly changes, beginning at
0° and going up to above 350". That which passes over first is a very
mobile, colourless ethereal liquid, from which the hydrocarbons whose
boiling points start from 0° may be extracted — namely, the hydrocarbons
C4H10, C,H12 (which boils at 30°), C6HU (boils at 62°), C7H16 (boils-
about 90°), tkc. Those fractions of the distillate of naphtha which boil

the main part (Lisenko, Markovinkoff, Beilstein), but doubtless (Mendeleeff) it also con-
tains saturated ones, CMH.,,,l+.2. The structure of the naphtha hydrocarbons is only known
for the lower homologues, but doubtless the distinction between the hydrocarbons of the
Pennsylvania!! and Baku naphthas, boiling at the same temperature (after the requisite
refining by repeated, methodical distillation, which can be very conveniently done by
means of steam, passing the steam through the dense mass — that is, by means of rectifi-
cation), depends not only on the predomination of saturated hydrocarbons in the
former, and naphthenes, C,, IL n, in the latter, but also on the diversity of composition and
structure of the corresponding portions of the distillation. The products of the Baku
naphtha are richer in carbon (therefore, in a suitably constructed lamp they ought to give-
a brighter light), they are of greater specific gravity, and have greater internal friction
(and are therefore more suitable for lubricating machinery) than the American produsta
collected at the same temperature.

52 The formation of naphtha fountains (which burst forth after the higher clay strata,
covering the layers of sand impregnated with naphtha have been bored through) is with-
out doubt caused by the pressure or tension of the combustible hydrocarbon gases
which accompany the naphtha, and are soluble in it under pressure. Sometimes these
naphtha fountains reach a height of 100 metres — for instance, the fountain of 1887, near
Baku. Naphtha fountains generally act periodically, and their force diminishes with the
lapse of time, which might be expected, because the gases which cause the fountains find
an outlet, and the naphtha issuing from the bore-hole carries away the sand which was,
partially choking it up.

CAIM'.ON AND THE HYDROCARBONS 363

above 130°, and contain hydrocarbons with C9, C,n, Cn, etc., enter into
the composition of the oily substance, universally used for lighting,
called kerosene or photogen or photonaphthalene and other names. The
specific gravity of kerosene is from O78 to 0-84, and it smells like
naphtha. Those products of the distillation of naphtha which pass off
below 130° and have a specific gravity below O75, enter into the com-
position of light petroleum (benzoline, ligroin, petroleum spirit, ifcc.) ;
which is used as a sol vent for india-rubber, for taking out grease spots,
A:r. Those portions of naphtha (which can only be distilled without
change by means of superheated steam, otherwise they are largely decom-
posed) which boil above 275° to 300° and have a specific gravity higher
than 0-85, form an excellent oil,53 safe as regards inflammability (which
is very importcant as diminishing the risks of fire), and may be used
in lamps as an effective substitute for kerosene.54 Those portions of
naphtha which pass over at a still higher temperature and have a higher
specific gravity than 0*9, which are found in abundance (about 30 p.c.) in
the Baku naphtha, make excellent lubricating or machine oils. Naphtha

65 This is a so-called intermediate (between kerosene and lubricating oils) oil or pyro-
miplit. Lamps are already being manufactured for burning it, but still require improve-
ment. Above all, however, iL requires a more extended market, and at present is likely
to want it owing to the following two reasons : (1) The products of the American petro-
leum, which are the most widely spread and almost universally consumed, contain but
little of this intermediate oil, and what there is is partly introduced into the kerosene and
partly into the lubricating oils ; (2) the Baku naphtha, which is capable of yielding a
great deal (up to 30 p.c.) of intermediate oil, is produced in enormous quantities, about 105
million poods (1887), but has no regular markets abroad, and for the consumption in
Russia (about 20 million poods of kerosene per annum) and for the limited export
(•20 millions poods per annum) into Western Europe (by the Trans-Caucasian Rail-
way) those volatile and more dangerous parts of the naphtha which enter into the composi-
tion of the American petroleum are sufficient, because Baku naphtha yields about 20 p.c of
such kerosene. For this reason pyronapht is not manufactured in sufficient quantities, and
tin- whole world is consuming the unsafe kerosene. When a pipeline has been laid from
Baku to the Black Sea (in America there are many which carry the raw naphtha to the
s*!a-shoiv, \\hciv it is made into kerosene and other products) then the whole mass of the
Baku naphtha will furnish safe illuminating oils, which, without doubt, will find an
imnu'iisr application. A mixture of the intermediate oil with kerosene or Baku oil (spe-
cific gravity O'Nl t<> IKS;,, may be considered (on removing the benzoline) to be the best
illuminating oil, because it is safe (flashing point from 40° to 60°), cheaper (Baku naphtha
gives as much as (50 p.c. of Baku oil), and burns perfectly well in lamps, differing
but little from those made for burning American kerosene (unsafe, flashing point
20° to 25° to 80°).

54 The substitution of Baku pyronapht, or intermediate oil, or Baku oil (see Note
58), would not only be a great advantage as regards safety from fire, but would also be
highly economical. A ton ((52 poods) of American crude petroleum costs there at the
roa-t considerably more than 24s. (12 roubles), and yields two-thirds of a ton of
kerosene suitable for ordinary lamps. A ton of raw naphtha in Baku costs less than
4s. (1 rouble 80 copecks), and with a pipe line to the shore of the Black Sea would not
cost more than 8 roubles, or 16s. And a ton of Baku naphtha will also yield as much as
two-thirds of a ton of kerosene, Baku oil, and pyronapht suitable for illuminating
purposes.

364 PEINCIPLES OF CHEMISTRY

has many important applications, and the naphtha industry is now of
great commercial importance, especially as naphtha and its refuse may
be used as fuel.55 Whether naphtha was formed from organic matter is
very doubtful, as it is found in the most ancient Silurian strata which
correspond with epochs of the earth's existence when there was little
organic matter ; it could not penetrate from the higher to the lower
strata (more ancient) as it floats on water (and water penetrates
through all strata). It therefore tends to rise to the surface of the earth,
and it is always found in highlands parallel to the direction of the
mountains.56 It is much more probable that its formation should be
attributed to the action of water, penetrating through the crevices
formed on the mountain slopes to the heart of the earth, on that ker-
nel of heated metallic matter which must be accepted as existing in
the interior of the earth. And as meteoric iron often contains carbon
(like cast iron), so, accepting the existence of such carburetted iron
at unattainable depths in the interior of the earth, it may be sup-

••5 Naphtha has been applied to heating purposes on a large scale in Russia only, not
only on account of the low cost of naphtha and of the refuse from the preparation of
kerosene, but also because the products of all the Baku naphtha do not find an outlet
for universal consumption. Naphtha itself and its various residues form excellent fuel,
burning without smoke and giving a high temperature (steel and iron may be easily
melted in the flame). A hundred poods of good coal (for instance, Don coal) used as
fuel for heating boilers are equivalent to 86 cubic feet (about 250 poods) of dry wood,
while only 70 poods of naphtha will be required ; and, moreover, there is no need for
stoking, as the liquid can be readily and evenly supplied in the required quantity. The
economical and other questions dealing with American and Baku petroleums have been
discussed more in detail in some separate works of mine (D. Mendeleeff) : (1) ' The
Naphtha Industry of Pennsylvania and the Caucasus,' 1870 ; (2) ' Where to Build
Naphtha Works,' 1880; (3) ' On the Naphtha Question,' 1883 ; (4) ' The Baku Naphtha
Question,' 1886.

56 As during the process of the dry distillation of wood, seaweed, and similar vege-
table debris, and also when fats are decomposed by the action of heat (in closed vessels),
hydrocarbons similar to those of naphtha are formed, it was natural that this should
have been turned to account to explain the formation of the latter. But the hypothesis
of the formation of naphtha from vegetable debris inevitably assumes coal to be the
chief element of decomposition, and naphtha is met with in Pennsylvania and Canada, in
the Silurian and Devonian strata, which do not contain coal, and correspond to an epoch
not abounding in organic matter. Coal was formed from the vegetable debris of the
Carboniferous, Jurassic, and other recent strata, but, judging from the composition and
structure, it was subjected to the same decomposition as peat ; neither could liquid
hydrocarbons have been formed to such an extent as we see in naphtha. If we ascribe
the derivation of naphtha to the decomposition of fat (adipose, animal fat), we encounter
three almost insuperable difficulties : (1) Animal remains would furnish a great deal of
nitrogenous matter, whilst there is but very little in naphtha ; (2) the enormous
quantity of naphtha already discovered as compared with the insignificant amount of fat
in the animal carcase ; (3) the sources of naphtha always running parallel to mountain
chains is completely inexplicable. Being struck with this last-mentioned circumstance
in Pennsylvania, and finding that the sources in the Caucasus surround the whole
Caucasian range (Baku, Tiflis, Gouria, Kouban, Taman, Groznoe, Dagestan), I developed
in 1876 the hypothesis of the mineral origin of naphtha expounded further on.

CARBON AND TI1K HYDROCARBONS 365

posed that naphtha was produced by the action of water penetrating
through the crevices of the strata during the upheaval of mountain
chains, r'7 because water with iron carbide ought to give iron oxide
and hydrocarbons.58 Direct experiment proves that the so-called
spiegeleisen (manganiferous iron, rich in chemically-combined car-
bon) when treated with acids gives liquid hydrocarbons,59 which in

57 During the upheaval of mountain ranges crevasses would be formed at the peaks
with openings upwards, and at the foot of the mountains with openings downwards.
These c-raeks in course of time fill up, but the younger the mountains (the Alleghany
mountains are, without doubt, more recent ; they were formed during the tertiary epoch)
the fresher the cracks ; through them water must gain access deep into the recesses of
the earth to an extent that could not occur on the level (on plains). The situation of
naphtha at the foot of mountain chains is the principal argument in my hypothesis.

Another fundamental reason is the consideration of the mean density of the earth.
Cavendish, Airy, Cornu, and many others who have investigated the subject by various
methods, found that, taking water =1, the mean density of the earth is nearly 5'5. As.
at the surface water and all rocks (sand, clay, limestone, granite, &c.) have a density less
than 3, it is evident (as solid substances are but slightly compressible even under the-
greatest pressure) that inside the earth there are substances of a greater density —
namely, not less than 7 or 8. What conclusion can. then be arrived at ? Something
heavy contained in the bosom of the earth must be scattered not only on its surface, but
in the whole solar system, because everything tends to show that the sun and planets
proceed from the same material, and, according to the hypothesis of Laplace and Kant,
it is most probable, and even ought to be held, that the earth and planets are but frag-
ments of the solar atmosphere which have had time to cool considerably and become
masses semi-liquid inside and solid outside, forming planets and satellites. The sun,
amongst other heavy elements, contains a great deal of iron, as shown by spectrum
analysis. There is also much of it in an oxidised condition on the surface of the earth.
Meteoric stones, carried as fragments of the planets in the solar system and sometimes
falling upon the earth, consisting of siliceous rocks similar to terrestrial ones, often con-
tain either dense masses of iron (for example, the Pallosovo iron preserved in the St.
Petersburg Academy of Sciences) or granular masses (for instance, the Okhansk meteo-
rite of 1886). For this reason it is possible that the interior of the earth contains much
iron in a metallic state. This might be expected from the hypothesis of Laplace, for
the iron must have been compressed into a liquid at that period when the other com-
ponent parts of the earth were still strongly heated, and oxides of iron could not yet be
formed. The iron was covered with slags (mixtures of silicates, like glass fused
together with rocky matter), which did not allow it to burn at the expense of the oxygen
of the atmosphere and water, just at that time when the temperature of the earth was
very high. Carbon was in the same state ; its oxides were also capable of dissociation
(Deville) ; it is also but slightly volatile, and has an affinity for iron, and iron carbide is
found in meteoric stones. On this account the supposition of the existence of iron
carbides in the interior of the earth was drawn by me from many indications, some of
which are confirmed by the fact that granular pieces of iron have been found in some
basalts (ancient lava) as well as in meteoric stones.

58 The following is the typical equation for this formation :

3FemCw + 4,,,H2O = 7nFe3O4 (magnetic oxide) +C5nH8m.

59 Cloez investigated the hydrocarbons formed when cast-iron is dissolved in hydro-
chloric acid, and found C,,H2n and others. I treated crystalline manganiferous cast-iron,
with the same acid, and obtained a liquid mixture of hydrocarbons exactly similar to
natural naphtha in taste, smell, and reaction. The occurrence of iron with carbon during
the formationrof the earth is all the more probable because those elements predominate
in nature which have small atomic weights, and among them the most widely-diffused,
the most difficultly-fusible, and therefore the most easily-condensed (Chap. XV.) are

366 PRINCIPLES OF CHEMISTRY

composition, appearance, and properties are completely identical with
naphtha.60

carbon and iron. They passed into the liquid state when all compounds were at a tem-
perature of dissociation.

69 Probably naphtha was produced during the upheaval of all mountain chains, but
only in some cases were the conditions favourable to its being preserved underground.
The water penetrating below formed there a mixture of naphtha and watery vapours,
and this mixture issued through fissures to the cold parts of the earth's crust. The
naphtha vapours, on condensing, formed naphtha, which, if there were no obstacles,
appeared on the surface of land and water. Here part of it soaked through formations
(perhaps the bituminous slates, schists, domites, &c., were thus formed), another part
was carried away on the water, became oxidised, evaporated, and was driven to the
shores (the Caucasian naphtha probably in this way, during the existence of the Aralo-
Caspian sea, was carried as far as the Sisran banks of the Volga, where many strata are
impregnated with naphtha and products of its oxidation resembling asphalt and pitch);
a great part of it was burnt in one way or another— that is, gives carbonic anludride and
water. If the mixture of vapours, water, and naphtha formed inside the earth had no
free outlet to the surface, it nevertheless would find its way through fissures to the
superior and colder strata, and there become condensed. Some of the format inns i clays)
which do not absorb naphtha were only washed away by the warm water, and formed
mud, which we also now observe issuing from the earth in the form of mud volcanoes.
All the suburbs of Baku and the whole of the Caucasus in the neighbourhood of the
" naphtha districts are full of such volcanoes, still from time to time in a state of eruption.
In old naphtha beds (such as the Pennsylvania!!) even these blow-holes are closed, and
the mud volcanoes have had time to be washed away. The naphtha and the gaseous
hydrocarbons formed with it under the pressure of the overlying earth and water im-
pregnated the layers of sand, which are capable of absorbing a great quantity of such
liquid, and if above this there were strata impermeable to naphtha (dense, clayey, damp
strata) the naphtha would accumulate in them. It is thus preserved from remote geo-
logical periods up to the present day, compressed and dissolved under the pressure of
the gases which burst out in places forming naphtha fountains. If this be granted, it
may be thought that in the comparatively new (geologically speaking) mountain chains,
'such as the Caucasian, naphtha is even now being formed. Such a supposition
may explain the remarkable fact that, in Pennsylvania, localities where naphtha had
been rapidly worked for five years have become exhaiisted^ and it becomes necessary to
constantly have recourse to sinking new wells in fresh places. Thus, from the year 185U,
the workings were gradually transferred along a line running parallel to the Alleghany
mountains for a distance of more than 200 miles, whilst in Baku the industry < lutes
from time immemorial (the Persians worked near the village of Ballaghana) and up to the
present time keeps to one and the same place. The amounts of the Pennsylvania!! and
Baku annual outputs are at present equal— namely, about 150 million poods (2£ million
tons). It may be that the Baku beds, as being of more recent geological formation, are
not so exhausted by nature as those of Pennsylvania, and perhaps in the neighbourhood
-of Baku naphtha is still being formed, which is partially indicated by the continued
activity of the mud volcanoes. As many varieties of naphtha contain in solution solid,
slightly volatile hydrocarbons like paraffin and mineral wax, the production of ozocerite,
or mountain wax, is accounted for in conjunction with the formation of naphtha.
Ozocerite is found in Galicia, also in the neighbourhood of Novorossisk, in the Caucasus,
and on the islands of the Caspian Sea (particularly in the Chileken and Holy Islands) ;
it is met with in large masses, and is used for the production of paraffin and rr/r.sr;/r,
for the manufacture of candles, and similar purposes.

As the naphtha treasures of the Caucasus are hardly broached (near Baku and near
Kouban and Novorossisk), and as naphtha finds manifold uses, the subject presents
most interesting features to chemists and geologists, and is worthy of the close atten-
tion of practical men.

367

CHAPTER IX

COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

CARBONIC anhydride (or carbonic acid or carbon dioxide, C02) was
the first of all gases distinguished from atmospheric air. Paracelsus
and Van Helmont, in the sixteenth century, knew that on heating
limestone a particular gas separated, which is also formed during the
alcoholic fermentation of saccharine solutions (for instance, in the
manufacture of wine) ; they knew that it was identical with the gas
which is produced by the combustion of charcoal, and that in some
cases it is found in nature. In course of time it was found that this
gas is absorbed by alkali arid saturates it, forming a salt which, under

In;. <!1. - Dumas and Stas' apparatus for determining the composition of carbonic anhydride.
( 'arbon, uraphite, or a diamond is placed in the tube H in the furnace, and heated in a stream of
oxygen displaced from the bottle by water flowing from A. The oxygen is purified from carbonic
anhydride and water in the tubes BCD. Carbonic anhydride, together with a certain amount of
carbon monoxide, is formed in E. The latter is converted into carbonic acid by passing the pro-
duets of combustion through a tube P, containing cupric oxide heated in a furnace. The cupric
oxide oxidises this CO into CO.,, forming metallic copper. The potash bulbs H and tubes I, J,
K retain the carbonic anhydride. Thus, knowing the weight of carbon taken and the weight of
the resultant carbonic anhydride (by weighing H, I, J, K before and after the experiment), the
composition of carbonic anhydride and the equivalent of carbon inav be determined.

the action of acid, again yields this same gas. Priestley found that
this gas exists in air, and Lavoisier determined its formation during
respiration, combustion, putrefaction, and during the reduction of
metals by charcoal ; he determined its composition, and showed that it
only contains oxygen and carbon. Berzelius, Dumas with Stas, and
Roscoe, determined its composition, showing that it contains twelve

368 PRINCIPLES OF CHEMISTRY

parts of carbon to thirty-two of oxygen. The composition by volume
of this gas is determined from the fact that during the combustion of
charcoal in oxygen, the volume remains unchanged ; that is to say,
carbonic anhydride occupies the same volume as the oxygen ivhich it
contains— that is, the atoms of the carbon are, so to speak, squeezed in
between the atoms of the oxygen. O2 occupies two volumes and is a
molecule of ordinary oxygen ; CO2 likewise occupies two volumes, and
expresses the composition and molecular weight of the gas. Carbonic
anhydride exists in nature, both in a free state and in the most varied
compounds. In a free state it is always contained (Chap V.) in the
air, and in solution is in all kinds of water. It is evolved from vol-
canoes, from mountain fissures, and in some caves. The well known
Dog grotto, near Agnano on the bay of Baioe, near Naples, is the
best known example of such an evolution. Similar sources of carbonic
anhydride are also found in other places. In France, for instance,
there is a well-known poisonous fountain in Auvergne. It is a round
hole, surrounded with luxurious vegetation and constantly evolving
carbonic anhydride. In the woods surrounding the Lacher See, near
the Rhine, in the neighbourhood of extinct volcanoes, there is a de-
pression constantly filled with this same gas. The insects which fly to
this place perish, animals being unable to breathe this gas. The birds
chasing the insects also die, and this is turned to profit by the local
peasantry. Many mineral springs carry into the air enormous quan-
tities of this gas. Yichy in France, Spriidel in Germany, and Narzan
in Russia (in Kislovodsk near Piatigorsk) are known for their car-
bonated gaseous waters. Much of this gas is also evolved in mines,
cellars, diggings, and wells. For this reason sometimes people descend-
ing into such places are suffocated and die. The evolution of carbonic-
anhydride in the earth is accounted for by the slow oxidation of
organic matter underground. The combustion, putrefaction, and fer-
mentation of organic substances give rise to the formation of carbonic
anhydride. It is also introduced into thk atmosphere during the respi-
ration of animals at all times and during the respiration of plants
in darkness and also during their growth. Very simple experiments
prove the formation of carbonic anhydride under these circumstances ;
thus, for example, if the air expelled from the lungs be passed
through a glass tube into a transparent solution of lime (or baryta)1
in water a white precipitate will soon be formed which contains an
insoluble compound of lime and carbonic anhydride. If a funnel
be held over a substance in a state of combustion — for instance,
over a lighted candle or brazier — and the air be collected from it by
means of an aspirator, the presence of a large amount of carbonic-

COMPOUNDS OF CARBON WITH OXYdKN AND MTKOdEN 369

anhydride in this air may be detected by its action on lime water.
By allowing the seeds of plants to grow under a bell jar, or in a
closed vessel, the formation of carbonic anhydride may be similarly
confirmed. By forcing an animal, for instance a mouse, to remain
under a bell jar, the quantity of carbonic acid which it evolves may
be exactly determined, and it will be found to be many grams per day
for a mouse. Such experiments on the respiration of animals have
been also made with great exactitude with large animals, such as
men, bulls, sheep, &c. By means of enormous hermetically-closed bell
receivers and the analysis of the gases evolved during respiration it
was found that a man expels about 900 grams (more than two
pounds) of carbonic anhydride per diem, and absorbs during this time
700 grams of oxygen.1 It must be remarked that the carbonic anhy-

1 The quantity of carbonic acid gas exhaled by a man during the twenty-four hours
is not evenly distributed ; during the night more oxygen is taken in than during the
day (by night, in twelve hours, about 450 grams), and more carbonic anhydride is sepa-
rated by day than during night-time and repose ; thus, of the 900 grams separated
during the twenty-four hours about 375 are given out during the night and 525 by day.
This depends on the separation of carbonic anhydride during any kind of work per-
formed by the man. During the day-time his activity in many respects is comparatively
greater than by night. Every movement is the result of some change of matter, because
force cannot be produced by itself (in accordance with the law of the conservation of
energy). Proportionally to the amount of carbon consumed an amount of force is stored
up in the organism ami is consumed in the various movements performed by animals.
This is proved by the fact that during work-time, in twelve hours, a man exhales 900
grams of carbonic anhydride instead of 525, absorbing the same amount of oxygen as
before. In a working day a man exhales by night almost the same amount of carbonic
an hydride as in a day of rest, so that during a total twenty-four hours' work a man
exhales about 1300 grams of carbonic anhydride and absorbs about 950 grams of
cxygen. Therefore in work the change of matter increases. The carbon expended on the
work is obtained from the food ; on this account the food of animals ought certainly to
contain carbonaceous substances capable of dissolving under the action of the digestive
fluids, and of passing into the blood, or, in other words, capable of being digested. Such
food for man and all other animals is formed of vegetable matter, or of parts of other
animals. The latter in every case obtain their carbonaceous matter from plants, in
which it is formed by the separation of the carbon from the carbonic anhydride taken up
during the day by the respiration of the plants. The green parts of plants during the
day absorb the carbonic anhydride of the air, and find it ample notwithstanding the
small proportion of it in the air, and give out oxygen. The volume of the oxygen exhaled
is almost equal to the volume of the carbonic anhydride absorbed ; that is to say, nearly
all the oxygen entering into the plant in the form of carbonic anhydride is separated in
a free state, whilst the carbon from the carbonic anhydride remains in the plant. At
the same time, the plant absorbs moisture by its leaves and roots. By a process which
is unknown to us, this absorbed moisture and carbon remaining from the carbonic acid
enter into the composition of the plants in the form of so-called carbohydrates, com-
posing the greater part of the vegetable tissues, starch and cellulose of the composition
C(jHloO5 being representatives of them. We may represent their composition as a com-
pound of carbon remaining from carbonic acid and water, 6C + 5H.,,O. In this way a
circulation of the carbon goes on in nature by means of vegetable and animal organisms,,
in which changes the principal item is the carbonic anhydride of the air.

VOL. I. B B

370 PRINCIPLES OF CHEMISTRY

dride of the air constitutes the fundamental food of plants (Chaps. III.,
V., and VIII.). Carbonic anhydride, in a state of combination with
most varied substances, is perhaps even more widely diffused in nature
than in a free state. Some of these substances are very stable,
and form a large portion of the earth's crust. In this state carbonic
anhydride enters into the composition of limestones. Limestones,
calcium carbonate CaCO^, were formed as precipitates in the seas
existing previously on the earth ; this is proved by their stratified
structure and the number of remains of sea animals which they
frequently contain. It may be concluded, judging from the enormous
quantity of these limestones, that the amount of carbon in the form of
carbonic anhydride in the atmosphere was in former periods much
greater than at present. Chalk, lithographic stone, limestone, marls (a
mixture of limestone and clay), and many other rocks are examples of
such sedimentary formations. Carbonates with various other bases —
such as, for instance, magnesia, ferrous oxide, zinc oxide, &c. — are
often found in nature. The shells of molluscs have also the composi-
tion CaCO3, and many limestones were exclusively formed from the
shells of minute organisms.

For the preparation of carbonic anhydride in laboratories and
often in manufactories, various kinds of calcium carbonate are used,
being treated with some acid ; it is, however, most usual to employ
the so-called muriatic acid — that is, an aqueous solution of hydro-
chloric acid, HC1 — because, in the first place, the substance formed,
calcium chloride, OaCl.2, is soluble in water and does not hinder the
further action of the acid on the calcium carbonate, and secondly
because, as we shall see further on, muriatic acid is a common product
of chemical works and one of the cheapest. For calcium carbonate,
either limestone, chalk, or marble is used.'2

CaCO3 + 2HCl=CaCl2 + H2O + CO2.

2 Other acids may be used instead of hydrochloric ; for instance, acetic, or even
sulphuric, although this latter is not suitable, because it forms as a product insoluble
calcium sulphate (gypsum) surrounding the untouched calcium carbonate, and thus pre-
venting a further evolution of gas. But if porous limestone — for instance, chalk — bo
treated with sulphuric acid diluted with an equal volume of water, the latter is absorbed,
and, acting on the mass of the salt, the evolution of carbonic anhydride continues evenly
for a long time. Instead of calcium carbonate, other carbonates may of course be used ;
for instance, washing-soda NaoCO3, which is often chosen when it is required to produce a
rapid stream of carbonic anhydride (for example, for liquefying it). But natural crystal-
line magnesium carbonate and similar salts are with difficulty decomposed by hydro-
chloric and sulphuric acids. When for manufacturing purposes — for instance, in pre-
cipitating lime in sugar-works— a large quantity of carbonic acid gas is required;
generally charcoal is burnt, and the products of combustion, rich in carbonic anhydride,
are pumped into the liquid containing the lime, and the carbonic anhydride is thus

Co.Ml'nt'NDS OF CAR 150 N WITH OXYGK.V AND NJTRoi iKA 371

The nature of the reaction in this case is the same as in the decom-
position of nitre by sulphuric acid ; only in the latter case a hydrate is
formed, but in the former an anhydride of the acid, because the
hydrate, carbonic acid, H2C03, is unstable and, as soon as it separates,
decomposes into water and its own anhydride. It is evident from the
explanation of the cause of the action of sulphuric acid on nitre that
not every acid can be employed for obtaining carbonic anhydride ;
namely, those will not set it free which chemically are but slightly
energetic, or those which are insoluble in water, or are themselves as
volatile as carbonic anhydride.3 But as almost all the known acids
are characterised by their great solubility in water and are less volatile
than carbonic anhydride, the latter is' evolved by the action of most
acids on its salts, and this reaction takes place at ordinary tempera-
tures, provided the acid be soluble in water and that the calcium salt
formed is also soluble in water.4

For the preparation of carbonic anhydride in laboratories marble is
generally used. It is placed in a Woulfe's bottle and treated with hydro-
chloric acid in an apparatus similar to the one used for the production
of hydrogen. The gas evolved carries away through the tube part of
the volatile hydrochloric acid, and it is therefore necessary to wash the
gas by passing it through another Woulfe's bottle containing water. If

al)sorl)«'tl. Another method is also practised, which consists in using the carbonic
anhydride separated during fermentation, or that evolved from lime-kilns. During the
fermentation of sweet-wort, grape-juice, and other similar saccharine solutions, the
glucose C6H13O6 changes under the influence of the yeast organism, forming alcohol
(2C._,H6O) and carbonic anhydride (2CO.2), which separates in the form of gas; if the
fermentation proceeds in closed bottles sparkling wine is obtained. When carbonic
acid gas is prepared for saturating water and other beverages it is necessary to use it
in a pure state. Whilst in the state in which it is evolved from ordinary limestones by
the aid of acids it contains, besides a certain quantity of acid, the organic matters
of the limestone; in order to diminish the quantity of these substances the densest
kinds of dolomites are used, which contain less organic matter, and the gas formed
is passed through various washing apparatus, and then through a solution of potas-
sium permanganate, which absorbs organic matter and does not take up carbonic
anhydride.

0 Hypochlorous acid, HC1O, and its anhydride, C12O, do not displace carbonic acid, and
hydrogen sulphide has the same relation to carbonic acid as nitric acid to hydrochloric —
an excess of either one displaces the other.

4 Thus, in preparing the ordinary effervescing powders, sodium bicarbonate (or
acid carbonate of soda) is used, and mixed with powdered citric or tartaric acid. In a
dry state these powders do not evolve carbonic anhydride, but when mixed with water the
evolution takes place briskly, which is due to the substance passing into solution. The
•-alts of carbonic acid may be recognised from the fact that they evolve carbonic acid
with a hissing noise when treated with acids. If vinegar, which contains acetic acid, be
pouivd upon limestone, marble, malachite (contains copper carbonate), etc., carbonic
anhydride is evolved with a hissing noise. It is necessary to remark that without the
presence of water neither hydrochloric acid, nor even sulphuric acid nor acetic acid, acts
•on limestone. We shall refer to this hereafter.

B B 2

372

PRINCIPLES OF CHEMISTRY

it be necessary to obtain dry carbonic anhydride, it must be passed
through chloride of calcium.5

Carbonic anhydride may also be prepared by heating many of the
salts of carbonic acid ; for instance, by heating magnesium carbonate,
MgCO3 (in dolomite), the separation is easily effected, particularly in the
presence of the vapours of water. The acid salts of carbonic acid (see
further on) readily give much carbonic anhydride when heated.
Carbonic anhydride, together with water, is produced during the com-
bustion of all organic compounds in a stream of oxygen or by heating
them with substances which readily part with oxygen to them — for
instance, with copper oxide, potassium chlorate (readily causing an
explosion),

FIG. 62. -Apparatus for the combustion of organic substances by igniting tiicrn with
oxide of copper.

The method of estimating the amount of carbon in organic com-
pounds is founded on this property. For this purpose a glass tube, closed
at one end, is filled with a mixture of the organic substance (about 0*2
grams) and copper oxide. The open end of the tube is fitted with a

5 The direct observations made (1876) by Messrs. Bogouski and Kayander lead to the
conclusion that the quantity of carbonic anhydride evolved by the action of acids on
marble (as homogeneous as possible) is directly proportional to the time of action, the
extent of surface, and the degree of concentration of the acid, and inversely proportional
to the molecular weight of the acid. If the surface of a piece of Carrara marble be equal
to one decimetre, the time of action one minute, and one cubic decimetre or litre contains
one gram of hydrochloric acid, then about 0'02 gram of carbonic anhydride will be
evolved. If the litre contains n grams of hydrochloric acid, then by experiment the
amount will be n x 0'02 of carbonic anhydride. Therefore, if the litre contains 86'5
( = HC1) grams, about 0'73 grams of carbonic anhydride (about half a litre) would be
evolved per minute. If nitric acid or hydrobromic acid be used instead of hydrochloric,
then, with a combining proportion of the acid, the same quantity of carbonic anhydride
will be evolved ; thus, if the litre contains 68 ( = HNO3) grams of nitric acid, or 81 ( = HBr)
grams of hydrobromic acid, the quantity of carbonic anhydride evolved will still be
0'78 gram.

COMPOUNDS OF CARBON WITH <>XY<;KN AND NITROGEN 873

cork and tube containing calcium chloride, for absorbing the water
formed by the oxidation of the substance. This tube is hermetically
connected (by a caoutchouc tube) with potash bulbs or other weighing
apparatus (Chap. 'V.) containing alkali destined to absorb the carbonic
anhydride. The increase in weight of this apparatus shows the amount
of carbonic anhydride formed during the combustion of the given
substance, and the quantity of carbon may be determined from this,
because three parts of carbon give eleven parts of carbonic anhydride.

Carbonic anhydride is colourless, has a slight smell and a feeble
acid taste ; its density in a gaseous state is twenty- two times greater
than that of hydrogen, because its molecular weight is forty-four.6
It is an example of those gaseous substances which have been
long ago transformed into all the three states. In order to obtain
liquid carbonic anhydride, the gas must be submitted to a pres-
sure of thirty-six atmospheres at 0°.7 Its absolute boiling point
= + 32°. 8 Liquid carbonic anhydride is colourless, does not mix with
water, but is soluble in alcohol, ether, and oils ; at 0° its specific gravity
is 0*83. When poured into a tube, which is then sealed up, liquid
carbonic anhydride is easily preserved, because a thick tube easily
resists the pressure which the liquid entails at an ordinary temperature —
namely, about fifty atmospheres. The boiling point of this liquid lies
at — 80° — that is to say, the pressure of carbonic acid gas at that
temperature does not exceed that of the atmosphere. At the ordinary
temperature the liquid remains as such for some time under ordinary
pressure, on account of its requiring a considerable amount of heat for
its evaporation. If the evaporation takes place rapidly, especially if
the liquid issues in a stream, such a decrease of temperature occurs that

6 As carbonic anhydride is one and a. half times heavier than air, it diffuses with
difficulty, and therefore does not easily mix with air, but sinks in it. This may be shown
in various ways ; for instance, it may be carefully poured from one vessel into another
containing air. If a lighted taper be plunged into the vessel containing carbonic
anhydride it is extinguished, and then, after pouring the gas into the other cylinder, it
will burn in the former and be extinguished in the latter. If a certain quantity of car-
bonic anhydride be poured into a vessel containing air, and soap-bubbles be introduced,
they will only sink as far as the line where the atmosphere of carbonic anhydride com-
mences, as this latter is heavier than the soap-bubbles filled with air. Naturally, after a
certain lapse of time, the carbonic anhydride will be diffused throughout the vessel, and
form a uniform mixture with the air, just as salt in water.

7 This liquefaction was first observed by Faraday, who sealed up in a tube a mixture
of a carbonate and sulphuric acid. Afterwards this method was very considerably im-
proved by Thilorier and Natterer, whose apparatus is described on page 289. It is,
however, necessary to remark here that in working with liquid carbonic anhydride it is
indispensable to have good liquefying apparatus, constant cooling, and in particular a
rapid preparation of large masses of pure carbonic anhydride.

3 Carbonic anhydride, having the same molecular weight as nitrous oxide, very much
resembles it when in a liquid state.

374 PRINCIPLES OF CHEMISTRY

a part of the carbonic anhydride is transformed into a solid snowy
mass. Water, mercury, and many other liquids freeze on coming into
contact with snowlike carbonic anhydride.9 In this form carbonic
anhydride may be preserved for a long time in the open air, because it
requires still more heat to turn it into a gas than when in a liquid
state. Solid carbonic anhydride, notwithstanding its very low tempera-
ture, can be safely placed on the hand, because it constantly evolves gas
which prevents its coming into actual contact with the skin ; but if a
piece be squeezed between the fingers, it produces a severe frost bite
similar to a burn. If the snowlike solid be mixed with ether a semi-
liquid mass is obtained, which may be used for artificial refrigeration.
This mixture may be used for liquefying many other gases — such as
chlorine, nitrous oxide, hydrogen sulphide, and others. The evapora-
tion of such a mixture proceeds with far greater rapidity under the
receiver of an air-pump, and consequently the refrigeration is more
intense. By this means many gases may be liquefied which resist
other methods — namely, olefiant gas, hydrochloric acid gas, and others.
Liquid carbonic anhydride in this case congeals in the tube into a
glassy transparent mass. Pictet availed himself of this method for
liquefying many permanent gases (see Chap. II.).

The capacity which carbonic anhydride has of being liquefied stands
in connection with its solubility in considerable quantity in water,
alcohol, and other liquids. Its solubility in water has been already
spoken of in the first chapter. Carbonic anhydride is still more soluble
in alcohol than in water, namely at 0° one volume of alcohol dissolves
4'3 volumes of this gas, and at 20° 2'9 volumes.

Aqueous solutions of carbonic anhydride, under a pressure of several
atmospheres, are now prepared artificially, because water saturated
with this gas is a good means of promoting digestion and quenching
thirst. For this purpose the carbonic anhydride is pumped by means
of a force pump into a closed vessel containing the liquid, and then
bottled off, taking special means to ensure rapid and air-tight corking.
Various effervescing drinks and artificially effervescing wines are thus
prepared. The presence of carbonic anhydride has an important

9 When a fine stream of liquid carbonic anhydride is discharged into a closed metallic
vessel, about one-third of its mass solidifies and the remainder evaporates. In employing
solid carbonic anhydride for making experiments at low temperatures, it is best to use it
mixed with ether, otherwise there will be few points of contact. If a stream of air be
blown through a mixture of liquid carbonic anhydride and ether, the evaporation proceeds
rapidly, and great cold is obtained. At present in some special manufactories (and for
making artificial mineral waters) carbonic anhydride is liquefied on the large scale, poured
into wrought-iron cylinders provided with screwed taps, and in this manner it can be
transported and safely preserved for a long time.

Co.MI'orNDs OF CARBON WITH OXYGEN AND NITROGEN 375

significance in nature, because by this means water acquires the
property of destroying and dissolving many substances which are not
acted on by pure water; for instance, calcium phosphates and carbonates
are soluble in water containing carbonic acid. If the water in the
interior of the earth is saturated with carbonic acid under pressure,
the quantity of calcium carbonate in solution may reach three grams
per litre, and on issuing at the surface, as the carbonic anhydride
escapes, the calcium carbonate will be deposited.10 Water charged
with carbonic anhydride brings about the destruction of many rocky
formations by removing the lime, alkali, &c., from them. This process
has been going on and continues on an enormous scale. Rocky
formations contain silica and the oxides of various metals, amongst
others oxides of aluminium, calcium, and sodium. Water charged
with carbonic acid dissolves both the latter, transforming them into
carbonates. The waters of the ocean ought, as the evolution of the
carbonic anhydride proceeds, to precipitate salts of lime ; these are
actually found everywhere on the surface of the ground in those places
which previously formed the bed of the ocean.

The presence of carbonic anhydride in solution in water is essential
to the nourishment and growth of water plants. Although carbonic
anhydride is soluble in water, yet no definite hydrate is formed ;11
nevertheless an idea of the composition of this .hydrate may be

10 If such water trickles through crevices and enters a cavern, the evaporation will
be slow, and therefore in those places from whence the water drips growths of calcium
carbonate will be formed, just like the icicles formed on the roof-gutters in winter-time.
Similar conical and cylindrical stony growths form the so-called stalactites or pendants
hanging from above and stalagmites formed on the bottom of caves. Sometimes these
two kinds meet together, forming entire columns filling the cave. Many of these caves
are remarkable for their picturesqueness ; for instance, the cave of Antiparos, in the
Grecian Archipelago. This same cause also forms spongy masses of calcium carbonate
in those places where the springs come to the surface of the earth. It is therefore very
evident that a calcareous solution is sometimes capable of penetrating plants and filling
the whole of their mass with calcium carbonate. This is one of the forms of petrified
plants. Calcium phosphate in solution in water containing carbonic acid plays an im-
portant part in nourishing plants, because all plants contain both lime and phosphoric
acid.

11 The crystallohydrate COo,8H.,O of Wroblewski (Chapter I. Note 67), in the first
place, is only formed under special conditions ; in the second place, it still requires con-
firmation ; and in the third place, it does not correspond with that hydrate HoCO^ which
should occur, judging from the composition of the salts.

It is easy to demonstrate the acid properties of carbonic anhydride by taking a long
tube, closed at one end, and filling it with this gas ; a test-tube is then filled with a
solution of an alkali (for instance, sodium hydroxide), which is then poured into the long
tube and the open end is corked. The solution is then well shaken in the tube, and the
corked end plunged into water. If the cork be now withdrawn under water the water
will fill the tube. The vacuum obtained by the absorption of the carbonic anhydride by
an alkali is so complete that even an electric discharge will not pass through it. This
method is often applied to produce a vacuum.

376 PRINCIPLES OF CHEMISTRY

formed from the composition of the salts of carbonic acid, because a
hydrate is nothing but a salt in which the metal is replaced by
hydrogen. As carbonic anhydride forms salts of the composition
K2CO3, Na2CO3, HNaCO:j, &c., therefore carbonic acid ought to have
the composition H2CO3 — that is, it ought to be formed from CO2 -f H.,0
Whenever this substance is formed, it decomposes into its component
parts — that is, into water and carbonic anhydride. The acid properties
of carbonic anhydride are demonstrated by its being directly absorbed
by alkaline solutions and forming salts with them. In distinction from
nitric, HNO3, and similar monobasic acids which with univalent metals
(exchanging one atom for one atom of hydrogen) give salts such as
those of potassium, sodium, and silver containing only one atom of the
metal (NaNO3, AgNO3), and with bivalent12 metals (such as calcium,
barium, lead) salts containing two acid groups — for example Ca(NO3)2,
Pb(NO3)2 — carbonic acid, H2C03, is bibasic, that is, contains two atoms
of hydrogen in the hydrate or two atoms of univalent metals in their
salts : for example Na2C03 is washing soda, a normal salt ; XaHCO3 is
the bicarbonate or an acid salt. Therefore, if M' be a univalent metal,
its carbonates in general are the normal carbonate M'.,C03 and the
acid carbonate M'HC03; or if M" be a bivalent metal (replacing H2) its
normal carbonate will be M'/CO3 ; these metals do not usually form acid
salts, as we shall see further on. The bibasic character of carbonic acid
is akin to that of sulphuric acid, H2SO4,13 but the latter, in distinction

12 The reasons for distinguishing the uni-, bi-, tri-, and quudri-valent metals will bo
examined hereafter on passing from the univalent metals (Na, K, Li) to the divalent ( .M;_.
Ca, Ba).

13 Up to the year 1840, or thereabout, acids were not distinguished by their basicity.
Graham, while studying phosphoric acid, H^PO.^ and Liebig, while studying many organic
acids, distinguished mono-, bi-, and tri-basic acids. Gerhardt and Laurent generalised
these relations, showing that this distinction extends over many reactions (for instance,
to the faculty of bibasic acids of forming acid salts with alkalis, KHO or NaHO, or with
alcohols KHO, &c.); but now, from the determination of a hard-and-fast conception as
to atoms and molecules, the basicity of an acid is determined by the nmnhcr of
hydrogen atoms contained in a molecule of the acid which can be exchanged for metals.
If carbonic acid forms acid salts, NaHCO5, and normal salts, Na,,CO5, it is evident that
the hydrate is H2CO5, a bibasic acid. Otherwise it is at present impossible to account for
the composition of these salts. But when C = 6 and O = 8 were taken, then the formula
CO2 expressed the composition, but not the molecular weight, of carbonic anhydride ; and
the composition of the normal salt would be Na2C2O6 or NaCO5, therefore carbonic acid
might have been considered as a monobasic acid. Then the acid salt would have been
represented by NaCO.^HCO.v Such questions were the cause of much argument and
difference of opinion among chemists about forty years ago. At present there cannot be
two opinions on the subject if the law of Avogadro-Gerhardt and its sequences be strictly
adhered to. Let us, however, remark here that the monobasic acids R(OH) were for a
long time considered to be incapable of being decomposed into water and anhydride, and
this property was ascribed to the bibasic acids B(OH)2 as containing the elements neces-
sary for the separation of the molecule of water H2O. This H2SO4 or SO2(OH)2, H2CO3,

CO.MI'OIXDS OF CAR] JON WITH oXYdKX AND MTI.'ndKN 377

from the former, is an example of the energetic or strong acids (such as
nitric or hydrochloric), whilst in carbonic acid we observe but feeble
development of the acid properties ; hence carbonic acid must be con-
sidered <> iri'cik acid. This conception must, however, betaken as only
comparative, as up to this time there is no definitely established rule for
measuring the energy14 of acids. The feeble acid properties of carbonic

or CO(OH).,, and other bibasic acids decompose into the anhydride, BO, and water,
H.2O. But as nitrous, HNO.,, iodio, HIO,-, hypochlorous, HC1O, and other monobasic
acids easily give their anhydrides X..O-, I.,Or), CLO, &c., that method of distinguish-
ing the basicity of acids, although it fairly well satisfies the requirements of organic
chemistry, must not be considered correct. It may also be remarked that up to the present
time not one of the bibasic acids has been found to have the faculty of being distilled with-
out being decomposed into anhydride and water (even H2SO4, on being evaporated and
distilled, gives SO- + H.,O), and the decomposition of acids into water and anhydride pro-
ceeds particularly easily in dealing with feebly energetic acids, such as carbonic, nitrous,
boric, and hypochlorous. Let us add that carbonic acid, as a hydrate corresponding to marsh
gas, C(HO)4 = CO.) + 2H.,O, ought to be tetrabasic. But in general it does not form such salts.
Basic salts, however, such as CuCO3CuO, may be regarded in this sense, because CCu.2O4
corresponds with CH^O.^, as Cu corresponds with Ho. Amongst the ethereal salts (alco-
holic derivatives) of carbonic acid corresponding cases are, however, observed ; for
instance, ethylic orthocarbonate, C(CoH5O)4 (obtained by the action of chloropicrin,
C(NO.,)C13, on sodium ethoxide, C.,H5ONa; boiling point, 158°; specific gravity, 0'92).
The name orthocarbonic acid for CH4O4 is taken from ortliophosplioric acid, PH3O4,
which corresponds with PH3 (see Chapter on Phosphorus).

K Long ago endeavours were made to find a measure of affinity of acids and bases,
because some of the acids, such as sulphuric or nitric, form comparatively stable salts, de-
composed with difficulty by heat and water, whilst others, like carbonic and hypochlorous
ticids, do not combine with feeble bases, and with many form salts which are easily decom-
posed. The same may be said with regard to bases, among which those of potassium, K.>O,
sodium, Na2O, and barium, BaO, may serve as examples of the most powerful, because
they combine with the most feeble acids and form a mass of salts of great stability, whilst
as examples of the feeblest bases alumina, A12O3, or bismuth oxide, BioO3, may be taken,
because they form salts easily decomposed by heat and water if the acid be volatile.
Such a division of acids and bases into the feeblest and most powerful is justified by all
evidence concerning them, and is quoted in this work. But in recent years the teaching
of this subject has acquired quite a new tone, which, in my opinion, cannot be accepted
without certain reservations and remarks, although it comprises many interesting features.
The fact is that Thomsen, Ostwald, and others proposed to express the measure of affinity
of acids to bases by figures drawn from data of the measure of displacement of acids in
aqueous solutions, judging (1) from the amount of heat developed by mixing a solution
of the salt with a solution of another acid (the avidity of acids, according to Thomsen) ;
(2) from the change of the volumes accompanying such a mutual action of solutions
(Ostwald) ; (3) from the change of the index of refraction of solutions (Ostwald), &c.
Besides this there are many other methods which allow us to form an opinion about the
distribution of bases among various acids in aqueous solutions. Some of these methods
will be described hereafter. It ought, however, to be remarked that in making investiga-
tions in aqueous solutions the affinity to water is generally left out of sight. If a base N,
combining with acids X and Y in presence of them both, divides in such a way that one-
third of it combines with X and two-thirds with Y, a conclusion is formed that the affinity,
or power of forming salts, of the acid Y is twice as great as that of X. But the presence
of the water is not taken into account. If the acid X has an affinity for water and for N
it will be distributed between them ; and if X has a greater affinity for water than Y,
then less of X will combine with N than of Y. If, in addition to this, the acid X is

378 PRINCIPLES OF CHEMISTRY

acid may, however, be judged from the accumulation of many indications.
With such energetic alkalis as soda and potash, carbonic acid forms.

capable of forming an acid salt NX2, and Y is not, the conclusion of the relative strength
of X and Y will be still more erroneous, because the X set free will form such a salt on
the addition of Y to NX. We shall see in Chapter X. that when sulphuric and nitric
acids in weak aqueous solution act on sodium they are distributed exactly in this way :
namely, one-third of it combines with the sulphuric and two-thirds with the nitric acid ;
but, in my opinion, this does not show that sulphuric acid, compared with nitric acid,
possesses but half the degree of affinity for bases similar to soda, and only demonstrates
the greater affinity of sulphuric acid for water compared with that of nitric acid. In this
way the methods of studying the distribution in aqueous solutions probably only shows
the difference of the relation of the acid to a base and to water. In general, it is impossible
to hope to be able to determine the direct relative degree of affinity of acids for bases by
studying aqueous solutions without taking into account the relation of the acids, ba •
and salts to water, and by regarding the water as a passive medium, because the water itself
forms saline and all other compounds with substances. This refers more especially to
those weak solutions by means of which investigations of this kind are most often con-
ducted, because the weak solutions contain a large mass of water, and its influence is
then great even when there is but little affinity, in accordance with the law of the action
of masses, and from the fact that water itself is a saline oxide.

In deference to these considerations, although the teaching of the distribution of salt-
forming elements in aqueous solutions is an object of great and independent interest, it
can hardly serve to determine the measure of affinity between bases and acids. Similar
considerations ought to be kept in view when determining the energy of acids by means of
the electrical conductivity of their weak solutions. This met hod, proposed by Arrhenius
(1884), and applied on an extensive scale by Ostwald (who developed it in great detail in
his Lehrbuch d. allgemeinen Chemie, v. ii., 1887), is founded on the fat-t that the re-
lation of the so-called molecular electrical-conductivity of weak solutions of various acids.

(I) coincides with the relation in which the same acids stand according to the distribution,

(II) found by one of the above-mentioned methods, and with the relation deduced for
them from observations upon the velocity of reaction, (III) for instance, according to the
rate of the splitting up of an ethereal salt (into alcohol and acid), or from the rate of the so-
called inversion of sugar — that is, its transformation into glucose — as is seen by comparing
the annexed figures, in which the energy of hydrochloric acid is taken as equal to 100 : —

I. II. III.

Hydrochloric acid, HC1 . . 100 100 100

Hydrobromic acid, HBr . . 101 98 105

Nitric acid, HNO3 ... 100 100 96

Sulphuric acid, H2SO4 . 65 41) 74

Formic acid, CH2O2 2 4 1

Acetic acid, C2H4O2 . . 1 2 1

Oxalic acid, C2H2O4. 20 24 18

Phosphoric acid, PH5O4 7 (5

The coincidence of these figures, obtained by so many various methods, presents a
most important and most instructive relation between phenomena of a different kind, but
in my opinion it does not permit us to assert that the degree of affinity existing between
bases and various acids is determined by all these various methods, because the influence
of the water must be taken into consideration. On this account, until the theory of
solution is more thoroughly worked out, this subject (which now ought to be treated
of in special treatises on chemical mechanics) must be treated with great caution.
But now we may hope to decide this question by exact methods, which we shall partially
touch on when speaking about the velocity of reaction.

There is no means of determining the degree of energy of carbonic acid by any one of
the above-mentioned methods,

COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 379

normal salts, soluble in water, but having an alkaline reaction and in
many cases acting themselves as alkalis.K) The acid salts of these alkalis,.
NaHCO3 and KHCO3, have a neutral reaction on litmus, although they,
like acids, contain hydrogen, which may be exchanged for metals. The
acid salts of such acids— as, for instance, of sulphuric acid, NaHSO4—
have a clearly-defined acid reaction, and therefore carbonic acid is un-
able to saturate the powerful basic properties of such alkalis as potash or
soda. Carbonic acid does not even combine at all with feeble bases, such
as alumina, A12O3, and therefore if a strong solution of sodium carbo-
nate, Xa.,CO3, be added to a strong solution of aluminium sulphate
A1.>(SO4)3, then (according to double saline decompositions) aluminium
carbonate, A12(CO3)3, ought to be formed, but the carbonic acid separates,
as this salt splits up in the presence of water into aluminium hydrox-
ide and carbonic anhydride: A12(CO3)3 + 3H2O = A12(OH)6 + 3CO2.
Therefore feeble bases are unable to retain carbonic acid even at
ordinary temperatures. For this reason in the case of bases of medium
energy, although they form carbonates, these are comparatively easily
decomposed by heating, as is shown by the decomposition of copper
carbonate, CuCO3 (see Introduction), and even calcium carbonate,
CaCO3. Only the normal (but not the acid) salts of such powerful bases,
as potassium or sodium are capable of standing a red heat without
decomposition. But the acid salts, for instance NaHCO3, decompose
even on heating their solutions (2NaHC03 = Na2CO3 + H2O + CO2),
evolving carbonic anhydride. The amount of heat given out by the
combination of carbonic acid with bases also shows its feeble acid
properties, because it is considerably less than with energetic acids.
Thus if a weak solution of forty grams of sodium hydroxide be satu-
rated (until the formation of a normal salt) with sulphuric or nitric
acid or another powerful acid, from thirteen to fifteen thousand
calories are given out, but with carbonic acid only about ten thousand
calories.16 The majority of carbonates are insoluble in water, and

15 Thus, for instance, in the washing of fabrics the caustic alkalis, such as sodium
hydroxide, in weak solutions, act in removing the fatty matter just in the same way as
carbonate solutions ; for instance, a solution of soda crystals, Na.2CO3. Soap, acts in the
same way, being composed of feeble acids, either fatty or resinous, combined with alkali.
On this account all such substances are applied to manufacturing purposes, and used
equally well in practice for bleaching and washing fabrics. Soda crystals or soap are
preferred to caustic alkali, because an excess of the latter may have a destructive effect
on the fabrics. It may be supposed that in aqueous solutions with soap or with soda,
crystals part of the base will form caustic alkali ; that is to say, the water will compete
with the weak acids, and the alkali will be distributed between them and the water.

10 Although carbonic acid is reckoned among the feeble acids, yet there are evi-
dently many others still feebler — for instance, hydrogen sulphide, prussic acid, hypo-
chlorous acid, many organic acids, ifcc. Bases like alumina, or such feeble acids as silica,,
when in combination with alkalis are decomposed in aqueous solutions by carbonic acid>

t herefore -ueli solutions as sodium. j u it a --mm. or a ninoni um carbona t es
form in snlutions of mo-t other salts MX < >y M X ,. in-< iluble ]>re
ei}>itates of carbf.-nates of tin- u'ivtm metals M lunivalent metal
rcjilai-iiiu1 I I * or M (bivalent metal ivplaein"; II, ) ihat is, M .,('<>.<, ]•
M ( ' >... llms a solution 'it barium chloride uives with sodium
carl M mat e a precipitate of barium carbonate, l>a(_'n.,. For tin's reason
in nature the waters i»uin^ from roekv Tonnat ions very often contain
carlionales : tor example, calcium, fern ms or magnesium carbonates, iVc.
( 'arbonic anhydride which. like \\"ater, is formed with the develop-
ment of a lai'u'c amount of heat is veiy .-table. For this reason very
few siib.-,tances are eaj.altle of <!••] >ri\ i nij; it of its oxx'^eii. lio\\-e\t-r,
ma ^iii'-iii 1 1 1. |iota>^ium and like metals, on heiim' heated, 'uirn in it,
•^ejiarat inur carbon and forming oxides. If a mixture of carbonic-
anh\'dride and hvdro^en be ]>assed t hrouudi a heated tube1, the format ion
of water and carbonic oxide will be observed ; ('< >., + 1 1 , =(.'( ) + 1 1 . .< >.

-!" 11 tlnit i-. will n uit tin- jin-M'iicc lit' wul i -r tln-y displact1 it, which clearly >lmws
• ' tlii- kind h"\v much dc|M-ii(is U|MUI the conditions of reaction and the
[IP pert • • : t he snli-tances formed. These relations, which at lir-t ML:! it appear coin-
:• • ••.. !•:.:;. Ice I.e-t uiidersl ..... 1 if we represent that two salt s. MX and X Y. in ^eiiend
ji'.e. iin n- or |e>s. two other -alts. .M Y and NX. and examine the properties of

t" -oine extent form -odium carhonute and silica. SiO : Idit the latter.
• . • ; the i-c-ma miiiu mass of -odium silicate i- ,i_ain decompo-ed
i vdride. -o that the matter terminate.- with the complete -eparalion of

ic Iln : ation of -odium carl ..... ale. In a t'll>ed state tile case is diiTercnt ;

•• eact /. itli CM to form carhc nic unhydride and -odium silicate,

ii.n \\ill ' • place, and the matter willend with the i-omplete -epara-

ile ami tin- formation ot -oilium silicate. If iml liin;_' i- -eparated
pi.en- of the reaction di-t rilmt ion takes place. '1'herefnre. alt lioiiLfh carlionic

• • ' ' ; • id. -till not for this rea-on, I ml only in virtue of it-, LTa-eons form.
M, -place it in -aline solutions. The small amount of energy of car-
l-r-n.iii forward l" account for it s l>ei m_r sepai at ed from salt s liy

• • c!d I I . i owe\.-r. not correct. Such a separation onh depends on the

pi-..|.e,-!i. ••-.. and M ,-. ,inp. .iin.U. Th u- a cH ic acid d i -pla ce- ca rl ..... ic anhydride

c..|. 'I - ........ • aiihwlride he pa-seil tliroii.L'li an alcoliolic -olulic ..... f

: • ' . d t.itc t hi ici ! !c ,ie:i| i . ! i P . . ; I M i 1 i i . • | ., .! . i -- . 1 1 1 1 1 , -;i 1 I M , | i a t , • -,( • p, i |'a 1 1 •-. 1 ii • i I l;_r

<|i ' • • ' ; - • ot 1 ...... n pot i nd I'c'imed and nf the react in;_f siilistal ices

• . < 1-1 n llui ' i • • : • ; •., ' • • i' i ••.nip" • it inn , ,i the measure ot

• •• . i • . d properties, carl.onic acid il.-n.mjiuM-s certain
i 1 In ive-l at ions of Secllem .!! on the -olllt ion
• •• • • .' ( hap. I . Note :;s,c. Still t he ivl.it i\e energy

• .; i. . tin-si CM ,.. . v, Inch ue -hall di^cii - fuitlier on \\ hen

ot d;-n il.nt ion and ..I \elocit\ "I r, action. '1'he principles of that
kind o I ..... •• pi ."n "! c In • n,; c.i i mutual ad i ..... idi a- on -I it now lo I ie accept ed were.

oii'.'ii! !"il. I'.erth'illet a! the commencement ot the present century, and sometimes
n oi ' |{< 1 1 et ! i \'. -.' t" v. Inch \M< v. : , I t urn our attention in the folloivin^

xns OF CARBON WITH OXYGEN AND NITROGEN 381

But only a portion of the carbonic acid gas undergoes this change, and
therefore the result will be a mixture of carbonic anhydride, carbonic
oxide, hydrogen, and water, which does not change under the action of
heat.17 Although, like water, carbonic anhydride is exceedingly stable,
still on being heated it partially decomposes into carbonic oxide and
oxygen. Deville showed that such is the case if carbonic anhydride
be passed through a long heated tube containing pieces of porcelain
and heated to 1300°. If the products of decomposition — namely,
the carbonic oxide and oxygen— be suddenly cooled, they can be
collected separately, although they partly reunite together. A similar
decomposition of carbonic anhydride into carbonic oxide and oxygen
takes place on passing a series of electric sparks through it (for
instance, in the eudiometer). With this an increase of volume
occurs, because two volumes of CO2 give two volumes of CO and one
volume of O. The decomposition reaches a certain limit (less than
one-third) and does not proceed further, so that the result is a
mixture of carbonic anhydride, carbonic oxide, and oxygen, which
is not altered in composition by the continued action of the sparks.
This is readily understood, as it is a reversible reaction. If the
carbonic anhydride be removed, then the mixture explodes when a
spark is passed and forms carbonic anhydride. If from an identical
mixture the oxygen (and not the carbonic anhydride) be removed,
and a series of sparks again be passed, the decomposition is renewed,

17 Hydrogen and carbon are near akin to oxygen as regards affinity, but it ought to
be considered that the affinity of hydrogen is slightly greater than that of carbon, be-
cause during the combustion of hydrocarbons the hydrogen burns first. Some idea of
this similarity of affinity may be formed by the quantity of heat evolved. Gaseous
hydrogen, Ho, on combining with an atom of oxygen, O = 16, develops 69000 heat-units
if the water formed be condensed to a liquid state. If the water remains in the form of
a gas (steam) the latent heat of evaporation must be subtracted, and then 58000 calories
will be developed. Charcoal taken in the solid form on combining with Oo = 32 develops
about 97000 calories, forming gaseous CO2. If the charcoal were gaseous like the
hydrogen, and only contained C.z in its molecule, much more heat would be developed,
and judging by other substances, whose molecules on passing from the solid to the gaseous
state absorb about 10000 to 15000 calories, it must be held that gaseous carbon on
forming gaseous carbonic anhydride would develop not less than 110,000 calories — that
is. approximately twice as much as is developed in the formation of water. And as in a
molecule of carbonic anhydride there is twice as much oxygen as in a molecule of water,
the oxygen develops approximately the same quantity of heat on combining with
hydrogen and carbon. That is to say, that here we find the same close, affinity (see
Chap. II. Note 7) determined by the quantity of heat as between hydrogen, zinc, and iron.
For this reason here also, as in relation to hydrogen and iron, we ought to expect an evi-
dent distribution of oxygen between hydrogen and carbon, if they are both in excess
compared with the amount of oxygen ; but if there be an excess of carbon it will decom-
pose water, whilst an excess of hydrogen will decompose carbonic anhydride. If similar
relations of mutual action are made clear in isolated cases, still the full theory of the
subject is wanting in the present condition of chemical knowledge.

383 PRINCIPLES OF rilK.MlSTRY

and terminates with the complete decomposition of the carbonic
anhydride. Phosphorus is used in order to attain the complete absorp-
tion of the oxygen. In these examples we see that a known mixture
of changeable substances is capable of arriving at a state of stable
equilibrium, destroyed, however, by the removal of one of the sub-
stances composing the mixture. This represents one of the instances
of the influence of mass. And as on rapid cooling the combustible
carbonic oxide does not succeed in combining with the oxygen, with
which it totally combines if slowly cooled, forming carbonic anhydride,
here, as in all chemical phenomena, the action of time is apparent — that
is, the existence of a definite velocity of reaction.

Although carbonic anhydride is decomposed on heating, yielding
oxygen, it is, nevertheless, like water, an unchangeable substance at
ordinary temperatures. The decomposition of carbonic anhydride, as
effected by plants, is on this account all the more remarkable ; in this
-case the whole of the oxygen of the carbonic anhydride is separated in
the free state. The mechanism of this decomposition is that the heat
and light absorbed by the plants are expended in the decomposition of
the carbonic anhydride. This accounts for the enormous influence of
temperature and light on the growth of plants. But it is at present
not clearly understood how this takes place, or by what separate inter-
mediate reactions the whole process of decomposition of carbonic
anhydride in plants into oxygen and the carbohydrates (Chapter VIII.)
remaining in them, takes place. It is known that sulphurous anhy-
dride (in many ways resembling carbonic anhydride) under the action
of light (and also of heat) forms sulphur and sulphuric anhydride, SO3,
and in the presence of water sulphuric acid. But no similar decompo-
sition has been directly obtained with carbonic anhydride, indeed
it forms the highest degree of oxidation of carbon ; perhaps for that
reason the oxygen separates. On the other hand, it is known that
plants always form and contain organic acids, and these must be
regarded as derivatives of carbonic acid, as is seen by all their
reactions, which we will shortly treat of. For this reason it might be
thought that the carbonic acid absorbed by the plants first forms
organic acids in them, and that these latter in their final trans-
formation form all the other complex organic substances of the plants.
Many organic acids are found in plants in considerable quantity ; for
instance, tartaric acid, C4H6O6, found in grape-juice and in the acid
juice of many plants ; malic acid, C4H6O5, not only found in unripe
apples but in still greater quantities in mountain ash berries ; citric
acid, C6H8O7, found in the acid juice of lemons, in gooseberries,
-cranberries, &c. ; oxalic acid, C.2H20.,, found in wood-sorrel and

ro.Mi'orxns OK CAKIION AVJTII O\Y<;I«;N AND NITROGEN 383

many other plants. Sometimes these acids exist in a free state in the
plants, and sometimes in the form of salts ; for instance, tartaric acid
is met with in grapes as the salt known as cream of tartar, but in the
impure state called argol, or tartar. C4H5KO6. In sorrel we find the
so-called salts of sorrel, or acid potassium oxalate, C2HKO4. There is
a very clear connection between carbonic anhydride and the above-
mentioned organic acids — namely, they all, under one condition or
another, yield carbonic anhydride, and can all be formed by means of it
from substances destitute of acid properties. The following examples
afford the best demonstration of this fact : if acetic acid, C2H4O2, the
acid of vinegar, be passed in the form of vapour through a heated tube,
it splits up into carbonic anhydride and marsh gas=CO2 + CH4. But
it can also be obtained conversely from those components into which it
decomposes. If one equivalent of hydrogen in marsh gas be replaced
(by indirect means) by sodium, and the compound CH3Na is obtained,
this directly absorbs carbonic anhydride, forming a salt of acetic acid,
CH;{Na + CO2 = C2H3Na02 ; from this acetic acid itself may be
easily obtained in a similar way to that by which nitric acid may
be obtained from nitre. Therefore acetic acid decomposes into marsh
gas and carbonic anhydride, and conversely is obtainable from them.
The hydrogen of marsh gas does not, like that in acids, show the
property of being directly replaced by metals ; the gas itself does not
show any acid character whatever, but on combining with the elements
of carbonic anhydride it acquires the properties of an acid. The investi-
gation of all other organic acids shows similarly that their acid character
depends on their containing the elements of carbonic anhydride. For
this reason there is no organic acid containing less oxygen in its mole-
cule than there is in carbonic anhydride ; every organic acid contains in
its molecule at least two atoms of oxygen. In order to express the rela-
tion between carbonic acid, H2CO3, and organic acids, and in order
to understand the reason of the acidity of these latter, it is simplest to
turn to that law of substitution which shows (Chapter VI.) the rela-
tion between the hydrogen and oxygen compounds of nitrogen, and
permits us (Chapter VIII.) to regard all hydrocarbons as derived
from methane. The facts of the matter are as follows : if we have a
given organic compound, A, which has not the properties of an acid, but
as the derivative of a hydrocarbon contains hydrogen combined
with carbon, then ACO2 will be a monobasic organic acid, A2CO2
a bibasic, A3CO2 a tribasic, and so on— that is, each molecule, CO2,
transforms one atom of hydrogen into that state in which it may be
replaced by metals, as in acids. Tljis already furnishes proof that in
organic acids it is necessary to recognise the group HCO2, or carboxyl.

384 PRINCIPLES OF CHE-MIST I! Y

If the addition of CO2 raises the basicity the removal of CO2 lowers
it. Thus from the bibasic oxalic acid, C2H2O4, orphthalic acid, C8H)5O4,
by eliminating CO2 (easily effected experimentally) we obtain the
monobasic formic acid, CH.2O2, or benzoic acid, C7H6O2., respectively.
The nature of carboxyl is directly explained by the law of substitution.
Judging from what has been stated in Chapters V. and VIII. concerning
this law, it should already be evident that CO2 is CH4) with the
exchange of H4 for O2, and the hydrate of carbonic acid, H2CO3, is
CO(OH)2, that is, methane, where two parts of hydrogen are replaced by
two parts of the water radicle (OH, hydroxyl), and the other two by
oxygen. Therefore the group CO(OH), or carboxyl, HCO2, is a part
of carbonic acid, and is equivalent to (OH), and therefore also to H.
That is, it is a univalent residue of carbonic acid capable of replacing
one atom of hydrogen. Carbonic acid itself is a bibasic acid, both
hydrogen atoms in it being replaceable by metals, therefore carboxyl,
which contains one of the hydrogen atoms of carbonic acid, represents
a group in which the hydrogen is exchangeable with metals. And
therefore if 1, 2 . . . n atoms of non-metallic hydrogen are exchanged
1, 2 ... n times for carboxyl we ought to obtain 1, 2 ... n-basic
acids. Organic acids are the products of the carboxyl substitution of
hydrocarbons.1* If in the saturated hydrocarbons, CMH2M+2, one part
of hydrogen is replaced by carboxyl the monobasic saturated (or fatty)
acids, CnH2w+l(CO2H), will be obtained, as, for instance, formic,

18 If CO-2 is the anhydride of a bibasic acid, and carboxyl corresponds with it, re-
placing the hydrogen of hydrocarbons, and giving them the character of comparatively
feeble acids, then SO3 is the anhydride of an energetic bibasic acid, and sulphoxyl,
SO.2(OH), corresponds with it, being capable of replacing the hydrogen of hydrocarbons,
and forming comparatively energetic sulphur oxyacids (sulphonic acids) ; for instance,
C6H5(COOH), benzoic acid, and C6H5(SO.2OH), benzenesulphonic acid, are derived from
C6H6. As the exchange of H for methyl, CH3, is equivalent to the addition of CHo, the
exchange of carboxyl, COOH, is equivalent to the addition of COo ; so the exchange of H
for sulphoxyl is equivalent to the addition of SO5. The latter proceeds directly, for
instance : C6H6 + SO3 = C0H5(SO.,,OH).

In Chapter VIII. we saw that the structure of hydrocarbons might and ought to be
represented by the presence of CH3, CHo, CH, and C— radicles of methane, starting
from methane itself ; therefore the atoms of hydrogen are retained by separate atoms of
carbon, which refers also to OH, COOH, and other radicles. Although with one atom of
carbon, H4, H3, and H2, or several, CH3, Cl, &c., may be retained, yet not more than one
of hydroxyl may be retained, and therefore there is no alcohol CH2(OH)., or C5H4(OH)4.
The carboxyl, as well as the sulphoxyl, substitution of hydrogen in hydrocarbons, leads to
the formation of acids, because the derivative acids H2CO3 and H2SO4 are bibasic.
Nitric acid, being monobasic, is unable to form a similar radicle. Its radicle, NO3, does
not contain hydrogen, and does not produce acidity, but has the same relation to the
acid NOa(OH) as carboxyl to carbonic acid.

As, according to the determinations of Thomsen, the heat of combustion of the vd/iotiM
of acids RCO-> is known where R is a hydrocarbon, and the heat of combustion of the
hydrocarbons R themselves, it may be seen that the formation of acids, RCO.>, from
R + CO2 is always accompanied by a small absorption or development of heat. We give

COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 385

HC02H, acetic acid, CH3C02H, . . . stearic acid, C17H35, CO2H, cfcc.
The double substitution will give bibasic acids, CBH2B(CO2H)(CO2H) ;
for instance, oxalic n=Q, malonic w=l, succinic acid n=2, &c. To
benzene, C6H6, correspond benzoic acid, C6H6(CO2H), phthalic acid
(and its isomerides), C6H4(CO2H)2, up to melitic acid, C6(CO2H)6, in
all of which the basicity is equal to the number of carboxyl groups. As
many isomerides exist in hydrocarbons, it is readily understood not
only that such can exist also in organic acids, but that their number
and structure may be foreseen. As in hydrocarbons the hydrogen may
be replaced by chlorine, hydroxyl, &c., evidently the same is possible
with organic acids. And, therefore, the number and transformations of
such compounds are exceedingly great. This complex and most interest-
ing branch of chemistry is treated separately in organic chemistry.

Carbonic Oxide. — This gas is formed whenever the combustion of
organic substances takes place in the presence of a large excess of incan-
descent charcoal ; the air first burns the carbon into carbonic anhydride,
but this in penetrating through the red-hot charcoal is transformed into
carbonic oxide, C02 + C = 2CO. By this reaction carbonic oxide is pre-
pared by passing carbonic anhydride through charcoal at a red heat. It
may be separated from the excess of carbonic anhydride by passing it
through a solution of alkali, which does not absorb carbonic oxide. This
reduction of carbonic anhydride explains why carbonic oxide is formed
in ordinary clear fires, where the incoming air passes over a large
surface of heated coal. A blue flame is then observed burning above
the coal ; this is the burning carbonic oxide. When charcoal is
burnt in stacks, or when a thick layer of coal is burning in a brazier,
and under many similar circumstances, carbonic oxide is also formed.
In metallurgical processes, for instance, when iron is smelted from the
ore, very often the same process of conversion of carbonic anhydride
into carbonic oxide occurs, especially if the combustion of the coal be
effected in high, so-called blast, furnaces and ovens, where the air
enters from the lower part and is compelled to pass through a thick
layer of coal. In this way, also, combustion with flame may be

the heats of combustion in thousands of calories, referring to the molecular weights of
the burning substances : —

B= H, CH4 C.2H6 C6H6

68-4 212 870 777

KCO2= 69'4 225 887 766

Thus H2 corresponds with formic acid, CH2O2 ; benzene, C6H6, with benzoic acid, C7H6O2-
The data for the latter are taken from Stohmann, and refer to the solid condition. For
formic acid Stohmann gives the heat of combustion as 59000 calories in a liquid state,
but in a state of vapour, 64'6 thousand units, which is much less than according to
Thorn sen.

VOL. I. C C

386

PRINCIPLES OF CHEM1STKY

obtained from those kinds of fuel which under ordinary conditions
burn without flame : for instance, anthracite, coke, charcoal. Heating
by means of a gas-producer — that is, an apparatus producing carbonic
oxide from fuel — is carried on in the same manner.19 In trans-
forming one part of charcoal into carbonic oxide 2420 heat units
are given out, and 011 burning to carbonic anhydride 8080 heat units.
It is evident that on transforming the charcoal first into carbonic
oxide we obtain a gas which in burning is capable of giving out 5660
heat units for one part of charcoal. This preparatory transforma-
tion of fuel into carbonic oxide, or producer gas containing a mixture
of carbonic oxide (about ^ by volume) and nitrogen (§ volume), in
many cases presents most important advantages, as it is easy to
completely burn gaseous fuel without an excess of air, which would
lower the temperature.20 In stoves where solid fuel is burnt it is im-
possible to effect the complete combustion of the various kinds of fuel
without admitting ^n excess of air. Gaseous fuel, such as carbonic oxide,

19 In gas-producers all carbonaceous fuels are transformed into inflammable gas. In
those which (on account of their slight density and large amount of water, or incom-
bustible admixtures which absorb heat) are not as capable of giving a high temperature

in ordinary furnaces — for instance, fir
cones, peat, the lower kinds of coal, &c.
— the same gas is obtained as with the
best kinds of coal, because the water
condenses on cooling, and the ashes and
earthy matter remain in the gas-pro-
ducer. The construction of a gas-pro-
ducer is seen from the accompanying
drawing. The fuel lies on the fire-bars O,
the air enters through them and the ash-
hole (drawn by the draught of the chimney
of the stove where the gas burns, or
else forced by a blowing apparatus), the
quantity of air being exactly regulated
by means of valves. The gases formed
are then led by the tube V, provided
with a valve, into the gas main, U. The
addition of fuel ought to proceed in
such a way as to avoid the separation of
carbonic anhydride ; hence the space A
is kept filled with the combustible and
covered with a lid.

20 An excess of air lowers the tem-
perature of combustion, because it be-
comes heated itself, as explained in
Chapter III. In ordinary furnaces the
excess of air is three or four timea

greater than the quantity required for perfect combustion. In the best furnaces
(with fire-bars, regulated air supply, and corresponding chimney draught) it is necessary
to introduce twice as much air as is necessary, otherwise the smoke contains much car-
bonic oxide.

FIG. 63. — Gas-producer for the formation of carbon
monoxide for heating purposes.

COMPOUNDS OF CARBON WITH OXYtiKN AND NITROGEN 387

is easily completely mixed with air and burnt without excess of it.
If, in addition to this, the air and gas required for the combustion be
previously heated by means of the heat which would otherwise be
uselessly carried off in the products of combustion (smoke)21 it is easy
to reach a high temperature, so high (about 1800°) that platinum may
be melted. Such an arrangement is known as a regenerative furnace?*
By means of this process not only may there be obtained the high
temperatures indispensable in many industries (for instance, glass-
working, steel-melting, &c.), but also great advantage23 as regards
the quantity of fuel, because the transmission of heat to the object to
be heated, conditions being equal, is determined by the difference of
temperatures.

The transformation of carbonic anhydride, by means of charcoal,

21 If in manufactories it is necessary, for instance, to maintain the temperature in
•A furnace at 1000°, the flame passes out at this or a higher temperature, and therefore
much fuel is lost in the smoke. For the draught of the chimney a temperature of
100° to 150° is sufficient, and therefore the remaining heat ought to be utilised. For this
purpose the flues are carried under boilers or other heating apparatus. The preparatory
heating of the air is the best means of utilisation, as by this means the highest
temperature is attained and the quickest heating and greatest economy are effected.

22 Regenerative furnaces were introduced by the Brothers Siemens about the year
1860 in many industries, and mark a most important progress in the use of fuel, espe-
cially in obtaining high temperatures. The principle is as follows : The products of com-
bustion from the furnace are led into a chamber, I, and heat up the bricks in it, and then
pass into the outlet flue ; when the bricks are at a red heat the products of combustion
are passed (by altering the valves) into another adjoining chamber, II, and air requisite
for the combustion of the generator gases is passed through I. In passing round about
the incandescent bricks the air is heated, and the bricks cool — that is, the heat of the
smoke is returned into the furnace. The air is then passed through II, and the smoke
through I. The regenerative burners for illuminating gas are founded on this same
principle, the products of combustion heat the incoming air and gas, the temperature is
higher, the light brighter, and an economy of gas is effected in lighting. Absolute per-
fection in these appliances has of course not yet been attained; amelioration is still
further possible, but dissociation imposes a limit because at a certain high temperature
combinations do not ensue, possible temperatures being limited by reverse reactions.
Here, as in a number of other cases, the ultimate conception of the subject ought to be
limited to the sphere of general external and visible advantages.

25 At first sight it appears absurd, useless, and paradoxical to lose nearly one-third of
the heat which fuel develops by turning it into gas. Actually the advantage is enormous,
especially for producing high temperatures, as is already seen from the fact that fuel rich
in oxygen (for instance, wood) when damp is unable, with any kind of hearth whatever, to
give the temperature required for glass-melting or steel-casting, whilst in the gas-producer
they furnish exactly the same gas as the driest and most carbonaceous fuel. In order to
understand the principle which is here involved, it is sufficient to turn one's attention to
the fact that the only heat employed with advantage is that which is concentrated in the
shape of products of combustion having a high temperature. A large amount of heat,
but having a low temperature, in many cases is of no use whatever. We are unable here
to enter into all the details of the complicated matter of the application of fuel, and
further particulars must be sought for in special technical treatises. The following
footnotes, however, contain certain fundamental figures for calculations concerning com-
bustion.

cc2

388 PRINCIPLES OF CHEMISTRY

into carbonic oxide (C + CO2=CO + CO), is considered a reversible
reaction, because at a high temperature the carbonic oxide splits up
into carbon and carbonic anhydride, as Sainte-Claire Deville showed by
using the method of the ' cold and hot tube.' Inside a tube heated in
a furnace another thin metallic (silvered copper) tube is fitted, through
which a constant stream of cold water flows. The carbonic oxide
coming into contact with the heated walls of the exterior tube forms
charcoal, and its minute particles settle in the form of lampblack on
(the lower part) of the cold tube, and, since they are cooled, they do not
act further on the oxygen or carbonic anhydride formed.24 A series
of electric sparks also decomposes carbonic oxide into carbonic anhy-
dride and carbon, and if the carbonic anhydride be removed by
alkali complete decomposition may be obtained (Deville). Aqueous
vapour, which is so similar to carbonic anhydride in many respects,
acts, at a high temperature, on charcoal in an exactly similar way,
C + H2O = H2 + CO. From 2 volumes of carbonic anhydride with
charcoal 4 volumes of carbonic oxide (2 molecules) are obtained, and
precisely the same from 2 volumes of water vapour with charcoal

21 The first product of combustion of charcoal is always carbonic anhydride, and not
carbonic oxide. This is seen from the fact that with a shallow layer of charcoal (less
than a decimetre if the charcoal be closely packed) carbonic oxide is not formed at all.
It is not even produced with a deep layer of charcoal if the temperature is not above 500°,
and the current of air or oxygen is very slow. With a rapid current of air the charcoal
becomes red hot, and the temperature rises, and then carbonic oxide appears (Lang 1888).
Naumann and Pistor determined that the reaction of carbonic anhydride with carbon
commences at about 550°, and that between water and carbon at about 500°. At the latter
temperature carbonic anhydride is formed, and only with a rise of temperature is car-
bonic oxide formed (Lang) from the action of the carbonic anhydride on the carbon, and
from the reaction CO2 + H2 = CO + H2O. Kathke (1881) showed that at no temperature
whatever is the reaction as expressed by the equation CO2 + C = 2CO complete ; a part of
the carbonic anhydride remains, and Lang determined that at about 1000° not less than
3 p.c. of the carbonic anhydride remains untransformed into carbonic oxide, even after
the action has been continued for several hours. The endothermal reactions, C + 2H2O =
CO2 + 2H2, and CO + H2O = CO2 + H2, are just as incomplete. This is made clear if we
note that on the one hand the above-mentioned reactions are all reversible, and therefore
bounded by a limit ; and, on the other hand, that at about 500° oxygen begins to combine
with hydrogen and carbon, and also that the lower limits of dissociation of water, carbonic
anhydride, and carbonic oxide lie near one another between 500° and 1200°. For water
and carbonic oxide the lower limit of the commencement of dissociation is unknown, but
judging from the accumulated data (according to Le Chatelier, 1888) that of carbonic
anhydride may be taken as about 1050°. Even at about 200° half the carbonic anhydride
dissociates if the pressure be small, about O'OOl atmosphere. At the atmospheric
pressure, not more than 0'05 of the carbonic anhydride decomposes. The influence of
pressure is hei-e evident, for the reason that the splitting up of carbonic anhydride into
carbonic oxide and oxygen is accompanied by an increase in volume (as in the case of
the dissociation of nitric peroxide. See Chapter VI. Note 46). As in stoves and
lamps, and also with explosive substances, the temperature is not higher than 2000° to
2500°, it is evident that although the partial pressure of carbonic anhydride is small, still
its dissociation cannot be considerable, and probably does not exceed 5 p.c.

ro.MIMH'Nns OF CARBON WITH OXYGEN' AND NITROGEN 389

4 volumes of a gas consisting of hydrogen and carbonic oxide (H2 + CO)
is formed. This mixture of combustible gases is called water gas.^
But aqueous vapour (and only when strongly superheated, otherwise
it cools the charcoal) only acts on charcoal to form a large amount of
carbonic oxide at a very high temperature (when carbonic anhydride
dissociates) ; it begins to react at about 500°, forming carbonic
anhydride, according to the equation C + 2H2O = CO2-f2H2. Besides
this, carbonic oxide on splitting up forms carbonic anhydride, and
therefore water gas always contains a mixture 2G in which hydrogen pre-

-•"' A molecular weight of this gas, or 2 volumes CO (28 grams), on combustion
(forming CO2) gives out 68,000 heat units (Thomsen 67960 calories). A molecular weight
of hydrogen, H2 (or 2 volumes), develops on burning into liquid water 69000 heat units
(according to Thomsen 68300), but if it forms aqueous vapour 58000 heat units. Char-
coal, resolving itself by combustion into the molecular quantity of CO2 (2 volumes),
develops 97000 heat units. From the data furnished by these exothermal reactions it
follows : (1) that the oxidation of solid charcoal into carbonic oxide develops 29000 heat
units ; (2) that the reaction C + CO2 = 2CO absorbs 39000 heat units ; (8) C + H2O = H2 + CO
absorbs (if the water be in a state of vapour) 29000 calories, but if the water be liquid
40000 calories (almost as much as C + CO2); (4) C + H2O = CO2 + 2H2 absorbs (if the
water be in a state of vapour) 19000 heat units ; (5) the reaction CO + H2O = CO2 + H2
develops 10000 heat units if the water be in the state of vapour.

Therefore it follows that 2 volumes of CO or H2 burning into CO2 or H2O develop
almost the same amount of heat, just as also the heat effects corresponding with the
equations

are nearly equal.

26 Water gas, obtained from steam at a white heat, contains about 50 p.c. of hydro-
gen, about 40 p.c. of carbonic oxide, about 5 p.c. of carbonic anhydride, the remainder
being nitrogen from the charcoal and air. Compared with producer gas, which contains
much nitrogen, this is a gas much richer in combustible matter, and therefore capable of
giving high temperatures, and is for this reason of the greatest utility. If carbonic anhy-
dride could be as readily obtained in as pure a state as water, there would be no differ-
ence in the heating powers of producer gas and water gas. As regards the utilisation of
the heat of the charcoal there is no difference, because on burning carbonic oxide gives
out almost as much heat as hydrogen — even more if the smoke has a temperature above
100°, and the water remains as vapour (Note 25). But water gas stands higher than
producer gas as regards the concentration of the fuel in its transformation into gas, and
therefore in places where a particularly high temperature is required (for instance, for
lighting by means of incandescent lime or magnesia, or for steel melting, &c.), and for
distributing through pipes at great distances, water gas is at present held in high esti-
mation, but when (in ordinary furnaces, re-heating, glass-melting, and other furnaces) a
very high temperature is not required, and there is no need to convey the gas in pipes,
producer gas is generally preferred on account of the simplicity of its preparation,
especially as for water gas such a high temperature is required that the plant soon
becomes damaged.

Water gas is prepared (there are very many systems, but the American patent of T.
Lowe, of Norristown, Pennsylvania, is much used) in a cylindrical generator, into which
heated air is introduced, in order, by partial combustion of the coke, to heat the remainder,
&c., to a white heat. The products of combustion containing carbonic oxide are utilised
in a contrivance for superheating steam, which is then passed over the white hot coke.
Water gas, or a mixture of hydrogen and carbonic oxide, is thus obtained. The practical

390 PRINCIPLES OF CHEMISTRY

dominates, the volume of carbonic oxide being comparatively less,
whilst the amount of carbonic anhydride increases as the temperature
of the reaction decreases (generally it is more than 3 per cent.).

Metals, like iron and zinc, which at a red heat are capable of
decomposing water with the formation of hydrogen, also decompose
carbonic anhydride with the formation of carbonic oxide ; so both
the ordinary products of complete combustion, water and carbonic
anhydride, are very similar in their reactions — and hydrogen may
be compared with carbonic oxide. The metallic oxides of the above-
mentioned metals, when reduced by charcoal, also give carbonic oxide.
Priestley obtained it by heating charcoal with zinc oxide. It must be
held that here carbonic anhydride is first produced, but that, combining
with carbon, it forms carbonic oxide. As free carbonic anhydride may
be transformed into carbonic oxide, so, in the same way precisely, may
that carbonic acid which is in a state of combination ; therefore, if
magnesium or barium carbonates (MgCO3 or BaCO3) be heated to a
red heat with charcoal, orlron or zinc, carbonic oxide will be produced —
for instance, it is obtained by heating an intimate mixture of 9 parts
of chalk and 1 part of charcoal in a clay retort.

Many organic substances 27 on being heated, or under the action of
different agents, yield carbonic oxide ; amongst these are many organic
or carboxylic acids. The simplest are formic and oxalic acids. Formic
acid, CH2O2, on being heated to 200°, easily decomposes into carbonic
oxide and water, CH2O2 = CO + H2O. This reaction may be effected

side of the question resolves itself into utilising all the heat given out by the charcoal,
and contained in the incandescent gas, for heating up the air, for the formation and super-
heating of the steam.

Water gas is sometimes called ' the fuel of the future,' because it is applicable to all
purposes, develops a high temperature, and is therefore available, not only for domestic
and industrial uses, but also for gas-motors (page 172) and for lighting. For the
latter purpose platinum, lime, magnesia, zirconia, and similar substances (as in the
Drummond light, Chapter III.), are rendered incandescent in the flame, or else the gas is
carbu retted — that is, mixed with the vapours of volatile hydrocarbons (generally benzene
or naphtha, naphthalene, or simply naphtha gas), which communicate to the pale flamo
of carbonic oxide and hydrogen a great brilliancy, owing to the high temperature ( 1 r \ ( • 1 . • ] > . • < 1
by the combustion of the non-luminous gases. As water gas, possessing these properties,
may be prepared at central works and conveyed in pipes to the consumers, may be pro-
duced from any kind of fuel, and ought to be much cheaper than ordinary gas, it may
actually be considered that in course of time (when practice will have determined the
cheapest and best way to prepare it) it will not only supplant ordinary gas but will with
advantage everywhere replace the ordinary forms of fuel, which in many respects air
inconvenient. At present its consumption spreads principally for lighting purposes, and
for use in gas-engines instead of ordinary illuminating gas.

•7 The so-called yellow prussiate, K4FeC6N6, on being heated with ten parts of strong
sulphuric acid forms a considerable quantity of very pure carbonic oxide completely free
from carbonic anhydride.

( o.ML'OlNDS <>F CARBON WITH OXYGEN AND NITKOGEN 391

by mixing formic acid with glycerin, because in a separate state it vola-
tilises much earlier, and therefore cannot be heated up to the required
temperature. The salts of formic acid, on being heated with sulphuric
acid, yield carbonic oxide. Usually, however, carbonic oxide is pre-
pared in laboratories, not from formic but from oxalic acid, C2H2O4,
the more so as formic acid is itself prepared from oxalic acid. The
latter acid is easily obtained by the action of nitric acid on starch,
sugar. Arc. : it is also found in nature. Oxalic acid is easily decom-
posed by heat ; its crystals first lose water, then partly volatilise,
but the greater part is decomposed. The decomposition is of the
following nature : it splits up into water, carbonic oxide, and car-
bonic anhydride,28 C2H2O4=H2O + CO2 + CO. This decomposition
is generally practically effected by mixing oxalic acid with strong
sulphuric acid, because the latter favours the decomposition by taking
up the water. On heating a mixture of oxalic and sulphuric acids a
mixture of carbonic oxide and carbonic anhydride is evolved. This
mixture is passed through a solution of an alkali in order to absorb
the carbonic anhydride, whilst the carbonic oxide passes on. Oxalic
acid in admixture with glycerin, on being heated first to 100° and
then to 140°, decomposes in a similar manner.

In its physical properties carbonic oxide resembles nitrogen ; this
is explained by the equality of their molecular weights. The absence
of colour and smell, the low temperature of the absolute boiling
point, -140° (nitrogen —146°), the faculty of solidifying at -200°
{nitrogen, -202°), the boiling point of —190° (nitrogen, —203°),
-and the slight solubility (page 78), of carbonic oxide are almost the
same as in those of nitrogen. The chemical properties of both
gases are, however, very different, and in these carbonic oxide re-
sembles hydrogen. Carbonic oxide burns with a blue flame, giving
2 volumes of carbonic anhydride from 2 volumes of carbonic oxide, just
as 2 volumes of hydrogen give 2 volumes of aqueous vapour. It
explodes with oxygen, in the eudiometer, like hydrogen.29 When

28 The decomposition of formic and oxalic acids, with the formation of carbonic oxide,
considering these acids as carboxyl derivatives, may be explained as follows : — The first
is H(COOH), and the second (COOH)2, or H2, in which one or both parts of hydrogen are
exchanged for carboxyl; therefore they are equal to H2 + CO2 and H2 + 2CO2; but Ho
reacts with CO2, as has been stated above, forming CO and H2O. From this it is also
evident that oxalic acid on losing CO2 forms formic acid, and also that the latter may
proceed from CO + H2O, as we shall see further on.

29 It is remarkable that, according to the investigations of Dixon, perfectly dry
carbonic oxide does not explode with oxygen when a spark of low intensity is used, but
an explosion takes place if there is the slightest admixture of moisture. L. Meyer,
however, showed that sparks of an electric discharge of considerable intensity pro-
duce an explosion. I think that this may be explained by the fact that water with

392 PRINCIPLES OF CHEMISTRY

breathed it acts as a strong poison, because it is absorbed by the
blood ; 30 this explains the action of charcoal fumes, the products of
the incomplete combustion of charcoal and other carbonaceous fuels.
Owing to its faculty of combining with oxygen, carbonic oxide acts as a
powerful reducing agent, taking up the oxygen from many compounds
at a red heat, and being itself transformed into carbonic anhydride.
The reducing action of carbonic oxide, however, is (like that of
hydrogen, Chapter II.) naturally confined to those oxides which easily
part with their oxygen — as, for instance, copper oxide — whilst the
oxides of magnesium or potassium are not reduced. Metallic iron itself
is capable of reducing carbonic anhydride into carbonic oxide, just as
it liberates the hydrogen from water. Copper, which does not de-
compose water, does not decompose carbonic oxide. If a platinum
wire heated to 300°, or spongy platinum at the ordinary tempera-
ture, be plunged into a mixture of carbonic oxide and oxygen, or of
hydrogen and oxygen, the mixture explodes. These reactions remind
one exceedingly of those which are peculiar to hydrogen. The follow-
ing important distinction, however, exists between them, namely : the
molecule of hydrogen is composed of H2, a group of elements divisible
into two like parts, whilst, as the molecule of carbonic oxide, CO,
contains unlike atoms of carbon and oxygen, in none of its reactions
of combination can it give two molecules of matter containing its
elements. This is particularly noticeable in the action of chlorine on
hydrogen and on carbonic oxide respectively ; with the former chlorine
forms hydrogen chloride, and with the latter it produces the so-called
carbonyl chloride, COC12 ; that is to say, the molecule of hydrogen, H2>
under the action of chlorine divides, forming two molecules of hydro-
chloric acid, whilst the molecule of carbonic oxide entirely enters
into the molecule of carbonyl chloride. This characterises the so-
called diatomic or bivalent reactions of radicles or residues. H is a
monatomic residue or radicle, like K, 01, and others, whilst carbonic
oxide, CO, is an indivisible (without decomposition) bivalent radicle,
equivalent to H2 and not to H, and therefore combining with X2 and

carbonic oxide gives carbonic anhydride and hydrogen, but hydrogen with oxygen givea
hydrogen peroxide (Chapter VII. p. 805), which with carbonic oxide forms carbonic
anhydride and water. The water, therefore, is renewed, and again serves the same
purpose. But it may be that here it is necessary to acknowledge a simple contact
influence.

50 Carbonic oxide is a very quick poison, because it is absorbed by the blood in the
same way as oxygen. In addition to this, the spectrum of the absorption of the blood
changes so that by the help of blood it is easy to detect the slightest traces of carbonic
oxide in the air. M. A. Kapoustin found that linseed oil (and therefore oil paints) are
capable of giving off carbonic oxide while drying (absorbing oxygen).

COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 39$

interchanir<';il>lr with H2. This distinction is evident from the annexed
comparison :

HH, hydrogen. CO, carbonic oxide.

HC1, hydrochloric acid. COC1V, carbonyl chloride.

HKO, potash. CO(KO)2, potassium carbonate.

HXH._>, ammonia. CO(NH2)2, urea.

HCH3, methane. CO(CH3)2, acetone.

HHO, water. CO(HO)2, carbonic acid.

Such monatomic (univalent) residues, X, like H, Cl, Na, NO2, NH4,
CH3, CO._,H (carboxyl), OH, and others, in accordance with the law
of substitution, combine together, forming compounds, XX' ; with
oxygen, or in general with diatomic (bivalent) residues, Y — for instance,
with O, CO, CH2, S, Ca, <fcc. — compounds XX'Y ; but diatomic-
residues, Y, sometimes capable of existing separately, combine together,
YY' and with X2 or XX', as we see from the transition of CO into
C02 and COC12. This combining faculty of carbonic oxide appears in
many of its reactions. Thus it is very easily absorbed by cuprous
chloride, CuCl, dissolved in fuming hydrochloric acid, forming a
crystalline compound, COCu2Cl2,2H2O, decomposable by water ; it
combines directly with potassium (at 90°), forming (KCO)K31 with
platinum dichloride, PtCl2, with chlorine, C12, &c.

But the compounds of carbonic oxide with the alkalis are still more
remarkable — for instance, with potassium or barium hydroxides, &c. —
although it is not directly absorbed by them, as it has no acid
properties. Berthelot (1861) showed that potash in the presence of
water is capable of absorbing carbonic oxide, but the absorption
takes place slowly, little by little, and it is only after being heated
for many hours that the whole of the carbonic oxide is absorbed by
the potash. The salt, CHK02, is obtained by this absorption ; it cor-
responds with an acid found in nature, namely, the simplest organic —
carboxylic — acid, formic acid, CH2O2. It can be extracted from the
potassium salt by means of distillation with dilute sulphuric acid,
just as nitric acid is prepared from sodium nitrate. The same acid
is found in ants, in nettles (when the points of the nettles enter

31 The molecule of metallic potassium (Scott, 1887), like that of mercury, contains one
atom, and it is probably in virtue of this that the molecules CO and K combine together.
But as in the majority of cases potassium acts as a univalent radicle, the polymeride
K.2C.>O3 is formed, and probably K10C10O10, because products containing CIQ are formed
by the action of hydrochloric acid. The black mass formed by the combination of
carbonic oxide with potassium explodes with great ease, and oxidises in the air. Although
Brodie and Lerch have greatly extended our knowledge of this compound, much still
remains unexplained.

394 PRINCIPLES OF CHEMISTRY

the skin they break, and the corrosive formic acid enters into the
body); it is obtained during the action of oxidising agents on many
organic substances ; it is formed from oxalic acid, and under many
conditions splits up into carbonic oxide and water. In the formation
of formic acid from carbonic oxide we observe an example of the
synthesis of organic compounds, such as are now very numerous, and
are treated of in detail in organic chemistry.

Formic acid, H(CHO2), carbonic acid, HO(CHO2), and oxalic acid
(CHO2)2, are the simple organic or carboxylic acids, R(CHO2) cor-
responding with HH and HOH. Proceeding from carbonic oxide, CO,
the formation of carboxylic acids is clearly seen from the fact that CO is
capable of combining with X2, that is, forming COX2. If, for instance,
one X is an aqueous residue, OH (hydroxyl), and the other X hydrogen,
then the simplest organic acid — formic acid, H(COOH) — is obtained.
As all hydrocarbons (Chapter VIII.) correspond with the simplest,
OH4, so all organic acids may be considered to proceed from formic
acid.

In a similar way it is easy to explain the relation to other com-
pounds of carbon of those compounds which contain nitrogen. By way
of an example we will examine one class of such nitrogenous com-
pounds. There are ammonium salts, R(CNH4O2) corresponding with
every carboxylic acid, R(CH02), which salts contain the elements of
water ; this, with the assistance of substances taking up the water, may
be separated with formation of a special class of nitrogenous organic,
or, as they are termed, cyanogen compounds, RCN,32 because the group
or monovalent residue, or radicle, ON, is called cyanogen. From this the
connection of the most varied organic compounds is apparent, and it is
not only expressed in relation to their composition but by a multitude
of reactions. It is most important to turn our attention to the two
following circumstances : (1) We have already noticed the various
mutual transformations of the simplest organic acids, clearly explained
by the existence of carboxyl in their composition. In a similar way, in
many reactions, the various cyanogen compounds undergo mutual
transformation, especially hydrocyanic acid, H(CN), corresponding
with formic acid, H(CHO2) ; cyanic acid, OH(CN), corresponding to
carbonic acid, OH(CHO2) ; and cyanogen, (CN)2, corresponding with
oxalic acid, (CHO2)2. The mutual transitions of cyanogen compounds
into one another were known much earlier than the doctrine of
carboxylic acids, and therefore Gay-Lussac acknowledged cyanogen,

52 The connection of the immense series of cyanogen compounds with the rest of the
hydrocarbons by means of carboxyl was enunciated by me, about the year 1860, at the
first Annual Meeting of the Russian Naturalists.

NDS OF CAEB03S WITH OXYGEN AND MTKoiiKN 395

CN, as the radicle of cyanic acid or hydrocyanic acid, HCN, form-
ing salts, for instance of mercury, which latter on being heated, as
Gay-Lussac showed, gives cyanogen itself, (GN)2. (2) As the ammo-
nium salt of a carboxylic acid, R(CNH4O2), contains the elements of
two molecules of water, and molecular quantities are those which act
in reactions ; then, previously to forming a cyanogen compound, or
nitri/c, by parting with two molecules water, an ammonium salt ought to
form an. amide by losing one molecule of water : R(CO2NH4) — H2O=
R(Ci!s H2O). Amides are therefore compounds containing the univalent
amide residue, NH2, of ammonia, or a compound, COX2, in which one
X=R and the other =NH2. Such are, for instance, formamide, or the
amide of formic acid, (CO)H(NH2), or H(CONH2). The amides com-
pose a most numerous series of nitrogen compounds, and are obtained
in many ways j 33 they are met with in plants and animals, and, as
has already been shown, by parting with water they form nitriles or
cyanogen compounds, so that they serve as transitional terms between
R(CO2NH4) and RON. The varieties, reactions, and properties of the
amides and nitriles of organic acids, and therefore also of cyano'gen
compounds, are, as carbon compounds, examined in greater detail in
organic chemistry, and here we will only dwell on the simplest of them ;
and, in order to clearly explain the derived ammonia compounds, we
will first dwell on the ammonium salts and amides of carbonic acids.

As carbonic acid isbibasic, its ammonium salts ought to have the fol-
lowing composition : acid carbonate of ammonium, H(NH4)CO3, and the
normal carbonate, (NH4)jCOj ; they represent compounds of one or two
molecules of ammonia with carbonic acid. The acid salt appears in the
form of a non-odoriferous and (when tested with litmus) neutral sub-
stance, soluble at the ordinary temperature in six parts of water, in-
soluble in alcohol, and obtainable in a crystalline form either without
water of crystallisation or with various proportions of it. If an
aqueous solution of ammonia be saturated with carbonic anhydride,
and then evaporated over sulphuric acid in the bell jar of an air-pump,
crystals of this salt are separated. Solutions of all other ammonium
carbonates, when evaporated under the air-pump, yield crystals of this
salt. A solution of this salt, even at the ordinary temperature, gives
oft" carbonic anhydride, as do all the acid salts of carbonic acid (for

55 Thus, for instance, oxamide, or the amide of oxalic acid, (CNH2O)2, is obtained in
the form of an insoluble precipitate on adding a solution of ammonia to an alcoholic
solution of ethyl oxalate (CO2C2H5)2, which is formed by the action of oxalic acid on
alcohol: (CHO2)2 + 2(C2H6)OH = 2HOH + (CO2C2H5)2. As the nearest derivatives of
ammonia, the amides \vitli alkalis yield ammonia and form the salt of the acid. The
nitriles do not, however, give similar reactions so readily.

396 PRINCIPLES OF CHEMISTRY

instance, NaHCO3), and at 38° the separation of carbonic anhydride
takes place with great rapidity. For this reason the formation of the
acid salt in the solution, and the evaporation, must be conducted at a
low temperature, and with an excess of carbonic acid, as the product of
dissociation of this salt. On losing carbonic anhydride and water,,
the acid salt is converted into the normal salt, 2(NH4)HCO3 =
H2O4-CO2-f-(N"H4)2CO3 ; the latter, however, decomposes in solution,,
and therefore can only be obtained in crystals, (NH4)2CO3,H2O, at
low temperatures, and from solutions containing an excess of ammo-
nia as the product of dissociation of this salt : (NH4)2CO3 =
NH3 + (NH4)HCO3. But the normal salt,34 according to the general
type, is capable of decomposing ivith separation of water, and forming
ammonium carbamate, NH4O(CONH2)=(NH4)2CO3— H2O ; this still
further complicates the history of the carbonates of ammonium. It is.
evident that, in reality, on changing the quantity of water, ammonia,
and carbonic acid, various intermediate salts will be formed con-
taining mixtures or combinations of those mentioned above, or, in
other words, various states of equilibrium of reactions reverse to one
another will be produced. Thus the ordinary commercial carbonate of
ammonia, obtained by distillation of a mixture of chalk and sulphate
of ammonia (Chapter VI.), or sal-ammoniac, 2NH4Cl + CaCO3 =
CaCl2 + (NH4)2CO3, does not remain as a normal salt, but, through loss-
of part of the ammonia, partly contains the acid salt, and, through loss-
of water, contains some carbamate, and most frequently presents the coin-
position NH40(CONH2) + 2OH(CO2NH4) = 4NH3 + 3C02 + 2H2O.
Indeed, this salt, from having under various conditions parted with
ammonia, carbonic anhydride, and water, does not present a constant
composition, and ought rather to be regarded as a mixture of acid salt
and amide salt. The latter must be recognised as entering into the
composition of the ordinary carbonate of ammonia, because it contains,
less water than is required for the normal or acid salt ;3S but on being
dissolved in water this salt gives a mixture of acid arid normal salts.
Ammonium carbamate itself, whose composition (and formation) is.
equal to 2NH3 + CO2, when dissolved in water, does not entirely
precipitate a calcium salt, for instance CaCl_;, which produces a normal
salt, (NH4)2CO3 + CaCl2 = 2NH4Cl + CaCO2, probably because a calcium
carbamate is produced which is soluble in water.

31 The acid salt (NH4)HCO3 on losing water ought to form the carboxylic acid,
(NH2)HCO2, or, better, OH(CNH2O), as we wrote it before ; but it is not formed, which
is accounted for by the instability of the acid salt itself. Carbonic anhydride is given
off and ammonia is produced, which gives a carboxylic ammonium salt.

35 In the normal salt, 2NH3 + CO2 + HaO, in the acid salt, NH3 + CO2 + H.,O, but in the
commercial salt to 3CO2 only 2H2O

NJ'S OF CARBON WITH OXVCEN AND NITROGEN 897

This ammonium carbamate is one of the most remarkable of the
ammonium carbonates, because it is the simplest in its composition,
and is easily and immediately formed by mixing two volumes of dry
ammonia with one volume of dry carbonic anhydride, 2JS~H34-CO2=
NH,O(CONH2) ; it is a solid substance, smells strongly of ammonia,
attracts moisture from the air, and entirely decomposes at 60°. The
fact of this decomposition may be judged 3(i by the density of its
vapour, which =13 (H = l) ; this exactly corresponds with the density
of a mixture of 2 volumes of ammonia and 1 volume of carbonic
anhydride. It is easily understood that such a combination will take
place with any ammonium carbonate under the action of salts which
take up the water — for instance, sodium or potassium carbonate.37 As
ammonia and carbonic anhydride in an anhydrous state only form one
compound, C022NH3,3H ammonium carbamate, as has been already
stated, may be regarded as COX2, where X has been replaced by the
residues NH2 and NH4O (that is, HO in which H is replaced by
NH,), and therefore is still capable of losing water and forming the
symmetrical amide CO(NH2)2. This must be termed carbamide. It
is identical with urea, CN2H4O, which, contained in the urine (about
2 per cent, in human urine), is for the higher animals (especially the
carnivorous) the ordinary product of excretion 39 of the nitrogenous
substances found in the organism. If ammonium carbamate be heated
to 140° (in a sealed -tube, Bazarnff), or if carbonyl chloride, COC12,
be treated with ammonia (Natanson), urea will be obtained, which

•Vi N auniiuni determined the following densities of the vapour of ammonium carbamate
(in millimetres of mercury) : —

-10° 0° +10° 20° 30° 40° 50° 60°

5 12 30 62 124 248 470 770

Horstmann and Isambert studied the densities corresponding with excess of NH3 or CO.,,
and found, as might have been expected, that with such excess the mass of the salt
formed (in a solid state) increases and the decomposition (transition into vapour)
decreases.

37 Calcium chloride enters into double decomposition with ammonium carbamate.
Acids (for instance sulphuric) take up ammonia, and set free carbonic anhydride ;
alkalis (for instance, potash) take up carbonic anhydride and set free ammonia, and
therefore, for removing water here only sodium or potassium carbonate may be
taken.

58 It must be imagined that the reaction takes place at first between equal volumes
(Chapter VII.) ; but then carbamic acid, HO(CNH2O), is produced, which, as an acid,
immediately combines with the ammonia, forming NH4O(CNH2O).

59 Urea is undoubtedly a product of the oxidation of complex nitrogenous matters
(albumin) of the animal body. It is found in the blood. It is absorbed from the blood by
the kidneys. A man excretes about 80 grams of urea per day. As a derivative of car-
bonic anhydride, into which it is readily converted, urea is in a sense a product of
oxidation.

398 PRINCIPLES OF CHEMISTRY

shows its direct connection with carbonic acid — that is, the presence of
carbonic acid and ammonia in it. From this it will be understood how
urea during the putrefaction of urine is converted into ammonium
carbonate, CN2H4O + H,O=CO2 + 2NH3.

Thus urea, both by its origin and reactions, is an amide of carbonic
acid. Representing as it does ammonia (two molecules) in. which the
hydrogen (two atoms) is replaced by the bivalent radicle of carbonic acid,
urea retains the property of ammonia of entering into combination,
with acids (for instance, with nitric acid, CISr2H4O,HNO3), with bases
(for instance, with mercury oxide), and with salts (for instance, sodium
chloride, ammonium chloride), but it has no alkaline properties. It is
soluble in water without change, but at a red heat loses ammonia and
forms cyanic acid, CNHO, and its polymeride, cyanuric acid, C3N3H3O3.
The first is a nitrile of carbonic acid — that is to say, is a cyanogen
compound. This conformity is evident from the fact that the acid
ammonium carbonate, OH(CNH4O.2), on parting with 2H2O, ought
to form cyanic acid, CNOH. There is a relation of direct poly-
merisation between cyanuric acid (a solid, crystalline, and very stable
substance) and cyanic acid (a liquid, very unstable substance, easily
changeable in several ways). Both have the same composition, and
they pass one into another at different temperatures. If crystals of
cyanuric acid be heated to a temperature, £°, then the vapour tension,
p, in millimetres of mercury (Troost and Hautefeuille) will be :

t. 160°, 170°, 200°, 250°, 300°, 350°.
p. 56, 68, 130, 220, 430, 1200.

The vapour contains cyanic acid, and, if it be rapidly cooled, it
condenses into a mobile volatile liquid (specific gravity at 0°=1*14).
If the liquid cyanic acid be gradually heated, it passes into a new
amorphous polymeride (cyamelide), which, on being heated, like
cyanuric acid, forms vapours of cyanic acid. If these fumes are heated
above 150°, they directly pass into cyanuric acid. Thus, at a tempera-
ture of 350°, the pressure does not rise above 1200 mm. on the addi-
tion of vapours of cyanic acid, because the whole excess is transformed
into cyanuric acid. Therefore, "the above-mentioned figures give the
tension of dissociation of cyanuric acid, or the greatest pressure which
the vapours of HOCN are able to attain at a given temperature, whilst
at a greater pressure, or by the introduction of a large mass of matter
into the given volume, the whole of the excess is converted into
cyanuric acid. The properties of cyanic acid which we have described
were principally observed by Wohler, and clearly show the faculty of
polymerism of cyanogen compounds. This is observed in many other

COMPOUNDS OF CAKUoN WITH OXY<-KX AND NITROGEN 399

cyanouv,'! di-rivatives, and is regarded as the consequence of the above-
mentioned explanation of their nature. All cyanogen compounds are
.•million in in salts, R(CNH4O2), deprived of water, 2H._,O ; therefore
their molecules ought to show the faculty of combining with two mole-
cules of water or with other molecules in exchange for it (for instance,
with H2S, or HC1, or 2H2, £c.), and are therefore capable of combining
together. The combination of molecules of the same kind to form
more complex ones is nothing else but polymerisation.40

Besides being a substance very apt to form polymerides, cyanic acid
presents many other features of interest, expounded in greater detail
in organic chemistry. First of all, the transition of ammonium cyanate
into urea ought to be considered. The reason is that the salts of
cyanic acid, which are easily split up, by the action of acids and even
of water, into ammonia and carbonic anhydride, are produced by the
oxidation of saline metals, as we shall see further on. Potassium
cyanate, KCNO, for instance, is most often obtained in this way.
Solutions of cyanates, by the addition of sulphuric acid, yield
cyanic acid, which, however, immediately decomposes : CNHO + H2O
=CO2 + NH3. A solution of ammonium cyanate, CN(NH4)O, behaves,
in the same manner, but only in the cold. On being heated it
completely changes, because it is transformed into urea. The composi-
tion of both substances is identical, CN2H4O, but the structure or dis-
position and the connection between the elements is different : in the
ammonium cyanate one atom of nitrogen exists in the form of cyanogen,
CN — that is, united with carbon — and the other as ammonium, NH4,
but, as cyanic acid contains the hydroxyl radicle of carbonic acid,
OH(CN), the ammonium in this salt is united with oxygen ; whilst in
urea both the nitrogen atoms are symmetrically and uniformly disposed
as regards the radicle CO of carbonic acid : CO(NH2)2. For this
reason, urea is much more stable than ammonium cyanate, and there-
fore the latter, on being slightly heated in solution, is converted into

40 Just as the aldehydes (for instance, C2H4O, page 145) are alcohols (for instance,
C2H6O) which have lost hydrogen.

The aldehydes are also capable of entering into combination with many substances,
and of polymerising, forming slightly- volatile polymerides, which depolymerise on heating.
Although there are also many similar phenomena (for instance, the transformation of
yellow into red phosphorus, the transition of cinnamene into metacinnamene, &c.) of
polymerisation, in no other case are they so clearly and simply expressed as in cyanic
acid. The details relating to this must be sought for in treatises on organic and
theoretical chemistry. If we touch on certain sides of this question it is principally with
the view of showing the phenomenon of polymerisation by typical examples (this pheno-
menon being met with more frequently than was formerly imagined, see Silica), and
also to show the faculty which cyanogen derivatives have of forming the most varied
compounds.

400 PRINCIPLES OF CHEMISTRY

urea. This remarkable isomeric transformation was discovered by
Wohler in 1828. It had an important historical signification, because
•at that time such an easy formation outside the organism of those
substances which are met with in it was quite unexpected, it being at
that time supposed that the vital force of the organism produced sub-
•stances which could not be formed outside it. But in addition to the
abolition of this presupposition, the easy transition of NH4OCN into
CO(NH2)2 is the best example of the transition of one state of
atomic equilibrium into another more stable one. Every carboxyl
acid, RCOOH, has its corresponding amide, RCONH2,41 and
•nitrile, as is the case with carbonic acid. In this way, formamide,
HCONH2, and hydrocyanic acid, HCN, as a nitrile, correspond
-with formic acid, HCOOH, and therefore ammonium formate,
HCOONH4, and formamide, under the action of heat and substances
which take up water (phosphoric anhydride), form hydrocyanic acid,
HCN, whilst, under many conditions (for instance, on combining with
hydrochloric acid in presence of water), this hydrocyanic acid forms
formic acid and ammonia. Although containing hydrogen in the
presence of two acid-forming elements — namely, carbon and nitrogen42
— hydrocyanic acid does not give an acid reaction with litmus (cyanic
acid has very marked acid properties), but forms salts, MCN, and
therefore presents the properties of a feeble acid, and for this reason is
called an acid. The small amount of energy which it has is shown
by the fact that the cyanides of the alkali metals — for instance, potas-
sium cyanide (KHO -f HCN = H2O + KCN) in solution — have a strongly

41 The majority of the amides corresponding with acids, being represented by the com-
position RNH2, very easily undergo the inverse change ; they combine with water, even
when simply boiled with it, and still more so under the action of alkalis and acids. Ammonia
is evolved by the action of alkalis on amides, naturally in virtue of the water combining
with the amide, in addition to which there is formed a salt of the acid, which had served to
form the amide according to the equation : RNH2 + KHO = RKO + NH3. The same thing
takes place under the action of acids, only, naturally, an ammonium salt of the given
acid is formed, and the acid, previously in the condition of an amide, st-jmrutes in the
free state: RNH2 + HC1 + H2O = RHO + NH4C1. Thus the amides, in the majority of
cases, easily pass back into ammonium salts in the presence of water, alkalis, and acids ;
but they differ in a marked way from them. None of the ammonium salts are distillable,
nor volatilise without change ; in most cases ammonia salts, on being heated, lose water
and form amides ; many amides volatilise without change, and sometimes are crystalline
volatile substances easily distilled. Such are, for instance, the amides of acetic, benzoic,
formic, and a large number of other organic acids. Judging from the preceding facts,
amides may be regarded as acids RHO, in which the hydroxyl HO is ivpliir.-d by the
ammonia radicle (amidogen) NH2.

42 If ammonia and methane (marsh gas) do not show any acid properties, that is in all
probability due to the presence of a large amount of hydrogen in bo'th ; but in hydro-
cyanic acid one atom of hydrogen is under the influence of two acid-forming elements.
Acetylene, C2H2, which contains but little hydrogen, presents acid properties in certain
respects, because its hydrogen is easily replaced by metals.

(•oMI'ol'NDS OF CARBON WITH OXYt.KN AND NITROGEN 401

alkaline reaction.43 If ammonia be passed over charcoal at a red heat,
especially in the presence of an alkali, or if gaseous nitrogen be passed
through a mixture of charcoal and an alkali (especially potash, KHO),
and also if a mixture of nitrogenous organic substances and alkali be
heated to a red heat, in all these cases the alkali metal combines with the
carbon and nitrogen, forming a metallic cyanide, MCN — for example,
KCN. Potassium cyanide is much used in the arts, and is obtained,
as above stated, under many circumstances — as, for instance, in iron
smelting, especially with the assistance of wood charcoal, the ash of
which contains much potash. The nitrogen of the air, the alkali of the
ash, and the charcoal are brought into contact at a high temperature
during iron smelting, and therefore, under these conditions, a consider-
able quantity of potassium cyanide is formed. In practice it is not usual
to prepare potassium cyanide directly, but a peculiar compound of it
containing potassium, iron, and cyanogen. This compound is potassium
ferrocyanide, and is also known as yellow prussiate of potash, because
with ferric salts it forms a blue colour — Prussian blue. This saline
substance, having the composition K4FeC6N6 + 3H2O, is prepared on
the large scale in cast-iron pots (the iron of which enters into the salt),
by heating with alkali leather cuttings, horn filings and shavings,
and similar animal matter, composed of complex organic substances
containing carbon and nitrogen ; the alkali which is added is com-
posed of potashes containing potassium ; thus all the necessary ele-
ments for producing the yellow prussiate are brought into contact at a
high temperature, and the above-mentioned substance is formed.
The name of cyanogen (/cvai/o?) is derived from the property which
the yellow prussiate possesses of forming, with a solution of a ferric
salt, FeX3, the familiar pigment Prussian blue. The yellow prussiate
is generally used as the source of the other cyanogen compounds
(because it is manufactured on a large scale), and we will therefore
describe the method of forming the cyanogen compounds derived
from it.

If yellow prussiate be mixed with two parts of water and three-

Solutions of cyanides — for instance, of those of potassium or barium — are decom-
carbonic acid. Even the carbonic anhydride of the air acts in a similar way,
and for this reason these solutions do not keep, because, in the first place, free hydro-
cyanic acid itself decomposes and polymerises, and, in the second place, with alkaline
liquids it forms ammonia and formic acid. Hydrocyanic acid does not liberate carbonic
anhydride from solutions of sodium or potassium carbonates. But a mixture of solutions
of potassium carbonate and hydrocyanic acid yields carbonic anhydride on the addition
of oxides similar to zinc oxide, mercuric oxide, &c. This is due to the great inclination
which the cyanides evince to form double salts. For instance, ZnK.>(CN)4 is formed,
which is a kind of soluble double salt.

VOL. I. D D

402 PRINCIPLES OF CHEMISTRY

quarter part of sulphuric acid, and the mixture be heated, it decom-
poses, volatile hydrocyanic acid separating. This was obtained for
the first time by Scheele in 1782, but it was only known to him in
solution. In 1809 Ittner prepared anhydrous prussic acid, and in
1815 Gay-Lussac finally settled its properties and showed that it con-
tains only hydrogen, carbon, and nitrogen, CNH. If the distillate be
redistilled (a weak solution of HCN), and the first part be collected,
the anhydrous acid may be prepared from this stronger solution. In
order to do this, pieces of calcium chloride are added to the concentrated
solution, and the anhydrous acid floats as a separate layer, because it
is not soluble in an aqueous solution of calcium chloride. If this layer
be then distilled over a new portion of calcium chloride at the lowest
temperature possible, the prussic acid may be obtained completely free
from water. It is, however, necessary to use the greatest caution in
work of this kind, because prussic acid, besides being extremely
poisonous, is exceedingly volatile.44

Anhydrous prussic acid is a very mobile and volatile liquid ; its
specific gravity is 0'697 at 18° ; at lower temperatures, especially when
mixed with a small quantity of water, it easily congeals ; it boils at 26°,
and therefore very easily evaporates, and at ordinary temperatures
may be regarded as a gas. On this account the greatest caution is
necessary in dealing with it, as an insignificant amount, when inhaled
or brought into contact with the skin, causes death. It is soluble in
all proportions in water, alcohol, and ether ; weak aqueous solutions
are used in medicine.45

44 The mixture of the vapours of water and hydrocyanic acid, evolved on heating yellow
prussiate with sulphuric acid, may be directly passed through vessels or tubes filled with
calcium chloride. These tubes must be cooled, because, in the first place, hydrocyanic
acid easily changes on being heated, and, in the second place, the calcium chloride, when
warm, would absorb less water. The mixture of hydrocyanic acid and aqueous vapour
on passing over a long layer of calcium chloride gives up water, and hydrocyanic acid
alone remains in the vapour. It ought to be cooled as carefully as possible in order
to bring it into a liquid condition. The method which Gay-Lussac employed for obtain-
ing pure hydrocyanic acid consisted in the action of hydrochloric acid gas on mercuric
cyanide. The latter may be obtained in a pure state if a solution of yellow prussiate be
boiled with a solution of mercuric nitrate, then filtered and crystallised by cooling ; the
mercuric cyanide is then obtained in the form of colourless crystals Hg(CN)2.

If a strong solution of hydrochloric acid be poured upon these crystals, and the mix-
ture of vapours evolved, consisting of aqueous vapour, hydrochloric acid, and hydrocyanic
acid, be passed through a tube containing, first marble (for absorbing the hydrochloric
acid), and then lumps of calcium chloride, on cooling, the hydrocyanic acid will be con-
densed. In order to obtain the hydrocyanic acid in an anhydrous form, the decomposition
of heated mercury cyanide by hydrogen sulphide may be made use of. Here the sulphur
and cyanogen change places, and hydrocyanic acid and mercury sulphide are formed :
Hg(CN)2 + H2S = 2HCN + HgS.

45 A weak (up to 2 p.c.) aqueous solution of hydrocyanic acid is obtained by the dis-

('(.^POUNDS OF CABBON WITH OXYGEN AND NITROGEN 403

The salts MCN — for instance, potassium, sodium, ammonium — as
well as the salts M(CX)2 — for instance, barium, calcium, mercury — are
soluble in water, but the salts of manganese, zinc, lead, and many others
are insoluble in water. They form double salts with potassium cyanide
and similar metallic cyanides, an example of which we will consider in
a further description of the yellow prussiate. Not only are some of
the double salts remarkable for their constancy and comparative
stability, but so also are the soluble salt HgC2N2, the insoluble silver
cyanide AgCN, and even potassium cyanide in the absence of water.
This salt,46 when fused, acts as a reducing agent with its elements, K
and C, and oxidises when fused with lead oxide, forming potassium
cyanate, KOCN", which establishes the connection between HCN and
OHCN — that is, between the nitriles of formic and carbonic acids —
which connection is the same as between the acids themselves, because
formic acid, on oxidation, yields carbonic acid. The relation which
exists between hydrocyanic acid, metallic cyanides, and the salts of
cyanic acid, as well as between all the above-mentioned compounds,
may be expressed by granting the existence of a radicle in them con-
taining carbon and nitrogen — cyanogen, CN. In this sense, prussic
acid is hydrocyanic acid, and cyanic acid a cyanogen hydroxide, in
exactly the same way as for chlorine we have hydrochloric acid, HC1,
and hypochlorous acid, C1OH. Free cyanogen ought to be represented
as prussic acid in which the hydrogen has been replaced by cyanogen,
(CN)2 or CNC1S". This composition, judging from what has been

tillation of certain vegetable substances. The so-called laurel water in particular enjoys
considerable notoriety from its containing hydrocyanic acid. It is obtained by the
steeping and distillation of laurel leaves. A similar kind of water is formed by the
infusion and distillation of bitter almonds. It is well known that bitter almonds are
poisonous, and have a peculiar characteristic taste. This bitter taste is due to the
presence of a certain substance called amygdalin, which can be extracted by alcohol.
This amygdalin decomposes in an infusion of bruised almonds, forming the so-called
bitter almond oil, glucose, and hydrocyanic acid :

+ H2O = C7H6O + CNH + 2C6H12O6.
Amygdalin in Water. Bitter Hydrocyanic Glucose.

bitter almonds. almond oil. acid.

If after this the infusion of bitter almonds be distilled with water, the hydrocyanic acid
and the volatile bitter almond oil are carried over with the aqueous vapour. The latter
is insoluble in water, or only sparingly soluble, while the hydrocyanic acid remains as an
aqueous solution. Bitter almond water is similar to laurel water, and is used, like the
former, in medicine, naturally only in small quantities, because a considerable amount
lias poisonous effects. Perfectly pure anhydrous hydrocyanic acid keeps without change,
just like the weak solutions, but the strong solutions only keep in the presence of other
acids; but in the presence of many admixtures these solutions easily give a brown
polymeric substance, which is also formed from a solution of potassium cyanide.

46 This salt will be described under potassium, as it is one of those which are often
used.

D D 2

404 PRINCIPLES OF CHEMISTRY

already stated, exactly expresses that of the nitrile of oxalic acid, and,
as a matter of fact, oxalate of ammonia and the amide corresponding
with it (oxamide), on being heated with phosphoric anhydride, which
takes up the water, yield cyanogen. This substance is also produced
by simply heating some of the metallic cyanides. Mercuric cyanide
is particularly adapted for this purpose, because it is easily obtained in
a pure state and is very stable in a separate form. If mercuric cyanide
be heated, it decomposes, in like manner to mercury oxide, into metallic
mercury and cyanogen : HgC2N2=Hg + C2N2.47 When cyanogen is
formed, part of it always polymerises into a dark brown insoluble
substance called paracyanogen, capable of forming cyanogen when
heated to redness.48 Cyanogen is an odorous, colourless, poisonous
gas, easily condensed by cooling into a colourless liquid, insoluble in
water and having a specific gravity of O86. It boils at about — 21°,
and therefore cyanogen may be easily condensed into a liquid by a
strong freezing mixture. At - 35° liquid cyanogen solidifies. This gas
is soluble in water and in alcohol to a considerable extent — namely, 1
volume of water absorbs as much as 4J volumes, and alcohol 23 volumes.
Cyanogen resists the action of heat without decomposing, but, under
the action of the electric spark, the carbon is separated, leaving a

47 For the preparation it is necessary to take completely dry mercuric cyanide, because
when heated in the presence of moisture it gives ammonia, carbonic anhydride, and
hydrocyanic acid. Instead of mercuric cyanide, a mixture of perfectly dry yellow prus-
siate and mercuric chloride may be used, then double decomposition and the formation
of mercuric cyanide take place in the retort. Silver cyanide also disengages cyanogen
on being heated.

48 Paracyanogen is a brown substance having the composition of cyanogen, formed
during the preparation of cyanogen by all methods, and remaining as a residue. Silver
cyanide, on being slightly heated, fuses, and on being further heated evolves a gas ; in
the residue a considerable quantity of paracyanogen is formed. Here it is remarkable
that exactly half the cyanogen becomes gaseous, and the other half is transformed into
paracyanogen. Metallic silver will be found in the residue with the paracyanogen ; it
may be extracted with mercury or nitric acid, which does not act on paracyanogen. If
paracyanogen be heated in a vacuum, it decomposes, forming cyanogen ; but here the
pressure p for a given temperature t exceeds the determined magnitude, so that the
phenomenon presents all the external appearance of a physical transformation into
vapour ; but, nevertheless, it is a complete change in the nature of the substance, but
limited by the pressure of dissociation, as we saw before in the transformation of cyanuric
acid into hydrocyanic, and as ought to be expected from the fundamental principles of
dissociation. Troost and Hautefeuille (1868) found that for paracyanogen

t = 530° 581° 600° (535°

p = 90 143 290 1089 mm.

However, already at 550° part of the cyanogen decomposes into carbon and nitrogen.
The reverse transition of cyanogen into paracyanogen commences at 350°, and at 600°
proceeds rapidly. And if the transition of the first kind is likened to evaporation, then
the reverse transition, or polymerisation, presents a likeness to the transition of vapours
into the solid state.

OF CAKDOX WITH OXYGEN AND NITROGEN 405

volume of nitrogen equal to the volume of the gas taken. As it con-
tains carbon it burns, and the colour of the flame is reddish-violet,
which is clue to the presence of nitrogen, all compounds of which impart
more or less of this reddish-violet hue to the flame. During the com-
bustion of cyanogen, carbonic anhydride and nitrogen are formed.
This also takes place in the eudiometer with oxygen and during the
action of cyanogen 011 many oxides at a red heat.

The relation of cyanogen to the metallic cyanides is seen not only
from the fact that it is formed from mercuric cyanide, but also by its
forming cyanide of sodium or potassium on being heated with sodium
or potassium, the sodium or potassium taking fire in the cyanogen. On
heating a mixture of hydrogen and cyanogen to 500° (Berthelot),49
or under the action of the silent discharge (Boilleau), hydrocyanic acid
is formed, so that the reciprocity of the transitions does not leave any
doubt in the matter that all the nitriles of the organic acids contain
cyanogemA^t as all the organic acids contain carboxyl, and in it the
element^^Harbonic anhydride. Besides the amides,50 the nitriles (or
cyanogen compounds, RON), and nitro-compounds (containing the
radicle of nitric acid, RNO2), there are a great number of other sub-
stances containing, at the same time, carbon and nitrogen, particulars
of which must be sought for in special works on organic chemistry.

4<J Cyanogen (like chlorine) is absorbed by a solution of sodium hydroxide, sodium
cyanide and cyanate being produced : C2N2 + 2NaHO = NaCN -f CNNaO + H2O. However,
the latter salt easily decomposes, just as also part of the cyanogen liberated by heat from
its compounds is subjected to a more complex transformation.

50 If, in general, compounds containing the radicle NH2 are called amides, some of the
(i))//' ties ought to be ranked with them; that is, the hydrocarbons, C,iH2m, in which the
hydrogen is replaced by NFL ; for instance, methylamine, CH3'NH2, aniline, C0H5'NH2,
&c. In general the amines represent ammonia in which part or all of the hydrogen is
replaced by hydrocarbon radicles — such as, for instance, trimethylamine N(CH5)3. They,
like ammonia, combine with acids and form crystalline salts. Related substances are
sometimes met with in nature, and bear the general name of alkaloids ; such are, for
instance, quinine in cinchona bark nicotine in tobacco, &c.

406 PRINCIPLES OF CHEMISTRY

CHAPTER X

SODIUM CHLORIDE — BERTHOLLET's LAWS— HYDROCHLORIC ACID

Ix the preceding chapters we have become acquainted with the most
important properties of the four elements, hydrogen, oxygen, nitrogen,
and carbon. They are sometimes termed the organogens, because they
enter into the composition of organic substances. Their mutual com-
binations may serve as types for all other chemical compounds — that is,
they present the same atomic relations (types, forms, or grades of
combinations) as those in which the other elements also combine
together.

Hydrogen, HH, or, in general, HR.

Water, H2O, „ „ H2R.

Ammonia, H3X, „ ,, H3R.

Marsh gas, H4C, „ ,, H4R.

One, two, three, and four atoms of hydrogen enter into these
molecules for one atom of another element. Xo compounds of one
atom of oxygen with three or four atoms of hydrogen are known ; hence
the atom of oxygen is without certain properties which are found in
the atoms of carbon and nitrogen.

The faculty of an element to form a compound of definite composi-
tion with hydrogen (or an element analogous to it) gives the possibility
of foretelling the composition of many other of its compounds. Thus,
if we know that an element, M, combines with hydrogen, forming HM,
and not forming H2M, H3M, HnMm, then we must conclude, on the
basis of the law of substitution, that this element will give compounds
M2O, M3N, MHO, MH3C, tfcc. Chlorine is an example of this kind.
If we know that another element, R, like oxygen, gives with hydrogen
a molecule, H2R, then we may expect that it will form compounds like
hydrogen peroxide, or the metallic oxides, or carbonic anhydride, or
carbonic oxide, and others. Sulphur is an instance of this kind.
Hence the elements may be classified according to their resemblance
to hydrogen, oxygen, nitrogen, and carbon, and in conformity with
this analogy it is possible to foretell, if not the properties (for example,

SODIUM CHLORIDE-BERTHOLLET'S LAWS 407

the acidity or basicity), at all events, the composition l of their com
pounds. This forms the substance of the conception of the valency or

1 But it is impossible to foretell all the compounds formed by an element from its
atomicity or valency, because the atomicity of the elements is variable, and furthermore
this variability is not identical for different elements. In CO2, COX.,, CH4, and the
multitude of carbon compounds corresponding with them, the C is quadrivalent, but in
CO either the carbon must be taken as bivalent or the atomicity of oxygen be accounted
as variable. And carbon, nevertheless, is an example of an element which preserves its
atomicity to a greater degree than most of the other elements. Nitrogen in NH5, NH2(OH),
N2O3, and even in CNH, must be considered as trivalent, but in NH4C1, NO2(OH), and
in all their corresponding compounds it is necessarily pentavalent. In N2O, if the
atomicity of oxgen = 2, nitrogen has an uneven atomicity (1, 3, 5), whilst in NO it is
bivalent. If sulphur be bivalent, like oxygen, in many of its compounds (for example,
H2S, SC12, KNS, &c.), then it could not be foreseen from this that it would form SO2,
SO3, SC14, SOClo, and a series of similar compounds in which its atomicity must be
acknowledged as greater than 2. Thus in SOo, sulphurous anhydride, there are a multi-
tude of points in common with CO.,, and if carbon be quadrivalent then the S in SO2 is
quadrivalent. Therefore the principle of the atomicity (valency) of the elements cannot
be established as the basis for the study of the elements, although it gives an easy possi-
bility for grasping many analogies. Although a definite atomicity cannot be considered
us a radical property of atoms and elements, it may, however, be made use of with great
advantage in investigating the compounds of such elements as carbon, because carbon in
all its usual compounds, and especially in the saturated compounds and those approach-
ing them, acts as a quadrivalent element. I consider the four following as the chief
obstacles to acknowledging the atomicity of the elements as a primary conception for the
consideration of the properties of the elements : 1. Such univalent elements as H, Cl,
tVc., appear in a free state as molecules Hx>, CL, &c., and are consequently like the uni-
valent radicles CH5, OH, CO2H, &c., which, as might be expected, appear as C2H6,
O2H2, C2O4H.> (ethane, hydrogen peroxide, oxalic acid), whilst on the other hand, potas-
sium and sodium (perhaps also iodine at a high temperature) contain only one atom,
K, Na, in the molecule in a free state. Hence it follows that free affinities may exist.
And then nothing obliges us not to admit free affinities in all unsaturated compounds ; for
example, it may be said that in CoH4 each atom of carbon is bound to the other by one
affinity, two other affinities holding the hydrogen, and that the fourth affinity of each
carbon atom is free. If such instances of free affinities be admitted, then all the possible
advantages to be gained by the application of the doctrine of atomicity (valency) are
lost. 2. There are instances — for example, Na2H — where univalent elements are
gathered together in molecules which are more complex than R2, and form molecules,
R3, R4, &c. ; which may again be either taken as evidence of the existence of free affini-
ties, or else necessitates such primary univalent elements as sodium and hydrogen being
considered as variable in their atomicity. 3. The periodic system of the elements, with
which we shall afterwards become acquainted, shows that there is a law or rule for the
variation of the forms of oxygen and hydrogen compounds ; chlorine is univalent with
respect to hydrogen, and septavalent with respect to oxygen'; sulphur is bivalent to
hydrogen, and sexavalent to oxygen; phosphorus is trivalent to hydrogen and penta-
valent in respect to oxygen — the sum is in every case equal to 8. Only carbon and its
analogues (for example, silicon) are quadrivalent to both hydrogen and oxygen. Hence the
property of the elements to change their atomicity lies in the essence of their nature, and
consequently atomicity cannot be considered as a fundamental property. 4. Crystallo-
hydrates (for instance, NaCl,2H2O, or NaBr,2H2O), double salts (for instance, PtCl4,2KCl,
H2SiF6, &c.), and similar complex compounds (and, according to Chap. L, solutions also)
demonstrate the capacity not only of the elements themselves, but also of their saturated
and limiting compounds, of entering into further combination. Therefore the admission
of a definite limited atomicity of the elements includes in itself an admision of limita-
tion which is not in accordance with the nature of chemical reactions.

408 PRINCIPLES OF CHEMISTRY

atomicity of the elements. Hydrogen is taken as the representative
of the univalent elements, giving compounds, RH, R(OH), R2O, RC1,
R3N, R4C, &c. Oxygen, in that form in which it gives water, is the
representative of the bivalent elements, forming RH2, RO, RC12,
RHC1, R(OH)C1, R(OH)2, R,C, RON, etc. Nitrogen in ammonia is
the representative of the trivalent elements, giving compounds RH3,
R2O3, R(OH)3, RC13, RN, RHC, &c. In carbon is expressed the
properties of the quadrivalent elements, RH4, RO.,, RO(OH)2, R(OH)4,
RHN, RC14, RHCU, &c. We meet with these forms of combination
or aspects of union of atoms in all other elements, some being analogous
to hydrogen, others to oxygen, and others to nitrogen or to carbon.
But besides these quantitative analogies or resemblances, which
are foretold by the law of substitution (Chapter VI.), there exist
among the elements qualitative analogies and relations which are not-
exhausted in the compounds of the elements which have been con-
sidered, but are most distinctly expressed in the formation of bases,
acids, and salts of different types and properties. Therefore, for a
complete study of the nature of the elements and their compounds it
is especially important to become acquainted with the salts as sub-
stances of a peculiar character, corresponding with acids and bases.
Common table salt, or sodium chloride, NaCl, may in every respect be
taken as a type of salts in general, and therefore we will pass to the
consideration of this substance, of hydrochloric acid, and of the base
sodium hydroxide, of the non-metal chlorine, and the metal sodium,
which correspond with it.

Sodium chloride, NaCl, or the familiar table salt, occurs in the
primary formations of the earth's crust,2 from which it is washed away
by the atmospheric waters, and is held in small quantities in all waters
flowing through these formations, and is in this manner collected in
the oceans and seas. The immense mass of salt in the oceans has been
accumulated by this process from the remote ages of the earth's crea-

2 The primary formations are those which do not bear any distinct traces of having
been deposited from water (have not a stratified formation, contain no remains of
animal or vegetable life), occur under the sedimentary formations of the earth, and uiv
everywhere uniform in composition and structure, which is generally distinctly crystal
line. If the origin of the earth were of a molten nature, the first primary formations
are those which formed the first solid crust of the earth. But even with this hypothesis
of the earth's origin, it is necessary to admit that the first aqueous deposits must have
re-formed the crust of the earth, and therefore under the head of the primary formations
must be understood the most ancient of the products of decomposition (mostly by atmo-
spheric, aqueous, and organic agency, &c.), from which all the rocks and substances of
the earth's surface have arisen. In speaking of the origin of one or another substance,
we can only, on the basis of facts, descend to the primary formation, of which granite,
gneiss, and trachyte may be taken as examples.

SODIUM CHLORIDE— BERTHOLLET'S LAWS 409

tion, because the water has evaporated from them while the salt has
remained in solution. The salt of sea water serves as the source not
only for its direct extraction, but also for the formation of other masses
of workable salt, such as rock salt and of saline springs and lakes.

The extraction of salt from sea water is carried on in several ways.
In southern climes, especially on the shores of the Atlantic Ocean, the
Mediterranean and Black Seas, the summer heats are taken advantage
of. A convenient, low-lying sea shore is chosen, and a whole series of
basins, communicating with each other, are constructed along it. The
upper of these basins are filled with sea water by pumping, or else
advantage is taken of high tides. These basins are sometimes separated
from the sea by natural sand-banks (limans) or by artificial means, and
in April the water already begins to evaporate considerably. As the
solution becomes more concentrated, it is run into the succeeding basins,
and the upper ones are supplied with a fresh quantity of sea water, or
else an arrangement is made enabling the salt water to flow by degrees
through the series of basins. It is evident that the bed of the basins
should be, as far as possible, impervious to water, and for this purpose
they are made of beaten clay. The crystals of table salt begin to
separate out when the concentration attains 28 p.c. of salt (which cor-
responds to 28° of Baume's hydrometer). It is raked off, and employed
for all those purposes to which table salt is applicable. In the majority
of cases only the first half of the sodium chloride which can be separated
from the sea water is extracted, because the second half has a bitter
taste, from the presence of magnesium salts which separate out together
with the table salt. But in certain localities — as, for instance, in the
estuary of the Rhone, on the island of Camarga 3 — the evaporation is
carried on to the very end, in order to obtain those magnesium and
potassium salts which separate out at the end of the evaporation of sea
water. Various salts are separated from sea water in its evaporation.
In the water of oceans (Chapter I.) there is held so large an amount of
sodium chloride that on evaporation this salt, notwithstanding its great
solubility in water, soon reaches saturation and separates out. From
100 parts of sea water there separates out, by natural and artificial
evaporation, about one part of tolerably pure table salt at the very
commencement of the operation ; the total amount held in solution

" The extraction of the potassium salts (or so-called summer salts) was carried on
at the Isle of Camarga about 1870, when I had occasion to visit that spot. If I mistake
not, this industry is now no longer carried on, because the deposits of Stassfurt provide
a much cheaper salt, owing to the evaporation and separation of the salt being there
carried on by natural means and only requiring a treatment and refining, which is also
necessary in addition for the ' summer salt ' obtained from sea water.

410 PRINCIPLES OF CHEMISTRY

being about 2^ p.c. The remaining portion of the table salt sepa-
rates out, intermixed with the bitter salts of magnesium which form
the chief admixture with table salt in sea water. These salts of
magnesium, owing to their degree of solubility and the small amount in
which they are present (less than 1 p.c.), only separate out, in the first
crystallisations, in traces with the table salt ; whilst, in the succeeding
crystallisations, they are deposited together with the table salt. Gypsum,
or calcium sulphate, CaSO42H2O, owing to its sparing solubility,
separates together with or even before the table salt. When about
half of the table salt has separated, then, on further evaporation, the
sea water gives a mixture of table salt and magnesium sulphate, and,
on still further evaporation, the chlorides of potassium and magnesium
begin to separate in a state of combination, forming the double salt
KMgCl3,6H2O, which occurs in nature as carnallite. This is a
crystallo-hydrated compound of KC1 and MgCl2.4 After the separa-
tion of this salt from sea water, there remains a mother liquor, con-
taining a large amount of magnesium chloride in admixture with
various other salts.5 The extraction of sea salt is usually carried on
for the purpose of procuring table salt, and therefore directly it begins
to separate mixed with a considerable proportion 6 of magnesium salts
(when it acquires a bitter taste) the remaining liquor is run back into
the sea.

The same process which is employed for artificially obtaining salt
in a crystalline form from sea water has been repeatedly accomplished
during the geological evolution of the earth on a gigantic scale ; up-
heavals of the earth have cut off portions of the sea from the remainder
(as the Dead Sea was formerly a part of the Mediterranean, and the Sea

4 The double salt, KCl,MgCl2, is only formed from solutions containing an excess
of magnesium chloride, because water decomposes this double salt, extracting the more
soluble magnesium chloride from it.

5 Owing to the fundamental property of salts of interchanging their metals, it
cannot be said that sea water contains this or that salt, but only that it contains certain
amounts of certain metals M (univalent like Na and K, and bivalent like Mg and Ca), and
haloids X (univalent like Cl, Br, and bivalent like SO4, CO3), which are disposed in
every possible kind of grouping ; for instance, K as KC1, KBr, K2SO4, Mg as MgCL,
MgBr2, MgSO4, and so on for all the other metals. In evaporation different salts separate
out consecutively only because they reach saturation. A proof of this may be seen in
the fact that a solution of a mixture of sodium chloride and magnesium sulphate (both
of which salts are obtained from sea water, as was mentioned above), when evaporated,
separates out crystals of these salts, but when refrigerated (if the solution be sufficiently
saturated) the salt Na2SO4,10H2O is first deposited because it is the first to arrive at
saturation at low temperatures. Consequently, this solution contains MgCl2 and Na-^SO.,,
besides MgSO4 and NaCl. So it is with sea water.

e The salt extracted from water is piled up in heaps and left under the action of
rain water, which purifies the salt, owing to its becoming saturated with table-salt and
then no longer dissolving it, but washing out the impurities.

SODIUM CHLORIDE— BERTHOLLETS LAWS 411

of Aral of the Caspian), and their water has evaporated and formed
(if the mass of the inflowing fresh water were less than that of the
mass evaporated) deposits of rock salt. It is always accompanied
by gypsum, because the latter is separated from sea water before the
sodium chloride. For this reason rock salt may always be looked for
in those localities where there are deposits of gypsum. But inasmuch
as the gypsum may remain on the spot where it has been deposited (as
it is a sparingly soluble salt), whilst the rock salt (as one which is very
soluble) may be washed away by rain or fresh running water, it may
sometimes happen that although gypsum is found still there may be
no salt ; but, on the other hand, where there is rock salt there will
always be gypsum. As the geological changes of the earth's surface
are still proceeding to the present clay, so in the midst of the dry land
salt lakes are met with, which are sometimes scattered over vast dis-
tricts formerly covered by seas now dried up. Such is the origin of
many of the salt lakes about the lower portions of the Volga and in the
Kirghiz steppes, where at a geological epoch preceding the present the
Aralo- Caspian Sea extended. Such are the Baskunchaksky (in the
Government of Astrakhan, 112 square kilometres superficial area), the
Eltousky (140 versts from the left bank of the Volga, and 200 square
kilometres in superficial area), and upwards of 700 other salt lakes
lying about the lower portions of the Volga. In those in which the
inflow of fresh water is less than that yearly evaporated, and in which
the concentration of the solution has reached saturation, the self-
deposited salt is found already deposited on their beds, or is being yearly
deposited during the summer months. Certain limans, or sea-side lakes,
of the Azoff Sea are essentially of the same character — as, for instance,
those in the neighbourhood of Henichesk and Berdiansk. The saline
soils of certain Central Asian steppes, which suffer from a want of
atmospheric fresh water, are of the same origin. Their salt originally
proceeded from the salt of seas which previously covered these localities,
and has not yet been washed away by fresh water. The main result of
the above described process of nature is the formation of masses of rock
salt, which are, however, being gradually washed away by the subsoil
waters flowing in their neighbourhood, and afterwards rising to the
surface in certain places as saline springs, which indicate the presence
of masses of deposited rock salt in the depths of the earth. If the sub-
soil water flows along a strata of salt for a sufficient length of time it
becomes saturated ; but in flowing in its further course along an im-
pervious strata (clay) it becomes diluted by the fresh water leaking
through the upper soil, and therefore the greater the distance of source
of a saline spring from the deposit of rock salt, the poorer will it be in

412 PRINCIPLES OF CHEMISTRY

salt. A perfectly-saturated brine, however, may be procured from the
depths of the earth by means of bore holes. The deposits of rock salt
themselves, which are sometimes hidden at great depths below the
earth's strata, may be arrived at by the guidance of bore-holes and the
arrangement of the strata of the district. Deposits of rock salt, about
35 metres thick and 20 metres below the surface, were discovered in this
manner in the neighbourhood of Briaustcheffky and Dekonoffky, in
the Bakhmut district of the Government of Ekaterinoslav. Large
quantities of most excellent rock salt are now (since 1880) won from
these deposits, whose presence was indicated by the neighbouring salt
springs (near Slaviansk and Bakhmut), and bore-holes which had been
sunk at these localities for procuring strong (saturated) brines. But
the Stassfurt deposits of rock salt, near Magdeburg in Germany, are
distinguished as being the first discovered in this manner, and for their
many remarkable peculiarities.7 The considerable distribution of saline
springs in this and the neighbouring districts suggested the presence
of deposits of rock salt in the near vicinity. Deep bore-holes sunk
'in this locality in fact did give a richer brine — even quite saturated
with salt. On sinking to a still greater depth, the deposits of salt
were themselves in the end arrived at. But the first salt which was
met with was a bitter salt unfit for consumption, and which was, there-
fore, called refuse salt (abraumsah). On sinking still deeper through
fresh bands of earth, vast beds of real rock salt were struck. In this
instance, the presence of these upper strata containing salts of potas-
sium, magnesium, and sodium, is an excellent proof of the formation of
rock salt from sea water. It is a self-evident fact that not only a case
of evaporation to the end — for instance, to the separation of carnallite —
but also the conservation of such soluble salts as those which separate
out from sea water after the sodium chloride, forms a very excep-
tional phenomenon, which is not repeated in all deposits of rock salt.
The Stassfurt deposits, therefore, are of particular interest, not only
from a scientific point of view, but also because they form a rich
source of potassium salts which have many practical uses. In Western

7 When the German savants pointed out the exact locality of the Stassfurt salt-
beds and their depth below the surface on the basis of information collected from
various quarters respecting bore-holes and the direction of the strata, and when the
borings, conducted by the Government, struck out a salt-bed which was bitter and unfit
for use, then there was a great outcry against science, and the doubtful result even
caused the cessation of the further work of deepening the shafts. It required a great
effort to persuade the Government to continue the work. Now, when the pure salt
encountered below forms one of the important riches of Germany, and when those
'refuse salts' have proved to be most precious (as a source of potassium and magnesium),
we should see in the utilisation of the Stassfurt deposits one of the conquests of science
for the common welfare.

SODIUM CHLORIDE— BERTHOLLET'S LAWS 413

Europe, deposits of rock salt have long been known at Wieliczka, near
•Cracow, and at Cardona in Spain. In Russia the following deposits
are known : (a) the vast masses of rock salt (3 square kilometres area
and up to 140 metres thick) lying directly on the surface of the earth
at Iletzky Zastchit, on the left bank of the river Ural, in the Govern-
ment of Orenburg ; (b) the Chingaksky deposit, 90 versts from the
river Volga, in the Enotaeft'sky district of the Government of Astra-
khan ; (c) the Kulepinsky (and other) deposits (whose thickness attains
150 metres), on the Araks, in the Government of Erivan in the
Caucasus ; (d) the Katchiezmansky deposit in the province of Kars ;
and (e) the Krasnovodsky deposit in the Trans-Caspian province.

A saturated brine, formed by the continued contact of subsoil
water with rock salt, is extracted by means of bore-holes, as, for
instance, in the Governments of Perm, Kharkoff, and Ekaterinoslav.
Sometimes, as at Wergtesgaden in Austria (at Salzkammerhutte),
spring water is run on underground beds of rock salt containing much
clay.

If a saline spring or the salt water pumped from bore-holes con-
tains but little salt, then the first concentration of the natural solution
is not carried on by the costly consumption of fuel, but by the cheaper
method of evaporation by means of the wind. For this purpose the
so-called graduators are constructed : they consist of long and lofty
sheds, which are sometimes several versts long, and generally extend
in a direction at right angles to that of the usual course of the wind in
the district. These sheds are open at the sides, and are filled with
brushwood as shown in fig. 6 4. Troughs, A B, c D, into which the salt
water is pumped, run along the top. On flowing from these troughs,
through the openings, a, the water spreads over the brushwood and
distributes itself in a thin layer over it, so that it presents a very large
surface for evaporation, in consequence of which it rapidly becomes con-
centrated in warm or windy weather. After trickling over the brush-
wood, the solution collects in a reservoir under the graduator, from
whence it is usually pumped up by the pumps P P', and again run a
second and third time through the graduator, until the solution reaches
a degree of concentration at which it becomes profitable to extract the
salt by direct heating. Generally the evaporation in the graduator
is not carried beyond a concentration of 15 to 20 parts of salt in
100 parts of solution. Strong natural solutions of salt, and also the
graduated solutions, are evaporated in large shallow metallic vessels,
which are either heated by the direct action of the flame from below
or from above. These vessels are made of boiler plate, and are called
salt pans. Various means are employed for accelerating the evapora-

414

PRINCIPLES OF CHEMISTRY

tion and for economising fuel, which are mainly based on an artificial
draught to carry off the steam as it is formed, and on subjecting the
saline solution to a preliminary heating by the waste heat of the steam
and furnace gases. Furthermore, the first portions of the salt which

FIG. 64. — Graduator for the evaporation of the water of saline springs.

crystallise out in the salt pans always contain gypsum, owing to the
water of saline springs always containing this substance. It is only
the portions of the salt which separate later that are distinguished by
their great purity. The salt is ladled out as it is deposited, and left to
drain on inclined tables and then dried, and in this manner the so-
called bay salt is obtained. Since it has become possible to discover
the saline deposits themselves, the extraction of table salt from the
water of saline springs by evaporation, which was before in general use,
has begun to be rejected, and is only able to hold its ground in cases
where fuel is cheap.

In order to understand the full importance of the extraction of
salt, it will be enough to mention that on the average 20 Ibs. of table

CHLORIDE— BERTHOLLET'S LAWS 415

salt are consumed per head of population, directly in food or for cattle.
In those countries where common salt is employed in technical pro-
cesses, and especially in England, almost an equal quantity is consumed
in the production of substances containing chlorine and sodium, and
especially in the manufacture of washing soda, &c., and of chlorine
compounds (bleaching powder and hydrochloric acid).

Although certain lumps of rock salt and crystals of bay salt some-
times consist of almost pure sodium chloride, still the ordinary com-
mercial salt contains various impurities, the most common of which are
magnesium salts. If the salt be pure, its solution gives no precipitate
with sodium carbonate, Na.2CO3, showing the absence of magnesium
salts, because magnesium carbonate, MgCO3, is insoluble in water.
Rock salt, which is ground for use, further generally contains a large
admixture of clay and other insoluble impurities.8 For common use
the bulk of the salt obtained is quite suitable without further purifica-
tion ; but some salts are purified by solution and crystallisation of
the solution after standing, in which case the evaporation is not carried
on to the end, and the impurities remain in the mother liquor or
in the sediment. When perfectly pure salt is required for chemical
purposes it is best to proceed as follows : a saturated solution of table
salt is prepared, and hydrochloric acid gas is passed through it ; this
precipitates the sodium chloride (which is not soluble in a strong solu-
tion of hydrochloric acid), while the impurities remain in solution. By
repeating the operation and fusing the salt (when adhering hydro-
chloric acid is volatilised) a pure salt is obtained, which is again
crystallised from its solution by evaporation.9

Pure sodium chloride, in the form of well-formed crystals (slowly
formed at the bottom of the liquid) or in compact masses (in which form
rock salt is sometimes met with), is a colourless and transparent sub-
stance resembling, but more brittle and less hard than, glass. 10 Common

8 The fracture of rock salt generally shows the presence of interlayers of impurities,
which are sometimes very small in weight, but visible owing to their refraction. In the
excellently laid out salt mines of Briansk, belonging to Mr. Letunoffsky, I counted (1888),
if my memory does not deceive me, on an average ten interlayers per metre of thickness,
between which the salt was in general very pure, and in places quite transparent. If this
be the case, then there would be 350 interlayers for the whole thickness (about 85 metres)
of the bed. They probably correspond with the yearly deposition of the salt. In this
case the deposition would have extended over more than 300 years. This should be
observable at the present day in lakes where the salt is saturated and in course of
deposition.

9 I have personally convinced myself that by this method not only the sulphates,
but also the potassium salts are entirely removed.

10 According to the determinations of Klodt, the Briansk rock salt withstands a
pressure of 840 kilograms per square centimetre, whilst glass withstands 1700 kilos.

416 PRINCIPLES OF CHEMISTRY

salt always crystallises in the cubic system, most frequently in cubes,
and more rarely in octahedra. Large transparent cubes of common salt,
having edges up to 10 centimetres long, are sometimes found in masses
of rock salt.11 By rapid evaporation common salt is deposited from its
solutions in the form of very small crystals, but with slow evaporation
the crystals attain a considerable size. When evaporated in open
spacer the salt often separates out on the surface12 as cubes, which
grow on to each other in the form of pyramidal square funnels. In
still weather, these clusters are able to support themselves on the sur-
face of the water for a long time, and sometimes go on increasing to a
considerable degree, but they sink directly the water penetrates inside
them. Salt fuses into a colourless liquid (sp. gr. 1*602, according to
Quincke) at about 774° (Carnelley), and if pure it solidifies into a non-
crystalline mass, and if impure into an opaque mass whose surface is
not smooth. In fusing, sodium chloride commences to volatilise (its

In this respect salt is twice as secure as bricks, and therefore immense masses may be
"extracted from underground workings with perfect safety, without having recourse
to brickwork supports, but only taking advantage of the properties of the salt itself.

n To obtain well-formed crystals, a saturated solution is mixed with ferric chloride,
and several small crystals of sodium chloride are placed at the bottom, and the solution
is allowed to slowly evaporate in a closed vessel. Octahedral crystals are obtained in the
presence of borax, urea, &c., in the solution. Very fine crystals are formed in a mass of
gelatinous silica.

12 If a solution of sodium chloride be slowly heated from above, where the evapora-
tion is accomplished, then the upper layer will become saturated before the lower and
cooler layers, and therefore crystallisation will then begin on the surface, and the crystals
first formed will be held up, having also dried from above, on the surface until they be-
come quite soaked. Being heavier than the solution the crystals are partially immersed
under it, and the following crystallisation, also proceeding on the surface, will only form
crystals along the side of the original crystals. The funnels are formed in this manner.
It will be borne on the surface like a boat (if there be no waves), because it will grow
more from the upper edges. We can thus understand this, at first sight, strange funnel
form of crystallisation of salt. In explanation why the crystallisation under the above
conditions begins at the surface and not at the lower layers, it must be mentioned that
the specific gravity of a crystal of sodium chloride = 2'16, and that of a solution saturated
at 25° contains 26'7 p.c. of salt and has a specific gravity at 25'4° of 1-2004 ; at 15° a
solution contains 26'5 p.c. of salt and has a sp. gr. 1'203 at 15'4°. Hence a solution satu-
rated at a higher temperature is specifically lighter, notwithstanding the greater amount
of salt it contains. Surface crystallisation cannot take place with many substances,
because their solubility increases more rapidly with the temperature than their specific
gravity decreases. In this case the saturated solution will always be in the lower layers,
where also the crystallisation will take place. Besides which, it may be added that, us a
consequence of the properties of water and solutions, when they are heated from above
(for instance, by the sun's rays) the warmer layers being the lightest remain above, whilst
when heated from below they rise to the top. For this reason the water at great depths
below the surface is always cold, as has long been known. These circumstances, as well
&s those observed by Soret (Chapter I. Note 19), explain the great differences of density,
temperature, and in the amount of salts held in the oceans at different latitudes (in polar
and tropical climes) and at various depths.

SODIUM CHLORIDE— BEBTHOLLETJ8 LA AYS 417

weight decreases), and at a white heat it volatilises with great ease and
entirely; but at the ordinary temperature it may, like all ordinary
salts, be considered as non-volatile, although as yet no exact experi-
ments have been made in this respect.

A saturated 13 solution of table salt (containing 26*4 per cent.) has
at the ordinary temperature a specific gravity of about l"2. The
specific gravity of the crystals is 2' 16. The salt which separates out
at the ordinary and higher temperatures contains no water of crystal-
lisation;14 but if the crystals are formed at a low temperature,
especially from a saturated solution cooled to —12°, then they present
a prismatic form, and contain two equivalents of water, NaCl, 2H20.
At the ordinary temperature, these crystals split up into sodium
chloride and its solution.15 Unsaturated solutions of table salt when
cooled below 0° give16 crystals of ice, but when the solution has a

15 By combining the results of Poggiale, Miiller, and Karsten (they are evidently
more accurate than those of Gray-Lussac and others), I found that a saturated solution at
t°, from 0° to 108°, contains 35'7 + 0'024!f + 0'0002j!2 grams of salt per 100 grams of water.
This formula gives a solubility at 0° = 35'7 grams ( = 26'3 p.c.), whilst according to Kar-
sten it is 36'09, Poggiale 35' 5, and Miiller 35'6 grams. The somewhat large diversity in
the data respecting so common a substance as sodium chloride shows the necessity of
fresh and most exact determinations.

14 Perfectly pure fused salt is not hygroscopic, according to Karsten, whilst the
crystallised salt, even when quite pure, attracts as much as 0'6 p.c. of water from moist
air, according to Stas. In the Briansk mines, where the temperature throughout the
whole year is about +10°, it may be observed, as Baron Klodt informed me, that in the
-;ummer during damp weather the walls become moist, while in winter they are dry.
This is in accordance with the fact that the vapour tension of solutions has a definite
magnitude, which is less than that of water.

If the salt contain impurities — such as magnesium sulphate, &c. — it is more hygro-
scopic. If it contain any magnesium chloride it partially effloresces in a damp atmosphere.
The crystallised and not perfectly pure salt decrepitates when heated, owing to its con-
taining water. The pure salt, and also the transparent rock salt, or that which has been
once fused, does not decrepitate. Fused sodium chloride gives a feeble alkaline reaction
with litmus, as has been shown by many observers, which is due to the formation of
sodium oxide (probably by the action of the oxygen of the atmosphere). According to
A. Stcherbakoff very sensitive litmus (washed in alcohol and neutralised with oxalic acid)
shows an alkaline reaction with even the crystallised salt.

It may be observed that rock salt sometimes contains cavities filled with a colourless
liquid. Certain kinds of rock salt emit an odour like that of hydrocarbons. These
phenomena have as yet received very little attention.

15 By cooling a solution of table salt saturated at the ordinary temperature to —15°
I first obtained well-formed tabular (six-sided) crystals, which on arriving at the ordinary
temperature disintegrated (with the separation of anhydrous sodium chloride), and then
prismatic needles up to 20 mm. long were formed from the same solution. I have not
yet investigated what is the reason of the difference in crystalline form. It is known
(Mitscherlich) that NaI,2!L>O also crystallises in plates or prisms. Sodium bromide also
crystallises with 2HoO at the ordinary temperature.

15 Notwithstanding the great simplicity (p. 91) of the observations on the formation
of ice from solution, still even for sodium chloride they cannot yet be considered as suffi-
ciently harmonious. According to Blagden and Raoult the temperature of the formation
VOL. I. BE

418 PRINCIPLES OF CHEMISTRY

composition NaCl,10H«,O it solidifies completely at a temperature of
— 23°. A solution of table salt saturated at its boiling point boils at
about 109°, and contains about 42 parts of salt per 100 parts of water.
Of all its physical properties the specific gravity of solutions of
sodium chloride is the one which has been the most fully investigated.
A comparison of the results of Kremers, Gerlach, Schmidt, Marignac,
Thomsen, Nicol, and Bender proves17 that the specific gravity (in
vacuum, taking water at 4° as 10000) at 15° in relation to p, or the
percentage amount of the salt in solution, is expressed by the equa-
tion S15 = 9991-6 + 71-lTp + 0-2140/A For instance, for a solution
200H.2O + NaCl, in which case _p=l-6, S15 = 10106. It is seen
from the curve that the addition of water to the solution produces a

contraction,18 and that the addition of salt f or the differential -,— J

causes, at 15°, a change of specific gravity which is expressed by the
straight line 71-17 + 0428^. At 0° and 100° (when the specific

of ice from a solution containing c grams of salt per 100 grams of water = — 0'6c to c = 10r
according to Rosetti = -0'649c to c = 8'7, according to De Coppet (to c = 10)= -0'762<r
+ 0'0084c2, and according to Guthrie a much lower figure. By taking Rosetti's figure and
applying the rule given on p. 91 (Chapter I. Note 49) we obtain —

i = 0-649 x ---'-= 2-05.
18-5

The data for strong solutions are not less contradictory. Thus with 20 p.c. of salt, ice
is formed at — 14'4° according to Karsten, —17° according to Guthrie, — 17'6° according
to De Coppet. Riidorff says that for strong solutions the temperature of the formation
of ice descends in proportion to the contents of the compound, NaCl,2H2O (per 100 grams
of water) by 0'342C per 1 gram of salt, and De Coppet shows that there is no proportion-
ality, in a strict sense, for either a percentage of NaCl or of NaCl,2H2O. The data respect-
ing the vapour tension and boiling point of solutions of sodium chloride are as untrust-
worthy as the preceding.

17 A collection of observations on the specific gravity of solutions of sodium chloride
and all other aqueous solutions which have been more or less investigated up to the
present time is given in my work cited in Chapter I. Note 50.

Solutions of common salt have also been frequently investigated as regards rate of
diffusion (p. 62j, but as yet there are no complete data in this respect. It may be men-
tioned that Graham and De Vries demonstrated that diffusion in gelatinous masses (for
instance, gelatin jelly or gelatinous silica) proceeds in the same manner as in water, which
may probably lead to a convenient and accurate method for the investigation of the
phenomena of diffusion. N. Umoff (Odessa, 1888) investigated the diffusion of common
salt by means of glass globules of definite density. Having poured water into a cylinder
over a layer of a solution of sodium chloride, he observed during a period of several
months the position (height) of the globules, which floated up higher and higher as the
salt permeated upwards. Umoff found that at a constant temperature the distances of
the globules (that is, the length of a column limited by layers of definite concentration)'
remain constant ; that at a given moment of time the concentration, q, of different layers
situated at a depth z is expressed by the equation B — K2 = log. (A— q), where A, B, and
K are constants ; that at a given moment the velocity of the different layers is propor-
tional to their depth, &c. The information respecting diffusion is considerably extended
by thede researches, but still this subject, from its importance for the theory of solutions
and of liquids in general, yet awaits a perfectly-detailed investigation.

18 If So be the specific gravity of water, and Sthe specific gravity of a solution containing

SODIUM CHLORIDE— BERTHOLLET'S LAWS » 419

gravity of water is 9998'7 and 9585) this augmentation is expressed if
7.V 1 and 65-7 be substituted for 71 '17, and 0-31 and O72 for 0'428.
In this manner the specific gravity19 at

0°

15°

30°

100°

= 5

10372

10353

10307

9922

10

10768

10728

10669

10278

15

11164

11107

11043

10652

20

11568

11501

11429

11043

when water at 0°= 10000.20

p p.c. of salt, then by mixing, for instance, equal weights of water and the solution, we shall
obtain a solution containing %p of the salt, and if it be formed without contraction, then its

211

specific gravity x will be determined by the equation - = — . -\ — , because the volume is

x S(> S

equal to the weight divided by the density. In reality, the specific gravity always comes out
greater than that calculated on the supposition of an absence of contraction, as may be
shown by substituting S by its parabolic expression, S = S0 + Ap + Bp2, and x by S0 + A^p +
B\p^. By this means it may be easily shown that the contraction, c, in the formation
of 100 grams of solution, is not in so simple a relation to the composition of the solution
as G. T. Gerlach (1888) takes it, on the supposition that c = Ap (100— j?), where A is a con-
stant for all solutions of a given substance. The magnitude of c is evidently determined
by the equation p,'B + (100 —p)/SH = 100/S + c, where B is the specific gravity of the sub-
stance dissolved, on the supposition of its being in a liquid state. If for sodium chloride
at 15° with ^ = 10 and ^ = 20, the mean observed specific gravities be taken as 10726 and
11501, then (as S0 = 9991'6) ^4 = 2851 x 10-'°, and 5 = 17476, and therefore for p = 5, the
calculated specific gravity will be 10377, and the observed is 10353, with a probable error
of not more than ±2 : hence the difference greatly exceeds the possible error. A similar un-
adaptability of the above-mentioned supposition is evinced in the investigation of all other
solutions. The hypothesis under consideration resembles in this respect the hypothesis of
Michel and Crafts or Grosjean, which are examined in my work on the specific gravities of
solutions. In a first rough approximation solutions may be regarded as mechanical aggre-
gates, and then a general law of their formation may be looked for, but a detailed study
of the subject necessitates the search for chemical reactions in them, and such a repre-
sentation of the nature of solutions leads to the conclusions enunciated in the first chapter
and more fully developed in my above-mentioned work. This naturally does not exclude
the desire to find laws to which solutions may be subjected, but under the inevitable
condition of the consideration of their chemical composition. Such are, for example, the
deductions of Van't Hoff, who does not, however, touch on the specific gravity of solu-
tions. With respect to the sp. gr. of weak solutions of metallic chlorides, it may, for
instance, be supposed that, having a composition RC1« + 200OHo, they would all have a
specific gravity at 15°/4C, approaching to 9951 + 2'585-M, where M is the molecular weight
of the metallic chloride dissolved. For instance, for SrCl.2, M=158, and the formula gives
S = 10861, and experiment 10364 ; for LiCl, M=42'5, and 5 = 10061, and experiment 10060.
But similar rules, without the existence of a complete theory of solutions, can only serve
as material for the construction of a theory, and too great an importance should not be
attached to them.

19 Generally the specific gravity is observed by weighing in air, dividing the weight
in grams by the volume in cubic centimetres, found from the weight of water displaced,
divided by its density at the temperature under which the experiment is carried on. If
we call this specific gravity St, then as a cubic centimetre of air under the usual condi-
tions weighs about 0'0012 gram, the sp. gr. in a vacuum 8=81^ — 0'0012 (S^-l), if the
density of water = 1.

20 If the sp. gr. S2 be found directly by dividing the weight of a solution by the

E E 2

420 PRINCIPLES OF CHEMISTRY

It should be remarked that Baume's hydrometer is graduated by
taking a 10 per cent, solution of sodium chloride as 10° on the scale,
and therefore it gives approximately the percentage amount of the salt
in a solution. Table salt is somewhat soluble in alcohol,21 but it is
insoluble in ether and in oils.

Table salt gives very few compounds 22 (double salts), and these are
very readily decomposed ; it is also decomposed with great difficulty
and its dissociation is unknown.23 But it is easily decomposed, both
when fused and in solution, by the action of a galvanic current. If the
dry salt be fused in a crucible and an electric current be passed through
it by immersing carbon or platinum electrodes in it (the positive elec-
trode is made of carbon and the negative of platinum or mercury), it is
decomposed into two substances : a malodorous gas called chlorine appears
at the positive pole and metallic sodium at the negative pole, which
shows that table salt consists of these two elements. Both of them act
on water at the moment of their evolution ; the sodium, as we already
know, evolves hydrogen from water and forms caustic soda, and the
chlorine evolves oxygen from water and forms hydrochloric acid, and
therefore on passing a current through a solution of table salt metallic
sodium will not be formed — but oxygen, chlorine, and hydrochloric acid
will appear at the positive pole, and hydrogen and caustic soda at the
negative pole. The presence of hydrochloric acid is easily recognised
by its acid properties, and the presence of caustic soda by its alkaline

weight of water at the same temperature and in the same volume, then the true sp. gr.
S referred to water at 4° is found by multiplying S2 by the sp. gr. of water at the tem-
perature of observation. All the necessary corrections for the specific gravity of liquids
are considered in my two works, On the Compounds of Alcohol with Water, 1865, and
The Investigation of Aqueous Solutions by their Specific Gravity, 1887.

It may not be superfluous to remark that the data respecting the sp. gr. for solutions
of sodium chloride near saturation do not sufficiently agree, and there is reason for
thinking that in strong solutions containing more sodium chloride than in NaCl,10H.,O
(^j = 24'58) a different curve should be adopted.

21 According to Schiff 100 grams of alcohol, containing p p.c. by weight of C2H6O,
dissolves at 15°—

_p = 10 20 40 60 80

25 22'6 13-2 5'9 1'2 grams NaCl.

22 Amongst the double salts formed by sodium chloride that obtained by Ditte (1870)
by the evaporation of the solution remaining after heating sodium iodate with hydro-
chloric acid until chlorine ceases to be liberated, is a remarkable one. Its composition is
NaIO3,NaCl,4H2O. Rammelsberg obtained a similar (perhaps the same) salt in well-
formed crystals by the direct reaction of both salts.

23 But it already gives sodium in the flame of a Bunsen's burner (see Spectrum
Analysis), doubtless under the reducing action of the elements carbon and hydrogen. In
the presence of an excess of hydrochloric acid in the flame (when the sodium would give
sodium chloride), there is no sodium formad in the flame and the salt does not communi-
cate its usual coloration.

SODIUM CHLORIDE- BERTHOLLET'S LAWS 421

reaction. Thus table salt, like other salts, is decomposed by the action
of an electric current into a metal and a haloid, and presents a compo-
sition of great simplicity in comparison with the composition of many
salts containing oxygen, which fact supports the hydrogen theory of
acids discussed in Chapter III. Naturally, like all other salts, it may
be formed from the corresponding base and acid with the separation
of water. In fact if we mix caustic soda (alkali) with hydrochloric
acid (acid) then table salt is formed, NaHO + HCl=NaCl + H2O.

AYith respect to the double decompositions of sodium chloride it
should be observed that they are most varied, and serve as the means
for obtaining nearly all the other compounds of sodium and chlorine.

The double decompositions of sodium chloride, as an example of the
double decompositions of salts, are almost exclusively based on the pos-
sibility of the metal sodium being exchanged for hydrogen and other
metals. But neither hydrogen nor any other metal is able to directly
displace the sodium from table salt. This would result in the separation
of metallic sodium, which itself displaces hydrogen and the majority of
other metals from their compounds, and is not, as far as is known, ever
separated by them. The replacement, then, of the sodium in sodium
chloride by hydrogen and different other metals is accomplished by the
transference of the sodium into some other sodium compound. If
hydrogen or another metal, M, were combined with an element X, then
the double decomposition NaCl + MX=NaX + MCl takes place. Such
double decompositions proceed under particular conditions, sometimes
completely and sometimes only partially, as we shall endeavour to explain.
In order to acquaint ourselves with the double decompositions of sodium
chloride, we will follow the methods actually employed in practice to
procure compounds of sodium and chlorine from table salt. For this
purpose we will first describe the treatment of sodium chloride by sul-
phuric acid for the preparation of hydrochloric acid and sodium sulphate.
We will then describe the substances obtained from hydrochloric acid
and sulphate of sodium. Chlorine itself, and nearly all the other com-
pounds of this element, may be procured from hydrochloric acid, whilst
sodium carbonate, caustic soda, metallic sodium itself and all its com-
pounds may be obtained from sodium sulphate.

Even in the laboratory of animal organisms table salt is subjected
to similar changes, furnishing the sodium, alkali, and hydrochloric acid,
which take part in the processes of animal life.

Its necessity as a constituent in the food of both human beings and
animals becomes evident when we consider that both hydrochloric acid
and salts of sodium are found in the substances which are separated out
from the blood into the stomach and intestines. So, for example,

422 PRINCIPLES OF CHEMISTRY

sodium salts are found in the blood and in the bile elaborated in the
liver, and acting on the food in the alimentary canal, whilst hydro-
chloric acid is found in the acid juices of the stomach. Chlorides of the
metals are always found in considerable quantities in the urine, and if
they are excreted they must be replenished in the organism ; and for
the replenishment of tne loss substances containing chlorine compounds
must be taken in food. Not only do animals consume those amounts
of sodium chlorine which are found in drinking water or in plants or
other animals, but experience has shown that many wild animals travel
long distances in search of salt springs, and that domestic animals,
which in their natural condition do not require table salt, willingly
take it, and that the functions of their organisms become much more
regular from doing so.

The action of sulphuric acid on sodium chloride, — If sulphuric acid
be poured over table salt, then even at the ordinary temperature, as
Glauber observed, an odorous gas, hydrochloric acid, is evolved. The
reaction which takes place consists in the sodium of the table salt and
the hydrogen of the sulphuric acid changing places.

NaCl + H2S04 HC1 + NaHS04

Sodium chloride. Sulphuric acid. Hydrochloric acid. Acid sodium sulphate.

At the ordinary temperature this reaction is not complete, but soon
ceases. If the mixture be heated, the decomposition proceeds until, if
there be sufficient table salt present, all the sulphuric acid taken is
converted into acid sodium sulphate. If there be an excess of acid it
will remain unaltered. If 2 molecules of sodium chloride (117 parts)
be taken per molecule of sulphuric acid (98 parts), then on heating the
mixture to a moderate temperature only one-half (58 '5) of the table
salt will suffer change. Complete decomposition, after which neither
hydrogen nor chlorine is left in the residual salt, proceeds (when 117
parts of table salt are taken per 98 parts of sulphuric acid) at a red heat
only. Then —

2NaCl + H2SO4 = 2HC1 + Na2SO4

Table salt. Sulphuric acid. Hydrochloric acid. Sodium sulphate.

This double decomposition is the result of the action of the acid
salt, NaHSO4, first formed on sodium chloride, because the acid salt,
as it contains hydrogen, itself acts like an acid, XaCl + JSTaHSO4 =
HC1 + Na2S04. By adding this equation to the first we obtain the
second, which expresses the ultimate reaction. Then all the hydrogen
of the sulphuric acid and chlorine of the salt are evolved as gaseous

SODIUM CHLORIDE— BERTHOLLKT s LAWS 423

hydrochloric acid, and the residue will be entirely free from them. Hence
in the above reaction, non- volatile or sparingly- volatile table salt and
sparingly-volatile sulphuric acid are taken, and as the result of their re-
action, after the hydrogen and sodium have exchanged places, there is
obtained non-volatile sodium sulphate and gaseous hydrochloric acid.
The fact of the latter being a gaseous substance forms the main reason
for the reaction proceeding to the very end. The mechanism of this
kind of double decomposition, and the cause of the course of the reac-
tion, are exactly the same as those we saw in the decomposition of
nitre (Chapter VI.) by the action of sulphuric acid. The sulphuric acid
in each displaces the other, volatile, acid.

Not only in these two, but in every instance, if a volatile acid can
be formed through the substitution for a metal of the hydrogen of sul-
phuric acid, then this volatile acid will be formed. From this it may be
concluded that the volatility of the acid should be considered as the
cause of the progress of the reaction ; and, indeed, if the acid be soluble
but not volatile, or if the reaction takes place in an enclosed space
where the resulting acid cannot volatilise, or at the ordinary tempera-
ture when it does not pass into an elastic state of vapour — then the
decomposition does not proceed to the end, but only up to a certain
limit. In this respect the explanations given at the beginning of this
century by the French chemist Berthollet in his work ' Essai de Statique
Chimique,' are very important. The doctrine of Berthollet starts from
the supposition that the chemical reaction of substances is accomplished
in consequence not only of the measure of affinity between the different
parts, but also under the influence of the relative masses of the reacting
substances and of those physical conditions under which the reaction
takes place. Two substances containing the elements MX and NY,
being brought into mutual contact, form by double decomposition the
compounds MY and NX, but the formation of these two new compounds
will not proceed to the end unless one of the resulting substances is
removed from the sphere of action. But it can only be removed if it
possesses different physical properties from those of the other substances
which are present together with it. Either it will be a gas while the
others are liquid or solid, or it will be an insoluble solid while the others
are liquid or soluble. The relative amounts of the resultant substance
depend, if nothing be separated out from their mutual contact, only on
the relative quantities of the substances MX and NY, and upon the
measure of attraction existing between the elements M, N, X, and Y ;
but however great their mass may be, and however considerable the
attraction, still in any case, if nothing be separated out from the sphere
of action, the decomposition will cease, a state of equilibrium will be

424 PRINCIPLES OF CHEMISTRY

established, and instead of two there will remain four substances in the
mass : namely, a portion of the original bodies MX and NY, and a
certain quantity of the newly-formed substances MY and NX, if it be
admitted that neither MN nor XY, nor any other substances, are pro-
duced, which may for the present 24 be admitted in the case of the
double decomposition of salts, for which M and N are metals and X
and Y haloids. As the ordinary double decomposition consists exclu-
sively in the exchange of metals, the above simplification is applicable
in this case. The sum total of the existing data concerning the double
decomposition of salts leads to the conclusion that from salts MX + NY
there always proceeds a certain quantity of NX and MY, as it should
be according to Berthollet's doctrine. A portion of the historical data
concerning this subject will be afterwards mentioned, and we will now
proceed to point out the observations made by Spring (1888), which
show that even in a solid state salts are subject to a similar interchange
of metals if under a condition of sufficiently close contact (which requires
time, a finely-divided state, and intimate mixture). Spring took two
non-hygroscopic salts, potassium nitrate, KNO3, and well-dried sodium
acetate, C2H3NaO2, and left a mixture of their powders for several
months in a desiccator. An interchange of metals took place, as was seen
from the fact that the resultant mass vigorously attracted the moisture
of the air owing to the formation of sodium nitrate, NaN03, and
potassium acetate, C2H3K02, both of which are highly hygroscopic. 24&

When Berthollet enunciated his doctrine the present views of atoms
and molecules had yet to be developed, but it is now necessary to sub-
mit the examination of the matter to these conceptions, and we will
therefore consider the reaction of salts, taking M and N, X and Y as

24 If M X and N Y represent the molecules of two salts, and if there be no third
substance present (such as water in a solution), the formation of X Y would also be pos-
sible ; for instance, cyanogen, iodine, &c., are capable of combining with simple haloids,
and with the complex groups which play the part of haloids in salts. Besides which the
salts MX and NY or MY with NX may form double salts. If the number of molecules-
be unequal, or if the valency of the elements contained be different, as in NaCl + H2SO4,
where Cl is a univalent haloid and SO2 is bivalent, then the matter may be complicated
by the formation of other compounds besides MY and NX, and when a solvent partici-
pates in the action, and especially if in a large proportion, then the phenomena must
evidently become still more complex ; and this is actually the case in reality. Therefore
in placing before the reader a certain portion of the existing store of matter concerning
the phenomena of double saline decompositions I cannot consider the theory of the sub-
ject as complete, and have therefore limited myself to a few data, the completion of which
must be looked for, without losing sight of what has been said above, in more detailed
works on the subject of theoretical chemistry.

246 "WTien the mixture of potassium nitrate and sodium acetate was heated by Spring
to 100° it was completely fused into one mass, although potassium nitrate fuses at about
840°, and sodium nitrate at about 320°.

SODIUM CHLOKIDE -BERTHOLLETS LA AYS 425

equivalent to each other- — that is, as capable of replacing each other * in
toto,' as Na or K, ^ Ca or ^Mg (bivalent elements), replace hydrogen.
And as, according to Berthollet's doctrine, when mMX of one salt
comes into contact with nNY of another salt a certain quantity xMY
and ccNX is formed, therefore there remains m — x of the salt MX, and
n — x of the salt XY. If m be greater than HJ and the mass of M and
N and X and Y be equivalent, then the maximum interchange could
lead to x=n, whilst from the salts taken there would ensue nMY +
nNX 4- (m — n)MX — that is, a portion of one of the salts taken would
remain unchanged only because the reaction could only proceed
between wMX and nNY. If x were actually equal to n or 0, the mass
of the salt MX would not have any influence on the modus operandi of
the reaction, which is essentially according to the teaching of Bergman,
who supposed double reactions to be independent of the mass but deter-
mined by affinity only. If M had more affinity to X than to Y, and N
more affinity to Y than to X, then, according to Bergman, there would
be no decomposition whatever, and x would equal 0. If the affinity
of M to Y and of N to X were greater than in the original grouping,
then the affinities of M for X and of N for Y would act, and, according
to Bergman's doctrine, complete interchange would take place— i.e., x
would equal n. According to Berthollet's teaching, a distribution of
M and N between X and Y will take place in every case, not only in
proportion to the measure of affinity, but also in proportion to the mass,
so that with a small affinity and a large mass the same action can be
produced as with a large affinity and a small mass. Therefore, (1) x

will always be less than n and their ratio - less than unity — that is,

n

the decomposition will be expressed by the equation, mMX + nN Y =
(m — x)MX + (n — a)NY + #MY + #NX ; (2) by increasing the massm.
we increase the decomposition — that is, the measure of x and the ratio

— ^ — until with an infinitely large quantity m the fraction - will
(TL — x) n

equal 1 and the fraction - — ^— be infinite, and the decomposition will

(n-x)

be complete, however small the affinities MY and NX may be ; and
(3) (if m=n) by taking MX + NY or MY + NX we arrive at one and
the same system in both cases : (n — cc)MX + (n— x)N Y + cc'MY + ccNX.
These direct consequences of Berthollet's teaching are verified in reality.
Thus, for example, a mixture of the solutions of sodium nitrate and
potassium chloride in all cases has the same sum of properties as a
mixture composed of the solutions of potassium nitrate and sodium
chloride, naturally under the condition of the mixed solutions being of

426 PEINCIPLES OF CHEMISTKY

identical composition. But this similarity of properties may either
proceed from the fact that one system of salts passes into the other (as
Bergman professed), in conformity with the predominating affinities
(for instance, from KCl + NaNO3 there proceeds KX03 + NaCl, if it be
admitted that the affinity of the elements of the latter system be greater
than in the former) ; or, on the other hand, because both systems by
the interchange of a portion of their elements give one and the same
state of equilibrium, as according to Berthollet's teaching. Experiment
proves the latter. But before citing the most historically important
experiments verifying Berthollet's doctrine, we must stop to consider the
conception of the mass of the reacting substances. Berthollet by mass
understood the direct relative quantity of substances ; but now it is
impossible to understand this term otherwise than as the number of
molecules, for they act as chemical units, and in the special case of double
saline decompositions it is better to take the number of equivalents.
Thus in the reaction NaCl-fH2S04, the salt is taken in one equivalent
and the H,SO4 in two. If 2NaCl + H2S04 act, then the number of
equivalents are equal, and so on. The influence of mass on the measure

of decomposition — forms the root of Berthollet's doctrine, and there-
fore we will first of all turn our attention to the establishment of this
conception with regard to the double decomposition of salts.

About 1840 H. Rose showed that water decomposes metallic sul-
phides like calcium sulphide, CaS, forming hydrogen sulphide, H2S,
notwithstanding the fact that the affinity of hydrogen sulphide, as an
acid, for lime, CaH2O2, as a base, causes them to react on each other,
forming calcium sulphide and water, CaS + 2H2O. Furthermore, Rose
showed that the greater the amount of water acting on the calcium
sulphide, the more complete is the decomposition. The results of this
reaction are evident from the fact that the hydrogen sulphide formed
may be expelled from the solution by heating, and that the resulting
lime is sparingly soluble in water. Rose clearly saw from this that
such feeble agents, in a chemical sense, as carbonic anhydride and
water, by acting in a mass and for long periods of time in nature on
the durable rocks, which resist the action of the most powerful acids,
are able to bring about chemical change — to extract, for example, from
rocks the bases, lime, soda, potash. The influence of the mass of water
on antimonious chloride, bismuth nitrate, &c., is essentially of the same
character. These substances give up to the water a mass of acid which
is greater according as the mass of the water acting on them is greater.'25

25 Historically the influence of the mass of water was the first well-observed pheno-
menon in support of Berthollet's teaching, and it should not now be forgotten. In double

SODIOTi CHLORIDE- -BERTHOLLET'S LAWS 427

Barium sulphate, BaS04, which is insoluble in water, when fused
with sodium carbonate, Na2CO3, gives, but not completely, barium
carbonate, BaCO3 (also insoluble), and sodium sulphate, Na2S04. If a
solution of sodium carbonate acts on precipitated barium sulphate, then
the same decomposition is also accomplished (Dulong, Rose), but it is
restricted by a limit and requires time. A mixture of sodium carbonate
.and sulphate is obtained in the solution and a mixture of barium carbo-
nate and sulphate in the precipitate. If the solution be decanted off and
a fresh solution of sodium carbonate be poured over the precipitate, then
a fresh portion of the barium sulphate passes into barium carbonate,
and so by increasing the mass of sodium carbonate it is possible to
entirely convert the barium sulphate into barium carbonate. If a
definite quantity of sodium sulphate be added to the solution of sodium
carbonate, then the sodium carbonate will have no action whatever
on the barium sulphate, because then an equilibrated system, deter-
mined by the reverse action of the sodium sulphate on the barium
carbonate and by the presence of the sodium carbonate and sulphate in
the solution, is at once arrived at. On the other hand, if the mass of
the sodium sulphate in the solution be great, then the barium carbonate
is reconverted into sulphate until a definite state of equilibrium is
attained between the reverse reactions, producing the barium carbonate
by the aid of the sodium carbonate or the barium sulphate by the aid
of the sodium sulphate.

Another most important conception of Berthollet's teaching consists
in the existence of a limit of exchange decom%)osition, or in the attain-
ment of a state of equilibrium. In this respect the determinations of
Malaguti (1857) are historically the most important. He took a
mixture of the solutions of equivalent quantities of two salts MX and
NY, and judged the amount of the resulting exchange from the
composition of the precipitate produced by the addition of alcohol.
When, for example, zinc sulphate and sodium chloride (ZnSO4 and
L'XaCl) were taken, then there were produced by exchange sodium
sulphate and zinc chloride. A mixture of zinc sulphate and sodium
sulphate was precipitated by an excess of alcohol, and it appeared from
the composition of the precipitate that 72 per. cent, of the salts taken
had been decomposed. When, however, a mixture of solutions of

decompositions taking place in dilute solutions where the mass of water is large, its in-
fluence, notwithstanding the weakness of affinities, must be great, according to the very
spirit of Berthollet's doctrine.

As explaining the action of the mass of water, the experiments of Pattison Muir (1879)
;ire very instructive. These experiments demonstrate that bismuth chloride is decom-
posed the more the greater the relative quantity of water, and the less the mass of hydro-
chloric acid forming one of the products of the reaction.

4-2H

sodium sulphate and zinc chloride were taken, tin- precipitate pre-
sented tin- sail if con i posit ion as 1 if fore I hat is. about -^ per cent, of the
-alts taken had been subjected to decomposition. In a similar experi-
ment with a mixture <if .sodium chloride and magnesium sulphate.
L'XaCl -f- MgS( >,, about half of the metals were subjected to decompo-
sition, which may be expressed by the equation 1 Na( '1 -f L'MgSO ,—
L'Xat'l + .MgSc >4-f Xa,,SO.j + MgCl, = L)Xa,S(J1^L)Mo-Cl,. A no less
clear limit expressed itself in other of .Mala^ut i's researches when lie
investigated the above-mentioned reversible reactions <f the insoluble
salts of barium. When, for example, barium carbonate and sodium
sulphate ( l'..i( '< )., -f- Na.^Sf >,) were taken, t lu-n about 7l' percent, of the
>alts were decomposed, that is. were converted into barium sulphate
and sodium carbonate. JUit when the two latter salts were taken,
then about 111 per cent, of the salts passed into barium carbonate
and -odium sulphate. Probably the end of the reaction was not
reached in cither case, because this would require a u'lvat time and a
ditlicultly attainable uniformity of conditions.

(Gladstone (1S.V>) took advantage of the colour of solutions of
different ferric salts for determining the measure of exchange between
metals. Thus u solution of ferric thioeyanate has an exceedingly
intense red colour, and by making a comparison between the colour of
the resulting solutions and the colour of solutions of known strength
it was possible to judge to a certain degree the quantity of the
thioeyanate formed. This colorinietrie method of determination has
an important significance1 as being the first in which a method was ap-
plied for determining the composition of a solution without the removal
of any of it s component parts. When Gladstone took equivalent quanti-
ties of ferric nitrate and potassium thiocvanate I'Y(X< ).,).{ -f- .">KCXS

then on'v 1 -I pel* cent, of the salts were subjected to decomposition.
( >n increasing the mass of the latter salt the quantity of ferric thio-
eyanate formed increased, but even when more than )>00 equivalents of
pota>sium thioeyanate were taken a portion of the iron still remained
as nitrate. It U e\ ident that the atlinitv acting between I'Y and
N(>,and between |\ ;uid ('NS on the one hand is greater than the
aflinit ies act ing between I'V and('NS, together with t he atlinit \' of 1\
for NO.,, on the other hand. The invest igat ion of the variation of
the fluorescence of quinine sulphate, as well as the variation of the
rotation of the plane of polarisat ion of nicotine, gave in the hands of
(Mad-tone manv proofs of the entire applicabilit v of IJerthollet's
doctrine, and in particular demonstrated the influence of mass which
forms the chief distinctive feature of the, in his time, but little appre-
ciated teaching of liert hollet .

CHLORIDE— BERTHOLLET'S LAWS 429

At the beginning of the year I860, the doctrine of the limit of
reaction and of the influence of mass on the process of chemical trans-
formations received a very important support in the researches of
Berthelot and P. de Saint-Gilles on the formation of the ethereal salts
RX from the alcohols ROH and acids HX, when water is also formed.
This conversion is essentially very similar to the formation of salts, but
differs in. that it proceeds slowly at the ordinary temperature, extend-
ing over whole years and is not complete — that is, it has a distinct
limit determined by a reverse reaction ; thus an ethereal salt RX with
-water gives an alcohol ROH and an acid HX — up to that limit
generally corresponding with two thirds of the alcohol taken, if the
action proceed between molecular quantities of alcohol and acid. Thus
common alcohol, C2H5OH, with acetic acid, HC2H3O2, gives the follow-
ing system rapidly when heated, or slowly at the ordinary temperature,
ROH + HX + 2RX + 2H2O, whether we start from 3RHO+3HX or
from 3RX + 3H20. The process and completion of the reaction in the
above-described instance are very easily observed, because the quantity
of free acid is easily determined from the amount of alkali requisite for
its saturation, as neither alcohol nor ethereal salt acts on litmus and
other reagents for acids. Under the influence of an increased mass
of alcohol the reaction proceeds further. If two molecules of
alcohol, RHO, be taken for every one molecule of acetic acid, HX, then
instead of 66 p.c., 85 p.c. of the acid passes into ethereal salt, and
with fifty molecules of RHO nearly all the acid is etherised. The
researches of Menschutkin in their details touched on many essential
aspects of the same subject, such as the influences of the composition
of the alcohol and acid on the limit and rate of exchange — but these, as
well as other details, must be looked for in special treatises on organic
and theoretical chemistry. In any case the study of etherincation
supplied chemical mechanics with clear and valuable data, which directly
confirm the two fundamental propositions of Berthollet : the influence
of mass, and the limit of reaction — that is, the equilibrium between
opposite reactions. The study of numerous instances of dissociation
which we have already touched on, and which we shall yet meet with
on several occasions, gave the same results. With respect to double
saline decompositions, it is necessary to further mention the researches
of Wiedemann on the decomposing action of the mass of water on the
ferric salts, which could be judged of by measuring the magnetism
of the solutions, because the ferric oxide (soluble colloid) set free by
the water is less magnetic than the ferric salts.

A very important epoch in the history of Berthollet's doctrine was
attained when, in 1867, the Norwegian chemists, Guldbergand Waage,

4:50

submitted it to ;ui algebraical formula. They called the active mass,
tin- number « if molecules occiin inu' in a given volume, and allowed, as
follows from the spirit of Berthollet's teaching, ilia! the action be-
tween tin- substances was ctjual to the product of the masses of the
reacting substances. Hence if the salts MX and NY be taken in
equivalent quantities (m— 1 and n=I) and the salts MY and XX are
not added to the mixture but proceed from it. then if /• represent the
coetlicient of the ratio of the action of .MX on NY and if /,' represent
the same coefficient for the pair .MY and NX. then we shall have at the
moment \vhen the decomposition equals .'' a measure of action for the
first pair : /• ( 1 —.».') ( 1 — .'') and for the second pair / ,<•,'', and a state of
equilibrium or limit will be reaehed when /• ( 1 — .,•)- = !. •'./•-, whence the
i-atio /• /,• =[".'' i 1 — .'•)']-. Therefore in the case of the action of alcohol
on an acid, when .''— r! the magnitude, /• // — I. that i-, the reaction of
alcohol on the acid is four times greater than that of the ethereal salt
on water. If the ratio /•'/'' be known, then ///'- Injlm-no of iiHtsu ///"//
//' cdsi/i/ <l< l< I'ml i«d from it. Thus if instead of one molecule of
alcohol two be taken, then the equation will be L\ -—.•'} ( 1 —./•) — />•',-•./•.
whence ./• = (.l-S.") or S5 p.c., which is close to tlie result of experiment.
If .">00 molecules of alcohol be taken, then ,<• proves to be approximately
100 p.c., which is also found to be the case bv e.vperiment.-1'

lUit it is impossible to subject the formation of salts to anv process
direct Iv analogous to that which is so conveniently ellected in etheriti-
catioii. Many eilbrts have, however, been made to solve the problem
of the measure of reaction in this case also. Thus, for example.
Khichinsky (isiir,^ Petrietl' ( 1 S^O), and many othei-s investigated the
distribution of metals and haloid groups in the case of one metal and
several haloids taken in excess, as acids ; or converselv with an excess of
bases, the distribution of these bases with relation to an acid ; in cases
when- a port ion ot the substances forms a precipitate and a port ion occurs
in -olution. llut such complex cases, although thev in general confirm
1 Jert hollet s teaching (for instance. ;i solution of silver nitrate gives a
port ion of silver oxide with lead oxide, and a sol ui ion of nit rate of lead
precipitate^ a portion of lead oxide under the action of -liver oxide, as
iVtrieH' demoiist rat ed), -till, owinu t o the complexity of the phenomena
(for instance, the formation ot basic and double salt-), thev cannot <rive

" Kn.in llif i)li««\<' it I-!!-'.'.- tli.it ,m c\cc - .,) ;,,•,. 1 ^lumld iiiMui-ncc tin- rcactuni lil<r

.; ..;,•.•!,..!. It tw«. im.li-i ill.- nf iin-tii: ni-iil In- lalii-n In MIM- mnli-ruli- <>f alcohol.

IH-II it i inli-cd houn i.y cx|'critni'iit tlnit ^1 |>.c. ol' nlcoln-l is ctlirnlird. If with ;i
ni--(. |,p-]M.ii(lfr,iin-i' "I' siciil or ot iilcolml ct-riiiiii <li > iv| ,,, nci. . arc O)IS.T\ r(|, then their
,111 emu 1 lie looked |'or ill tin' ilicoinjilete re •endilalice ot t lie condil ion- and outside
ntllleliee ..

SODIUM CHLORIDE— BERTHOLLET'S LAAVS 431

simple results. But much more instructive and complete are researches
similar to those made by Pattison Muir (1876), who took the simple
case of the precipitation of calcium carbonate, CaCO3, by the mixture of
solutions of calcium chloride and sodium or potassium carbonate, and
found in this case that not only was the rate of action (for example, in
the case of CaCl2 + Na2CO3, 75 per cent, of CaCO3 was precipitated
in five minutes, 85 per cent, in thirty minutes, and 94 in two
days) determined by the temperature, relative mass, and amount of
water (the mass of water decreases the rate and limit), but that the
limit of decomposition was also dependent on these influences. How-
ever, even in researches of this kind the conditions of reaction are
complicated by the non-uniformity of the media, inasmuch as a portion
of the substance is obtained or remains in the precipitate, so that the
system is heterogeneous. The investigation of double saline decompo-
sitions offers many difficulties, which cannot be considered as yet
entirely overcome. Many efforts have, however, long since been made.
In virtue of their historical interest, I will cite two of these efforts,
which are due to Thomsen (1869) and Ostwald (1876).

Thomson applied a thermo-chemical method to exceedingly dilute
solutions without taking the water into further consideration. He
took solutions containing 100H.2O per NaHO, and sulphuric acid con-
taining J,H2SO4 + 100H2O. If these solutions be mixed in such
proportions that atomic proportions of acid and alkali would act, then
for forty grams of caustic soda (which answers to its equivalent)
there should be employed 49 grams of sulphuric acid, and then
+ 15689 heat units would be evolved. If the normal sodium sulphate
so formed be mixed with n equivalents of sulphuric acid, then a certain

amount of heat is absorbed, namely a quantity equal to ,— - rrrrox heat

units. An equivalent of caustic soda, in combining with an equivalent
of nitric acid, evolves +13617 units of heat, and the augmentation of
the amount of nitric acid entails an absorption of heat for each equiva-
lent equal to —27 units • so also in combining with hydrochloric acids
+ 13740 heat units are absorbed, and for each equivalent of hydro-
chloric acid beyond this amount there are absorbed 32 heat units.
Thus sulphuric acid evolves a somewhat greater quantity of heat than
nitric and hydrochloric acids ; the difference being approximately equal
to 2000 heat units per equivalent of caustic soda. From this it might
be concluded that nitric acid or hydrochloric acid would not act on
sodium sulphate. In reality, Thomsen found this not to be the case.
Nitric and hydrochloric acids decompose sodium sulphate to a much
more considerable extent than sulphuric acid decomposes sodium

432 PRINCIPLES OF CHEMISTRY

•chloride or sodium nitrate. The following are the data from which
Thomsen deduced this conclusion. He mixed any one of these three
neutral salts with an acid which is n.ot contained in it ; for instance, he
mixed a solution of sodium sulphate with a solution of nitric acid and
•determined the number of heat units then absorbed. An absorption of
heat ensued because a normal salt was taken in the first instance, and
the mixture of all the above normal salts with acid produced an absorp-
tion of heat. The amount of heat absorbed enabled him to obtain an
insight into the process taking place in this intermixture, for sulphuric
acid added to sodium sulphate absorbs a considerable quantity of heat,
whilst hydrochloric and nitric acids absorb a very small amount of heat
in this case. By mixing an equivalent of sodium sulphate with various
numbers of equivalents of nitric acid Thomsen observed that the
amount of heat absorbed increases more and more as the amount of
•nitric acid was increased ; thus when HN03 was taken per JNa2SO4,
1752 heat units were absorbed per equivalent of soda contained in the
jsodium sulphate. When twice as much nitric acid was taken 2026
. heat units, and when three times as much 2050 heat units were ab-
sorbed. Had the double decomposition been complete in this case when
•one equivalent of nitric acid was taken, then the heat evolved would
be determined by the sum of 13617 — 15689 — 1650/1'S, or would equal
— 2989 heat units, if it be admitted that sulphuric acid when mixed
with NaNO3 absorbs as much heat as when mixed with iNa2SO4.
But as in reality only 1752 heat units were absorbed instead of 2989
units, therefore a displacement of only about two- thirds of the sulphuric
acid had taken place — that is, the ratio k : k' for the reaction
^Na2SO4 + HNO3 and NaNO3-f ^H.2SO4 is equal, as for ethereal salts,
to 4. By taking this figure and admitting the above supposition,
Thomsen found that for all mixtures of sodium with nitric acid, and of
-sodium nitrate with sulphuric acid, the amounts of heat followed
Guldberg and Waage's law ; that is, the limit of decomposition reached
was greater the greater the mass of acid added. The relation of hydro-
chloric to sulphuric acid gave the same results. Thus on mixing
4 with HC1, the thermal result of experiment was — 1682, and
calculation — 1690 ; when mixed with 2HC1 the experimental result
— 1878 and by calculation— 1870, with 4HC1 the result of experi-
ment was - 1 896 and by calculation — 1 9 1 7. When NaCl + |H2SO4 was
taken, then experiment showed an evolution of + 244 heat units, whilst
from calculation it should have be'en +257 heat units; on doubling
the quantity of acid experiment gave +336 and calculation +292.
The slight differences between the results obtained by experiment and
•calculation are due to the unavoidable errors of calorimetric determi-

snlUT.M CHLORIDE IIKUTHOLLET'S LAWS 433

nations. Therefore the researches of Thomseu fully confirm the
hypotheses of (iuldberg and Waage and the doctrine of Berthollet.27

-" ThoiMsen concludes his investigation with the words: (a) 'When equivalent
quantities <>f XaHO,HXO5 (or HC1) and ^H.2SO4 react on one another in an aqueous solu-
tion, then two-thirds of the soda combines with the nitric and one-third with the sulphuric
arid ; ifn this snl)division repeats itself, whether the soda be taken combined with nitric
or with sulphuric acid ; ('<•) and therefore nitric acid has double the tendency to combine
with the base that sulphuric acid has, and therefore in the wet way it is a stronger acid
than the latter.'

'It is therefore necessary,' Thomsen afterwards remarks, 'to have an expression
indicating the tendency of an acid for the saturation of bases. This idea cannot be
expressed by the word affinity, because by this term is most often understood that force
which it is necessary to overcome in order to decompose a substance into its component.
parts. This force should therefore be measured by the amount of work or heat employed
for the decomposition of the substance. The above-mentioned phenomenon is of an
entirely different nature,' and Thomsen introduces the term avidity, by which he desig-
nates the tendency of acids for neutralisation. ' Therefore the avidity of nitric acid with
respect to soda is twice as great as the avidity of sulphuric acid. An exactly similar
result is obtained with hydrochloric acid, so that its avidity with respect to soda is also
double the avidity of sulphuric acid. Experiments conducted with other acids showed
that not one of the acids investigated had so great an avidity as nitric acid; some had a
greater avidity than sulphuric acid, others less, and in some instances the avidity = 0.'
The reader will naturally clearly see that the path chosen by Thomsen deserves being
worked out, because his results concern important questions of chemistry, but great faith
cannot be placed in the deductions as yet arrived at by Thomsen, because great com-
plexity of relations is to be seen in the very method of his investigation. It is especially
important to turn attention to the fact that all the reactions investigated are reactions
•of double decomposition. In them A and B do not combine with C and distribute them-
selves according to their affinity or avidity for combination, but reversible reactions are
induced. MX and NY give MY and NX, and conversely ; therefore, the affinity or avidity
for combination is not here directly determined, but only the difference or relation of the
affinities or avidities. The affinity of nitric acid not only for the water of constitution,
but also for that serving for solution, is much less than that of sulphuric acid. This is
seen from thermal data. The reaction N2O5 + H.>O gives +3600 heat units, and the
solution of the resultant hydrate, 2NHO5, in a large excess of water evolves +14986 heat
units. The formation of SO3 + H.,O evolves +21308 heat units, and the solution of
H.,,SO4 in an excess of water 178(50 — that is, sulphuric acid gives more heat in both cases.
The interchange between Na.,SO4 and 2HNO5 is not only accomplished at the expense of
the production of NaNO3, but also at the expense of the formation of H.jSO.,, hence the
affinity of sulphuric acid for water plays its part in the phenomena of displacement.
Therefore in determinations like those made by Thomsen the water does not form a
medium which is present without participating in the process, but surely plays its own
part. No less essential is the circumstance that sodium sulphate is able to combine with
An excess of sulphuric acid to form the acid salt, whilst the salts of nitric and hydrochloric
acids do not have this property. But with that method of interpretation of Guldberg
and Waage's deduction which is given in the text, the adaptability of their formula to
the phenomena observed by Thomsen may be expected, notwithstanding these incidental
-actions, although the idea of the relative tendencies of acids for combining with alkalis
cannot be deduced from the coincidence of the results of experiment with those of calcu-
lation. If sodium oxide (Na^O) were brought into contact with sulphuric anhydride and
nitric anhydride (SO3 and N.^Os) the distribution would probably be different, and even if
stronger solutions than those employed by Thomsen were taken perhaps the result would
•be different, more especially judging from what is said in the following note. (Compare
Also Chapter IX. Note 14.)

VOL. I. F F

434 PRINC 1TLF.S OF CHEMISTRY

Whilst retaining essentially the methods of Thomsen, Ostwald
determined the variation of the sp. gr. (and afterwards of volume) pro-
ceeding in the same dilute solutions, on the saturation of acids by
bases, and in the decomposition of the salts of one acid by the other,
and arrived at conclusions of just the same nature as Thomsen did.
Ostwald's method will be clearly understood from an example. A
solution of caustic soda, containing an almost molecular (40 grams)
weight per litre, had a specific gravity of 1 '04051. The specific gravities,
of solutions of equal volume and equivalent composition of sulphuric and
nitric acids were 1*02970 and 1-03084 respectively. On mixing the
solutions of NaHO and H2SO4 there was formed a solution of ]S"a2SO4
of sp. gr. 1 '02959 ; hence there ensued a decrease of specific gravity
which we will term Q, equal to 1-04051 + 1-02970— 2(1-02959) = 0-01103.
So also the specific gravity after mixture of the solutions of NaHO
and HXO;< was 1-02633, and therefore Q = 0'01869. When one
volume of the solution of nitric acid was added to two volumes of the
solution of sodium sulphate, a solution of sp. gr. 1-02781 was obtained,
and therefore the resultant decrease of sp. gr.

Q1 = 2(l-02959) + 1-03084 -3(1-02781) = 0-00659.

Had there been no chemical reaction between the salts, then according
to Ostwald's reasoning the specific gravity of the solutions would not
have changed, and if the nitric acid had displaced the sulphuric acid Q.,
would be=0'01869 — 0-01103 = 0-00766. It is evident that a portion
of the sulphuric acid was displaced by the nitric acid. But the
measure of displacement is not equal to the ratio between Q, and Q.2,
because a decrease of sp. gr. also occurs on mixing the solution of sodium
sulphate with sulphuric acid, whilst the mixing of the solutions of
sodium nitrate and nitric acid only produces a slight variation of sp. gr.
which falls within the limits of error of experiment. Ostwald deduces
from similar data the same conclusions as Thomsen, and thus re-
confirms the formula deduced by Guldberg and Waage, and the teach-
ing of Berthollet.28

28 The participation of water is seen still more clearly in the methods adopted by
Ostwald than in those of Thomsen, because in the saturation of solutions of acids by
alkalis (which Kremers, Reinhold, and others had previously studied) there i> observed,
not a contraction, as might have heen expected from the quantity of heat which is then
evolved, but an expansion, of volume (a decrease of specific gravity, it' we calculate as
Ostwald did in his first investigations). Thus by mixing UNO grains oi a solution of
sulphuric acid of the composition SOj + 100TL.O, occupying a volume of lsir> c.e., with M
corresponding quantity of a solution '2(NaHO -I- 5HoO), whose volume =17'.):: <•.<•.. we
obtain not 3G08 but 8638 c.c., an expansion of 25 c.c. per gram molecule of the resulting
salt, Na2SO4. It is the same in other cases. Nitric and hydrochloric acids -ive a still
greater expansion than sulphuric acid, and potassium hydroxide than sodium hydroxide.

sol>irM CHLORIDE— BERTHOLI.KTS LAWS 435

The majority of the above-mentioned researches were carried on in
aqueous solutions, and as water is itself a saline compound, and is
able to combine with salts and enter into double decomposition with
them, such reactions taking place in solutions in reality present very
complex cases. In this sense the reaction between alcohols and acids
is much more simple, and therefore its significance in confirmation
of Berthollet's doctrine is of particular importance. The only cases
which can be compared with these reactions from their simplicity are
those exchange decompositions investigated by G. G. Gustavson, and
which take place between CC14 and RBr,, on the one hand and CBr4
and RC1,( on the the other. This case is convenient for investigation
in the sense that the RC1,, and RBr,, taken belong (like BC13, SiCl4,
TiCl4, POC13, and SnCl4) to the number of substances which are de-
composed by water, whilst CC14 and CBr4 are not decomposed by
water ; and therefore, by heating, for instance, a mixture CCl4 + SiBr4
it is possible to arrive at a conclusion as to the amount of interchange
by treating the product with water, which decomposes the SiBr4 left
unchanged and the SiCl4 formed by the exchange, and therefore
by determining the composition of the product acted on by the water
it is possible to form a conclusion as to the amount of decomposition.

whilst a solution of ammonia gives a contraction. The relation to water must be considered
;is the cause of these phenomena. When sodium hydroxide and sulphuric acid dissolve
in water they develop heat and give a vigorous contraction ; the water is separated from
such solutions with great difficult y. After mutual saturation they form the salt Na.>SO4,
which retains the water but feebly and evolves but little heat with it, which, in a word,
has little affinity for water. The water in the saturation of sulphuric acid by soda is, so
to say, displaced from a stable combination and passes into an unstable combination ;
hence an expansion (decrease of sp. gr.) takes place. It is not the reaction of the acid on
the alkali, but the reaction of water, that produces the phenomenon by which Ostwald
desires to measure the degree of salt formation. The water, which escaped attention,
itself has affinity, and influences those phenomena which are being investigated. Further-
more, in the given instance its influence is very great because its mass is large. When it
is not present, or only present in small quantities, the affinity of the base to the acid leads
to contraction, and not expansion. Na.,,O has asp. gr. 2'8, hence its volume = 22; the
:!•. of SO5 is 1".) and volume 41, hence the sum of their volumes is 63, and for
N:I. .so, the -p. gr. is '2-C,r, and volume 53'6, consequently a contraction of 10 c.c. per gram
molecule of salt. The volume of HoSO4 = 53'3, that of 2XaHO = 37'4; there is produced
2H.X), volume = 36 + Xa, SO,, volume f,:l r,. There react 90'7 c.c., and on saturation there
results 89*6 c.c. ; consequently contraction again ensues, although less, and although this
reaction is one of substitution and not of combination. Consequently the phenomena
studied by Ostwald hardly depend on the measure of the reaction of the salts, but more
on the relation of the substances dissolved to water. In substitutions, for instance,
•JNaXO5 + HoSO, = 2HNOr> + Na,,SO4, the volumes vary but slightly; in the above example
they are 2(38-8) + 53'3 and 2(41'2) + 53'6 ; hence 131 volumes act, and 136 volumes are pro-
duced. It may be thought, therefore, on the basis of what has been said, that on taking
water into consideration the phenomena studied by Thomseii and Ostwald prove to be
much more complex than they at first appear, and that this method can scarcely lead to
a right judgment as to the distribution of acids between 1.

vrfi

436 PRINCIPLES OF CHEMISTRY

The mixture was always formed with equivalent quantities — for in-
stance, 4BC13 + 3CBr4. It appeared that there was no exchange what-
ever on simple intermixture, but that it proceeds, and then slowly,
when the mixture is heated (for example with the above-cited mixture
at 123° 4-06 per cent, of Cl was replaced by Br after 14 days' heating,
and 6'83 per cent, after 28 days, and 10*12 per cent, when heated at
150° for 60 days) A limit was always reached which corresponded
with the complemental system ; for example, in the given instance to the
system 4BBr3 + 3CCl4. So in this last 89*97 per cent, of bromine in
the BBr3 was replaced by chlorine ; that is, there was obtained 89 '97
molecules of BC13 and there remained 10*02 molecules of BBr3, and
therefore the same state of equilibrium was reached as that given by
the system 4BC13 + 3CBr4. Both systems gave one and the same state
of equilibrium at the limit, as follows from Berthollet's doctrine. 28b

-8b G. G. Gustavson's researches, which were conducted in the laboratory of the
St. Petersburg University in 1871-72, are among the first in which the measure of
the affinity of the elements for haloids appears with perfect clearness in the limit of
substitution and in the rate of reaction. The researches conducted by A. L. Potilitzin
(of which mention will be made in Chapter XI. Note 6(i) in the same laboratory touch on
another aspect of the same problem which has not yet made much progress, notwith-
standing its importance and the fact that the theoretical side of the subject (thanks
especially to Guldberg and Van't Hoff ) has since been rapidly pushed forward. It would
be very important if the researches of Gustavson touched on the influence of mass, and
were more fully supplied with data concerning velocities and temperatures, because of
the great significance which the instance considered has for the understanding of double
saline decompositions in the ' absence of water.'

Furthermore, Gustavson showed that the greater the atomic weight of the element
(B, Si, Ti, As, Sn) combined with chlorine the greater the amount of chlorine replaced
by bromine by the action of CBr.j, and consequently the less the amount of bromine re-
placed by chlorine by the action of CC14 on bromine compounds. For instance, for
chlorine compounds the percentage of substitution (at the limit) is —

BC1- SiCl, TiCl4 AsCl3 SnCl4

10-1 12-5 43'6 71-8 77'5

It should be observed, however, that Thorpe, on the basis of his experiments, denies
the universality of this. I may mention one conclusion which it appears to me may be
drawn from the above cited figures of Gustavson, if they further verify themselves even
within narrow limits. If CBr4 be heated withKCl,, then an exchange of the bromine by
chlorine takes place. But what would be the result if it were mixed with CC14 ? Judging
by the magnitude of the atomic weights, B = 11, C = 12, Si = 28, about 11 p.c. of the chlorine
would be replaced by bromine. But what does this signify ? I think that this shows the
existence of a movement of the atoms in the molecule. The mixture of CC14 and CBr4
does not remain in a state of static equilibrium ; not only are the molecules contained in
it in a state of movement but also the atoms in the molecules, and the above figures show
the measure of their translation under these conditions. The bromine in the CBr4 is,
within the limit, substituted by the chlorine of the CC14 in a quantity of about 11 out of
100 ; that is, a portion of the atoms of bromine previously to this moment in combination
with one atom of carbon pass over to the other atom of carbon, and the chlorine passes over
from this second atom of carbon to replace it. Therefore, also, in the homogeneous mass
CC14 all the atoms of Cl do not remain constantly combined with the same atoms of

so PI I'M CHLORIDE— BERTHOLLET'S LAWS 437

Thus we now find ample confirmation from various quarters for the
following rules of Berthollet, on referring them to double saline decom-
positions : 1. From two salts MX and NY containing different haloids
and metals there result from their reaction two others, MY and NX,
but such a substitution will not proceed to the end unless one product
passes from the sphere of action. 2. This reaction is limited by the
existence of an equilibrium between MX, NY, MY, and NX, because a
reverse reaction is quite as possible as the direct reaction. 3. This limit
i* « letermined by both the measure of the active affinities and by the rela-
tive masses of the substances measured by the number of the reacting
molecules. 4. Other conditions being constant, the chemical action is
proportional to the product of the chemical masses in action. 2!)

Thus if the salts MX and NY after reaction partly formed salts MY
and NX, then a state of equilibrium is reached and the reaction ceases ;
but if one of the resultant compounds, in virtue of its physical properties,
passes from the sphere of action of the remaining substances, then the
reaction will continue. This exit from the sphere of action depends on
the physical properties of the substance and on the conditions under
which the reaction takes place. Thus, for instance, the salt NX may,
in the case of reaction between solutions, separate as a precipitate, as
an insoluble substance, while the other three substances remain in solu-
tion, or it may pass into vapour, and in this manner also pass away
from the sphere of action of the remaining substances. Let us now
suppose that it passes away in some form or other from the sphere of
action of the remaining substances — for instance, that it is transformed
into a precipitate or vapour — then a fresh reaction will set in and a

carbon, and there is an exchange of atoms between different molecules in a homogeneous
medium ditto. This hypothesis may, in my opinion, explain certain phenomenaof disso-
ciation, but in mentioning it I do not consider it possible to linger on it. I will only
observe that a similar hypothesis suggested itself to me in my researches on solutions,
and that Pfaundler essentially enunciated a similar hypothesis, and in recent times a
like view is beginning to diffuse itself with respect to the electrolysis of saline solutions.
29 Berthollet's doctrine can hardly be in any way undermined when it is shown that
there are cases in which there is no decomposition between salts, because in principle
the affinity may be so small that a large mass would still give no observable displacements.
The fundamental condition of the adaptability of Berthollet's doctrine, as well as
Deville's doctrine, of dissociation lies in the reversibility of reactions. As there are prac-
tically unreversible reactions (for instance, CC14 + 2H.,O = CO., + 4HC1), and as non- volatile
substances do exist, so whilst accepting the doctrine of reversible reactions and retaining
the doctrines of the evaporation of liquids, it is possible to admit the existence of non-
volatile substances, and in just the same way of reaction without any visible conformity
to Berthollet's doctrine. This doctrine evidently approaches nearer than the opposite
doctrine of Bergman to the solution of the complex problems of chemical mechanics,
for the successful resolution of which at the present time the richest fruits are to be
expected from the working out of data concerning dissociation, the influence of mass,
and the equilibrium and velocity of reactions.

438 PRINCIPLES OF CHEMISTRY

reformation of the salt XX. If it be removed, then, although the
quantity of the elements X and X in the mass will be diminished, still,
according to Berthollet's law, a certain amount of XX should be again
formed. When this substance is again formed, then, owing to its
physical properties, it will again pass away ; hence the reaction, in
consequence of the physical properties of the resultant substance, is
able to proceed to completion notwithstanding the possible weakness of
the attraction existing between the elements entering into the compo-
sition of the resultant substance XX. Naturally, if the resultant
substance is also formed of elements having a considerable degree of
affinity, then the complete decomposition is considerably facilitated.

ISuch a representation of the modus operaitdi of chemical trans-
formations is applicable with great clearness to a number of re-
actions studied in chemistry, and, what is especially important, the
application of this aspect of Berthollet's teaching does not in any way
require the determination of the measure of affinity acting between the
substances present. For instance, the action of ammonia on solutions
of salts ; the displacement, by its means, of basic hydrates soluble in
water ; the separation of volatile nitric acid by the aid of non-volatile
sulphuric acid, as well as the decomposition of table salt by means of
sulphuric acid, when gaseous hydrochloric acid is formed — may be
taken as examples of reactions which proceed to the end, inasmuch as
one of the resultant substances is entirely removed from the sphere of
action, but they in no way indicate the measure of affinity.80

30 Common salt not only enters into double decomposition with acids but also trith
every salt. However, as clearly follows from Berthollet's doctrine, this form of decom-
position will only in a few cases render it possible for new metallic chlorides to be ob-
tained, because the decomposition will not be carried on to the end unless the metallic-
chloride formed separates from the mass of the active substances. Thus, for example.
if a solution of common salt be mixed with a solution of magnesium sulphate, double
decomposition ensues, but not completely, because all the substances remain in the solu-
tion. In this case the decomposition may result in the formation of sodium sulphate and
magnesium chloride, substances which are soluble in water; nothing is disrupted,
and therefore the decomposition 2NaCl + MgSOt = MgClo-r Xa-jSO, cannot proceed t<> tin-
end. However, the sodium sulphate formed in this manner may be separated by free/ing
the mixture. Sulphate of sodium is sparingly soluble in the cold, and therefore if a suffi-
ciently strong solution of common salt and magnesium sulphate lie taken, the resulting
sodium sulphate will separate out in the cold as crystals containing water of crystallisa-
tion. The complete separation of the sodium sulphate will naturally not take place, owing
to a portion of the salt remaining in the solution in the cold. Nevertheless, this kind ot
decomposition is made use of for the preparation of sodium sulphate from the residue-
left after the evaporation of sea-water, which contain a mixture of magnesium sulphate
and common salt. Such a mixture is encountered at Stassfurt in a natural form. The
separation is sometimes carried on by means of artificial cooling by refrigerating
machines. It might be said that this form of double decomposition is only accomplished
with a change of temperature; but this would not be true, as may be concluded from
other analogous cases. Thus, for instance, a solution of copper sulphate is of a blue

CIILORJDE-BEKTHOLLErs LAWS 439

As a proof that double decompositions like the above are actually
;ir«-o mpli>h«'d in the sense of Berthollet's doctrine, the fact may be cited
that table salt may be entirely decomposed by nitric acid, and nitre may
be completely decomposed by hydrochloric acid, just as they are decom-
IIOMM! by sulphuric acid ; but this only takes place when, in the first
instance, an excess of nitric acid is taken, and in the second instance with
an excess of hydrochloric acid for a given quantity of the sodium salt,
and if the resultant acid passes off. If sodium chloride be put into a
porcelain evaporating basin, nitric acid be added to it, and the mix-
ture be heated, then both hydrochloric and nitric acids are expelled by
the heat. Thus the nitric acid partially acts on the sodium chloride,
but on heating, as both acids are volatile, they are both converted into
vapour ; and therefore the residue will contain a mixture of a certain
quantity 'of the sodium chloride taken and of the sodium nitrate formed.
If a fresh quantity of nitric acid be then added, reaction will again set
in, a certain portion of hydrochloric acid is again evolved, and on heat-
ing is expelled together with nitric acid. If this be repeated several
times, it is possible to expel all the hydrochloric acid, and to obtain
sodium nitrate only in the residue. If, on the contrary, we take
sodium nitrate and add hydrochloric acid to it in an aqueous solution,
then reaction again sets in, a certain quantity of the hydrochloric acid
displaces a portion of the nitric acid, and on heating the excess of
hydrochloric acid passes away with the nitric acid formed. On repeat-
ing this process, it is possible to displace the nitric acid with an excess
of hydrochloric acid, just as it was possible to displace the hydrochloric
acid by an excess of nitric acid. The influence of the mass of the sub-
stance in action and the influence of volatility is here very distinctly

colour, while a solution of copper chloride is green. If we mix both salts together the
green tint is distinctly visible, so that by this means the presence of the copper chloride
in the solution of copper sulphate is clearly seen. If now we add a solution of common
salt to a solution of copper sulphate a green coloration is obtained, which indicates the
formation of copper chloride. In this instance it is not separated, but it is immediately
formed on the addition of common salt, as it should be according to Berthollet's doctrine.
The complete formation of a metallic chloride from common salt can only occur,
judging from the above, when it separates from the sphere of action. The salts of silver
are instances in question, because the silver chloride is insoluble in water; and therefore
if we add a solution of sodium chloride to a solution of a silver salt, then silver chloride
and the sodium salt of that acid which was in the silver salt are formed. The amount
of silver chloride formed immediately separates from the solution, because it is in-
soluble in water, and the reaction in consequence is further directed to the end — that is,
either the whole quantity of the silver is separated or all the chlorine passes into silver
chloride. This method is made use of for separating silver from its solutions, and also
for determining the quantity of chlorine. It must not, however, be forgotten that silver
chloride is slightly soluble in water and still more so in a solution of sodium chloride, and
that, therefore, a certain trace of silver escapes precipitation.

440

PKINCIPLES

CHEMISTRY

seen. Hence it may be affirmed that sulphuric acid does not displace
hydrochloric acid because of an especially high degree of affinity, but
that this reaction is only carried on to the end because the sulphuric-
acid is not volatile whilst the hydrochloric acid which is formed is
volatile.

The preparation of hydrochloric acid in the laboratory and on a
large scale is based upon these data. In the first instance, an excess-
of sulphuric acid is employed in order that the reaction may proceed
easily and at a low temperature, whilst on a large scale, when it is
necessary to economise every material, equivalent quantities are taken
in order to obtain the normal salt Na.2SO4 and not the acid salt (p. 422),
which would require twice as much acid. It may be remarked that
dry hydrochloric acid is a gas which is very soluble in water. It is
most frequently used in practice in this state of solution under the name
of muriatic acid.31

PlG. 65. — Section of a salt eake furnace. B, pan in \vhic
are first mixed and heated. C, muffle for the ultimate

sulphuric acid

In chemical works the decomposition of sodium chloride by means of
sulphuric acid is carried on on a very large scale, chiefly with a view to
the preparation of normal sodium sulphate, the hydrochloric acid being
a bye-product. The furnace employed is termed a salt cake furnace.
It is represented in fig. 65, and consists of the following two parts : the

31 The apparatus shown in fig. 46 (p. 250) is generally employed for tin- pivp
of small quantities of hydrochloric acid. Common salt is plaet-d in the retort ; tin
generally previously fused, as it otherwise froths and boils over the appara t u >. \V]

SODIUM CHLORIDE— BERTHOLLET'S LAWS 441

pan B and the roaster C, or enclosed space built up of large bricks a
and enveloped on all sides by the smoke and flames from the fire grate,
F. The ultimate decomposition of the salt by the sulphuric acid is
accomplished in the roaster. The hydrochloric acid gas evolved in this
reaction escapes through the tube O as will presently be described. But
the first decomposition of sodium chloride by sulphuric acid does not
require so high a temperature as the ultimate decomposition, and is.
therefore carried on in the front and cooler portion, B, whose bottom
is heated by gas flues. When the reaction in this portion ceases
and the evolution of hydrochloric acid stops, then the mass, which
contains about half of the sodium chloride yet undecomposed and
the sulphuric acid in the form of acid sodium sulphate, is removed
from B and thrown into the roaster C, where the action is com-
pleted. Normal sodium sulphate, which we shall afterwards describe,
remains in the roaster. It is employed both directly in the manu-
facture of glass, and in the preparation of other sodium compounds — for
instance, in the preparation of soda ash, as will afterwards be described.
For the present we will only turn our attention to the hydrochloric
acid evolved in B and C. In chemical works, where sulphuric acid of
60° Baume (22 p. c. of water) is employed, 117 parts of sodium chloride
are taken to about 125 parts of sulphuric acid.

The hydrochloric acid gas evolved is subjected to condensation by
dissolving it in water.32 If the apparatus in which the decomposition

apparatus is placed in order sulphuric acid mixed with water is poured down the thistle
funnel into the retort. About one and a half times the weight of the salt of strong sul-
phuric acid is usually taken, and it is diluted with a small quantity of water (half) if it be
desired to retard the action, as in using strong sulphuric acid the action immediately
begins with great vigour. The mixture, at first without the aid of heat and then at a
moderate temperature (in a water-bath), evolves hydrochloric acid. Commercial hydro-
chloric acid contains many impurities ; it is usually purified by distillation, the middle
portions being collected. It is purified from arsenic by adding FeCL>, distilling, and
rusting aside the first third of the distillate. If free hydrochloric acid gas be required, it
is passed through a vessel containing strong sulphuric acid to dry it, and it is collected
over a mercury bath.

Phosphoric anhydride absorbs hydrogen chloride (Bailey and Fowler, 1888;
2P2O5 + 8HC1 = POC1.-; + 3HPO5) at the ordinary temperature, and therefore the gas cannot
l>c dried by this substance.

52 As at works which treat common salt in order to obtain sodium sulphate, the
hydrochloric acid is sometimes held in no value, they would readily allow it to escape
into smoke and into the atmosphere, which would greatly injure the air of the neighbour-
hood and destroy all vegetation, and therefore in all countries there are laws forbidding
the works to proceed in this manner, and requiring the absorption of the hydrochloric acid
by water at the works themselves, and not permitting the solution to be run into rivers
and streams, whose waters it would spoil. It may be remarked that the absorption of
hydrochloric acid presents no particular difficulties (the absorption of sulphurous acid
is much more difficult), because hydrochloric acid has a great affinity for water and gives
a hydrate which boils above 100°. Hence, even steam and hot water, as well as weaker
solutions, can be used for absorbing the acid. However, Warder (1888) showed that weak

442 PRINCIPLES OF CHEMISTRY

I

is accomplished were hermetically closed, and only presented one outlet,
then the escape of the hydrochloric acid would only proceed through
the escape pipe intended for this purpose. But as it is impossible to
construct a perfectly hermetically closed furnace, it is necessary to
increase the draught by artificial means, or to oblige the hydrochloric
acid gas to pass through those arrangements in which it is condensed.
This is done by connecting the ends of the tubes through which the
hydrochloric acid gas escapes from the furnace with high chimneys, where
the strong draught proceeding from the combustion of the fuel carries
off the hydrochloric acid and air which is mixed with it. This causes a
current of hydrochloric acid gas to pass through the absorbing apparatus
in a definite direction. Here it encounters a current of water flowing
in the opposite direction, by which it is absorbed. It is not cus-
tomary to cause the acid to pass through the water, but only to bring
it into contact with the surface of the water. The absorption apparatus
consists of large earthenware vessels having four orifices, two above and
two lateral ones in the wide central portion of each vessel. The upper
orifices serve for connecting the vessels together, and the hydrochloric
acid gas escaping from the furnace passes through these tubes. The
water for absorbing the acid enters at the upper, and flows out from the
lower, vessel, and passes through the lateral orifices in the vessels. The
water flows from the chimney towards the furnace, and it is therefore
evident that the outflowing water will be the most saturated with acid,
of which it actually contains about 20 per cent. The absorption in
these vessels is not complete. The ultimate absorption of the hydro-
chloric acid is carried on in the so-called coke towers. These are high
chimneys, like that shown in fig. 66. They are frequently divided into
two portions, or consist of two adjacent chimneys. A lattice work of
bricks, C, is laid 011 the bottom of these towers, on which coke is
piled up to the top of the tower. Water, distributing itself over the
coke, trickles down to the bottom of the tower, and in so doing absorbs
the hydrochloric acid gas rising upwards.

It will be readily understood that hydrochloric acid may be

solutions of composition HoO + ;jHCl when boiled (the residue will be almost HC1,HH.X))
evolve (not water but) a solution of the composition HoO + 445/i*HCl ; for example, <>n dis-
tilling HCl,10HoO, HCl,28HoO is first obtained in the distillate. As the strength of the
residue becomes greater, so also does that of the distillate, and therefore in order to com-
pletely absorb hydrochloric acid it is necessary in the end to have recourse to \vuter.

As in Russia the manufacture of sodium sulphate from sodium chloride lias not yet
been sufficiently developed, and as hydrochloric acid is required for many technical pur-
poses (for instance, for the preparation of zinc 'chloride, which is employed for soaking
railway sleepers), therefore the salt is often treated mainly for the manufacture of hydro-
chloric acid.

SODIUM CHLORIDE BERTHOLLETS LAAVS

443

obtained from all other
metallic chlorides.33' It is

53 Thus the metallic chlorides,
which an- decompo^-d toa._
or lessdegree by water^ornespond
with feeble bas»-s. Such are, for
example, MgCL, A1C1.-, SbCl-,
Bid.-. The decomposition of
magnesium chloride (and also
carnallite) by sulphuric acid pro-
ceeds at the ordinary temperature
and inaij l>' i-injilui/ct? as a con-
venient incihoil far t/t>' ^roilnc-
tion qf hydrochloric acid. The
vapour of common salt in the pre-
sence of steam and silica (which
is capable of combining with so-
dium oxide) gives hydrochloric
acid and sodium silicate. Hydro-
chloric acid is also produced in
many reactions of decomposition
of carbon compounds containing
chlorine and hydrogen; for in-
-tance, when the vapour of ethyl
chloride is passed through a red-
hot tube, it gives defiant gas and
hydrochloric acid. It is also pro-
duced by the ignition of certain
metallic chlorides in a stream
of hydrogen, especially of those
metals which are easily reduced
and difficultly oxidised — for in-
stance, silver chloride. Lead
chloride, when heated to redness
in a stream of steam, gives hy-
drochloric acid and lead oxide.
In general 2MC1« + nH->O fre-
quently gives MoO;, + 2,(HC1, and
MCLj+H,, sometimes M-f/tHCl,
although both these reactions
also proceed in the opposite di-
rection, that is, M..O,, + ?i2HCl fre-
quently forms 2MCln + nU..,O and
sometimes M + »HC1 = MCI,, +
Hn, where 31 is a metal. The
multitude of the cases of forma-
tion of hydrochloric acid are un-
derstood from the fact that it is
a substance which is compara-
tively very stable, resembling
water in this respect, and even
most probably more stable than
water, because, at a high tem-
perature and even under the
action of light, chlorine decom-

FIG. 66. —Coke tower.

141

frequently f'i>rnif(I in other reactions, many of which we shall meet
with in the further course .if this work. It is. for instance, formed
by i lie action of water on suljiliur chlom It-, phosphorus chloride, anti-
ni« ni y clili >ride, A:C.

////'//•"'7, '"i'r- >"-i>/ is a roll >urless u'a- havinu' a pungent suffocating
odour and an acid taste. This ^as fume- in ail1 and attract-- moisture.
1 1"' -a use it forms ya pour contain in;: a compound of hydrochloric acid and
svatrr. 1 1 ydrochloric acid is lioiietied by cold, and under a pressure of
in atmosphere- into a colourless li.juid of .-p. ur. (H.MJS at d ,:i ' boiling
point — •">•"' and absolute boiling point -f ~)'2. \\'c ha\r already seen
i ( 'hap. l.i t Itat 1 i\'d rod iloric acid comliines \'erv ener^vt icallv //•/'/// ?/v///-/'.
and in >o doinu' c\ol\csa considei'able ainoiint of heat. 'J'he solution
saturateil in tin- cold attains a density 1 "2-\. On heatiiiLT such a solu-
tion containing ahoiit -I-"' ]>arts of acid per \()() parts, the hydrochloric
acid ua- i> expelled with only a slight admixture of aijueou^ \a])otir.
r.u' it is inipossilile to entirely separate the \\hole of the hydrochloric
acid tVoin the water liv thi> niean>, as could be done in the case of an
'a nn non iacal Dilution. The t emperal ure reijiiired foi1 1 he e\ olut ion of the
L;';I- rises and reaches 1 lU 111, and after this remains constant — that
i<. a >olution having a constant boiling point is obtained (p. !»S), wliich.
howe\'er. doc> not ( Roscoe and I )ittmar) present a constant composition
under ditl'ei-eni pre^ures. because the hydrate is decomposed in distil-
lation, as is >een from the del erm in at ions of its \ a pour dens it v ( I Jineau ).
• ) udu'ii r_: from the fact.- ( 1 i that \\ it 1 1 decrease* of the pressure under which
t he di-t illat ion proceed- the solution of constant boiling point ajiproaches
to a couipo-it ion of '2~> ]i.c. of hydrochloric acid.:i' ('2) that bv passing a
-t ream of dr\- ;dr t hrouu'h a -olut ion of hydrochloric acid t here is obtained
in the residue a -olut ion which a!-o approaches to I1-'1 p.c. of acid, more
nearl\' a- the t emj ,era t u re fa 1 ls.:!'' ( .'i i t hat man v of t he propert ies of solu-
1 ion - of hydrochloric acid varv dist i net I v according as t hey contain more
or Ies- than L'.'I p.c. of h\ drochloric acid (for instance, antimonious
-ulphide -i \ e- h\ droLTen sn !phide \\ it h a st roi i^er acid, but i- not acted
(in by a weaker solution, al.-o a stronger solution fume> in the air,
,vc. ,, and ( I i that t he com} osil ion H ( '],G H .,( ) correspond.- with L'.ViM p.c.

I' ilil-.-rl ;irl i. II. ,i \s,. ,ll;ill ;i 1 1 rl'\\ ; I H I - ( I. •-( 'I'll ir .

-H ti:.' i r. ..I i.Min.i liy.lniclil..rii- sicid at tl O'lHlM, at
.!. ( 'li.ij.i, ill. N,,),. :; i .

I >;'::. - r ;il .1 HV- III .• nl l]ir«'f ;i t n K ,-, ,1 |i •]•( •- till' -' "111 tli H I

CHLOBIDE— BEBTHOLLET^ LAWS 445

HC1 — judging from all these data, and also from the loss of tension
which occurs in the combination of hydrochloric acid with water,
it may be said that they form a definite Jtydrtite of the composition
HC1,6H20. The conclusions to be drawn from the densities of solu-
tions of hydrochloric acid also point to the same fact, as will presently
be explained. Besides this hydrate there exists also a crystallo-hyclrate
HC1,2H2O,37 which is formed by the absorption of hydrochloric acid
by a saturated solution at a temperature of — 23°. It crystallises and
melts at -18°.:w

•"' This crystallo-hydrate (obtained by Pierre and Puchot, and investigated by Rooze-
l)()oin i is analogous to XaCl,2H.,O. The crystals HC1,2H.,O at -22° have a specific gravity
T4C), the vapour tension (dissociation) of the solution having a composition HC1,2H2O
at -24° =7(50, at -19° =1010, at -18° = 1057, at -17° = 1112 mm. of mercury. In a solid
-tatt- tin- crystallo-hydrate at — 17'7° has the same tension, whilst at lower temperatures
it is much less : at —24° about 150, at —19° about 580 mm., at — 17*2° about 10 atmo
•>pheres; at —13° about 150 atmospheres. A mixture of fuming hydrochloric acid with
snow reduces the temperature to —38°.

•~8 According to Roscoe at 0° a hundred grams of water at a pressure^ (in millimetres
of mercury) dissolves —

p = 100 200 300 500 700 1000

Grams HC1 65'7 70'7 73'8 7*-2 Hl'7 s.Vf,

At a pressure of 7<>0 millimetres and temperature t, a hundred grams of water dissolves

t = 0 8° 16° -24 3 40° 60°

Grams HC1 82'5 78'3 74'2 70'0 (i3'3 5<M

Roozeboom (1886) showed that at t° solutions containing c grams of hydrogen chloride per
100 grams of water may (with the variation of the pressure p) be formed together with
the crystallo-hydrate HCl,2H.jO :

t = -28-8° -21° -19° —18° -l- 17'7°

C = 84-2 8(5-8 92'(5 !)8'4 101'4

p = -334 580 900 1073 mm.

The last composition answers to the melted crystallo-hydrate HC1,2H.»O, which splits up
at temperatures above — 17'7°, and at a constant atmospheric pressure when there are
no crystals —

t = -24° —21° -18° —10° 0

c = .101-2 98-8 95-7 89'8 84'2

From these data it is seen that the hydrate HC1,2H._,O can exist in a liquid state, which
is not the case for the hydrates of carbonic and sulphurous anhydrides, chlorine, &c.

According to Marignac, the specific heat c of a solution HC1 + wH2O (at about 30°,
taking the specific heat of water = 1) is given by the expression —

C(36'5 + w!8) = 18/rc - 28'89 + 140/w - 2<>8/ w2
if in be not less than 6'25. For example, for HC1 + 25H->O C = 0'877.

According to Thomsen's data, the amount of heat Q, expressed in thousands of calories,
evolved in the solution of 3(5'5 grams of gaseous hydrochloric acid in ?«H2O or 18m
grains of water —

m =2 4 10 50 400

Q = 11-4 14-3 16-2 17-1 17'3

In these quantities the latent heat of liquefaction is included, which, judging by
analogy (page 821), must be taken as 5-9 thousand calories per molecular quantity of
hydrogen chloride.

The researches of Scheffer (1888) on the rate of diffusion (in water) of solutions of

The mean (from the observations t f I" re, Kremer>. Kolbo, Herthe-
lot. Marianne, and Kohlraiisch ) specific uravities at 1 -V . taking water
at it- maximum density ( I ) as IOIMKI, for solutions containing ]> per
cent, of hydrogen eldnride. are u'iven in tin- following tal)le in the
columns, under the svmlio] x. Their possible error is not less than it ±
and is scarcelv greater than ± 1".

K'lM-j 1 Hi' in li.-.

In lul'.Hi lull'i* .",()

i:> Ki74! K»74i» :',:»

•_M i 1 1 1 'i 1 1 1 1 1 " '.") 4 1 1

L nder the letter .S iii the third coltimn is Driven the specific LjTavitv
calculated according to the formula N = '.|(J(.i I •(> -f (!>• j \\/, 4- ()-().")7 !//-'.
until /< = I'-ViM'i, \\hich answers to the hydrate II C'l.ii 1 1 .,( ) mentioned

above. Abo\e this percentaLTe S=.(M^~r\ -f (').")• 1 I )/, — ( I'L'4 ( )y/-. A com-
parison of the results obtained by experiment and calculation (,s with
>'i sj,o\\s tha; thecur\'e is in perfect harnioiiv \\pith the mean result
of observation. The necessity of t^'o cttr\'es is alread\- indicated by
the tact that the rise of specitic u'ra\'it\" \\"ith an increase of iiercentau'e

liet \\ecii |( and I1' p.c. tlie ineaii ditl'erence of x, correspi mdinu' \vitli
1 p.c. = (!>•>. lietxveen l'1 • and .'lo ji.c. = .")!'• 1 . and liet\veen MO and
I11 p.<-. = 17''\ judu'inu' from the results of experiment. The inter-
medi;i'c solution, IK 'l.fill .,« >. i> furtlier dist inu'uislied l»v the fact that
the van at ion oi l he scci tic L; ra \ it v \\ il h the va ria t ion of t em e rat lire is

II. n-i,:i;,! CMTIV |H,I,«[- will, HCI.C.H .0,

• !••. . -i l.\ .1 |).ir,ilM,h, ,.| tin- Iliinl

., • • • r.I. nr. uith r.-ality.rith.'r

i i .i,-i,|. ali.-n- tin-

CIILHKIDE-BERTHOLLET'S LAWS 447

water, the sporitir gravity at 1° (or the differential ' j rises with a

rise of temperature, for the difference between £0 — £15 and *S'ir,~.S'j(1
(i.e. the difference between the specific gravities at 0°, 15°, and 30°)
varies as follows : "

]>= 0 5 10 15 20

,S'0-,S',- = 7-1 23 38 52 64

Slt,-S80=33-9 42 50 59 67

Whilst for solutions which contain a greater proportion of hydrogen
chloride than HC1,6H2O, these coefficients decrease with a rise of
temperature ; for instance, for 30 p.c. of hydrogen chloride £0 — £15=88
and £1.5 — £3o=87 (according to Marigiiac's data). In the case of
HC1,6H.20 these differences are constant, and equal 76.

Thus the formation of two definite hydrates, HC1,2H2O and
HC1,6H2O, between hydrochloric acid and water may be accepted
upon the basis of many facts. But both of them, inasmuch as they
occur in a liquid state, dissociate with great facility into hydrogen
chloride and water, and are completely decomposed when distilled.

All solutions of hydrochloric acid present the properties of an
energetic acid. They not only transform blue vegetable colouring-
matter into red, and disengage carbonic acid gas from carbonates, &c.,
but they also entirely saturate bases, even such energetic ones as pot-

uniform, then the modulus of expansion must be taken as —ds/dtS0] attains a magnitude
ii (1(10147 at about 48°, it might be thought that at 48° all solutions of hydrochloric acid
would have the same modulus of expansion, but in reality this is not the case. At low
and the ordinary temperatures the modulus of expansion of aqueous solutions is greater
than that of water, and the greater it is the greater the amount of the substance dissolved
(for this reason the temperature of maximum density, when the modulus = 0, is lowered
by solution). But for water the modulus of expansion ( — ds/dtS,,} rises rapidly with the
temperature, whilst for solutions the increase is much slower (or even falls, as for
sulphuric acid or fuming hydrochloric acid), and therefore at a certain temperature t the
modulus of solutions becomes equal to that of water. This temperature may be termed
the ' characteristic temperature.' For solutions of sodium chloride it is about 58°, for
lithium chloride 80°, for potassium nitrate about 80°, for weak solutions of sulphuric
acid about 68°. In order to illustrate this by an example I will insert the magni-
tudes of denomination of expansion (multiplied by 10000) of solutions of sodium
chloride : —

0° 20° 50° 60° 80° 100°

Water . . . -0(55 2'07 8'64 5'11 6'25 7'<>'.)
10 p.c. NaCl . 2'8 8-4 4'8 5'0 5'7 6'8

20 p.c. . . . 8-(5 4-0 4'5 5'0 5'4 5'8

41 The figures cited above may serve for the direct determination of the variation of
the specific gravity of solutions of hydrochloric acid with the temperature, because we
may take S, = S,> - t (A-Bt). Thus, knowing that at 15° the specific gravity of a
in p.c. solution of hydrochloric acid = 10492, we find that at f° it = 10580 -
Whence also may be found the modulus of expansion (Note 40).

448 PRINCIPLES OF CHEMISTRY

ash, lime, <kc. In a dry state, however, hydrochloric acid does not alter
vegetable dyes, and does not accomplish many double decompositions
which easily take place in the presence of water. This is explained by
the fact that the gaso-elastic state of the hydrochloric acid prevents its
entering into reaction. However, incandescent iron, zinc, sodium, &c.,
act on gaseous hydrochloric acid, displacing the hydrogen and leaving
half a volume of hydrogen against one volume of hydrochloric acid gas ;
this reaction may serve for determining the composition of hydrochloric
acid. Combined with water the hydrochloric acid gas is deprived of its
elasticity, and loses a considerable amount of heat in its passage into
a liquid and combined state. It then acts as an acid which much
resembles nitric acid 42 in its energy and in many of its reactions ; how-
ever, the latter contains oxygen, which is disengaged with great ease,
and so very frequently acts as an oxidiser, which hydrochloric is not
capable of doing. The majority of metals (even those which do not
displace the H from H.2SO4 but which, like copper, decompose it to the
limit of SO2) displace the hydrogen from hydrochloric acid. Thus
hydrogen is disengaged by the action of zinc, and even of copper and
tin. Only a few metals withstand its action ; for example, gold and
platinum. Lead is only acted on feebly in compact masses, because the
lead chloride formed is insoluble and prevents the further action of the
acid on the metal. The same is to be remarked with respect to the
feeble action of hydrochloric acid on mercury and silver, because the
compounds of these metals, AgCl and HgCl, are insoluble in water.
Metallic chlorides are not only formed by the action of hydrochloric
acid on the metals, but also by many other methods ; for instance, by
the action of hydrochloric acid on the carbonates, oxides, and hydrox-
ides, and also by the action of chlorine on metals and certain of their
compounds. Metallic chlorides have a composition MCI ; for example,
NaCl, KC1, AgCl, HgCl, if the metal replaces hydrogen equivalent for
equivalent, or, as it is said, if it be monatomic or univalent. In the case
of bivalent metals, they have a composition MC12 ; for example, CaCl.2,
CuCl2, PbCl2, HgCl2, FeCl2, MnCl2. The composition of the haloid
salts of other metals presents a further variation ; for example, A1C13,
Fe2Cl6, PtCl4, &c. Many metals, as will have been remarked in the
preceding examples, give several degrees of combination with chlorine,
as with hydrogen. In their composition the metallic chlorides differ

42 Thus, for instance, with feeble bases they evolve in dilute solutions (Chapter III.
Note 58) almost equal amounts of heat ; their relation to sulphuric acid is quite identical.

' They both form fuming solutions as well as hydrates ; they both form solutions of con-
stant boiling point ; in the solutions of both (with HCl,6HoO and with NHO5,5H2O) th«-

•direction of the differential da/ dp varies, &c.

M CHLORIDE— BERTIK >LLKT S LAWS 449

from the corresponding oxides in that the O is replaced by C12, as
should follow from the law of substitution, because oxygen gives OH.,
and is consequently bivalent, whilst chlorine forms HC1, and is there-
fore univalent. So, for instance, ferrous oxide, FeO, corresponds with
ferrous chloride. FeCl2, and the oxide Fe.2O3 with ferric chloride, which
is also seen from the origin of these compounds, because FeCl2 is ob-
tained by the action of hydrochloric acid on ferrous oxide or carbonate
and FeCl 3 by its action on ferric oxide. The action of hydrochloric
acid, like that of every acid, on basic oxides, MO, and in general on
.M < >„„ consists in a double decomposition, water, raH2O, and a metallic
chloride, MHC12JI,, being formed. Hydrochloric acid, HC1, acts in the
same manner on the hydroxides of the bases M,4(OH)2m -t- 2wHCJ
= '2mH.,O -t- M,,Cl2m, and on carbonates, for instance, Na2CO3 + 2HC1
= 2XaCl 4- H.2O + CO2. In a word, all the typical properties of
acids are shown by hydrochloric acid, and all the typical properties
of salts in the metallic chlorides derived from it. Acids and salts
composed like HC1 and M,,Cl.2m without any oxygen bear the name of
haloid salts ; for instance, HC1 is a haloid acid, Nad a haloid salt,
chlorine a halogen. Certain higher degrees of combination of chlorine
with the metals disengage chlorine when heated to redness ; for in-
stance, the salt Cud., is converted into CuCl. The capacity of hydro-
chloric acid to give, by its action on bases, MO, a metallic chloride,
MCI.,, and water, is limited at high temperatures by the reverse re-
action MC12 -f H.,O = MO + 2HC1, and the more pronounced are
the basic properties of MO the feebler is the reverse reaction, while
for feebler bases such as A12O3, MgO, <tc., this reverse reaction pro-
ceeds with ease. In this manner Deville obtained crystals of the
amorphous oxides Fe.2O3, SnO2, ttc., by passing a slow current of
hydrogen chloride over them at a red heat ; Fe2ClG and SnCl4 were
momentarily formed, but were again decomposed by the water liberated
and the liberated oxides were separated as crystals. Metallic chlorides
corresponding with the peroxides either do not exist, or are easily decom-
posed with the disengagement of chlorine. Thus there is no compound
BaCl4 corresponding with the oxide BaO2. Metallic chlorides having
the general aspect of salts, like their representative sodium chloride,
are, as a rule, easily fusible, more so than the oxides (for instance, CaO
is infusible whilst CaCl2 is easily fused), and many other salts. Under
the action of heat many are more stable than the oxides, many are even
converted into vapour ; thus corrosive sublimate, HgCl2, is particularly
volatile, whilst the oxide HgO decomposes at a red heat. Silver chloride,
AgCl, is fusible and is decomposed with difficulty, whilst Ag2O is easily
decomposed. The majority of the metallic chlorides are soluble in
VOL. i. G G

450 PRINCIPLES OF CHEMISTRY

water, but silver chloride, cuprous chloride, mercurous chloride, and
lead chloride are sparingly soluble in water, and are therefore easily
obtained as precipitates when a solution of the salts of these metals is
mixed with a solution of any chloride or even with hydrochloric acid.
The metal contained in a haloid salt may often be replaced by another
metal, or even by hydrogen, just as is the case with a metal in an.
oxide. Thus copper displaces mercury from a solution of mercuric
chloride, HgCl2 4- Cu = CuCl.2 + Hg ; thus hydrogen at a red heat dis-
places silver from silver chloride, 2AgCl + H.2 = Ag2 + 2HC1. These,
and a whole series of similar reactions, form ^ the typical methods of
double saline decompositions. The measure of decomposition and the
conditions under which one or the other side of reactions of double
saline decompositions proceeds (for instance, whether a metal and an
acid give hydrogen and a salt or the reverse) are determined by the
properties of the compounds which are in action, and which are able
to be formed at the temperature, <fcc., as was shown when speaking of
sodium chloride, and as will be frequently found hereafter.

If hydrochloric acid enters into double decomposition with basic
oxides and their hydrates, this is only due to its acid properties ; and
for the same reason it rarely enters into double decomposition with
acids and acid anhydrides. Sometimes, however, it combines with the
latter, as, for instance, with the anhydride of sulphuric acid, forming
the compound SO3HC1 ; and in other cases it acts on acids, giving up
its hydrogen to their oxygen and forming chlorine, as will be seen in
the following chapter.

Hydrochloric acid, as may already be concluded from the compo-
sition of its molecule, belongs to the monobasic acids, and does not,
therefore, give true acid salts (like HNaSO4 or HNaCO3) ; nevertheless
many metallic chlorides, formed from powerful bases, are capable of
combining with hydrochloric acid, just as they combine with water,
or with ammonia, and as they give double salts. Compounds have long
been known of hydrochloric acid with auric, platinic, and antimo-
nious chlorides, and other similar metallic chlorides corresponding
with very feeble bases. But Berthelot, Engel, and others have shown
that the capacity of HC1 for combining with MnClm is much more
frequently encountered than was before supposed. Thus, for instance,
dry hydrochloric acid when passed into a solution of zinc chloride
(containing an excess of the salt) gives in the cold (0°) a compound
HCl,ZnCL2,2H2O,and at the ordinary temperature HCl,2ZnCl2,2H2O,
just as it is able at low temperatures to form the cry stallo- hydrate
ZnCl2,3H2O (Engel, 1886). Similar compounds are obtained with
CdCl2, CuCl2, HgCl2, Fe.2Cl6J &c. (Berthelot, Ditte, Cheltzoff, Lachinoff,

SODIl'M < IILOKIDE— EERTHOLLETS LAWS

and others). These compounds with hydrochloric acid are generally
more soluble in water than the metallic chlorides themselves, so that
whilst hydrochloric acid decreases the solubility of MClm, correspond-
ing with energetic bases (for instance, sodium or barium chlorides), it
increases the solubility of the metallic chlorides corresponding with
feeble bases (cadmium chloride, ferric chloride, tfec.). Silver chloride,
which is insoluble in water, is soluble in hydrochloric acid. Hydro-
chloric acid also combines with certain unsaturated hydrocarbons (for
instance with turpentine, C10H162HC1) and their derivatives. Sal-
ammoniac, «>r ammonia hydrochloride, NH4C1 = NH3,HC1, also belongs
to this class of compounds.43 If dry hydrogen chloride be mixed
with dry ammonia, a solid compound formed from equal volumes of
each is immediately formed. The same compound is formed on mixing
solutions of the two gases. It is also produced by the action of
hydrochloric acid on ammonium carbonate. Sal-ammoniac is usually
prepared by the last method in practice.44 The specific gravity of sal-
ammoniac is 1-55. We have already seen (p. 253) that sal-ammoniac,

4r> When an unsaturated hydrocarbon, or, in general, an unsaturated compound,
assimilates to itself the molecules CL, HC1, SO5, HoSO4, &c., the cause of the reaction
is most simple. As nitrogen, besides the type NX3 to which NH3 belongs, forms com-
pounds of the form NX5 — for example, NO2(OH) — therefore the formation of the salts of
ummonium should be understood in this sense. NH- gives NH4C1 because NX5 is
capable of giving NX5. But as saturated compounds — for instance, HC1, H2O, NaCl,
Arc. — are also capable of combination even between themselves, therefore it is impossible
; ( » deny the capacity of HC1 also for combination. SO3 combines with HoO, and also with
HC1 and the unsaturated hydrocarbons. It is impossible to see the limit here, which it
\vas lately wished to establish, by distinguishing atomic from molecular compounds, and
regarding, for instance, PC15 as an atomic compound and PC15 as a molecular one, only
because it easily splits up into molecules PC15 and Clo.

44 Sal-ammoniac is prepared from ammonium carbonate, obtained in the dry distilla-
tion of nitrogenous substances (Chapter VI.), by saturating the resultant solution with
hydrochloric acid. A solution of sal-ammoniac is thus produced, it is evaporated, and
in the residue a mass is obtained containing a mixture of various other, especially tarry,
products of dry distillation. The sal-ammoniac is generally purified by t sublimation.
For this purpose iron vessels covered with semispherical metallic covers are employed,
or else simply clay crucibles covered by other crucibles. The upper portion, or helmet,
of the apparatus of this kind will have a lower temperature than the lower portion, which
is under the direct action of the flame. The sal-ammoniac volatilises when heated, and
settles on the cooler portion of the apparatus. It is thus purified from many impurities,
and is obtained as a crystalline crust, generally several centimetres thick, in which form
it is generally sold.

In order to demonstrate the very instructive formation of solid sal-ammoniac from
the gases NH3 + HC1 we may proceed as follows : A thin glass tube is filled with ammonia
and closed with a cork, and placed in a thick glass cylinder into which a rapid stream of
hydrogen chloride is introduced and is then closed with a cork; the inside tube is then
broken by violently shaking the apparatus. The ammonium chloride is obtained in white
Hakes. A glass rod, or paper moistened with hydrochloric acid, also gives a distinct
dond of sal-ammoniac when held over the mouth of a bottle containing a strong solution
of ammonia.

o o 2

452 PRINCIPLES OF CHEMISTRY

like all other ammonium salts, easily decomposes ; for instance, by
volatilisation with alkalis, and even partially when its solution is
boiled. But at higher temperatures ammonia decomposes with the
liberation of hydrogen, and therefore ammonia is then able to act
as a reducing agent — as, for example, on metallic oxides. The other
properties and reactions of sal-ammoniac, especially in solution, fully
bring to mind what has been already mentioned in speaking of sodium
chloride. Thus, for instance, with silver nitrate it gives a precipitate
of silver chloride ; with sulphuric acid it gives hydrochloric acid and
ammonium sulphate, and it forms double salts with certain metallic
chlorides and other salts.45

45 The solubility of sal-ammoniac in 100 parts of water (according to Alluard) is —

0° 10° 20° 30° 40° 60° 80° 100° 110°

28-40 32-48 37'28 41'72 46 55 64 73 77

A saturated solution boils at 115-8°. The specific gravity at 15°/4° of solutions of sal-
ammoniac (water 4° = 10000) = 9991'6 + 31-26^ — 0'085p2, where p is the amount by weight
of ammonium chloride in 100 parts of solution. With the majority of salts the differen-
tial dsjdp increases, but here it decreases with the increase of p. For (unlike the
sodium and potassium salts) a solution of the alkali phis a solution of acid occupy a
greater volume than that of the resultant ammonium salt. In the solution of solid'
ammonium chloride a contraction, and not expansion, generally takes place. It may
further be remarked that solutions of sal-ammoniac have an acid reaction even when
prepared from the salt remaining 'after prolonged washing of the sublimed salt with
water (A. Stcherbakoff ).

453

CHAPTER XI

THE HALOGENS : CHLORINE, BROMINE, IODINE, AND FLUORINE

ALTHOUGH hydrochloric acid, like water, is one of the most stable
substances, it is nevertheless decomposed not only by the action of a
.galvanic current l but also by a rise of temperature. Sainte-Claire Deville
showed that decomposition already occurs at 1 300°, because a cold tube
(p. 388) covered with an amalgam of silver absorbs chlorine in a red-
hot tube, and the escaping gas contains hydrogen. Meyer and Langer
(1885) observed the decomposition of hydrochloric acid at 1690° in a
platinum vessel ; the decomposition in this instance was proved not
only from the fact that hydrogen permeated through the platinum
(p. 141), owing to which the volume was diminished, but also from
chlorine being obtained in the residue (the. hydrogen chloride was
mixed with nitrogen), which liberated iodine from potassium iodide.2
The usual method for the preparation of chlorine consists in the
abstraction of the hydrogen by oxidising agencies.

The acid properties of hydrochloric acid were known when
Lavoisier pointed out the formation of acids by the combination of
water with the oxides of the non-metals, and therefore there was reason
for thinking that hydrochloric acid was formed by the combination of
water with the oxide of some element. Therefore when Scheele
obtained chlorine by the action of hydrochloric acid on manganese
peroxide he considered it as the acid contained in common salt. When
it became known that chlorine gives hydrochloric acid with hydrogen,
Lavoisier and Berthollet supposed it to be a compound with oxygen of
<in anhydride contained in hydrochloric acid. They supposed that
hydrochloric acid contained water and the oxide of a particular radicle

1 The decomposition of fused sodium chloride by an electric current has been proposed
in America and Russia (N. X. Beketoff) as a means for the preparation of chlorine and
sodium. A strong solution of hydrochloric acid is decomposed into equal volumes of
chlorine and hydrogen by the action of an electric current.

2 To obtain so high a temperature (at which the best kinds of porcelain soften) Langer
.Did Meyer employed the dense graphitoidal carbon from gas retorts, and a strong draught.
They determined the temperature by, the alteration of the volume of nitrogen in the
platinum vessel, for it does not permeate through platinum, and is not altered by heat.

454 PRINCIPLES OF CHEMISTRY

and that chlorine was a higher degree of oxidation of this radicle
murias (from the Latin name of hydrochloric acid, aciduni niuriati-
cum). It was only in 1811 that Gay-Lussac and Thenard in France
and Davy in England arrived at the conclusion that the substance
obtained by Scheele does not contain oxygen, nor under any conditions,
give water with hydrogen, and that there is no water in hydrochloric
acid gas, and therefore concluded that chlorine is an elementary
substance. They named it * chlorine ' from the Greek word ;(Aoy)09,
signifying a green colour, because of the peculiar colour by which this
gas is characterised.

An aqueous solution of hydrochloric acid is generally employed for
the evolution of chlorine. The hydrogen has to be abstracted from the
hydrochloric acid. This is accomplished by nearly all oxidising
substances, and especially by those which are able to evolve oxygen at
a red heat (besides bases, such as mercury and silver oxides, which are
able to give salts with hydrogen chloride); for example, manganese
peroxide, potassium chlorate, chromic acid, itc. The decomposition
essentially consists in the oxygen of the oxidising substance displacing
the chlorine from 2HC1, forming water, H.2O, or taking up the hydrogen
and setting the chlorine free, as is sometimes said, 2HC1 + O (disengaged
by the oxidising substances) =H2O + C12. Even nitric acid partially
produces a like reaction, but as we shall afterwards see its action is
more complicated, and it is therefore not suitable for the preparation of
pure chlorine. But other oxidising substances which do not give any
other volatile products with hydrochloric acid may be employed for
the preparation of chlorine. Among these may be mentioned :
potassium chlorate, acid potassium chromate, sodium manganate,
manganese peroxide, &c. Manganese peroxide is commonly employed
in the laboratory, and on a large scale, for the preparation of chlorine.
The chemical process which proceeds in this case may be represented
as follows : an exchange takes place between 4HC1 and MnO2, in
which the manganese takes the place of the four atoms of hydrogen, or
the chlorine and oxygen exchange places that is, MnCl, and 2H2O are
produced. The chlorine compound, MnCl4, obtained is very unstable ;
it splits up into chlorine, which as a gas passes from the sphere of
action, and a lower compound containing less chlorine than the
substance first formed, and which remains in the apparatus in which
the mixture is heated, MnClt = MnCL2 + Cl.,.3 The action of hydro-

3 This representation of the process of the reaction is the most natural. However,,
this decomposition is generally represented as if chlorine gave only one degree of comlii
nation with manganese, MnCLj, and therefore directly reacts in the following manner —
.^ + 4HG1 = MnCL + 2ILO + Clo, in which case it is, as it wen-, supposed that

TIIK HALodKNs 455

chloric acid requires a temperature of about 100°. In the laboratory
the preparation <>f r///(>,-hte is carried on in flasks, heated over a water
bath, by acting on manganese peroxide with hydrochloric acid or a

manganese peroxide, MnO >, breaks up into manganous oxide, MiiO, and oxygen, both of
which react with hydrochloric acid, the manganous oxide acting as abase, MnO + 2HC1 =
MnCl , H ._,(), and at the same time '2HC1 + O = ELO + CL. In reality a mixture of
oxygen and hydrochloric acid does give chlorine at a red heat, and this may also take
place at the moment of its evolution in this case.

All the oxides of manganese (Mn.,O-, MnO.,, MnO-,, Mn-jOy), with the exception of man-
ganou s ox ide. MnO, disengage chlorine from hydrochloric acid, because manganous chloride,
MnCL, is the only compound of chlorine and manganese which exists as a stable compound,
all the higher chlorides of manganese being unstable and evolving chlorine. The reaction
between hydrochloric acid and salts containing much oxygen is explained according to the
law of substitution by the double decomposition and instability of the higher chlorides;
thus, for example, by taking potassium dichromate, K,Cr.>O7, a compound, K,Civ>Cl|.,,
might be formed, but it is unknown, and if it is formed it in all probability immediately
decomposes into chlorine, (5C1, potassium chloride, 2KC1, and chromium chloride, Cr.^Clg.
Hence we here encounter two circumstances : (1) a substitution between oxygen and chlo-
rine, and (2) the instability of those higher chlorine compounds. Both these circum-
stances have a very important signification for the comprehension of the relation of
such elements as chlorine and oxygen. As (according to the law of substitution) in the
substitution of oxygen by chlorine, CL takes the place of O, therefore the chlorine com-
pounds will contain in themselves more atoms than the corresponding oxygen compounds.
It is not surprising, therefore, that certain of the chlorine compounds corresponding with
oxygen compounds do not exist, or if they are formed are very unstable. And further-
more, an atom of chlorine is heavier than an atom of oxygen, and therefore a given
element would have to retain a large mass of chlorine if in the higher oxides the oxygen
were replaced by chlorine. For this reason equivalent compounds of chlorine do not
exist for all oxygen compounds. Many of the former are immediately decomposed, when
formed, with the evolution of chlorine. Therefore chlorine is evolved by the action of
hydrochloric acid on compounds which contain much oxygen. From this it is evident that
there should exist such chlorine compounds as would evolve chlorine as peroxides evolve
oxygen, and indeed a large number of such compounds are known. Amongst them may
be mentioned antimony pentachloride, SbCls, which splits up into chlorine and antimony
trichloride when heated. Cupric chloride, corresponding with copper oxide, and having
a composition CuClo, similar to CuO, when heated parts with half its chlorine, just a-
barium peroxide evolves half its oxygen. This method may even be taken advantages of
for the preparation of chlorine and cuprous chloride, CuCl. The latter attracts oxygen from
the atmosphere, and in so doing is converted from a colourless substance into a green
compound whose composition is CuoCLO. With hydrochloric acid this substance gives
cupric chloride (CuoCloO + 2HC1 = H.^O + 2CuCl3), which has only to be dried and heated,
and it again evolves chlorine. Thus, in solution, and at the ordinary temperature, the
compound CuCl., it, ((instant, but when heated it splits up. On this property is founded
Deacon's process for the preparation of chlorine from hydrochloric acid by the aid of air
and copper salts, by passing a mixture of air and hydrochloric acid at about 440° over
bricks soaked with a solution of a copper salt (a mixture of solutions of CuSO4 and Na^SOj).
CuClo is then formed by the double decomposition of the salt of copper and the hydro-
chloric acid ; the CuClo liberates chlorine, and the CuCl forms CuoCLO with the oxygen of
the air, which again gives CuClo with 2HC1, and so on.

Magnesium chloride, which is obtained from sea-water, carnallite, &c., may serve not
only as a means for the preparation of hydrochloric acid, but also of chlorine, because
(Weldon-Pechiuv's process) its basic salt (magnesium oxychloride) when heated in the
air gives magnesium oxide and chlorine. Chlorine is now prepared on a large scale by
this method.

456 PRINCIPLES OF CHEMISTRY

mixture of common salt and sulphuric acid 4 and washing the gas with
water to remove hydrochloric acid. The action of hydrochloric acid on
bleaching powder (p. 161) gives chlorine without the aid of heat :
CaCl2O2 + 4HCl=CaCl, + 2H,O + 2Cl.2, and is therefore also taken
advantage of in the laboratory for the preparation of chlorine/'
Chlorine cannot be collected over mercury, because it combines with it
as with many other metals, and it is soluble in water ; however, it is
but slightly soluble in hot water or brine. Owing to its great weight,
chlorine may be directly collected in a dry vessel by introducing the
gas-conducting tube into the bottom of the vessel. The chlorine will
lie in a heavy layer at the bottom of the vessel, displace the air, and
the extent to which it fills the vessel may be followed by its colour.6

4 The following proportions are then taken by weight : 8 parts of powdered manga-
nese peroxide, 4 parts of salt (best fused, to prevent its frothing), and 9 parts of sulphuric-
acid previously mixed with 5 parts of water. It is heated in a salt bath, so as to obtain a
temperature above 100°. The corks in the apparatus must be soaked in paraffin (other-
wise they are corroded by the chlorine;, and black india-rubber tubing must be taken, and
not vulcanised (which contains sulphur, and becomes brittle under the act i< m of the chlorine).
The reaction which proceeds may be expressed thus: MiiO.^, ; '2XaCl + 2H.>SO4 =
MnSO4 + NaoSO4 -f 2H.2O + C12. The preparation of C1.2 from manganese peroxide and
hydrochloric acid was discovered by Scheele, and from sodium chloride by Berthollet.

6 The reaction of hydrochloric acid on bleaching powder is very violent if all the
acid be added at once ; it should be poured in drop by drop (Merme, Kammerer).
C. Winkler proposed mixing bleaching powder with one quarter of burnt and powdered
gypsum, and having damped the mixture with water, to press aiad cut it up into cubes and
dry at the ordinary temperature. These cubes can be used for the preparation of chlorine
in the same apparatus as that used for the evolution of hydrogen and carbonic anhydride
— the disengagement of the chlorine proceeds uniformly.

A mixture of potassium dichromate and hydrochloric acid evolves chlorine perfectly
free from oxygen (V. Meyer and Langer).

6 Chlorine is manufactured on a large scale from manganese peroxide and hydrochloric-
acid. It is most conveniently prepared in the arrangement shown in fig. (57, which con-
sists of a three-necked earthenware vessel whose central orifice
is the largest. A clay or lead funnel, furnished with a number
of orifices, is placed in the central wide neck of the vess.-l.
Roughly-ground lumps of natural manganese peroxide are placed
in the funnel, which is then closed by the cover N, and luted
with clay. One orifice is closed by a clay stopper, and is used for
the introduction of the hydrochloric acid and withdrawal of the
residues. The chlorine disengaged passes along a leaden gas-
conducting tube placed in the other orifice. A row of these
vessels is surrounded by a water-bath, to ensure their being
uniformly heated. Manganese chloride is found in the residue.
rineon a large scale. In Weldon's process lime is added to the acid solution of man-
ganese chloride. A double decomposition takes place, resulting

in the formation of manganous hydroxide and calcium chloride. When the insoluble
manganous hydroxide has settled, a further excess of milk of lime is added (to make
a mixture 2Mn(OH)2 + CaO + xCaCl.>, which is found to be the best proportion, judging
from experiment), and then air is forced through the mixture. The hydroxide is then
converted from a colourless to a brown substance, containing peroxide, MnOo, and oxide
of manganese, Mn2Oj. This is due to the manganous oxide absorbing oxygen from the

THE HALOGENS 457

Chlorine is a //</* of a yellowish green colour, and has a very
suffocating and characteristic odour. On lowering the temperature to
— .~)0° or increasing the pressure to six atmospheres (at 0°) chlorine
condenses7 into a liquid which has a yellowish green colour and a
density of 1'3, and boils at — 34°. The density and atomic weight of
chlorine is 35*5 times greater than that of hydrogen, hence the molecule
contains C1.2.8 At 0° one volume of water dissolves about H volumes
of chlorine, at 10° about three volumes, at 50° again 1^ volumes.9
Such a solution of chlorine is termed ' chlorine water ' ; and is employed
in a diluted form in medicine and laboratory practice. It is prepared

air. Under the action of hydrochloric acid this mixture evolves chlorine, because of all
the compounds of chlorine and manganese the chloride MnClo is the only one which is
staMe (see Note 3). Thus one and the same mass of manganese may be repeatedly used
for the preparation of chlorine. The same result is attained in other ways. If manga-
nous oxide be subjected to the action of oxides of nitrogen and air (Coleman's process),
then manganese nitrate is formed, which at a red heat gives oxides of nitrogen (which are
again used in the process) and manganese peroxide, which is thus renewed for the fresh
evolution of chlorine.

7 Davy and Faraday liquefied chlorine in 1823 by heating the crystallo-hydrate
CLMtLO in a bent tube (like that shown in fig. 45, p. 248), in warm water, while the other
«nd of the tube was immersed in a freezing mixture. Meselan condensed chlorine in
freshly-burnt charcoal (placed in a glass tube), which when cold absorbs an equal weight
of chlorine. The tube was then fused up, the bent end cooled, and the charcoal heated,
by which means the chlorine was expelled from the charcoal, and the pressure increased.

8 Judging from Ludwig's observations (1868), and from the fact that the coefficient of
expansion of gases increases with their molecular weight (Chapter II. Note 26, for hydrogen
= 0'367, carbonic anhydride =0*373, hydrogen bromide — 0'386),it might be expected that
the expansion of chlorine would be greater than that of air or of the gases composing it.
V. Meyer and Langer(lSKf>) having remarked that at 1400° the density of chlorine (taking its
expansion as equal to that of nitrogen) =29, consider that the molecules of chlorine split
up and partially give molecules Cl, but it might be thought that the decrease in density
observed only depends on the increase of the coefficient of expansion.

9 Investigations on the solubility of chlorine in water (the solutions evolve all their
chlorine on boiling and passing air through them) show many different peculiarities.
First (ray-Lussac, and then Pelouze, determined that the solubility increases between
0° and 8° to 103 (from l£ to 2 vols. of chlorine per 100 vols. of water atO3 to 8 to 2J at 10°).
In the following note we shall see that this is not due to the breaking- up of the hydrate at
about 8J to 10°, but to its formation below 9°. Roscoe observed an increase in the solu-
bility of chlorine in the presence of hydrogen — even in the dark. Berthelot determined
an increase of solubility with the progress of time. Schb'nebein and others suppose that
chlorine arts <>n water, forming hypochlorous and hydrochloric acids fHClO + HCl).

The equilibrium between chlorine, steam as a gas, between water, liquid chlorine, ice,
and the solid crystallo-hydrate of chlorine is evidently very complex. Guldberg (1870)
gave a theory for similar states of equilibrium, which was afterwards developed by Rooze-
boom (1887), but it would be inopportune here to enter into its details. It will be enough
in the first place to mention, that there is now no doubt (according to the theory of heat,
and the direct observations of Ramsay and Young) that the vapour tension at one and the
same temperature are different for the liquid and solid states of substances; secondly, to
call attention to the following note ; and, thirdly, to state that, in the presence of the
crystallo-hydrate, water between 0'24° and + 28'7° (when the hydrate and a solution may
occur simultaneously) dissolves a different amount of chlorine than it does in the absence
of the crystallo-hydrate.

458 PRINCIPLES OF CHEMISTRY

by passing chlorine through a series of Woulfe's Lotties, or into an
inverted retort filled with water. Under the action of light, chlorine
water gives oxygen and hydrochloric acid. At 0° a saturated solution
of chlorine yields a crystallo-hydrate, CL;,8H.2O, which easily splits up
into chlorine and water when heated, so that if it be fused up in a tube
and heated to 35° two layers of liquids are formed — a lower stratum
of chlorine containing a small quantity of water, and an upper stratum
of water containing a small quantity of chlorine.10

Chlorine explodes with hydroyen, if a mixture of equal volumes be
exposed to the direct action of the sun's rays 1 1 or brought into contact
with spongy platinum, or a strongly-heated substance, or subjected to
the action of an electric spark. The explosion in this case takes place

10 According to Faraday's data the hydrate of chlorine contains CL,10H.,O, but Rooze-
boom (1885) showed that it is poorer in water — CL,8!L,O. At. first small, almost
colourless, crystals are obtained, but they gradually form (if the temperature be below
their critical point 28'7°, above which they do not exist) large yellow crystals, like those of
•potassium chromate. The specific gravity is T23. The hydrate is formed if there be
more chlorine in a solution than it is able to dissolve under the dissociation pressure
corresponding with a given temperature. In the presence of the hydrate the percentage
amount of chlorine at 0° = 0'5, 93 = 0'9, and at 20° = 1'82. At temperatures below 9° the
solubility (determined by Gay-Lussac and Pelouze, see Note 9) is dependent on the forma-
tion of the hydrate; whilst at higher temperatures under the ordinary pressure the
hydrate cannot be formed, and the solubility of chlorine falls, as it does for all gases
(Chapter L). If the crystallo-hydrate is not formed, then below 9° the solubility follows
the same rule (6° 1'07 p.c. Cl, 9° 0'95 p.c.). According to Roozeboom the chlorine evolved
by the hydrate presents the following tensions of dissociation at 0° = 249 mm., at 4° = 398,
at 8° = 620, at 10° = 797, at 14° = 1400 mm. In this case a portion of the crystallo-hydvate
remains solid. At 9' 6 the tension of dissociation is equal to the atmospheric pressure. At
a higher pressure the crystallo-hydrate may form at temperatures above 9° up to 28'7°,
when the vapour tension of the hydrate equals the tension of the chlorine. It is evident
that the equilibrium which is established is on the one hand a case of a complex hetero-
geneous system, and on the other hand a case of the solution of solid and gase<m>
substances in water.

The crystallo-hydrate or chlorine water must be kept in the dark, or the access of light
be prevented by coloured glass, otherwise oxygen is evolved and hydrochloric acid
formed.

11 The chemical action of light on a mixture of chlorine and hydrogen was discovered
by Gay-Lussac and Draper (1809). It has been investigated by many, and especially l>y
Draper, Bunsen, and Roscoe. Electric or magnesium light, or the light emitted by the
combustion of carbon bisulphide in nitric oxide, and in general that which forms photo-
graphic images, acts in the same manner as sunlight, according to the intensity. At
temperatures below — 12° light no longer brings about reaction, or at all events does not
give an explosion. It was long supposed that chlorine that had been subjected to tin-
action of light was afterwards able to act on hydrogen in the dark, but it was shown that
this only takes place with moist chlorine, and depends on the formation of oxides of chlo-
rine. The presence of foreign gases, and even of excess of chlorine or of hydrogen, very
much enfeebles the explosion, and therefore the experiment is conducted with a detonat-
ing mixture prepared by the action of an electric current on a strong solution isp. gr. 1'15)
of hydrochloric acid, in which case the water is not decomposed— that is, no oxygen is
mixed with the chlorine.

THK HALorjKNS 459

from exactly the same reasons — i.e. the evolution of heat and expansion
of the resultant product as in detonating gas (Chap. III.). Diffused
light acts in the same way, but slowly, whilst the direct sunlight excites
an explosion.12 The hydrochloric acid gas produced by the reaction of
chlorine on hydrogen occupies (at the same temperature and pressure)
a volume equal to the sum of the original volumes ; that is, a reaction
of substitution here takes place: H., + C12 = HC1 + HC1, the atoms of
chlorine and hydrogen change places — there were two molecules and
two molecules are also obtained, although different molecules were first
taken and like molecules are formed. In this reaction twenty-two
thousand heat units are evolved for one part by weight of hydrogen.13

These relations show that the affinity of chlorine for hydrogen is
very great and analogous to the affinity between hydrogen and oxygen.

- The quantity of chlorine and hydrogen \vliicheombineispropovtional to the intensity
of the light — not of all the rays, but only those so-termed chemical (actinic) rays which
produce chemical action. Hence a mixture of chlorine and hydrogen, when exposed to
the action of light in vessels of known capacity and surface, may be employed as a means
for estimating the intensity of the chemical rays (as an actinometer), the influence of the
heat rays being previously destroyed, which may be done by passing the rays through
water. Investigations of this kind (photo-chemical) showed that chemical action is.
chiefly limited to the violet end of the spectrum, and that even the invisible ultra-violet
rays produce this action. A colourless gas flame contains no chemically-active rays, the
flame coloured given by a salt of copper evinces more chemical action than the colourless
flame, but the flame brightly coloured yellow by salts of sodium has no more chemical
action than that of the colourless flame.

As the chemical action of light becomes evident in plants, photography, the bleaching
of tissues, and the fading of colours in the sunlight, and as a means for studying the
phenomenon is given in the reaction of chlorine on hydrogen, this subject has been the
most fully investigated in photo-chemistry. The researches of Bunsen and Roscoe in
the fifties and sixties are the most complete in this respect. Their actinometer contains
hydrogen and chlorine, and is enclosed by a solution of chlorine in water. The hydro-
chloric acid is absorbed as it forms, and therefore the variation in volume indicates the
progress of the combination. As was to be expected, the action of light proved to be
proportional to the time of exposure and intensity of the light, so that it was possible to
conduct detailed photometrical investigations respecting the time of day and season of
the year, various sources of light, its absorption, &c. This subject is considered in detail
in special works, and we only stop to mention one circumstance, that a small quantity of
a foreign gas decreases the action of light; for example, 3^ of hydrogen by 38 p.c.,
cfaj "f oxygen by 10 p.c.. , (\n of chlorine by 60 p.c., <tc. According to the researches
of Klimenko and IVkatoros ils.siM, the photo-chemical alteration of chlorine water is
retarded by the presence of traces of metallic chlorides, and this influence varies with
different metals.

As much heat i> evolved in the reaction of chlorine on hydrogen, and as this reaction,
being exothermal, may proceed by itself, therefore the action of light is essentially the
same as that of heat — that is, it brings the chlorine and hydrogen into the condition
necessary for the reaction — it, as we may say, shakes the original equilibrium ; this is the
work done by the luminous energy. It seems to me that the action of light on the mixed
gas«-s should be understood in this sense, as Pringsheim (1877) pointed out.

15 In the formation of steam (from one part by weight of hydrogen) 29000 heat units
are evolved. The following are the quantities of heat (thousands of units) evolved in

460 PRINCIPLES OF CHEMISTRY

Thus u on the one hand by passing a mixture of steam and chlorine
through a red-hot tube, or by exposing water and chlorine to the sun-
light, oxygen is disengaged, whilst on the other hand, as we saw above,
oxygen in many cases displaces chlorine from its compound with
hydrogen, and therefore the reaction H2O + C12=2HC1 + O belongs to
the number of reversible reactions, and hydrogen will distribute itself
between oxygen and chlorine if it comes into contact with both these
elements. These relations determine many of the properties of
•chlorine — that is, its relation to substances containing hydrogen and its
reactions in the presence of water, to which we shall turn our attention
after having pointed out the relation of chlorine to other elements.

Many metals when brought into contact with chlorine immediately
•combine with it, and form those metallic chlorides which correspond
with hydrogen chloride and with the oxide of the metal taken. With
a large surface of metal, and if it be slightly heated, this combination
may proceed rapidly with the evolution of heat and light ; that is,
metals are able to burn in chlorine. Thus, for example, sodium lft burns
in chlorine, thus synthesising common salt. Metals in the form of
powders burn without the aid of heat, and become highly incandescent
in the process ; for instance, antimony, which is a metal easily con-
verted into a powder.16 Even such metals as gold and platinum,17

the formation of various other corresponding compounds of oxygen and of chlorine (from
Thomson's, and, for Na.2O, Beketoff 's results) :

f 2XaCl, 195 ; CaCL, 170 ; HgCl,, 03 ; 2AgCl, r,i).

1 NaoO, 100 ; CaO, 131 ; HgO, 42 ; Ag._,O, 6.

f 2AsCl3, 143; 2PC15, 210 ; CC14, 21 ; 2HC1, 44 (gas).

\ AsoO3, 155 ; PoOs, 370 ; CO.,, 97 ; H2O, 58 (gas).

With the first four elements the formation of the chlorine compound gives the most
heat, and with the four following the formation of the oxygen compound evolves the
greater amount of heat. The first four chlorides are true salts formed from HC1 and the
oxide, whilst the remainder have other properties, as is seen from the fact that they are not
formed from hydrochloric acid and the oxide, but give hydrochloric acid with water. If
affinity be measured by heat, then in the former the affinity for chlorine is greater than
that for oxygen, whilst in the latter it is the reverse. But as the physical states of the
substances are different, and as chlorine displaces oxygen as well as oxygen displaces
chlorine, the affinity cannot be determined from thermo-chemical data without a number
of corrections which are still subject to doubt.

14 This has been already pointed out in Chap. III. Note 5.

15 Sodium remains unaltered in perfectly dry chlorine at the ordinary temperature,
and even when slightly warmed ; but the combination is exceedingly violent at a red heat.

16 An instructive experiment on combustion in chlorine may be conducted as follows :
leaves of Dutch metal (used for gilding instead of gold) are placed in a glass globe, and a
gas-conducting tube furnished with a glass cock is placed in the cork closing it, and the
air is pumped out of the globe. The gas-conducting tube is then connected with a vessel
containing chlorine, and the cock opened; the chlorine rushes in, and the metallic leaves
are consumed.

17 The behaviour of platinum to chlorine at a high temperature (1400°) is very

THK HALOGENS 461

which do not combine directly with oxygen and give very unstable
compounds with it, unite directly with chlorine to form metallic-
chlorides. Either chlorine water or aqua regia may be employed for
this purpose instead of gaseous chlorine. These dissolve gold and
platinum, converting them into metallic chlorides. Aqua regia is a
mixture of 1 part of nitric acid with 2 to 3 parts of hydrochloric acid.
This mixture converts into soluble chlorides not only those metals
which are acted on by hydrochloric and nitric acids, but also gold and
platinum, which are insoluble in either acid separately. This action
of aqua regia depends on the fact that nitric acid in acting 011 hydro-
chloric acid deprives it of its hydrogen, and evolves chlorine little by
little as the action proceeds. If the chlorine evolved be transferred to-
a metal, then a fresh quantity is formed from the remaining acids and
combines with the metal.18 Thus the aqua regia acts in virtue of the
chlorine which it contains and disengages.

The majority of non-metals also react directly on chlorine ; sulphur
and phosphorus burn in it, and combine with it directly when heated
or even at the ordinary temperature. Only nitrogen, carbon, and oxygen
do not form any direct compounds with it. The chlorine compounds,
formed by the non-metals — for instance, phosphorus trichloride, PC13,
and sulphurous chloride, Arc., do not have the properties of salts, and
if the metallic chlorides M,,Cl.2m correspond with bases M}iOw and their
hydrates M,,(OH).,m, then, as we shall afterwards see more fully, the-
chlorides of the non-metals bear the same relation to acid anhydrides-
and acids :

NaCl FeCl2 SnCl4 PC13 HC1
Na(HO) Fe(HO)2 Sn(HO)4 P(HO)3 H(HO)

remarkable, because platinous chloride, PtCL, is then formed, whilst this substance de-
composes at a much lower temperature into chlorine and platinum. Therefore, when
chlorine comes into contact with platinum at such high temperatures it forms fumes of
platinous chloride, and they on cooling decompose with the liberation of platinum, so
that the phenomenon appears to be dependent on the volatility of platinum. Deville
proved the formation of platinous chloride by inserting a cold tube inside a red-hot one
(as in the experiment on carbonic oxide, p. 388). However, Meyer was able to observe-
the density of chlorine in a platinum vessel at 1(590°, at which temperature chlorine does
not exert this acjtion on platinum, or at least only to an insignificant degree.

18 When left exposed to the air aqua regia disengages chlorine, and afterwards it no
longer acts on gold. Gay-Lussac, in explaining the action of aqua regia, showed that when
heated it evolves, besides chlorine, the vapours of two chloranhydrides — that of nitric
acid, NOoCl (nitric acid, NOoOH, in which HO is replaced by chlorine, see chapter on
Phosphorus), and that of nitrous acid, NOC1 (ibid.} — but these do not act on gold. The
formation of aqua regia may therefore be expressed by 4NHO3 + 8HC1 = 2NO2C1 -I- 2NOC1 +
tfH2O + 2C1.2. The formation of the chlorides NO2C1 and NOC1 is explained by the fact
that the nitric arid is deoxidised, gives the oxides NO and NO..,, and they directly combine
with chlorine to form the above anhydrides.

462 PRINCIPLES OF CHEMISTRY

As the above-mentioned relation in composition exists between
many chlorine compounds and their corresponding hydrates and as
furthermore some (acid) hydrates are obtained from chlorine compounds
by the action of water, for instance

PC13 + 3H2O = P(HO)3 + 3HC1

Phosphorus w - Phosphorous Hydrochloric

trichloride acid acid ;

whilst other chlorine compounds (basic) are formed from hydroxides
and hydrochloric acid, with the liberation of water, for instance

XaHO + HC1 = NaCl + H,O ;

therefore this intimate connection between the hydrates and chlorine
compounds is endeavoured to be expressed by calling the latter ch /<>/•-
anhydrides. In general terms, if the hydrate be basic, then,

M(HO) + HC1 = MCI + H20

hydrate + hydrochloric acid — choranhydride + water

and if the hydrate ROH be acid, then

RC1 + H,O = R(HO) + HC1

Choranhydride + water = hydrate -,- hydrochloric acid.

In this manner a distinct equivalency is remarked between the
compounds of chlorine and the so-called hydroxyl radicle (HO), which
is also expressed in the analogy existing between chlorine, C12, and
hydrogen peroxide, (HO).,.

As regards the chloranhydrides corresponding with acids and non-
metals, they bear but little resemblance to metallic salts. They are
nearly all volatile, and have a powerful suffocating smell which irritates
the eyes and respiratory organs. They react on water like many
anhydrides of the acids, with the evolution of heat and liberation of
hydrochloric acid, forming acid hydrates. For this reason they cannot
usually be obtained from hydrates — that is, acids — by the action of
hydrochloric acid, as then water would be formed together with them,
and water decomposes them, converting them into hydrates. There are
many intermediate chlorine compounds between true saline metallic
chlorides like sodium chloride and true acid chloranhydrides, just as
there are all kinds of transitions between bases and acids ; feeble bases
not unfrequeiitly present feeble acid properties. These relations will
become gradually clearer as we become acquainted with elements of
different character, and for the sake of greater clearness we will not
enter into an intimate acquaintance with the saline character of acid
.chloranhydrides, and will only remark that compounds of this type are

TIIK IIALOfiKNS 463

not only obtained from chlorine and non-metals, but also from many
lower oxides, by the aid of chlorine. Thus, for example, CO, NO,
N( ).,, S( )2, and other lower oxides which are capable of combining with
•oxygen may also assimilate to themselves a corresponding quantity of
chlorine. Thus COC1.2, NOCL XO2C1, SO2C12, <fcc., are obtained. They
correspond with the hydrates CO(OH)2, NO(OH), NO2(OH), SO,(OH)2,
Arc., and to the anhydrides CO2, X,O3, N20A, SO3, etc. In this we should
evidently see two sides of the matter : (1) chlorine combines with that
with which oxygen is able to combine, because it is in many respects
equally if not more energetic than oxygen and replaces it in the propor-
tion Cl2 : 0 or (Cl : OH) ; (2) that highest limit of possible combination
whicli is proper to a given element or grouping of elements is very
easily and often attained by combination with chlorine. If phosphorus
gives PC13 and PC15, it is evident that PC15 is the higher form of
combination compared with PC13. To the form PC15 or in general PXr,,
correspond PH4I, PO(OH)3, POC13, tfcc. If chlorine does not always
directly give compounds of the highest possible forms for a given
dement, then generally the lower forms combine with it in order to
reach or approach the limit. This is particularly clear in hydrocarbons,
where we see the limit C,,H2/l+2 very distinctly. The unsaturated
hydrocarbons are sometimes able to combine with chlorine with the
greatest ease and thus reach the limit. Thus ethylene, C2H4, combines
with CL2 forming the so-called Dutch liquid or ethylene chloride,
C2H4C1.2, because it then reaches the limit C,(X2,,+2. In all like cases
the combined chlorine is able by reactions of substitution to give a
hydroxide and a whole series of other derivatives. Thus a hydroxide
called glycol, C2H4(OH)2, is obtained from C4H4C12.

In this way chlorine whilst entering with great ease into combina-
t ion with simple gases, in a number of cases converts lower forms of
combination into higher. Very often chlorine in the presence of water
acts directly as an oxidising agent. A substance A combines with
chlorine and gives, for example, AC12, and this in turn a hydroxide
A(OH)2, which on losing water forms AO. Hence the chlorine
oxidised the substance A. This frequently happens in the simulta-
neous action of water and chlorine: A + H2O + C12=2HC1 + AO.
Examples of this oxidising action of chlorine may frequently be
observed, both in chemical practice and technical processes. Thus
chlorine, for instance, in the presence of water oxidises sulphur and
metallic sulphides. In this case the sulphur is converted into
sulphuric acid, and the chlorine into hydrochloric acid or a metal-
lic chloride if a metallic sulphide be taken. A mixture of carbonic
oxide and chlorine passed into water gives carbonic anhydride

464 PKINCIPLES OF CHEMISTRY

and hydrochloric acid. Sulphurous anhydride is oxidised by
chlorine in the presence of water into sulphuric acid, just as it is
by the action of nitric acid: SO2 + 2H2O + Cla=H2SO4 + 2HCl.
The oxidising action of chlorine in the presence of water is taken
advantage of in practice for the rapid bleaching of tissues and fibres.
The colouring matter of the fibres is altered by oxidation and con-
verted into a colourless substance, but the chlorine afterwards
acts on the tissue itself. Bleaching by means of chlorine therefore
requires a certain amount of technical skill in order that the chlorine
should not act on the fibres themselves, but that its action should be
limited to the colouring matter only. The fibre for making writing
paper, for instance, is bleached in this manner. The property of
chlorine of bleaching was discovered by Berthollet, and forms an
important acquisition to the arts, because it has in the majority of
cases replaced that which before was the universal method of bleach-
ing, namely, exposure to the sun of the fabrics damped with water,
which is still employed for linens, <fcc. Time and great trouble, and
therefore money also, have been considerably saved by this change.19

The power of chlorine for combination is intimately connected with
its capacity for substitution, because, according to the law of substitu-
tion, if chlorine combines with hydrogen, then it also replaces hydrogen,
and furthermore the combination and substitution are accomplished in
the same quantities. Therefore the atom of chlorine which combines
with the atom of hydrogen is also able to replace the atom of hydrogen.
We mention this property of chlorine not only because it illustrates
the adaptation of the law of substitution in clear and historically
important examples, but more especially because reactions of this kind
explain those indirect methods of the formation of many substances
which we have often mentioned and to which recourse is had in many
cases in chemistry. Thus chlorine does not act on carbon,20 oxygen,
or nitrogen, but nevertheless its compounds with these elements may
be obtained by the indirect method of the substitution of hydrogen
by chlorine.

As chlorine easily combines with hydrogen and does not act on

19 The oxidising property of chlorine shows itself when it destroys the majority of
organic tissues, and proves fatal to organisms. This property of chlorine is taken
advantage of in quarantine stations. But the simple fumigation by chlorine must !><•
carried on with great care in dwelling places, because chlorine disengaged into the
atmosphere renders it harmful to the health by attacking the respiratory organs and the
tissues of the lungs.

20 A certain propensity of carbon to attract chlorine must be seen in the immense
absorption of chlorine by charcoal (Note 7), but, as far as is at present known (no one
has tried, if I do not mistake, to have recourse to the aid of light), no combination thjen
takes place between the chlorine and carbon.

TIIK HALOdKNs 465

• -HI -boil, it decomposes hydrocarbons (and many of their derivatives) at
^i high temperature, depriving them of their hydrogen and liberating
the earbon, as, for example, is clearly seen when a lighted candle is
placed in a vessel containing chlorine. The flame becomes smaller but
•continues for a certain time, a large amount of soot is obtained, and
hydrochloric acid is formed. In this case the gaseous and incandescent
substances of the flame are decomposed by the chlorine, the hydrogen
•combines with it, and the carbon is disengaged as soot.21 This action
•of chlorine on hydrocarbons, Arc., proceeds otherwise at lower tempera-
tures, as we will proceed to consider.

A very important epoch in the history of chemistry was formed by
the discovery of Dumas and Laurent that chlorine is able to displace
•and replace hydrogen. This discovery is important from the fact that
chlorine proved to Le an element which combines with great ease
simultaneously with both the hydrogen and the element with which
the hydrogen was combined. This clearly proved that there is no
•opposite polarity between elements forming stable compounds. Chlorine
•does not combine with hydrogen because it has opposite properties, as
Dumas and Laurent stated previously, accounting hydrogen to be
•electro-positive and chlorine electro -negative ; this is not the reason of
their combining together, because the same chlorine which combines
with hydrogen is also able to replace it without altering many of the
properties of the resultant substance. This substitution of hydrogen
by chlorine is termed metalepsis. The mechanism of this substitution
is very constant. If we take a hydrogen compound, preferably a
hydrocarbon, and if chlorine act directly on it, then there is produced
•on the one hand hydrochloric acid and on the other hand a compound
•containing chlorine in the place of the hydrogen — so that the chlorine
divides itself into two equal portions, one portion is evolved as hydro-
chloric acid, and the other portion takes the place of the hydrogen thus
liberated. Hence this metalepsis is always accompanied by the formation
•>f Itifilrochloric acid.'22 The scheme of the process is as follows :
CMHIMX + C12 C^H^CIX + HC1

Hydrocarbon. Free chlorine. Product of metalepsis. Hydrochloric acid.

21 The same takes place under the action of oxygen, with the difference that
it burns the carbon, which chlorine is not able to do. If chlorine and oxygen compete
together at a high temperature, the oxygen will unite with the carbon, and the chlorine
with the hydrogen ; if pure hydrogen be taken with a sufficient quantity of chlorine, it
will all combine with the chlorine without forming any water with the oxygen.

-- Tliis division of chlorine into two portions may at the same time be taken as a clear
'irmation of the conception of molecules. According to Avogadro-Gerhardt's law, the
molecule of chlorine (p. 808) contains two atoms of this substance ; one atom replaces
tin- hydrogen, and the other combines with it.

VOL. I.

466 PRINCIPLES OF CHEMISTRY

Or, in general terms —

RH + Cl, = RC1 + HC1.

The conditions under which metalepsis takes place are also very
constant. In the dark chlorine does not usually act on hydrogen com-
pounds, but the action commences under the influence of light. The
direct action of the sun's rays is particularly propitious to metalepsis.
It is also remarkable that the presence of traces of certain substances 2^
promotes the action (especially of iodine, aluminium chloride, antimony
chloride, &c.). A trace of iodine added to the substance subjected to-
metalepsis often produces the same effect as sunlight.24

If marsh gas be mixed with chlorine and the mixture ignited, then
the hydrogen is entirely taken up from the marsh gas and hydrochloric-
acid and carbon formed, but there is no metalepsis. 2fi But if a
mixture of equal volumes of chlorine and marsh gas be exposed to the
action of diffused light, then the greenish yellow mixture gradually
becomes colourless, and hydrochloric acid and the first product of
metalepsis, namely, methyl chloride, are formed :

CH4 + C12 = CH3C1 + HC1

Marsh gas. Chlorine. Methyl chlorii1 >. Hydrochloric acid.

25 Such carriers or media for the transference of chlorine and the halogens in general'
were long known to exist in iodine and antimonious chloride, and have been most fully
studied by Gustavson and Friedel, of the Petroffsky Academy — the former with respect
to aluminium bromide, and the latter with respect to aluminium chloride. Gustavson
showed that if a trace of metallic aluminium be dissolved in bromine (it floats on bromine,,
and when combination takes place much heat and light are evolved), the latter becomes
endowed with the property of entering into metalepsis, which it is not able to do of its
own accord. When pure, for instance, it acts very slowly on benzene, C6H6, but in the
presence of a trace of aluminium bromide the reaction proceeds violently and easily, so that
each drop of the hydrocarbon gives a mass of hydrobromic acid, and of the product of meta-
lepsis. Gustavson showed that the modus operandi of this instructive reaction is based
on the property of aluminium bromide to enter into combination with hydrocarbons and
their derivatives. The details of this and all researches concerning the metalepsis of
the hydrocarbons must be looked for in works on organic chemistry.

24 As small admixtures of iodine, aluminium bromide, &c., aid the metalepsis of large
quantities of a substance, just as nitric oxide aids the reaction of sulphurous anhydride
on oxygen and water, so the matter is essentially the same in both cases. Effects of this-
kind (which should also be explained by a chemical reaction proceeding at the surfaces)
only differ from true contact phenomena in that the latter are produced by solid bodies
and are accomplished at their surfaces, whilst in the former all is in solution. Probably
the action of iodine is founded on the formation of iodine chloride, which reacts more
easily than chlorine.

25 Metalepsis belongs to the number of delicate reactions — if it may be so expressed—
as compared with the energetic reaction of combustion. Many cases of substitution are
of this kind. Reactions of metalepsis are accompanied by the evolution of heat, but in a
less quantity than that evolved in the formation of the resulting quantity of the halogen
acids. Thus the reaction C2H6 + C12 = C2H3C1 + HC1, judging from the data given by

THK JIAL<X;KNS 467

The volume of the mixture remains unaltered. The methyl
chloride which is formed is a gas. If it be separated from the hydro-
chloric acid (it is soluble in acetic acid, in which hydrochloric is but
sparingly soluble) and be again mixed with chlorine then it may be
subjected to a further metalepsical substitution — the second atom of
hydrogen may be substituted by chlorine, and a liquid substance
CH-jClo, called methylene chloride, will be obtained. In the same
manner the substitution may be carried on still further, and CHC13, or
chloroform, and, lastly, carbon tetrachloride, CC14, will be produced.
Of these substances the best known is chloroform, owing to its being
formed in many cases from organic substances (by the action of bleaching
powder) and to its being used in medicine as an. anaesthetic • chloroform
boils at 62° and carbon tetrachloride at 78°. They are both colourless
odoriferous liquids, heavier than water. The progressive substitution
of hydrogen by chlorine is thus evident, and it can be clearly seen that
the double decompositions are accomplished between molecular quanti-
ties of the substance — that is, between equal volumes in a gaseous state.
Chloroform may be obtained directly from marsh gas, but this, judging
from the above, will be the third product of the metalepsis of marsh gas-
Between it and marsh gas there are two more intermediate products,
and the first of them is produced in the action of one molecule of marsh
gas on one molecule of chlorine.

Carbon tetrachloride, which is obtained by the metalepsis of marsh
gas, is not obtained directly from chlorine and carbon, but it may be
obtained from certain compounds of carbon — for instance, from carbon
bisulphide — if its vapour mixed with chlorine be passed through a
red-hot tube. Both the sulphur and carbon then combine with the
chlorine. It is evident that by ultimate metalepsis a corresponding
carbon chloride may be obtained from any hydrocarbon — indeed, the
number of chlorides of carbon already known is very large.

As a rule, the fundamental chemical characters of hydrocarbons are
not changed by metalepsis ; that is, if a neutral substance be taken, then
the product of metalepsis is also a neutral substance, or if an acid be
taken the product of metalepsis also has acid properties. Even the
crystalline form not unfrequently remains unaltered after metalepsis.
The metalepsis of acetic acid, CH3'COOH, is historically the most
important. It contains three of the atoms of the hydrogen of marsh
gas, the fourth being replaced by carboxyl, and therefore by the action
of chlorine it gives three products of metalepsis (according to the mass
of the chlorine and conditions under which the reaction takes place),

Thomson, evolves about 20000 heat units, whilst the formation of hydrochloric acid
evolves 22000 units.

HH 2

468 PRINCIPLES OF CHEMISTRY

mono-, di-, and tri-chloracetic acids— CH.2GTCOOH, CHC1/COOH, and
CC13'COOH ; they are all, like acetic acid, monobasic. Without enter-
ing into the further description of cases of like reaction (they must be
looked for in organic chemistry), it is necessary to turn special attention
first to the fact that in this manner by an indirect method, carbon
•compounds (for instance, CC14, C.2C14, C(iCl(!, &c.) are formed, which
cannot be obtained directly from the elements, and secondly, that the
resulting products of metalepsis, in containing an element which so
•easily acts on metals as chlorine, give the possibility of attaining a
further complexity of molecules for which the original hydrocarbon is
often in no way capable. Thus on treating with an alkali (or first
with a salt and then with an alkali, or with a basic oxide and water,
<kc.) the chlorine forms a salt with its metal, and the hydroxyl radicle
takes the place of the chlorine — for example, CH3'OH is obtained from
CHjjOL By the action of metallic derivatives of hydrocarbons — for
instance, CH3Na — the chlorine also gives a salt, and the hydrocarbon
radicle — for instance, CH3 — takes the place of the chlorine. Thus, or
in a similar manner, CH3'CH3 or C2H6 is obtained from CH3C1 or
C6H5'CH3 from C6H6. The products of metalepsis also often react on
ammonia, forming hydrochloric acid (and thence NH4C1) and an
amide ; that is, the product of metalepsis with the ammonia radicle
NH.2 in the place of chlorine. Thus by means of metalepsical substitu-
tion a method is found in chemistry for an artificial and general means
of the formation of complex carbon compounds from more simple
compounds which are often totally incapable of direct reaction.
Besides which, this key opened the doors of that secret edifice of the
structure of complex organic compounds into which man had up to
then feared to enter, supposing it to be the dwelling of the spirits of
organisms under the influence of whose magical force that was united
which by other means could not be brought together.26

26 With the predominance of the representation of compound radicles (this doctrine
dates from Lavoisier and Gay-Lussac) in organic chemistry, it was a very important
moment in its history when it became possible to gain an insight into the structure of
the radicles themselves. It was clear, for instance, that ethyl, C2H5, or the radicle of
common alcohol, C2H5'OH, passes, without changing, into a number of ethyl derivatives,
but its relation to the still simpler hydrocarbons was not clear, and occupied the attent ion
of science in the ' forties ' and ' fifties,' Having obtained ethyl hydride, C.^HsH = CoHg, it
•was looked on as containing the same ethyl, just as methyl hydride, CH4 = CH3H, was
considered as existing in methane. Having obtained free methyl, CH5CH5 = C.^H6, from
it, it was considered as a derivative of methyl alcohol, CH5OH, and as only isomeric with
ethyl hydride. By means of the products of metalepsis it was proved that this is not a
case of isomerism but of strict identity, and it therefore became clear that ethyl is
methylated methyl, C2H5 = CH2CH3. In its time a still greater impetus was -i\( n 1>\
the study of the reactions of monochloracetic acid, CHoCl'COOH, or CO(CH2C1)(OH). j
Jt appeared that metalepsical chlorine, like the chlorine of chloranhydrides — for instance

'I' UK HALOGENS 469

It is not only hydrocarbons which are subjected to metalepsis.
Certain other hydrogen compounds, under the action of chlorine, also
give corresponding chlorine derivatives in exactly the same manner ;
for instance, ammonia, caustic potash, caustic lime, and a whole series
of alkali »•' substances.27 In fact, just as the hydrogen in marsh gas
can be replaced by chlorine and form methyl chloride, so the hydrogen
in caustic potash, KHO, ammonia, NH3, and calcium hydroxide,,
CaH2O2 or Ca(OH)2, may be replaced by chlorine and give potassium
hypochlorite, KC10, calcium hypochlorite, CaCl2O2, and the so-called
chloride of nitrogen, NC13. Not only is the correlation in composition
the same as in the substitution in marsh gas, but the whole mechanism
of the reaction is the same. Here, also, two atoms of chlorine act :
one takes the place of the hydrogen whilst the other is evolved as
hydrochloric acid, only in the former case the hydrochloric acid evolved
remained free, and in the latter, in presence of alkaline substances,
the hydrochloric acid formed reacts on them. Thus, in the action of
chlorine on caustic potash, the hydrochloric acid formed acts on
another quantity of caustic potash and gives potassium chloride and
water, and, therefore, not only KHO + C1.2=HC1 + KC10, but also
KHO + HC1= H20 -}- KC1, and therefore the result of both simultaneous
phases will be 2KHO + C12=H2O + KC1 + KC1O. We will here enter
into certain special cases.

The action of chlorine on ammonia may either result in the entire
breaking up of the ammonia with the evolution of gaseous nitrogen, or
in a product of metalepsis (as with CH4 and H2O). With an excess
of chlorine and the aid of heat the ammonia is decomposed, with the
disengagement of free nitrogen.28 This reaction is evidently accom-

of methyl chloride, CH3C1, or ethyl chloride, C2H5C1— is capable of substitution ; for
instance, glycollic acid, CH2(OH)(CO.2H), orCO(CH2'OH)(OH), was obtained from it, and
it appeared that the OH in the group CH2(OH) reacted like that in alcohols, and it
became clear, therefore, that it was necessary to examine the radicles themselves by
analysing them from the point of view of the bonds connecting the constituent atoms.
Whence arose the present doctrine of the structure of the carbon compounds. (See
Chapter VIII. Note 42.)

27 By embracing many instances of the action of chlorine under metalepsis we not
only explain the indirect formation of CC14, NC15, and C12O by one method, but we alsQ
arrive at the fact that the reactions of the metalepsis of the hydrocarbons lose that
exclusiveness which was often ascribed to them. Also by subjecting the chemical repre-
sentations to the law of substitution we may foretell metalepsis as a particular case of a
general law.

28 This may be taken advantage of in the preparation of nitrogen. If a large excess
of chlorine water be poured into a beaker, and a small quantity of a solution of ammonia
be added, then, after shaking, nitrogen is evolved. If chlorine act on a dilute solution
of ammonia, then the volume of nitrogen doos not correspond with the volume of the
chlorine taken, because ammonium hypochlorite is formed. If ammonia gas be passed
through a-fine orifice into a vessel containing chlorine, then the reaction of the formation

470 PRINCIPLES OF CHEMISTRY

panied by the formation of sal-ammoniac, 8
But if the ammonium salt be in excess, then the reaction proceeds with
the replacement of the hydrogen in the ammonia by chlorine. The
essence of the matter is that NH3 + 3C1, forms NC1;J -f 3HC1.29 The
resulting product of metalepsis, or chloride of nitrogen, NC13, dis-
covered by Dulong, is a liquid having the property of decomposing
with excessive ease not only when heated, but even under the action
of mechanical influences, as by a blow or the contact of certain solid
substances. The explosion which accompanies the decomposition is due
to the fact that the liquid chloride of nitrogen gives gaseous products,
nitrogen and chlorine ; a large volume of gas is evolved instantaneously
and causes the explosion. It is even dangerous to prepare this sub-
stance in any considerable quantity. Whenever an ammoniacal sub-
stance comes into contact with chlorine great care must be taken,
because it might be a case of the formation of such products and a very
dangerous explosion. The liquid product of the metalepsis of ammonia
may be most safely prepared in the form of small drops by the action
of a galvanic current on a slightly warm solution of sal-ammoniac ;
chlorine is then evolved at the positive pole, and this chlorine, acting
on the ammonia, gradually forms the product of metalepsis, which
floats on the surface of the liquid (because it is borne up by the gas),
and if a layer of turpentine be poured on to it these small drops, on

of nitrogen is accompanied by the emission of light and the appearance of a cloud of sal-
ammoniac. In all these instances there must be an excess of chlorine.

29 The hydrochloric acid formed combines with ammonia, and therefore the result is
4NH3 + 3C12 = NC13 + 3NH4C1. Consequently, more ammonia enters into the reaction,
but the metalepsical aspect of the reaction in reality only takes place with an excess of
ammonia in the form of the salt. If bubbles of chlorine be passed through a fine tube
into a vessel containing ammonia gas, then each bubble gives rise to an explosion. If,
however, chlorine be passed into a solution of ammonia, then the reaction first directs
itself towards the formation of nitrogen, because chloride of nitrogen acts on ammonia
like chlorine. But when sal-ammoniac begins to form, then the reaction directs itself
towards the formation of chloride of nitrogen. The first action of chlorine on a solution
of sal-ammoniac always consists in the formation of chloride of nitrogen, which reacts on
ammonia thus : NC13 + 4NH3 = N2 + 3NH4C1. Therefore, so long as the liquid is alka-
line from the presence of ammonia the chief product will be nitrogen. The reaction
NH4C1 + 8C12 = NC13 + 4HC1 is reversible ; with a dilute solution it proceeds in the above-
described direction (perhaps owing to the affinity of the hydrochloric acid for the excess
of water), but with a strong solution of hydrochloric acid it takes the opposite direction
(probably in virtue of the affinity of hydrochloric acid for ammonia). Therefore there
must exist a very interesting case of equilibrium between ammonia, hydrochloric acid,
chlorine, water, and chloride of nitrogen which has not yet been investigated. The re-
action NC13 + 4HC1 = NH4C1 + 3C12 enabled Deville and Hautefeuille to determine the
composition of chloride of nitrogen. When slowly decomposed by water, chloride of
nitrogen gives, like a chloranhydride, nitrous acid or its anhydride 2NC13 + 3HoO
= N2O3 + 6HC1. From these observations it is evident that chloride of nitrogen presents
great chemical interest, which is strengthened by its analogy with trichloride of phos-
phorus.

THE HALOGENS 471

coming into contact with the turpentine, give feeble explosions, which
are in no way dangerous, owing to the small mass of the substance
formed. The drops of chloride of nitrogen may, with great precaution,
be collected for investigation in the following manner. The neck of a
f mmel is immersed in a basin containing mercury, and, first, a saturated
solution of common salt is poured into the funnel, and above it a solu-
tion of sal-ammoniac in 9 parts of water. Chlorine is then slowly
passed through the solutions, when drops of chloride of nitrogen fall
into the salt water. It is a yellow oily liquid of sp. gr. 1*65, which
boils at 71°, and breaks up into N + C13 at 97°. The contact of phos-
phorus, turpentine, india-rubber, etc., causes an explosion, which is
.sometimes so violent that a small drop will pierce through a thick
board. The exceeding facility of the decomposability of chloride of
nitrogen is connected with the fact that it is formed with an absorption
of heat, which it evolves when decomposed, to the amount of about
38000 heat units for NC13, as Deville and Hautefeuille determined.

Chlorine, when absorbed by a solution of caustic soda (and also of
other alkalis) at the ordinary temperature causes the replacement of
the hydrogen in the caustic soda by the chlorine, with the formation of
sodium chloride by the hydrochloric acid formed, so that the reaction
may be represented in two phases, as was described above. In this
manner, sodium hypochlorite, NaCIO, and sodium chloride are simul-
taneously formed : 2NaHO + Cl2=NaCl + NaCIO + H2O. The resultant
solution is termed * eau de Javelle.' An exactly similar reaction takes
place when chlorine is passed over dry hydrate of lime at the ordinary
temperature : 2Ca(HO)2 + 2Cl2=CaCl2O2 + CaCl2 + 2H2O. A mixture
of the product of metalepsis and calcium chloride is obtained. This
mixture is employed in practice on a large scale, and is termed ' bleach-
ing powder,' owing to its acting, especially when mixed with acids, as a
bleaching agent on tissues, so that it resembles chlorine in this respect,
but is preferable to chlorine, because the destructive action of the
-chlorine may be moderated in this case, and because it is much more
-convenient to deal with a solid substance than with gaseous chlorine.
Bleaching powder is also called chloride of lime, because this substance
is obtained from chlorine and hydrate of lime, and contains30 both

50 Quicklime, CaO (or calcium carbonate, CaCO5), does not absorb chlorine when cold,
but at a red heat, in a current of chlorine, it forms calcium chloride, with the evolution
of oxygen. This reaction corresponds with the decomposing action of chlorine on
methane, ammonia, and water. Slaked lime (calcium hydroxide, CaH2O2) also, when dry,
does not absorb chlorine at 100°. The absorption proceeds at the ordinary temperature
(below 40°). The dry mass thus obtained contains not less than three equivalents
of calcium hydroxide to four equivalents of chlorine, so that its composition is
{Ca(HO)2]3Cl4. In all probability a simple absorption of chlorine by the lime at first
tiakes place in this case, as may be seen from the fact that even carbonic anhydride, when

472

PRINCIPLES OF CHEMISTRY

these substances. It may be prepared in the laboratory by passing a
stream of chlorine through a cold mixture of water and lime (milk of
lime). The mixture must be kept cold, as otherwise 3Ca(ClO)2 passes
into 2CaCl2-fCa(ClO3)2. In the manufacture of bleaching powder in
large quantities at chemical works, the purest possible slaked lime is.

FIG. 68.- -Apparatus for the manufacture of l>le:irlihi'j- powder (at -mall work-o by the' action of
chlorine, which is generated in the vessels 0, on lime, which is churned him 31.

taken and laid in a thin layer in large flat chambers, M (whose walls.
are made of Yorkshire flags or tarred wood, on which chlorine has no
action), and into which chlorine gas is introduced by lead tubes. The
distribution of the plant is shown in the annexed drawing (fig. 68).

acting on the dry mass obtained as above, disengages all the chlorine from it, forming
only calcium carbonate. But if the bleaching-powder be obtained by a wet method, or if
it be dissolved in water (it is very soluble), and carbonic anhydride be passed into it,
then chlorine is no longer disengaged, but chlorine oxide, C12O, and only half of the
chlorine is converted into this oxide while the other half remains in the liquid as-
calcium chloride. From this it may be supposed that calcium chloride is formed by the
action of water on bleaching powder, and this is proved to be the case by the fact that
small quantities of water extract much calcium chloride from bleaching-powder. If a
large quantity of water act on bleaching-powder there remains an excess of calcium
hydroxide, a portion of which is not subjected to change. The action of the water may
be expressed by the following formulae : From the dry mass Ca^(HO)6Cl, there is formed
lime, Ca(HO)2, calcium chloride, CaCl2, and a saline substance, Ca(ClO)2. Ca5H6O6Cl4
= CaH2O2 + CaCloOo + CaCl2 + 2H2O. The resulting substances are not equally soluble ;:
water first extracts the calcium chloride, which is the most soluble, then the compound
Ca(ClO)2, and calcium hydroxide is ultimately left. A mixture of calcium chloride and
hypochlorite passes into solution. On evaporation there remains Ca2O2Cl4,8H2O. The
dry bleaching-powder does not absorb more chlorine, but the solution is able to absorb
it in considerable quantity. If the liquid be boiled, a considerable amount of chlorine
monoxide is evolved. After this calcium chloride alone remains in solution, and the
decomposition may be expressed as follows: CaCl2 + CaCl2O2 + 2Cl2 = 2CaCL + 2CLO.
Chlorine monoxide may be prepared in this manner.

It is sometimes said that bleaching-powder contains a substance, Ca(OH)->CL, like
calcium peroxide, CaO2, in which one atom of oxygen is replaced by (OH) ... and the other
by Clo ; but, judging from what has been said above, this can only be admitted in the-
dry mass, and not in solutions.

After being kept for some time, bleaching-powder sometimes decomposes, with the-
evolution of oxygen (page 161) ; the same takes place when it is heated.

TIIK HALOGENS 47$

The products of the metalepsis of alkaline hydrates, XaCIO and
Ca(CK)).,, which are held in solutions of Mavelle salt,' and bleaching
powder (they are not obtained free from metallic chlorides), must be
counted as salts, because their metals are liable to substitution. . But
the hydrate HC1O corresponding with these salts, or hypochlorous^
acid, is not obtained in a free or pure state, for two reasons : in the
first place, because this hydrate, as a very feeble acid, splits up (like
H2CO3 or HXO;>) into water and the anhydride, or chlorine monoxide,
(<LO = 2HC1O — H.2O ; and, in the second place, because, in a number
of instances, it evolves oxygen with great facility, forming hydrochloric
acid : HC1O=HC1 + O. Both hypochlorous acid and chlorine monoxide
may be regarded as the product of the metalepsis of water, because
HOH corresponds with C1OH and C1OC1. Hence, in many instances,
bleaching salts (a mixture of hypochlorites and chlorides) break up,
with the evolution of (1) chlorine, under the action of an excess of a
powerful acid, capable of evolving hydrochloric acid from sodium or
calcium chlorides, and which is most simple under the action of
hydrochloric acid itself, because (p. 456) NaCl + NaCIO + 3HC1
=2NaCl + HCl + CL2 + H,O; (2) oxygen, as we saw in Chapter III.
(p. 161) — the bleaching properties and, in general, oxidising action
of bleaching salts is based on this evolution of oxygen (or chlorine) ;
oxygen is also disengaged on heating the dry salts — for instance,
NaCl + NaClO==2NaCl + O ; (3) and, lastly, chlorine monoxide, which
contains both chlorine and oxygen. Thus, if a little sulphuric, nitric, or
similar acid (in order that hydrochloric acid should not yet be produced)
be added to a solution of a bleaching salt (which has an alkaline reac-
tion, owing either to an excess of alkali or to the feeble acid properties
of HC1O), then the hypochlorous acid set free gives water and chlorine
monoxide. If carbonic anhydride (or boracic or a similar very feeble
acid) act on the solution of a bleaching salt, then hydrochloric acid is.
not evolved from the sodium or calcium chlorides, but the hypochlorous
acid is displaced and gives chlorine monoxide,31 because hypochlorous
acid is one of the most feeble acids (p. 371). An excellent method for
the preparation of chlorine monoxide is based on these feeble acid pro-
perties of hypochlorous acid. Zinc oxide and mercury oxide, under

31 For this reason it is necessary that in the formation of bleaching powder the chlorine
should be free from hydrochloric acid, and even the lime from calcium chloride. An
excess of chlorine, in acting on a solution of bleaching-powder, may also give chlorine
iiK.nu7.ide, because calcium carbonate also gives chlorine monoxide under the action of
chlorine. This reaction may be brought about by treating freshly-precipitated calcium
carbonate with a stream of chlorine in water: aClo + CaCOj^CO.^ + CaCLj + CLO. From
this \\e may conclude that, although carbonic anhydride displaces hypochlorous anhy-
dride, it may be itself displaced by an excess of the latter.

-474 PRINCIPLES OF CHEMISTRY

the action of chlorine in the presence of water, do not give a salt of
.hypochlorous acid, but form a chloride and hypochlorous acid, which
fact shows the incapacity of this acid to combine with the given bases.
Therefore, if such oxides as those of zinc or mercury be shaken up in
water, and chlorine be passed through the turbid liquid,32 a reaction
occurs which may be expressed in the following manner : 2HgO -f 2C12
=Hg2OCl2-f C12O. In this case, a compound of mercury oxide with
anercury chloride, or the so-called mercury oxychloride, is obtained :
Hg2OCl2=HgO + HgCl2. This is insoluble in water, and is not affected
by hypochlorous anhydride, so that the solution will contain hypo-
•chlorous acid only, but the greater part of it splits up into the anhydride
•and water.

A solution of hypochlorous anhydride is also obtained by the action
•of chlorine on many salts ; for example, in the action of chlorine on a
solution of sodium sulphate the following reaction takes place :
Na2SO4 + H2O + Cl2 = NaCl + HClO + NaHSO4. Hence here the
hypochlorous acid is formed, together with HC1, at the expense of the
reaction of chlorine on water, for C12 +H2O=HC1 + HC1O. If the
crystallo-hydrate of chlorine be mixed with mercury oxide, then the
hydrochloric acid formed in the reaction gives mercury chloride, and
hypochlorous acid remains in solution. A dilute solution of hypo-
chlorous acid or chlorine monoxide may be concentrated by distillation,
•and if a substance which takes up water (without destroying the acid)
— for instance, calcium nitrate — be added to the stronger solution
then the anhydride of hypochlorous acid — i.e. chlorine monoxide — is
disengaged.

In the bleaching salts and hypochlorous salts which correspond
with chlorine monoxide and contain the two elements oxygen and
chlorine, which both act in an oxidising manner, we see a characteristic
•example of a compound of elements which, in the majority of cases, act
•chemically in an analogous manner. Chlorine monoxide, as prepared
from an aqueous solution by the abstraction of water or by the action
of dry chlorine on cold mercury oxide, is, at the ordinary temperature,
a gas or vapour which condenses into a red liquid boiling at + 20° and
giving a vapour whose density (43 referred to hydrogen) shows that

52 Dry red mercury oxide acts on chlorine, forming dry hypochlorous anhydride
(chlorine monoxide) (Balard) ; when mixed with water, red mercury oxide acts feebly on
chlorine, and when freshly precipitated it evolves oxygen and chlorine. An oxide of
mercury which easily and abundantly evolves chlorine monoxide under the action of
-chlorine in the presence of water may be prepared as follows : the oxide of mercury,
precipitated from a mercuric salt by an alkali, is heated to 300° and cooled (Pelouze). If
•a salt, MC1O, be added to a solution of mercuric salt, HgX2, then mercuric oxide is
liberated, because the hypochlorite is decomposed.

TIIK HALOGENS 475

:'2 vols. of chlorine and 1 vol. of oxygen give 2 vols. of chlorine
monoxide. In an anhydrous form the gas or liquid easily explodes,
splitting up into chlorine and oxygen. This explosiveness is determined
by the fact that heat is evolved in the decomposition to the amount of
.about 15000 heat units for C12O.33 The explosion may even take place
accidentally, and also in the presence of many oxidisable substances
(for instance, sulphur, organic compounds, etc.), but the solution,
•although unstable and showing strong oxidising properties, does not
give any explosion.34

Hypochlorous acid, its salts, and chlorine monoxide serve as a
transition between hydrochloric acid, chlorides, and chlorine, and a
whole series of compounds containing the same elements combined
"with a still greater quantity of oxygen. The higher oxides of chlorine,
-even in their origin, are closely connected with hypochlorous acid and
its salts :

C12, NaCl, HC1, hydrochloric acid.

C12O, NaCIO, HC1O, hypochlorous acid.

C12O3, NaClO2, HC1O2, chlorous acid.35

C1,O5, NaClO3, HC1O3, chloric acid.

C12O7, NaClO4, HC1O4, perchloric acid.

When heated, solutions of hypochlorites undergo a remarkable
-change. Themselves so unstable, they, without the addition of any-

33 All explosive substances are of this kind — ozone, hydrogen peroxide, chloride of
nitrogen, nitro-compounds, &c. Hence they cannot be formed directly from the elements
•or their simplest compounds, but, on the contrary, decompose into them. In a liquid
state chlorine monoxide even explodes on contact with powdery substances, or when
rapidly agitated — for instance, if a file be rasped over the vessel in which it is.

34 A solution of chlorine monoxide, or hypochlorous acid, does not explode, owing to
the presence of the mass of water. In dissolving, chlorine monoxide evolves about 9000
heat units, so that its store of heat becomes less.

The capacity of hypochlorous acid for entering into combination with the unsaturated
hydrocarbons (Carius and others) is very often taken advantage of in organic chemistry.
'Thus its solution absorbs ethylene, forming the chlorhydrin C2H4C1'OH.

The oxidising action of hypochlorous acid and its salts is not only applied to bleaching
but also to many reactions of oxidation. Thus it converts the lower oxides of manganese
into the peroxide.

35 Chlorous acid, HClOo (judging from the data given by Millon, Brandau, and
others) in many respects resembles hypochlorous acid HC1O, whilst they both differ from
•chloric and perchloric acids in their degree of stability, which is expressed, for instance,

in their bleaching properties ; the two higher acids do not bleach, but both the lower ones
•do so (oxidise at the ordinary temperature). On the other hand, chlorous acid is ana-
logous to nitrous acid, HNO2. The anhydride of chlorous acid, C12O3, is not known in a
pure state, but it probably occurs in admixture with chlorine dioxide, C1O2, which is ob-
tained by the action of nitric and sulphuric acids on a mixture of potassium chlorate with
;such reducing substances as nitric oxide, arsenious oxide, sugar, &c. All that is at pre-
rsent known is that pure chlorine dioxide C1O2 (see Notes 89-48) is gradually converted
into a mixture of hypochlorous and chlorous acids under the action of water (and alkalis)

476 PRINCIPLES OF CHEMISTRY

thing, yield two fresh salts which are both much more stable ; one
contains more oxygen than MC1O, the other contains none at all.

3MC1O = MC1O3 + 2MC1

hypochlorite chlorate chloride.

Part of the salt — namely, two- thirds of it — parts with its oxygen in
order to oxidise the remaining third.36 From an intermediate sub-
stance, RX, there proceed two extremes, R and RX3, just as from
nitrous anhydride there proceed nitric oxide and nitric anhydride (or
nitric acid): 3N2O:5=X,>O5 + 4NO. The resulting salt MC]O3 cor-
responds with chloric acid and potassium chlorate, KC1O3. It is
evident that a similar salt is obtained directly by the action of chlorine
on an alkali if its solution be heated, because RC1O will be first formed,
and then RC1O3 ; for example, 6KHO + 3C1.2=KC1O3 + 5KC1 + 3H2O.
Chlorates are formed thus ; for instance, potassium chlorate, which is

that is, it acts like nitric peroxide, NO2 (giving HNO5 and HNO.,>), or as a mixed anhydride,
~2C1O2 + H2O = HC1O; + HC1O2. The silver salt, AgClOo, is sparingly soluble in water.
The investigations of Garzarolli-Thurnlackh and others seem to show that the anhydride
CloO5 does not exist.

36 Hydrochloric acid, which forms an example of compounds of this kind, is a satu-
rated substance which does not combine directly with oxygen, but in which, nevertheless,
a considerable quantity of oxj^gen may be inserted between the elements forming it.
The same may be observed in a number of other cases. Thus, for instance, oxygen may
be added or inserted between the elements, sometimes in considerable quantities, in the
saturated hydrocarbons ; for instance, in C5H8, three atoms of oxygen produce an alcohol, .
glycerin or glycerol C5H5(OH)5. We shall meet with similar examples hereafter. This
is explained generally by regarding oxygen as a bivalent element — that is, as capable of
combining with two different elements, such as chlorine, hydrogen, &c. On the basis ot
this view, it may be inserted between each pair of combined elements ; the oxygen will
then be combined with one of the elements by one of its affinities and with the other
element by its other affinity. This view does not, however, express the entire substance
of the matter, even when applied to the compounds of chlorine. Hypochlorous acid,
HOC1 — that is, hydrochloric acid in which one atom of oxygen is inserted — is, as we have
already seen, a substance of small stability ; it would therefore be expected that on the
addition of a fresh quantity of oxygen a still less stable substance would be obtained,
because, according to the above view, the chlorine and hydrogen, which form such a
stable compound together, are then still further removed from each other. But it appears
that chloric and perchloric acid, HC1O5 and HC1O4, are much more stable substances.
Furthermore, the addition of oxygen has also its limit, it can only be added to a certain
extent. If the above representation were true and not formal, then a limit to the com-
bination of oxygen could not be looked for, and the more it entered into one uninter-
rupted chain the more stable would be the resultant compound. But not more than four
atoms of oxygen can be added to hydrogen sulphide, nor to hydrochloric acid, nor to
hydrogen phosphide. This peculiarity must lie in the properties of oxygen itself; fom
atoms of oxygen seem to have the power of forming a kind of radicle which retains two
or several atoms of different other substances — for example, chlorine and hydrogen,
hydrogen and sulphur, sodium and manganese, phosphorus and metals, &c., forming
comparatively stable compounds, NaClO4, Na2SO4, NaMnO4, Na.-,PO4, &c. See Chapter X
Note 1.

477

•easily separated from potassium chloride, because it is sparingly soluble
in cold water.37

If dilute sulphuric acid be added to a solution of potassium chlorate,
then chloric <t<-i<l is liberated; but it cannot be separated by distillation,
.as it is decomposed in the process. To obtain the free acid sulphuric
jicid must be added to a solution of barium chlorate.38 The sulphuric
.acid JLCIVCS a precipitate of barium sulphate with the barium, and free
chloric acid remains in solution. The solution may be evaporated
under the receiver of an air-pump. This solution is colourless, has no
.smell, and acts as a powerful acid (it saturates sodium hydroxide,
decomposes sodium carbonate, gives hydrogen with zinc, tfcc.) ; when

37 If chlorine be passed through a cold solution of potash, then a bleaching compound,
potassium chloride and hypochlorite, HC1 + KC1O, is formed, but if it be passed through
a hut solution potassium chlorate is formed. As this is sparingly soluble in water, it
chokes the gas-conducting tube, which should therefore be widened out at the end.

Potassium chlorate is usually prepared on a large scale from calcium chlorate, which
is prepared by passing chlorine (as long as it is absorbed) into water containing lime, the
mixture being kept warm. A mixture of calcium chlorate and chloride is thus formed
in the solution. Potassium chloride is then added to the warm solution, and on cooling
a precipitation of potassium chlorate is formed as a substance which is sparingly soluble
in cold water, especially in the presence of other salts. The double decomposition taking
place is Ca( ClO-)o + 2KC1 = CaCL + '2KC1O3. On a small scale in the laboratory potassium
chlorate is best prepared from a strong solution of bleaching powder by passing chlorine
through it and then adding potassium chloride.

Potassium chlorate crystallises easily in large colourless tabular crystals. Its solu-
bility in 100 parts of water at 0°= 3 parts, 20°= 8 parts, 40°= 14 parts, 60°= 25 parts,
80° = 40 parts. For comparison we will cite the following figures showing the solubility of
potassium chloride and perchlorate in 100 parts of water: potassium chloride at 0°=28
(arts, -JO - = >55 parts, 40° = 40 parts, 100°= 57 parts ; potassium perchlorate at 0° about 1
>art . -' * about 1 ^ parts, 100° about 18 parts. When heated potassium chlorate melts (the
nelting point has been given as from 335°-376° ; according to the latest determination by
.'arnellev, :'>r>(.) ) and decomposes with the evolution of oxygen, potassium perchlorate
>eing at first formed, as will afterwards be described. A mixture of potassium chlorate
and nitric and hydrochloric acids brings about oxidation and chlorination in solutions.
It deflagrates when thrown upon incandescent carbon, and when mixed with sulphur (£
by weight) it sets light to it, even when struck, in which case an explosion takes place.
The same occurs with many metallic sulphides and organic substances. Such mixtures
are inflamed by a drop of sulphuric acid. All these effects are due to the large amount
of oxygen contained in potassium chlorate, and to the ease with which it is evolved. A
mixture of two parts of potassium chlorate, one part of sugar, and one part of yellow
prussiate of potash acts like gunpowder, but it burns very rapidly, and therefore bursts
the guns, and also it has a very strong oxidising action on their metal. The sodium salt,
Na( 1< >.-. is much more soluble than the potassium salt, and it is therefore more difficult
to free it from sodium chloride, &c. The barium salt is also more soluble than the
potassium salt; 0°= 24 parts, 20°= 37 parts, 80°= 98 parts of salt per 100 of water.

•~3 Barium chlorate, Ba(ClO;j)2,HoO, is prepared in the following way : impure chloric
acid is first prepared and saturated with baryta, and the barium salt purified by crystal-
lisation. The impure free chloric acid is obtained by converting the potassium in potas-
sium chlorate into an insoluble salt. This is done by adding tartaric or hydrofluosilicic
acid to a solution of potassium chlorate, because potassium tartrate and potassium silico-
fluoride are very sparingly soluble in water. Chloric acid is easily soluble in water.

•ITS PIMNCII'I.F.S OF CHK.MJSTKV

heated above 4U , however, it decompt ises, forming chlorine, oxv^eiu
and perchloric acid : HK'H >;, = L(J1< '!<>.,+ 1 1 ,< ' + ('!, -f< >:.. When con-
siderably concentrated the acid acts as an exceedingly energetic oxi-
diser. so tliat organic substances, when brought into contact with it.
burst into llainc. Iodine, sulphurous acid, and similar substances
liable to oxidation form higher oxidation products and reduce the
chloric acid to hydrochloric acid. Hydrochloric acid gas gives chlorine
with chloric acid in the same manner as it acts on the lower acids :

Hcio:{ + :>Jici=:m./> + :tt.'i,.

l>y cautiotis]\- acting on potassium chlorate with sulphuric acid, the
dioAdi' (I'Mvr'u' /» ro,.-i<!<>),C\( >,.:!'' is obtained (J)avy, Millon). This gas
is easily li-|tietied in a freezing mixture, and the liquid boils at +10 .
The vapour density (about '-\~> if 11 = 1) shows that t he molecule of this
substance is ( i< >.,.'" In a gaseous or liquid state it very easily explodes
(tor instance, at i)(>"5 or by contact with organic compound.s or fineK"
divided substance.--. Are.), forming Cl., and ( ). and it therefore in many
instances. " acts as an oxidising agent, although (like nitric peroxide)
.it may be itself further oxidised.1'-' In dissolving in water or alkalis,
chloric peroxide gives chlorous and hvpochlorous acids 2C1( )., + 2KH( >
= K ('!<).,+ KC10, + H,O and therefore, like nitric ]»ero.xide. the di-
oxide may lie regarded as an intermediate oxide netween the (unknown)
anhydrides of chlorous and chloric acids : H'H »., = O.,< ), + ('!.,< ).. »3

"b IHM ;_<Ta.niH of -ulpliuric acid arc conlcd in a niixturr of ice and ^alt. and IT.

grains of pou'dcrcd potassium chlorate ai'c gradually added to the acid, \vliicli is then

cai'etully distil le<l al 'Ji i to •}' ' . t lie \ a] n >u r L;'I \'en i ill' lieinjjf condensed in a t I'eexini,' mix t lire.

]'ota>siuni perchlorate i- then t'ormed ; :;K(']O- -Jl I. S( ). •- -Jl\" HS( >,-- K( '1O,- "2('\( ),

II ( ). The reaction may result in an explosion, ('ah'crt and I hivies obtained chloric

jieroxide \\itlimii tin- least danger liy heatinu: a mixture of oxalic acid and potassium

chlorate in ;i test tube in a water-bath. In thi> case 12K( 'K >-, :!( '.,] I ,( ),'JH..( )

•J( ' ,11 1\< i; 'JCO , 'J('K), si I ,( ). The reaction is still further facilitated by the addi-

tion of a ~ni, ill <|iiantitv of sulphuric acid.

'" llv analogy \\itli nitric peroxide it would lie expected that at low temperatures a
doubling of the molecule into ('!,<), would take place, as the reactions in which it
act-, as the mixed anhydride of H ('!<>, ami H <'!<>.-, point out.

•" Owin^r to the formation of this chlorine dioxide, a mixture of pota-siuin chlorate

and sU^rar i -• inflamed b\ a drop ol siilpimric add. 'I'ln- jiropertx' was formerly made

11-1: ot for makiiiL' matches, and i- now sometimes employed for setting tire to explosive

i i • means of an arrangement in which the acid is caused to fall on the mixture

e moment re.piired. An interestin • • (pi-riiin-iit on the combustion of phosphorus

l,e conducted with chlorine dioxide. Pieces of phosphorus and ot

(thron-h ., Ion- funnel : the phosphoric then I Minis at lli- ex pen -e of the chlorine di1,xide.
•i - I'ot.i ,iiiin permanganate oxidises chlorine dioxide into chloric acid (Fiirsti.
"• The eiichl.irine obtaine<l b\ Davy b\ ^i-ntlv heat in. Lr potassium chlorate with hy<lro-

i ; 1'eliali a mixture of ,-hlorine dioxide and tree chh.rine. The li.piid and

. o: .i- Note :;:,i. uhich Millon con idered t o !„• ( '1 .()-. pr, .balily contains

.1 mi vt lire oj ( |( ) , vapour del i sit \ :!."M, ( 'I ,O:i i who-,e \ apoiir density sliouhl l»e ."j'.h, and

e lil, nine s a poui' 1 1 1 •!;',! \ ','t'i'~i . because it \ apoiir den -it s wa -determined to be about III.

THK ii A LOO F.NS

As the salts of chloric acid, HC1O3, are produced by the oxidation of
the salts of hypochlorous acid, so, in a similar manner, the salts of per-
chloric acid, HC1O4, are produced by the oxidation of the salts of chloric
acid, HC1O3. But this is the highest form of the oxidation of HC1.
Perchloric acid, HC1O4, is the most stable of all the acids of chlorine.
When fused potassium chlorate begins to swell up and solidify, after
having parted with one-third of its oxygen ; then potassium chloride
and potassium perchlorate are formed, 2KClO3 = KClO4 + KCl-f O2.

The formation of this salt is clearly remarked in the preparation-
of oxygen from potassium chlorate, owing to the fact that the potas-
sium perchlorate fuses with greater difficulty than the chlorate, and
therefore appears in the molten salt as solid grains. (Under the action,
of certain acids — for instance, sulphuric and nitric — potassium chlorate
also gives potassium perchlorate.) It may be easily purified, because it
is but sparingly soluble in water, although all the other salts of per-
chloric acid are very soluble and even effloresce in the air. It is a
remarkable fact that the perchlorates, although they contain mor&
oxygen than the chlorates, are decomposed with greater difficulty, and,
even when thrown on ignited charcoal, give a much feebler deflagration
than the chlorates. Sulphuric acid (at a temperature not below 100°)
evolves volatile and, to a certain extent, stable perchloric acid from
potassium perchlorate. Neither sulphuric nor any other acid will
further decompose perchloric acid as it decomposes chloric acid. Of
all the acids of chlorine, perchloric acid alone can be distilled.44 The
pure hydrate HC1O445 is a colourless and exceedingly caustic substance

44 If a solution of chloric acid, HC1O5, be first concentrated over sulphuric acid under
the receiver of an air-pump and afterwards distilled, then chlorine and oxygen are evolved
and perchloric acid formed: 4HC1O5 = 2HC1O4 + C13 + 8O + H2O. Eoscoe accordingly
directly decomposed a solution of potassium chlorate by hydrofluosilicic acid, decanted it
from the precipitate of potassium silicofluoride, K.>SiF6, concentrated the solution of chloric
acid, and then distilled it, perchloric acid being then obtained (see following footnote).
That chloric acid is capable of passing into perchloric acid is also seen from the fact that
potassium permanganate is decolorised, although slowly, by the action of a solution of
chloric acid. On decomposing a solution of potassium chlorate by the action of an elec-
tric current, potassium perchlorate is obtained at the positive electrode (where the oxygen
is evolved). Perchloric acid is also formed by the action of an electric current on solu-
tions of chlorine and chlorine monoxide. Perchloric acid was obtained by Count Stadion
and afterwards by Serullas, and was studied by Roscoe and others.

45 Perchloric acid, which is obtained in a free state by the action of sulphuric acid on
its salts, may be separated from a solution very easily by distillation, because it is volatile,
although it is partially decomposed by distillation. The solution obtained after distilla-
tion may be concentrated by evaporation in open vessels. In the distillation the solution
reaches a temperature of 200°, and then a very constant liquid hydrate of the composi-
tion HC1O4,2H2O is obtained in the distillate. If this hydrate be mixed with sulphuric
acid, it begins to decompose at 100°, but, nevertheless, a portion of the acid passes over
into the receiver without decomposing, forming a crystalline hydrate HC1O4,H.,O which
melts at 50°. On carefully heating this hydrate it breaks up into perchloric acid, which

480 PRINCIPLES OF CHEMISTRY

which fumes in the air and has a sp. gr. 1'78 at 15° (sometimes, after
•being kept for some time, it decomposes with a violent explosion). It
•explodes violently when brought into contact with charcoal, paper,
wood, and other organic substances. If a small quantity of water be
added to this hydrate, and if it be subjected to cold, then a crystallo-
hydrate, ClHO4,H2Oj separates out. It is much more stable, but the
liquid hydrate HC1O4,2H2O is still more so. The acid dissolves in
water in all proportions, and its solutions are distinguished for their
stability.46 When ignited both the acid and its salts are decomposed,
with the evolution of oxygen.47

distils over below 100°, and into the liquid hydrate HC1O4,2H.2O. The acid HC1O4 may
also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate,
•and carefully distilling and subjecting the crystals of the hydrate HC1O4,H2O obtained
in the distillate to a fresh distillation. A liquid of the composition HC1O4 then passes
over. Perchloric acid, HClO4,when taken separately, does not distil, and is decomposed
in distillation until the more stable hydrate HClO4,H.jO is formed ; this is decomposed in
•distillation into HC1O4 and HClO4,2HoO, which latter hydrate distils without alteration.
This forms an excellent example of the influence of water on stability, and of the pro-
perty of chlorine to give compounds of the type C1X7, of which all the above hydrates,
C105(OH), C1O2(OH)3, and C1O(OH)5, are members. Probably further research will lead
to the discovery of a hydrate C1(OH)7.

46 According to Roscoe the sp. gr. of perchloric acid = 1*782 and of the hydrate
HClO4,HoO in a liquid state (50°) 1*811 ; hence a considerable contraction takes place in
the combination of HC1O4 with HoO.

47 The decomposition of salts analogous to potassium chlorate has been more fully
studied in recent years by Potilitzin and P. Frankland. Professor Potilitziii, by decom-
posing, for example, lithium chlorate LiClO3, found (from the quantity of lithium chloride
and oxygen) that at first the decomposition of the fused salt (368°) is accomplished
according to the equation, 8LiClO3 = 2LiCl + LiClO4 + 5O, and that towards the end the
remaining salt is decomposed thus: 5LiClO3 = 4LiCl + LiClO4-f 10O. The phenomena
observed by Potilitzin obliged him to admit that lithium perchlorate is capable of de-
composing simultaneously with lithium chlorate, with the formation of the latter salt and
oxygen ; and this was confirmed by direct experiment, which showed that lithium chlorate
is always formed in the decomposition of the perchlorate. Potilitzin turned particular
attention to the fact that the decomposition of potassium chlorate and of salts analogous
to it, although exothermal (Chapter III. Note 12), not only does not proceed by itself, but
requires time and a rise of temperature in order to attain completion, which again shows
that chemical equilibria are not expressed by the heat effects of reactions only.

P. Frankland and J. Dingwall (1887) showed that at 448° (in the vapour of sulphur)
a mixture of potassium chlorate and pounded glass is decomposed almost in accordance
with the equation 2KC1O3 = KC1O4 + KC1 + O.,, whilst the salt by itself evolves about half
as much oxygen, in accordance with the equation, 8KC1O3 = 5KC1O4 + 3KC1 + 2O2. The
decomposition of potassium perchlorate in admixture with manganese peroxide proceeds
to completion, KC1O4 = KC1 + 2O2. But in decomposing by itself the salt at first gives
potassium chlorate, approximately according to the equation 7KC1O4 = 2KC103 + 5KC1
+ 11O.2. Thus there is now no doubt that when potassium chlorate is heated, the per-
chlorate is formed, and that this salt, in decomposing also with the evolution of oxygen,
gives the former salt.

I may further remark that the decomposition of potassium chlorate as a reaction
evolving heat from this very reason easily lends itself to the contact action of manganese
peroxide and other similar admixtures ; for such very feeble influences as those of contact
may evince themselves, as is observed either in those cases (for instance, detonating gas,

THE HALOGENS 481

On comparing chlorine as an element not only with nitrogen and
•jarbon but with all the other non-metallic elements (chlorine has so
little analogy with the metals that a comparison would be superfluous),
we find in it the following fundamental properties of the halogens or
salt-producers. With metals chlorine gives salts (such as sodium
chloride, &c.) ; with hydrogen a very energetic and monobasic (con-
taining one H in its molecule) acid HC1, and the same chlorine is able
by metalepsis to replace the hydrogen ; with oxygen it forms oxides of
•in acid character. These properties of chlorine are possessed1 by three
•other elements, bromine, iodine, and fluorine. They are members of
one natural family. Each representative has its peculiarities, its indi-
vidual properties, and points of distinction in combination and in the
free state — otherwise they would not be independent elements ; but
the repetition in all of them of the same chief signs of the family
-enables one to foretell from one element the properties of another,
and thus abbrev^Bk; an acquaintance with all the differences of their
•elementary propeMfc and a system isation of the elements themselves.

In order to have a guiding thread in forming comparisons between
the elements, attention must be turned to those of their properties and
signs in which they differ most from each other, because it is only
under this condition that the comparison ceases to be artificial. And
the atomic weights of the elements must be counted as their most
elementary property ; it being a quantity which is most undoubtedly
•established, and which acts in all the manifestations of the element.
The halogens are endowed with atomic weights —

F = 19, 01 = 35-5, Br=80, 1 = 127.

All the properties, physical and chemical, of the elements and their
corresponding compounds must evidently be in a certain dependence
on this fundamental point, if the grouping in one family be natural.
And we find in reality that, for instance, the properties of bromine,
whose atomic weight is almost the mean between those of iodine and-
ohlorine, occupy a mean position between those of these two elements.
The second measurable property of the elements is their equivalence or
their capacity for forming compounds of definite forms. Thus carbon

hydrogen peroxide, &c.), when the reaction is accompanied by the evolution of heat, or
when (for instance, H2 + I2, &c.) little heat is absorbed or evolved. In these cases it is
evident that the existing equilibrium is not very stable, and that a feeble alteration in it
proceeding at the surfaces of contact may suffice to cause its destruction. In order to
conceive the modus operandi of contact phenomena, it is enough to imagine, for instance,
that at the borders of contact the movement of the atoms in the molecules changes from
a circular to an elliptical path. Momentary and transitory compounds may be formed,
but their formation cannot alter the explanation of the phenomena.

VOL. I. I I

482 PBINCJPLKS OF CHEMISTRY

or nitrogen in this respect differs widely from the halogens. Although
the form C1O2 corresponds with KO._, and C02, yet the latter is the
highest oxide of carbon, whilst that of nitrogen is N.,O-, and for chlorine,
if there was an anhydride of perchloric acid, its composition would be
C1.2O7, which is quite different from that of carbon. In respect to
the forms of their compounds the halogens, like all elements of out-
family or group, are perfectly analogous to each other, as is seen from
their hydrogen compounds :

HF, HC1, HBr, HI.

It is the same with their oxygen compounds. Only fluorine does
not give any oxygen compounds. The iodine and bromine compounds
corresponding with HC1O3 and HC1O, are HBrO3 and HBrO4, HIO;j
and HIO4. On comparing the properties of these acids we can even
foresee t;he fact that fluorine will not form any oxygen compound. In
fact, iodine is easily oxidised— for instance, by nitric acid — whilst
chlorine is not directly oxidised. The oxygen acids of iodine are com-
paratively more stable than the acids of chlorine ; and, generally
speaking, the affinity of iodine for oxygen is much greater than that of
chlorine. Here also bromine occupies an intermediate position. In
fluorine we may expect a still smaller affinity for oxygen than in
chlorine— and up to now it has not been combined with oxygen. If
any oxygen compound of fluorine be obtained, it will be exceedingly
unstable. The relation of these elements to hydrogen is the reverse
of the above. Fluorine has so great an affinity for hydrogen that it
decomposes water at the ordinary temperature ; whilst iodine has so
little affinity for hydrogen that hydriodic acid, HI, is formed with diffi-
culty, is easily decomposed, and acts as a reducing agent in a number
of cases.

From the form of their compounds the halogens are nnivalent
elements with respect to hydrogen and septivalent with respect to
oxygen, if N be trivalent to hydrogen (it gives NH3) and quinquivalent
to oxygen (it gives N.2O5), and if C be quadrivalent to both H and O as
it forms CH4 and CO.;.

As not only their oxygen compounds, but also their hydrogen com-
pounds, have acid properties, the halogens are elements of an exclusively
acid character. Such metals as sodium, potassium, barium only give
basic oxides. In the case of nitrogen, although it forms acid oxides,
still in ammonia we find that capacity to give an alkali with hydrogen
which indicates a less distinctly acid character than in the halogens.
In no other elements are the acid characters so strongly developed as
in the halogens.

TIN-: H.\LM<;F,.\S 483

In describing certain peculiarities characterising the halogens, we
shall at every step encounter a confirmation of the above-mentioned
general relations.

As fluorine decomposes water with the evolution of oxygen (which
forms ozone if the temperature be not high), F., + H2O = 2HF + O,
therefore for a long time all efforts to obtain it in free state by means
of methods similar to those for the preparation of chlorine proved fruit-
less. Thus by the action of hydrofluoric acid on manganese peroxide, or
by decomposing a solution of hydrofluoric acid by an electric current,
either oxygen or a mixture of oxygen and fluorine were obtained instead
of fluorine. Probably a certain quantity of fluorine 48 was set free by
the action of oxygen or an electric current on incandescent and fused
calcium fluoride, but it then, at the high temperature, acted even on
platinum, and was therefore absorbed, leaving only oxygen. When
chlorine acted on silver fluoride, AgF, in a vessel of natural fluor spar,
CaF.,, fluorine was also liberated ; but it was mixed with chlorine, and
it was impossible to study the properties of the resultant gas. Brauner
also obtained fluorine by igniting cerium fluoride, 2CeF4 = 2CeF3 + F.2 ;
but this, like all preceding efforts, only showed fluorine to be a gas
which decomposes water, and is capable of acting in a number of
instances like chlorine, but gave no possibility of testing its properties.
It was evident that it was necessary to avoid as far as possible the
presence of water and a rise of temperature ; this Moissan succeeded
in doing in 1886. He decomposed anhydrous hydrofluoric acid, liquefied
at a temperature of — 23° and contained in a U-shaped tube (to which
a small quantity of potassium fluoride had been added to make it a
better conductor), by the action of a powerful electric current (twenty
Bunsen's elements in series). Hydrogen was then evolved at the
negative pole, and fluorine appeared at the positive pole (of iridium
platinum) as a colourless gas which decomposed water with the forma-
tion of ozone and hydrofluoric acid, and combined directly with silicon
(forming silicon fluoride, SiF4), boron (forming BF3), sulphur, <kc. But

4S It is most likely that in this experiment of Fremy's, which corresponds with the
action of oxygen on calcium chloride, fluorine was set free, but that a converse reaction
a 1 *o proceeded, CaO + F2 = CaF., + O— that is, the calcium distributed itself between the
i and ihiorine. MnK,. which is capable of splitting up into MnF., and Fo, is without
doubt formed by the action of a strong solution of hydrofluoric acid on manganese per-
oxide, but under the action of water the fluorine gives hydrofluoric acid, and probably
this is aided by the allinity of the manganese fluoride and hydrofluoric acid. In all the
efforts (Davy, Knox, Louget, Fremy, Gore, and others) made to decompose fluorides
(those of lead, silver, calcium, and others) by chlorine, there were doubtless also cases
of distribution, a portion of the metal combined with chlorine and a portion of the fluorine
was evolved; but it is improbable that there were pure results. Probably Fremy obtained
fluorine, but it was not pure.

ii 2

484 PKINCIPLES OF CHEMISTRY

the action of fluorine on metals at the ordinary temperature is com-
paratively feeble, because the metallic fluoride formed coats the
remaining mass of the metals ; it is, however, completely absorbed by
iron. Hydrocarbons (for instance, naphtha), alcohol, £c., immediately
absorb fluorine, with the formation of hydrofluoric acid. Fluorine when
mixed with hydrogen easily explodes violently, forming hydrofluoric
acid.49

Among the compounds of fluorine calcium fluoride, CaF2, is rather
widely spread in nature as fluor spar,50 whilst cryolite, or aluminium
sodium fluoride, Na3AlF6, is found more rarely (in large masses in
Greenland). Cryolite, like fluor spar, is also insoluble in water, and
gives hydrofluoric acid with sulphuric acid. Small quantities of fluorine
have also in a number of cases been found in the bodies of animals, in
the blood, urine, and bones. If fluorides occur in the bodies of animals,
they must have been introduced by food, and must occur in plants and
water. And in reality river, and especially sea, water always contains
a certain, although small, quantity of fluorine compounds.

49 According to Moissan, fluorine is disengaged by the action of an electric current
on fused hydrogen potassium fluoride, KHF2. The present state of chemical knowledge
is such that the notion of an element with its properties is much more general than the
notion of that element in the free state. It is profitable and agreeable to learn that even
fluorine in the free state has not succeeded in eluding experiment and research, that the
efforts to isolate it have been crowned with success, but the sum total of the general
chemical data concerning fluorine as an element gain but little by this result. The gain
will be augmented, however, if it be now possible to subject fluorine to a comparative
study in relation to oxygen and chlorine. There is particular interest in the phenomena
of the distribution of fluorine and oxygen, or fluorine and chlorine, competing under dif-
ferent conditions and relations.

*° It is called spar because it very frequently occurs as crystals of a clearly laminar
structure, and is therefore easily split up into pieces bounded by planes. It is called fluor
spar because when used as a flux it renders ores fusible, owing to its reacting with silica,
SiO2 + 2CaF2 = 2CaO + SiF4 ; the silicon fluoride escapes as a gas and the lime combines
with a further quantity of silica, and gives a vitreous slag. Fluor spar occurs in mineral
veins and rocks, sometimes in considerable quantities. It always crystallises in the cubic
system, sometimes in very large semitransparent cubic crystals, which are colourless or
of different colours. It is insoluble in water. It melts under the action of heat, and
crystallises on cooling. The sp. gr. is 8'1. When steam is passed over incandescent
fluor spar, lime and hydrofluoric acid are formed : CaF2 + H2O = CaO + 2HF. A double
decomposition is also easily produced by fusing ftuor spar with sodium or potassium
hydroxides, or potash, or even with their carbonates ; the fluorine then passes over to the
potassium or sodium, and the oxygen to the calcium. In solutions — for example,
Ca(NO3)2 + 2KF = CaF2 (precipitate) 4-2KNO3 (in solution)— the formation of calcium
fluoride takes place, owing to its very sparing solubility. 26000 parts of water dissolve
one part of fluor spar. Fluorine in the form of calcium fluoride also enters into the
composition of certain minerals : but in general the amount of fluorine in minerals is
inconsiderable. Apatite is a mineral whose chief mass consists of calcium phosphate.
Apatites sometimes contain no fluorine whatever, but only chlorine ; whilst in other
instances a certain amount of fluorine enters into their composition, in which case the
atomic proportion of chlorine is proportionately diminished.

THE HALOGENS 485

Hydrofluoric acid, HF, cannot be obtained from fluor spar in glass
retorts, because glass is acted on by and destroys the hydrofluoric acid.
It is prepared in lead vessels, and when it is required pure, in platinum
vessels, because lead in reality also acts on hydrofluoric acid, only very
feebly at the surface, and when once a coating of fluoride and sulphate
of lead is formed there is no further action. Powdered fluor spar
and sulphuric acid evolve hydrofluoric acid (which fumes in the air)
even at the ordinary temperature, CaF2 + H2SO4 = CaSO4 + 2HF. At
1 30° fluor spar is completely decomposed by sulphuric acid. The acid is
then evolved as vapour, which may be condensed by a freezing mixture
into an anhydrous acid. The condensation is aided by pouring water
into the receiver of the condenser, as the acid is easily soluble in cold
water.

In the liquid anhydrous form hydrofluoric acid boils at +19°, and
its sp. gr. at 12>8° = 0'9849.51 It dissolves in water with the evolution
of a considerable amount of heat, and gives a solution of constant
boiling point which distils over at 120° ; hence the acid is able to
combine with water. The specific gravity of the compound is 1*15, and
its composition HF,2H.,O.52 With an excess of water a dilute solu-
tion first distils over. The aqueous solution and the acid itself must
be kept in platinum vessels, but the dilute acid may be conveniently
preserved in vessels made of various organic materials, such as gutta-
percha, or even in glass vessels having an interior coating of paraffin.
Hydrofluoric acid does not act on hydrocarbons and many other sub-
stances, but it acts in a highly corrosive manner on metals, glass, porce-
lain, and the majority of rock substances/53 It also corrodes the skin,

51 According to Gore, hydrofluoric acid remains liquid when cooled to —34°. Fremy
obtained anhydrous hydrofluoric acid by decomposing lead fluoride at a red heat, by
hydrogen, or by heating the double salt HKF.2, which easily crystallises (in cubes) from
a solution of hydrofluoric acid, half of which has been saturated with potassium hydroxide,
or carbonate.

5- This "composition corresponds with the crystallo-hydrate HC1,2H.2O. All the
properties of hydrofluoric acid call to mind those of hydrochloric acid, and therefore the
comparative ease with which hydrofluoric acid is liquefied (it boils at +19°, hydrochloric
acid at —35°), must be explained by a polymerisation taking place at low temperatures,
as will be afterwards explained, H.>F2, being formed, and therefore in a liquid state it
differs from hydrochloric acid, for which a phenomenon of a similar kind has not yet been
observed.

53 The corrosive action of hydrofluoric acid on glass and similar siliceous compounds
is based upon the fact that it acts on silica, SiO.i, as we shall consider more fully in
describing that compound, forming gaseous silicon fluoride, SiO.> + 4HF = SiF4 + 2H.2O.
Silica, on the other hand, forms the binding (acid) element of glass and of the mass of
mineral substances forming the salts of silica. When it is removed the bond is
destroyed. This is made use of in practice, and the laboratory, for etching designs and
scales, <fec., on glass. In engraving on glass the surface is covered with a varnish com-
posed of four parts of wax and one part of turpentine. This varnish is not acted on by

486 PRINCIPLES OF CHEMISTRY

and is distinguished by its poisonous properties, so that in working with
the acid a strong draught must be kept up, to prevent the possibility of
the fumes being inhaled. The non-metals do not act on hydrofluoric
acid, but all metals— with the exception of mercury, silver, gold, and
platinum, and, to a certain degree, lead — decompose it with the evolution
of gaseous hydrogen. With bases it directly gives metallic fluorides,
and behaves in many respects like hydrochloric acid. There are, how-
ever, several distinct individual differences, which are furthermore
much greater than those between hydrochloric, hydrobromic, and
hydriodic acids. Thus the silver compounds of the latter are in-
soluble in water, whilst silver fluoride is moderately soluble. Calcium
fluoride, on the contrary, is insoluble in water, whilst calcium chloride,
bromide, and iodide are not only soluble, but attract water with great
energy, owing to which gases are frequently dried by passing them
over calcium chloride. Neither hydrochloric, nor hydrobromic, nor
hydriodic acid acts on sand and glass, whilst hydrofluoric acid corrodes
them, forming gaseous silicon fluoride. The other halogen acids only
form normal salts, KC1, NaCl, with Na or K, whilst hydrofluoric acid
gives acid salts, for instance HKF2 (and by dissolving KF in liquid
HF, KHF22HF is obtained). This latter property is in close connec-
tion with the fact that at the ordinary temperature the vapour density
of hydrofluoric acid is nearly 20, which corresponds with a formula
H2F2, as Mallet (1881) showed ; but a depolymerisation occurs with a
rise of temperature, and the density approaches 10, which answers to
the formula HF.54

The analogy between chlorine and the other two halogens, bromine

hydrofluoric acid, and it is soft enough to allow designs being drawn upon it whose lines
lay bare the glass. The drawing is made with a steel point, and the glass is afterwards
laid in a lead trough in which a mixture of fluor spar and sulphuric acid is placed.
The sulphuric acid must be taken in considerable excess, as otherwise transparent lines
are obtained (owing to the formation of hydrofluosilicic acid). After being exposed
for some time, the varnish is removed (melted), and the design drawn by the steel
point is found reproduced in dull lines. The drawing may be also made by the direct
application of a mixture of a silicofluoride and sulphuric acid, which forms hydrofluoric
acid. In the laboratory a solution of hydrofluoric acid or its fumes is employed for
decomposing siliceous substances which are insoluble in ordinary acids, or else they are
fused with the salts KHP2 or NH4F.

54 Mallet (1881) determined the density at 30° and 100°, previous to which Gore
(1869) had determined the vapour density at 100 3, whilst Thorpe and Hambly (1888)
made fourteen determinations between 26° and 88°, and showed that within this limit of
temperature the density gradually diminishes, just like the vapour of acetic acid.
The capacity of HF to polymerise into H2F2 is probably connected with the property
of many fluorides of forming acids with HF — for example, KHF2 and HaSiF6. We
saw above that HC1 has the same property (forming, for instance, HaPtClc, &c., p. 450),
and hence this property of hydrofluoric acid does not stand isolated from the properties
of the other halogens.

THE HALOGENS 487

mid iodine, is much more perfect. Not only have their hydrates or
halogen acids much in common, but they themselves resemble chlorine
in many respects,"'5 and even the properties of the corresponding
metallic compounds of bromine and iodine are very much alike. Thus
the chlorides, bromides, and iodides of sodium and potassium crystallise
in the cubic system, and are soluble in water ; the chlorides of calcium,
aluminium, magnesium, and barium are soluble in water, like the
bromides and iodides of these metals. The iodides and bromides of
silver and lead are sparingly soluble in water, like the chlorides of
these metals. Besides which even the oxygen compounds of bromine
and iodine present a very strong analogy to the corresponding compounds
of chlorine. A hypobromous acid is known corresponding with hypo-
chlorous acid. The salts of this acid have the same bleaching property

i5 For instance, the experiment with Dutch metal foil (Note 16) may be made with
bromine just as well as with chlorine. A very instructive experiment on the direct com-
bination of the halogens with metals may be made by throwing a small piece (a shaving) of
aluminium into a vessel containing liquid bromine ; the aluminium, being lighter, floats
on the bromine, and after a certain time reaction sets in accompanied by the evolution
of heat, light, and fumes of bromine. The incandescent piece of metal moves rapidly
over the surface of the bromine in which the resultant aluminium bromide is dissolved.
It was in this manner that Gustavson prepared that mixture of bromine and aluminium
bromide which reacts by metalepsis with the greatest ease in those cases when bromine
by itself is not able to bring about metalepsis, or else only acts very slowly, as, for
instance, with benzene, C6H6. When drops of this hydrocarbon are added to bromine
containing aluminium bromide, they immediately give a mass of hydrobromic acid and
of the product of metalepsis. Gustavson showed that the cause of this facility of
reaction must be looked for in the capacity of aluminium bromide to form an unstable
compound with the products of reaction which are formed. For the sake of comparison
we will proceed to cite several thermochemical data (Thomsen) for analogous actions of
(1) chlorine, (2) bromine, and (3) iodine, with respect to metals; the halogen being
expressed by the symbol X, and the sign plus connecting the reacting substances. All
the figures are given in thousands of calories, and refer to molecular quantities in
.grams and to the ordinary temperature : —

123

K.j + X3 211 191 160

195 172 138

59 45 28

83 68 48

Hg +X2 63 51 34

Ca + X3 170 141

Ba + X3 195 170

Zn + X3 1)7 76 49

Pb +X3 83 64 40

Al +X3 161 120 70

We may remark that the latent heat of vaporisation of the molecular weight Br3 is about
7'ii, and of iodine 6'0 thousand heat units, whilst the latent heat of fusion of Br3 is about
•O'S, and of I3 about 3'0 thousand heat units. From this it is evident that the difference
between the amounts of heat evolved does not depend on the difference in physical state.
For instance, the vapour of iodine in combining with Zn to form ZnI2 would give
-48 + 8 + 3, or about sixty thousand heat units, or li times less than Zn + Cla,

488 PRINCIPLES OF CHEMISTRY

as the salts of hypochlorous acid. The higher oxygen acids of iodine,
bromine, and chlorine are closely analogous to each other. Iodine was
discovered in 1811 by Courtois in kelp, and was soon investigated by
Clement, Gay-Lussac, and Davy. Bromine was discovered in 1826 by
Balard in the mother liquor of sea water.

Bromine and iodine, like chlorine, occur in sea water in combina-
tion with metals. However, the amount of bromides, and especially
of iodides, in sea water is so small that their presence can only be
discovered by means of sensitive reactions.56 In the extraction of salt
from sea water the bromides remain in the mother liquor, and can be
obtained therefrom ; this method is, however, rarely employed on a
large scale, as other and richer sources are known for the extraction of
this substance. Iodine and bromine also occur combined with silver, in
admixture with silver chloride, as a rare ore which is mainly found in
America. Certain mineral waters (those of Kreuznach and Schonebeck)
contain metallic bromides and iodides, always in admixture with an
excess of sodium chloride. Those upper strata of the Stassfurt rock-
salt (Chapter X.) which are a source of potassium salts also contain
metallic bromides,57 which collect in the mother liquors left after the
crystallisation of the potassium salts ; and this now forms the chief
source (together with certain American springs) of the bromine in
common use. Bromine may be easily liberated from a mixture of
bromides and chlorides, owing to the fact that chlorine displaces
bromine from its compounds with sodium, magnesium, calcium, <fec. A
colourless solution of bromides and chlorides turns an orange colour
after the passage of chlorine, owing to the disengagement of bromine.5*
Bromine may be extracted on a large scale by a similar method, but it

56 One litre of sea water contains about 20 grams of chlorine, and about 0'07 grams-
of bromine. The Dead Sea contains about ten times as much.

57 But there is no iodine in Stassfurt carnallite.

58 The chlorine must not, however, be in large excess, as otherwise the bromine
would contain chlorine. Commercial bromine not unfrequently contains chlorine, as
bromine chloride ; this is more soluble in water than bromine, which may thus be freed
from it. To obtain pure bromine the commercial bromine is washed with water, dried
by sulphuric acid, and distilled, the portion coming over at 58° being collected ; the
greater part is then converted into potassium bromide and dissolved, and the re-
mainder is added to the solution in order to separate iodine, which is removed by
shaking with carbon bisulphide. By heating the potassium bromide thus obtained with
manganese peroxide and sulphuric acid, bromine is obtained quite free from iodine,,
which, however, is not present in certain kinds of commercial bromine (the Stassfurt,,
for instance). By treatment with potash, the bromine is then converted into a mixture
of potassium bromide and bromate, and the mixture (which is in th<> proportion given in
the equation) is distilled with sulphuric acid, bromine being then evolved: 5KBr + KBrO5

+ 6H<jSO4 = 6KHSO4 + 8H2O + 3Br2. After dissolving the bromine in a strong solution
of calcium bromide and precipitating with an excess of water, it loses all the chlorine it-
contained, because chlorine foi'ms calcium chloride with CaBr2.

THE HALOGENS 489

is simpler to add a small quantity of manganese peroxide arid sulphuric
acid to the mother liquid direct. This sets free a portion of the chlorine,.
and this chlorine liberates the bromine. Bromine is a dark brov'ii
liquid, giving brown fumes, and having a poisonous suffocating smell,
from whence its name (from the Greek /fyw/xos, signifying evil smelling).
The vapour density of bromine shows that its molecule is Br2. In the
cold bromine freezes into brown-grey scales like iodine. The melting
point of pure bromine is — 7*05°. r'9 The density of liquid bromine at 0°
is 3-187, and at 15° about 3-0. The boiling point of bromine is about
58'7° ; it is generally purified by distillation. Bromine, like chlorine, i&
soluble in water; 1 part of bromine at 5° requires 27 parts of water,
and at 15° 29 parts of water. The aqueous solution of bromine is of
an orange colour, and when cooled to —2° yields crystals containing 10
molecules of water to 1 molecule of bromine.60 Alcohol dissolves a

39 There has long existed a difference of opinion as to the melting point of pure
bromine. By some (Regnault, Pierre) it was given as between — 7° and — 8°, and by others
(Balard, Liebig, Quincke, Baumhauer) as between — 20° and — 25°. There is now no doubt,
thanks more especially to the researches of Ramsay and Young (1885), that pure bromine-
melts at about —7°. This figure is not only established by direct experiment (Van der
Plaats confirmed it) but also by means of the determination of the vapour tensions. For
solid bromine the vapour tension^, in mm. at t was found to be —

p= 20 25 30 85 40 45mm.

t= -16-6° -14° -12° -10° - 8-5° - 7°

For liquid bromine —

p= 50 100 200 400 600 760mm.

t= - 5-0° + 8'2° 23-4° 40-4° 51'9° 58'7°

These curves intersect at — 7'05. Besides which, in comparing the vapour tension of
many liquids (for example, those given in Chapter II. Note 27), Ramsay and Young
observed that the ratio of the absolute temperatures (£ + 273) corresponding with equal
tension varies for every pair of substances in rectilinear proportion in dependence upon t>
and, therefore, for the above pressure p, Ramsay and Young determined the ratio of
t + 273 for water and bromine, and found that the straight lines expressing these ratios,
for liquid and solid bromine intersect also at 7'05° ; thus, for example, for solid bromine —

p= 20 25 30 85 40 45

273 + £ = 25(>-4 259 261 263 264'6 266

278 + £' = 295-8 >>W) 302'1 804'8 807'2 809'3

c = 1-152 1-154 1-157 1-159 1'lGl 1'168

where t' indicates the temperature of water corresponding with a vapour tension^;, and
where c is the ratio of 273 + 1' to 278 + 1. The magnitude of c is evidently expressed with
great accuracy by the straight line c = ri708 + 0'0011f. In exactly the same way we find
the ratio for liquid bromine and water to be cl = T1585 + 0'00057^. The intersection of
these straight lines in fact corresponds with — 7'0<P, which again confirms the melting
point given above for bromine. In this manner it is possible with the existing store of
data to accurately establish and verify the melting point of substances. Ramsay and
Young established the thermal constants of iodine by exactly the same method.

60 The observations made by Paterno and Nasini (by Raoult'a method, Chapter I.
Note 49) on the temperature of the formation of ice (-1-115°, with 1-891 grams of bromine

greater (juantitv of bromine, and ether a -\i\\ greater amount. Hut
after a certain time products of the action of the bromine on these
organic substances are formed in the solutions. Aqueous .solutions of
the bromides also absorb a large amount of bromine.

With respect to iml/nc, it is almost exclusively extracted from the
mother liquors after the crystallisation of natural .sodium nitrate (Chili
saltpetre) and the ashes of the sea-weed cas! upon the shores of I'Yance.
( i real r>ritain. and Spain, sometimes in considerable quantities, bv the
hi^'h tides. The majority of these sea-weeds are of ihe genera /'/'''/'x,
Luini iittrin, ivc. The fused ashes of these sea-weeds are called 'kelp'
in Scotland and ' varee ' in Normandy. A somewhat considerable
quantity of iodine is contained in these sea-weeds. After being burnt
(or subjected to dry distillation ) an a>h is left which chief! v contains
salts of potassium, >odium. and calcium. 'Ihe metals occur in the sea-
weed as salts of organic acids. ( Mi beinu' burnt ihe.se organic salts are
deeompo>ed formin"1 carbonates of potassium and sodium. llence,

1 <3 A

sodium carbonate is found in the ash of sea plants. The ash is dissolved
in hot water, and on evaporation sodium carbonate and other salts
separate, but a portion of the substances remains in solution. These
mother liqm >rs left after the separation of the sodium carbonate contain
chlorine, bromine, and iodine in combination with metals, the chlorine
and iodine bein^ in excess of the bromine. I:)UIMI kilos, of kelp give
a'oout I ( if H i kilos, of sodium carbonate and !•> kilos, of iodine.

The liberation of the iodine from the mother liquor is accomplished
with comparative ease, because, chlorine disengages iodine from potas-
sium iodide and its other combinations with the metals. Not only
chlorine, but also sulphuric acid, liberates iodine from sodium iodide.
Sulphuric acid, in acting on an iodide, sets hydriodic acid free, but the
latter easily decomposes, especially in t he presence of substances capable
of evolving oxvgen, such as chromic acid, nitrous acid, and even ferric
salts.'1' <>wino; to its sparing solubility in water, the iodine liberated

r-i . i; .

diir.-n nt -..!\.-ni
];. i;- . - i :.-.', i

. N.,t. LI. IIL. 'I'h.- t.-i |" itiin ni
>;••_> : • . , :. iy <,!' r.r..lnj| .( ) IT.)
'• In ..jeiienil. iill I M I, H .( ). ii tin- , _-en proceed from a -iili-tancc- from which

;• i , . \\ , ... , ,| .,.,|. I-', ,r t.lii- re, i , in CM) n pi, m |tl- corre-;pi,iidiii;_' u i! li lln- higher i-ta^'t-s of
... 'd.M ;, ' it ion t'rei|iieni K ;_ is . ,i I»SM r ta;_re \s hen treated u ilh hydriodir acid.

. i , i i . .1 hi-i id.-. ,i nd i.-i-i i o> i,l.-. !•'.•< ). :i Inu.-r ,,xide : the fonm-r

I-',. A III,, liter s1 l-',-X .. and Ihi- |o - -a-.- from tlic II,-H,T to ilie

. /, ,, i , .. ; -, . j '. . . i , , .driodii- acid. < ompi muds of copper

,.- ,],.. ( ,,M , . ( ,,.\ . , • ' • ;:,) . . . . oi !ii, 1 1 id.- Ciiv' ). <.r TuX. I-! ven sulphuric; acid,

TlIK HALOCKNS 491

separates as a precipitate. To obtain pure iodine it is enough to distil
it, and neglect the first and last portions of the distillate, the middle
portion only being collected. Iodine passes directly from a state of
vapour into a crystalline form, and settles on the cool portions of the
apparatus in tabular crystals, having a black grey colour and metallic
lustre.63

The specific gravity of the crystals of iodine is 4-95. It melts at
114° and boils at 18-4°. Its vapour is formed at a much lower tempera-
ture, and is of a violet colour, from whence iodine receives its name.
The smell of iodine calls to mind the characteristic smell of hypochlorous
acid ; it has a sharp sour taste. It destroys the skin and organs of the
body, and is therefore frequently employed for cauterising and as an
irritant for the skin. In small quantities it turns the skin brown, but
the coloration disappears after a certain time, partly owing to the
volatility of the iodine. Water dissolves only -,,,1,,-,, part of iodine. A
"brown solution is thus obtained, which bleaches, but much more feebly
than bromine and chlorine. Water which contains salts, and especially
iodides, in solution dissolves iodine in considerable quantities, and the
resultant solution is of a dark brown colour. Pure alcohol dissolves a
small amount of iodine, and in so doing acquires a brown colour, but
the solubility of iodine is considerably increased by the presence of a
small quantity of an iodine compound — for instance, ethyl iodide — in the
alcohol.*'3 Ether dissolves a larger amount of iodine than alcohol ; but

winch corresponds to the higher stage SO-, is able to act thus, forming the lower oxide
S(_).j. The liberation of iodine from hydriodic acid proceeds with still greater ease under
the action of substances capable of disengaging oxygen. In practice, many methods
are employed for liberating iodine from acid liquids containing, for example, sulphuric
acid and hydriodic acid. The higher oxides of nitrogen are most commonly used; they
then pass into nitric oxide. Iodine may even be disengaged from hydriodic acid by the
•vet ion of iodic acid, ivc. But there is a limit in these reactions of the oxidation of hydri-
odic acid because, under certain conditions, especially in dilute solutions, the iodine set
free is itself able to act as an oxidising agent — that is, it evinces in itself the character
of chlorine, and of the halogens in general, to which we shall again have occasion to
refer.

52 For the ultimate purification of iodine, Stag dissolves it in a strong solution of
potassium iodide, and precipitates it by the addition of water (see Note 58).

' '•" The solubility of iodine in solutions containing oxides, and compounds of iodine in
general, may serve, on the one hand, as an indication that solution is called forth by
analogy (p. 70), and, on the other hand, as an indirect proof of that view as to solutions
which was cited in Chapter I., because in many instances unstable highly-iodised com-
pounds, resembling crystallo-hydrates, have been obtained from such solutions. Thus
iodide of tetramethylammonium N(CH-)4I, combines with I2 and I4. Even a solution of
iodine in a saturated solution of potassium iodide presents traces of the formation of a
definite compound KI3. Thus, an alcoholic solution of KI3 does not give up iodine to
carbon bisulphide, although this solvent takes up iodine from an alcoholic solution of
iodine itself (Girault, Jiirgensen, and others). The instability of these compounds resem-
bles the instability of many crystallo-hydrates, for instance of HC1,2H2O.

492 PRINCIPLES OF CHEMISTRY

iodine is particularly soluble in liquid hydrocarbons, in carbon bisulphide.,
and in chloroform. A small quantity of iodine dissolved in carbon
bisulphide tints it rose-colour, but in a somewhat larger amount it
gives a violet colour. Chloroform (quite free from alcohol) is also
tinted rose colour by a small amount of iodine. This gives an
easy means for detecting the presence of free iodine in small quan-
tities. The coloration which free iodine gives with starch may
also, as has already been frequently mentioned, serve for the detection
of iodine.

If we compare the four elements, fluorine, chlorine, bromine, and
iodine, we see in them an example of analogous substances which,
arrange themselves by their physical properties in the same order as
they stand in respect to their atomic and molecular weights. If the-
weight of the molecule be large, the substance has a higher specific-
gravity, a higher melting and boiling point, and a whole series of pro-
perties depending on this "difference in its fundamental properties. A
large atomic weight should determine a larger affinity between the-
molecules, and consequently a greater difficulty in their division and a
greater attraction between them. We meet a very evident example-
in question in polymeric compounds ; for instance, in the hydrocarbons-
expressed by the formula, CrtH2B : C2H4, C3HG are gases, those having a
greater molecular weight — for instance, C5H10, C7H14, &c. — areliquids,
and those of a still greater molecular weight are solids. The same rela-
tion is found in the above-named four elements. Chlorine in a free
state boils at about — 35°, bromine boils at 60°, and iodine only above
180°. According to Avogadro-Gerhardt's law, the vapour densities
of these elements in a gaseous state are proportional to their atomic
weights, and here, at all events approximately, the densities in a liquid
(solid) state are also almost in the ratio of their atomic weights.
Dividing the atomic weight of chlorine (35-5) by its sp. gr. in a liquid
state (1'3), we obtain a volume = 27, for bromine (80/3'1) 26, as also
for iodine (127/4-9) 26.'J1

The metallic bromides and iodides are in the majority of cases and

64 The equality of the atomic volumes of the halogens themselves is all the more
remarkable because in all the halogen compounds the volume augments with the substitu-
tion of fluorine by chlorine, bromine, and iodine. Thus, for example, the volume of
sodium fluoride (obtained by dividing the weight expressed by its formula by its specific-
gravity) is about 15, of sodium chloride 27, of sodium bromide 82, and of sodium
iodide 41. The volume of silicon chloroform, SiHCl,-, and of the corresponding bromine,
and iodine compounds, are 82, 108, and 122 respectively. So in solutions, for example,
NaCl + 200H2O has a sp. gr. (at 15°/4°) of 1'OIOG, consequently the volume of the solution
8658-5/1-0106 = 8620, hence the volume of sodium chloride in solution =3620-8603 (this is
the vol. of 200 H2O) = 17. So also in similar solutions, NaBr = 26 and Nal = 85.

THE HALOGENS 493

Tespects analogous to the corresponding chlorides,05 but chlorine displaces
the bromine and iodine from them, and bromine liberates iodine from
iodides, which is taken advantage of in the preparation of these halogens.
However, the researches of Potilitzin showed that a reverse displace-
ment of chlorine by bromine may occur both in solutions of and in
ignited metallic chlorides in an atmosphere of bromine vapour — that is,
•a distribution of the metal (according to Berthollet's doctrine) takes
place between the halogens, although, however, the larger portion still
unites with the chlorine, which shows its greater affinity for metals as
•compared with that of bromine and iodine.66 The latter, however,

fis But the density (and even volume, Note 64) of a bromine compound is always
greater than that of a chlorine compound, whilst that of an iodine compound is still
greater. The order is the same in many other respects. For example, an iodine com-
pound has a higher boiling point than a bromine compound, Arc.

66 A. L. Potilitzin showed that in heating various metallic chlorides in a closed tube, with
•an equivalent quantity of bromine, a distribution of the metal between the halogens always
•occurs, and that the amounts of chlorine replaced by the bromine in the ultimate product
•are proportional to the atomic weights of the metals taken and inversely proportional to
their equivalency. Thus, if NaCl + Br be taken, then out of 100 parts of chlorine, 5'54 are
replaced by the bromine, whilst if AgCl + Br be taken, the 27'28 parts are replaced.
These figures are in the ratio 1 : 4'9, but the atomic weights Na : Ag = l : 4'7. In general
terms, if a chloride MCln be taken, then it gives with ?iBr a percentage substitution
= 4M/M2, where M is the atomic weight of the metal. This law was deduced from observa-
tions on the chlorides of Li, K, Na, Ag (n= I), Ca, Sr, Ba, Co, Ni, Hg, Pb (n = 2), Bi (n = 8),

In these determinations of Potilitzin we see not only a brilliant confirmation of
Berthollet's doctrine, but also the first effort (there have been no similar determinations
since 1879) to directly determine the affinities of elements by means of displacement. The
chief object of these researches consisted in proving whether a displacement occurs in
those cases where heat is absorbed, and in this instance it should be absorbed, because
the formation of all metallic bromides is attended with the evolution of less heat than
that of the chlorides, as is seen by the figures given in Note 55. Among the results of
•other researches made by Potilitzin in the same direction, it is necessary to turn atten-
tion to the following.

If the mass of the bromine be increased, then the amount of chlorine displaced also
increases. For example, if masses of bromine of 1 and 4 equivalents act on a molecule of
sodium chloride, then the percentages of the chlorine displaced will be 6'08 p.c. and
12'46 p.c. ; in the action of 1, 4, 9, 16, 25, and 100 molecules of bromine on a molecule of
barium chloride, there will be displaced 7'8, 17'6, 23'5, Sl'O, 35'0, and 45'Op.c. of chlorine.
If an equivalent quantity of hydrochloric acid act on metallic bromides in closed tubes, and
in the absence of water at a temperature of 800°, then the percentages of the substitution
•of the bromine by the chlorine in the double decomposition taking place between univalent
metals are inversely proportional to their atomic weights. For example, NaBr + HC1 gives
at the limit 21 p.c. of displacement, KC1 12 p.c. and AgCl 14^ p.c. Essentially the same
takes place in an aqueous solution, although the phenomena is complicated by the parti-
cipation of the water. The reactions proceed of themselves in one or the other direction
at the ordinary temperature but at different rates. In the action of a dilute solution (1
equivalent per 5 litres) of sodium chloride on silver bromide at the ordinary tempera-
ture, 2*07 p.c. of bromine is replaced in six and a half days, with potassium chloride
1*5 p.c. With an excess of the chloride the magnitude of the substitution increases ;
with four equivalents it is 4'95 p.c. in nine days. In the action of 9 grams of sodium
•chloride on I'l grams of silver bromide, 9'G9 p.c. of bromine is replaced in thirteen days.

494 PRINCIPLES OF CHEMISTRY"

sometimes behave with respect to metallic oxides in exactly the same-
manner as chlorine. Gay-Lussac, by igniting potassium carbonate in
iodine vapour, obtained (as with chlorine) an evolution of oxygen and
carbonic anhydride, K2C03 + I., = 2KI + CO., -f- O, only the reac-
tions between the halogens and oxygen are more easily reversible with
bromine and iodine than with chlorine^ Thus, at a red heat oxygen
displaces iodine from barium iodide (besides which the reaction is
here complicated by iodine being more easily oxidised than chlorine).
Aluminium iodide burns in a current of oxygen (Deville and Troost),
and a similar, although not so highly developed, relation exists for
aluminium choride, and shows that the halogens have a distinctly
smaller affinity for metals which only form feeble bases. This refers to
a still greater extent to non-metals, which form acids and evolve much
more heat with oxygen than the halogens. But in all these instances,
the affinity (and amount of heat evolved) of iodine and bromine is less
than that of chlorine, probably because the atomic weights are greater,
whilst the properties of the atoms of all the halogens are analogous.
The smaller store of energy in iodine and bromine is seen still more
clearly in the relation of the halogens to hydrogen. In a gaseous state
they all enter, with more or less ease, into direct combination with
gaseous hydrogen — for example, in the presence of spongy platinum,
forming halogen acids, HX — but the latter are far from being equally
stable ; hydrogen chloride is the most stable, hydrogen iodide the least
so, and hydrogen bromide occupies an intermediate position. A very
strong heat is required to only partially decompose hydrogen chloride,

These conversions also proceed with the absorption of heat. The reverse reactions
evolving heat proceed incomparably more rapidly, but also to a certain limit ; for example,
in the reaction AgCl + RBr the following percentages of silver bromide are formed in
different times :

hours 2 3 22 96 120

K 79*82 87'4 88'22 94'21

Na 83-68 90'74 91'70 95'49

Consequently, the conversions which are accompanied by this evolution of heat proceed
with very much greater rapidity than the reverse conversions. If the rates at the
commencement of the first reactions be placed side by side with the amounts of heat
evolved by them, then a perfect conformity is seen to exist between the data. In the
reaction between AgCl and KBr 3'5 thousand heat units are evolved, and the rate of
this reaction in the first two hours is expressed by the formation of 79"8 p.c. of bromide of
silver; if NaBr act on AgCl, then 4*3 thousand heat units are evolved, and the rale
during the first two hours is 83'2 p.c. This conformity between the rate of reaction and
the thermal figures is also remarked for other compounds. This shows that the thennal
figures are not equivalent to the entire work of affinity, but are only proportional to the
first rates of reactions. This explains why it is possible upon their basis to foresee the
direction of the dominating reaction in a complex medium, but impossible to foretell in
what direction the reaction will proceed.

THE HALOC.KNs 495

whilst hydrogen iodide is decomposed by light at even the ordinary
temperature, and very easily by a red heat. And therefore the reaction
IL> + H.2 = HI + HI is very easily reversible, and consequently has a
limit, and hydrogen iodide easily dissociates.67 Judging by the direct
measurement of the heat evolved, the conversion of 2HC1 into H2 + C1.,
requires the expenditure of 44000 heat units (because the formation of
HC1 evolves 22000 heat units). The decomposition of 2HBr into
H.2 + Br._, requires, if the bromine is obtained in a gaseous state, a
consumption of only about 24000 units, and in the decomposition
of 2HI into H._, + !•> as vapour about 3000 heat units are evolved^9*
which, without doubt, stands in causal connection with the great

r>7 The (lifixocidtion of hydriodic r/r?V71ms been studied in detail by Hautefeuille and
Lemoine, from whose researches we will extract the following information. The decompo-
sition of hydriodic acid is distinct, but proceeds slowly at 180° ; the rate and limit of de-
composition increases with a rise of temperature The reverse action is the same — that
is. T.-fHo forms 2HI, not only under the influence of spongy platinum (Corenwinder),
which also accelerates the decomposition of hydriodic acid, but it also proceeds by itself,
although slowly. The limit of the reverse reaction remains the same with or without
spongy platinum. An increase of pressure has a very powerful accelerative effect on the
rate of the reaction of the formation of hydriodic acid, and therefore spongy platinum l.y
condensing gases has the same effect as increase of pressure. At the atmospheric pressure
the decomposition of hydriodic acid reaches the below-mentioned limit at 250° in several
months, and at 440° in several hours. The limit at 250° is about 18 p.c. of decomposition
— that is, out of 100 parts of hydrogen previously combined in hydriodic acid, about
10 p.c. may be disengaged at this temperature (this hydrogen may be easily measured,
and the measure of dissociation determined), but not more ; the limit at 440° is about
26 p.c. If the pressure under which 2HI passes into H., + L,be 4A atmospheres, then the
limit is 24 p.c. ; under a pressure of i atmosphere the limit is 29 p.c. The small influence
of pressure on the dissociation of hydriodic acid (compared with N.>C\>, Chap. VI. Note
4)'>) is due to the fact that the reaction 2HI = I2 + H3 is not accompanied by a change of
volume, and probably the existing differences are determined by deviations from Boyle-
Mariotte's law. In order to show the influence of time, we will cite the following figures
referring to 350° : 1. Reaction H.> + 1_>, 3 hours, 88 p.c. of hydrogen remained free ; 8 hours,
69 p.c. ; 34 hours, 48 p.c. ; 70 hours, 29 p.c.; and 327 hours, 18'5 p.c. of hydrogen remained
in a free state. 2. The reverse decomposition of 2HI, 9 hours, 8 p.c. of hydrogen set free,
and after 250 hours lsT> p.c. of hydrogen became free — that is, the limit was reached. The
addition of extraneous hydrogen diminishes the limit of the reaction of decomposition, or
innvasrs the formation of hydriodic acid from iodine and hydrogen, as would be expected
from Berthollet's doctrine (Chap. X.). Thus at 440° 26 p.c. of hydriodic acid is decom-
posed if there be no admixture of hydrogen, while if H., be added, then at the limit half
as small a mass of HI is decomposed. Therefore, if an infinite mass of hydrogen be
taken there will be no decomposition of the hydriodic acid. Light aids the decompo-
sition of hydriodic acid very powerfully. At the ordinary temperature 80 p.c. is decom-
posed under the influence of light, whilst under the influence of heat alone this limit
corresponds with a very hrjfh temperature. The distinct action of light, spongy platinum,
and of imperfect cleanliness in glass (especially of sodium sulphate, which destroys
hydriodic acid), not only render the investigations difficult, but also show that in reactions
like 2HI = Io + H.,, which are accompanied by slight heat effects, all foreign and feeble
influences may deeply affect the process of the phenomenon (Note 47).

68 The thermal determinations of Thorn sen (at 18°) gave in thousands of calories,
C1 + H= +22, HCl + Aq (that is, a large amount of water dissolves HC1) = + 17'3, and

496 PRINCIPLES OF CHEMISTRY

stability of hydrogen chloride, the easy decomposability of hydrogen
iodide, and the intermediate properties of hydrogen bromide. From
this it would be expected that chlorine is capable of decomposing water
with the evolution of oxygen, whilst iodine has not the energy to
produce this disengagement,69 although it is able to liberate the oxygen
from the oxides of potassium and sodium, as the affinity of these metals
for the halogens is very considerable. For this reason oxygen, especially
in compounds from which it can be evolved (for instance, C1HO, CrO3,
tic.), easily decomposes hydrogen iodide. A mixture of hydrogen iodide
and oxygen burns in the presence of an ignited substance, forming
water and iodine. Drops of nitric acid in an atmosphere of hydrogen
iodide produce the disengagement of violet fumes of iodine and brown
fumes of nitric peroxide. In the presence of alkalis and an excess of
water, however, iodine is able to produce oxidation like chlorine — that is,
it decomposes water ; the action is here aided by the affinity of hydrogen
iodide for the alkali and water, just as sulphuric acid helps zinc to decom-
pose water. But the relative instability of hydriodic acid is best seen in
comparing the acids in a gaseous state. If the halogen acids be dissolved
in water, they evolve so much heat that they approach much nearer
to each other in properties. This is seen from thermochemical data,
because in the formation of HX in solution (in a large excess of water)
from the gaseous elements there is evolved for HC1 39000, for HBr 22000,

therefore H + Cl + Aq = -f 39'3. In taking molecules, all these figures must be doubled.
Br + H = +8-4; HBr + Aq = 19'9; H + Br + Aq = + 28'3. According to Berthelot 7' 2 go
to the vaporisation of Br.,, hence Bro + Hj = 16'8 + 7'2 = +24, if Br2 be taken as vapour
for comparison with CL. H + I= — 6'0, HI + Aq = 19'2 ; H + I + Aq= +13'2, and, accord-
ing to Berthelot, the heat of fusion of Io = 3'0, and of vaporisation 6'0 thousand heat units,
and therefore Io + H2 = — 2(6'0) + 3 + 6 — — 3'0, if the iodine be taken as vapour. Berthelot,
on the basis of his determinations, gives, however, + 0'8 thousand heat units. Similar
contradictory results are often met with in thermochemistry with the imperfection of
the existing methods, and depend on the necessity of obtaining the fundamental figures
by an indirect method. Thus Thomsen decomposed a dilute solution of potassium iodide
by gaseous chlorine; the reaction gave + 26'2, whence, having first determined the heat
effects of the reactions KHO + HC1, KHO + HI and Cl + H in aqueous solutions, it was
possible to find H + I + Aq ; then, knowing HI + Aq, to find I + H. It is evident that the
unavoidable errors may multiply themselves.

69 One would think, however, that on the basis of Berthollet's doctrine, and the obser-
vations of Potilitzin (Note 66), a certain trace of slow decomposition of water by iodine
may exist. In this sense the observations of Dossios and Weith on the fact that the
solubility of iodine in water increases after lapse of several months, will be comprehen-
sible. Hydriodic acid is then formed, and it increases the solubility. If the iodine be
extracted from such a solution by carbon bisulphide, then, as the authors showed, after
the action of nitrous anhydride, iodine may be again found in the solution by means of
starch. One can easily imagine that a number of like reactions, requiring much time and
taking place in small quantities, have up to now eluded the attention of investigators
who even still doubt the universal application of Berthollet's doctrine, or only see the
thermochemical side of reactions, or else neglect to pay attention to the element of time
and the influence of mass.

THE HALOGENS 497

and for HI 18000 heat units.70 But this is especially evident from
the fact that solutions of hydrogen bromide and iodide in water have
many points in common with solutions of hydrogen chloride, both in
their capacity to form hydrates, fuming solutions of constant boiling
point, and in their capacity to form haloid salts, &c., by reacting on
bases.

In consequence of what has been said above, it follows that hydro-
bromic and hydriodic acids, being substances which are but slightly
stable, cannot be evolved in a gaseous state under many of those condi-
tions under which hydrochloric acid is formed. Thus if sulphuric acid
in solution acts on sodium iodide, all the same phenomena take place as
with sodium chloride (a portion of the sodium iodide gives hydriodic
acid, and all remains in solution), but if sodium iodide be mixed with
strong sulphuric acid, then its oxygen decomposes the hydriodic acid
set free with liberation of iodine, H2SO4 + 2HI = 2H,O + SO.2 + I2.
This reaction proceeds in the reverse direction in the presence of a mass
of water (2000 parts of water per 1 part of SO2), in which not only the
affinity of hydriodic acid for water evinces itself, but also the direct
participation of water in the direction of chemical reactions accomplished
through its medium.71 Therefore, having a halogen salt, it is easy to
obtain gaseous hydrochloric acid (by the action of sulphuric acid), but
neither hydrobromic nor hydriodic acid can be so obtained free (as
gases).72 Other methods are requisite for their preparation ; but above
all there must be conditions for the removal of oxygen, which is so easily
able to destroy these acids. Therefore hydrogen sulphide, phosphorus,
&c., which themselves easily take up oxygen, are introduced as means for
the conversion of bromine and iodine into hydrobromic and hydriodic
acids in the presence of water. For example, in the action of phosphorus
the essence of the matter is that the oxygen of the water goes to the
phosphorus, and the reaction of the remainders leads to the formation
of hydrobromic or hydriodic acid ; but the matter is complicated by the
reversibility of the reaction, the affinity for water, and other circum-
stances which are understood by following Berthollet's doctrine.
Chlorine (also bromine) directly decomposes hydrogen sulphide, forming
hydrochloric acid and liberating sulphur, both in a gaseous form and in
solutions, whilst iodine only decomposes hydrogen sulphide in weak

70 On the basis of the data in Note 68.

71 A number of like cases confirm what has been said in Notes 26 and 28 of
Chapter X.

72 This is prevented by the reducibility of sulphuric acid. If volatile acids be taken
they pass over, together with the hydrobromic and hydriodic acids, when distilled ;
whilst many non-volatile acids which are not reduced by hydrobroraic and hydriodic

acids only act feebly (like phosphoric acid), or do not act at all (like boric acid).
VOL. I. K K

•His

solutii ins \\hen ii.-> alhniiv tor hydrogen is aided bv the atlinitv of
livdroifeli indidi' ii>r \\alcr. In a gaseous slate Iodine docs not act
on hydrofoil sulphide,"'' whilst siil]iln;r is able to decompose gaseous
hydri"dic arid, forming hydrogen sulphide, and a compound of sulphur
a lid iodine \\ it h \\ hieli s\ at cr forms hydriodie acid.' '

If hydrogen sulphide in1 pa»ed through water containing iodine, the
reaction II S - I ., = l!l I I -f- S proceeds so long as the solution he
dilute : or \\itli a greater mass of iodine, so l.m-- as a solution of iodine
in hvdriodir acid ha\ inu a composit ion of nearly l! 1 1 I +11- -t-(.'ll.,<)
(according to I '.incau i be not formed. I 1 \ -dro^vn sulphide t hen no longer
act- on >uch a .-.oliition. Ayhatevcr be the mass of the free iodine. There-
fore, only weak solution.-* of hxdriodic acid can he ohtained bv passing
hvdrii^eji Milplnde mio \\ater \\ith iodine.1'

To obtain7' u,"a>eous h\ drobroinic and hvdriodic acids it is most

: till fit] itr ti>l' Mllplmr lili- licit! nl t'll-inll '- »''•':. ;ill(l llic llcUt (if

:'•: ... • -., II - 1-7 : M ( : II: II Mr -Jl. HIH! H , • I...

I-; lifiicc the l',.nii:iti<.n i'f II S u:\i- I'-- lu-;it than llial ot IK'l :i)i<l

! i i ; •. i ..:: i'...., i i.; i ii ; i:,:i < •: 1 1 i . I ii <liiutr Milutiuii- 11 , > Aq '.c:'.. and (-(iiiM-inu-ntly

: i • , f > II ,S ,-M.I\I-> Inn little hfiit with

• •, . • ••• -'ilutidis chluriiH'. IIIMI .inc. ami indiiii' ilccdiniiost-

• . . • I • • • •- 1 1 1 < I y < > t t h i • > i 1 1 1 [ 1 1 1 • >

1 i . ii - c\ idt-iit li'diu the examples cited >n

•;.''•.,. intii the condition^ ni' chemical e(|iiilihn'a, it

I,!,. 1,, i i , , ,;,!. l;\- l'dlld\\ !i:._- the I'ddtMeps of

I ..;.-:.: • tin |i|-ol)lelll \\ ill he reached : I'll! tile Udl'li ill

. , .,,.,- dci iiMnii--, pn-cnl tlifiiiM-lvfs

•,,•,;.'!.'. il, ml.! place the-e [irdlileins to (lie

:,.,... I i .. i-_' el the I • • \ \: : -hi .1 Id i urn the reader's att< ntion to jiriihleiiis

i : Id arrive ;it ! heir >d|utidii.

. . . i k. I am lidt al.le t«, t,,ni-li mi all tlice\i>t-

' in .

'i in i- i ' : • , • . • i anhydride, hi a dilute ->nlu-

• • iodine. ( >n ci mceiit ra I ion a rex eive

I . ; ! . nt hii i ! e liiit part iciilar cases of

i • tin . i . i,i chemic.il mechii iiic-- \\eiv more exact and

, . : '.;.;,.:,,, ,,| |,n paralm.. ,/•////

• - . ' ' !••! Ii pi-rat Ill'e. pi , - -lire, ma— e . iVc.l The --I lld\

,,. , .,,;',.,: mi ! hdd i,| ] . t ,di t hen tore one o) the path-- for the stud v i if elii'iniciil

\', ;, 7 i d t In ' ' ' : led li\ the possiliilil \ of (lie format ion

i rodin tin.

THE HALOGENS 499

convenient to take advantage of the reactions between phosphorus,
the halogens, and water, when the latter is present in small quantity
(otherwise the halogen acids formed are dissolved by the water), and
the halogen is gradually added to the phosphorus moistened with
water. Thus if red phosphorus be placed in a flask and moistened with
water, and bromine be added drop by drop (from a closed funnel with
a glass tap), hydrobromic acid is abundantly and uniformly dis-
engaged.76 Hydrogen iodide is prepared by adding 1 part of common
(yellow) dry phosphorus to 10 parts of dry iodine in a glass flask. On
revolving the flask, reaction (light and heat are evolved) proceeds quietly
between them, and when the mass of the iodide of phosphorus which is
formed has cooled, water is added drop by drop (from a funnel with a
tap) and hydrogen iodide is evolved even without the aid of heat.
These methods of preparation will be at once understood when it is
remembered (p. 462) that phosphorus chloride gives hydrogen chloride
with water. It is exactly the same here — the oxygen of the water
passes over to the phosphorus, and the hydrogen to the iodine ; for
example, PI3 + 3H2O =PH3O3 + 3HI.77

In a gaseous form hydrobromic and hydriodic acids are closely

equilibria in all their complexity is naturally very great, but it falls in the background
before the primary interests of discovering practical methods for the isolation of sub-
stances, and the means of employing them for the requirements of man. It is only after
the satisfaction of these requirements that interests of the other order arise, which
themselves reflect the interests of the primary nature. For these reasons, whilst con-
sidering it opportune to point out the theoretical interests of chemical equilibria, the
chief attention of the reader is directed in this work to the primary chemical interests.

76 Hydrobromic acid is obtained by the action of bromine on paraffin heated to 180°.
Gustavson proposed preparing it by the action of bromine (it is best if it be added in
drops together with traces of aluminium bromide) on anthracene (a solid hydrocarbon
from coal tar). Balard prepared it by passing bromine vapour over moist pieces of
common phosphorus. The liquid tribromide of phosphorus, directly obtained from
phosphorus and bromine, also gives hydrobromic acid when treated with water. Bromide
of potassium or sodium, when treated with sulphuric acid in the presence of a piece of
phosphorus, also gives hydrobromic acid, but hydriodic acid is decomposed by this
method. In order to free hydrobromic acid from bromine vapour it is passed over moist
phosphorus and dried either by phosphoric anhydride or calcium bromide (calcium
chloride cannot be used, as hydrochloric acid would be formed). Neither hydrobromic
nor hydriodic acids can be collected over mercury, on which they act, but they may be
directly collected in a dry vessel by letting the gas-conducting tube pass down to the
bottom of the vessel, both gases being much heavier than air.

77 But, generally, more phosphorus is taken than is required for the formation of PI-,
because, otherwise, a portion of the iodine is distilled over. If less than one-tenth part
of iodine be taken, then much phosphonium iodide, PH4I is formed. This proportion
was established by Gay-Lussac and Kolbe. Hydriodic acid is also prepared in many
other ways. Bannoff dissolves two parts of iodine in one part of a previously-prepared
strong (sp. gr., T67) solution of hydriodic acid, and pours it on to red phosphorus in a
retort. Personne takes a mixture of fifteen parts of water, ten of iodine, and one of red
phosphorus, which, when heated, disengages hydriodic acid mixed with iodine vapour,
which is removed by passing it over moist phosphorus (Note 76). It must be remembered,

K K 2

analogous In hydrochloric a i Mil : 1 hey are 1 1 1 i Uet let ! by pressure and col< 1,
thev fuiiif in the air. fiirm Solutions and hydrates of con-taut boiling
point, and react on nu'tal.s, oxides and salts. A-C.<S ( )nlv the relatiyely
casv decompo-abihty of hydrobromic acid, and especially of hydriodic
aciil. clearly distinguish the-e acids from hydrochloric acid. For this
rea-on. hydriodic acid acts in a number of cases as a deoxidiser or
reduce]', and frequently e\vn sei'yes as a means for the transmission of
hydrogen. Thus I'ertheloT. Ha ever. \Vreden.and others, by heating
unsaturated hydrocarbons in a -olution of hydriodic acid, obtained their
compounds with hydrogen nearer to the limit (' II ., or even the
-aturated compounds. For example, ben/ene. <,!!,. \\hen heatr-d in a
cli.-etl tube \vith ; -tronu' -oluiion of h\dri"dic acid, n'iyes he.xylene,
(' ,11, . The easy decomposability of hydriodic a<-id accounts for the
fact that iodine due- not act 1 >y metalepsis on h \-drocarbons, for the
li\ . ! ro^'ei i iiiilule libei'atetl \\iih the product of metalepsis. |JI. formed,
^i\-es iodine and the ii\-dro^-en compound. RH.baek auain. And there-
fore, to obtain t he products of iodine substitution, eit her iodic acid. 1 1 1( ).,
( Kekult'). or mercury oxide, Hijf< ) ("\Ves.e]sky), is added, as the\'imme-
diately react on the liydro-vn iodide, thus : II |< >, + 5II I =.°>H,< > + .^I,.
in-. 1 lu( » — L'l I I = I li:! +H.,<>. From these consideration- it will be
readily understood that iodine acts like chlorine (or bromine) on am
lu oni a and sodium hydroxide, for in this case the hydriodic acid produced
fonn- N 1 1 , 1 and Na I . Thus with t i net lire of iodine, or even the solid
* • . en!, a -olntion ''i ammonia immediately forms a lii^ldv-oxplo^ive

that IV, i • ' ' M'i" ' ' ' lift wren tin- lt\ ilriciilic acid

• . i ,, I.IMI.I- I'M ,1 and I'l . Miv InnniMl.

• ' ' • • ' ;|H. ivacti,,!! IH t\V.MMI | -1 1 ,- 1 .1 i . .1' 1 1- all ! iixlilli- alld ttnt.T

.1 '. ... \l>' ' • Hi |,n.c.-i-U ,[uici!\. l.iii iii-v,Ttlir!«->- n-cjuiri-

1 he I'racl ion prncrciU \\ itlmllt t lie

linn I1 M I II .( ) IMI-O, .Mil.

I' Wiitrr arr place, 1 in a LVtiii-l. and a

• ' . I . \ ' : 1 ! 1 1 • a 1 t i r - :

TIIK HAUMIKNS 501

solid black product of metalepsis, generally known as iodide of nitrogen,
although it probably contains hydrogen, 3NH3 + 2I2 = 2XH4I + NHI2.
However, the composition of the product is variable, and with an excess
of water XT;, seems to be formed. Iodide of nitrogen is no less explosive
than nitrogen chloride. In the action of iodine on sodium hydroxide
no bleaching compound is formed (whilst bromine gives one), but a
direct reaction is always accomplished with the formation of an iodate
GNaHO + 3I2 = 5NaI + 3H2O + NaIO3 (Gay-Lussac). Solutions of
other alkalis, and even a mixture of water and oxide of mercury, act in
the same manner. This direct formation of iodic acid, HIO3 = IO2(OH),
shows the propensity of iodine to give compounds of the type IX5.
Indeed, the propensity of iodine to form compounds of a higher type
clearly evinces itself in many ways. But it is most important to turn
attention to the fact that iodic acid is easily and directly formed by the
action of oxidising substances on iodine. Thus, for instance, strong
nitric acid directly converts iodine into iodic acid, whilst it has no
oxidising action on chlorine.79 This shows a greater affinity in iodine for
oxygen than in chlorine, and this conclusion is confirmed by the fact
that iodine displaces chlorine from its oxygen acids,80 and that in the
presence of water chlorine oxidises iodine.81 Even ozone or a silent
discharge passed through a mixture of oxygen and iodine vapour is able
to directly oxidise iodine 82 into iodic acid. It is disengaged from solu-

79 The oxidation of iodine by strong nitric acid was discovered by Connell ; Millon
showed that it is accomplished, although more slowly, by the action of the hydrates of
nitric acid up to HNO3,H2O, but that the solution HNOr,,2H2O, and weaker solutions,
do not oxidise, but simply dissolve, iodine. The participation of water in reactions is
seen in tin's instance. It is also seen, for example, in the fact that dry ammonia com-
bines directly with iodine— for instance, at 0° forming the compound I2,4NH5— whilst
iodide of nitrogen is only formed under the action of water.

-° Bromine also displaces chlorine — for instance, from chloric acid, directly forming
bromic acid. If a solution of potassium chlorate be taken (75 parts per 400 parts of
water), and iodine be added to it (80 parts), and then a small quantity of nitric acid,
chlorine is disengaged on boiling, and potassium iodate is formed in the solution. In
this instance the nitric acid first evolves a certain portion of the chloric acid, and the
latter, with the iodine, evolves chlorine. The iodic acid thus formed acts on a further
quantity of the potassium chlorate, sets a portion of the chloric acid free, and in this
manner the action is kept up. Potilitzin (1887) remarked, however, that not only do
bromine and iodine displace the chlorine from chloric acid and potassium chlorate, but
also chlorine displaces bromine from sodium bromate, and, furthermore, the reaction does
not proceed as a direct substitution of the halogens, but is accompanied by the formation
of free acids; for example, 5NaClO3 + 8Br2 + 8H2O = 5NaBr + 5HClO5 + HBrO5.

J1 If iodine be stirred up in water, and chlorine passed through it, then the iodine is
dissolved; the liquid becomes colourless, and contains, according to the mass of water
and chlorine, either the compounds IHC12, or IC13, or HIO5. If there be a small amount
of water, then the iodic acid may separate out directly in crystals, but a complete con-
version (Bornemann) only occurs when not less than ten parts of water are taken to
one part of iodine— IC1 + 3H2O + 2C12 = IHO5 + 5HC1.

s- S( -liiinebein and Ogier proved this. Ogier found that at 45° ozone immediately

502 PRINCIPLES OF CHEMISTRY

tions as a hydrate, HIO3, which loses water at 170°, and gives an
anhydride, I2O,5. Both these substances are crystalline (sp. gr. I2O5
5*037, HIO3 4*869 at 0°), colourless, soluble in water,83 decompose at a
red heat into iodine and oxygen, are in many cases powerfully oxidising —
for instance, they oxidise sulphurous anhydride, hydrogen sulphide, car-
bonic oxide, &c. — form chloride of iodine and water with hydrochloric
acid, and with bases form salts, not only normal MIO3, but also
acid, for example, KIO3HIO3, KIO32HIO3. With hydriodic acid
iodic acid immediately reacts, disengaging iodine, HIO3 + SHI =
3H2O + 31.2.

As with chlorine, so with iodine, a periodic acid, HIO4, is formed.
This acid is produced, as its salts, by the action of chlorine on alkaline
solutions of iodates, and also by the action of iodine on chloric acid.84

oxidises iodine vapour, first forming the oxide L>O5, which is decomposed by water or on
heating into iodic anhydride and iodine. Iodic acid is formed at the positive poles
(Riche) when a solution of hydriodic acid is decomposed by a galvanic current. It is
also formed in the combustion of hydrogen mixed with a small quantity of hydriodic acid
(Salet).

85 Kammerer showed that a solution of sp. gr. 2'127 at 14°, containing 2HIO3,9fL>O,
entirely solidified in the cold. On comparing solutions HI + ?«HoO with HIO5 + wH.>O,
we find that the specific gravity increases but the volume decreases (it is the same in
their passage to solutions HIO5 + raH2O), whilst in the passage of solutions HCl + ??iH2O
to HClO3 + ?nH2O the specific gravity augments, and so does the volume, which is re-
marked also in certain other cases (for example, H3PO5 and H-PO 4). Observations on the
specific volume of solution of iodic and periodic acids were made by Thomson at 17°/17°.
He expresses-for HIO3 + ?«H2O by 18w + 39-l-l3-lw/(m + 18), and for H5IO6-i-wHoO
by 18m + 23'8, which shows the absence of contraction in mixing these acids with water
(see Mendeleeff, Investigation of Aqueous Solutions, p. 368).

84 If sodium iodate be mixed with a solution of sodium hydroxide, heated, and chlorine
be passed through the solution, then a sparingly soluble salt separates out, which corre-
sponds with periodic acid, and has the composition Na4I2O9,3H2O.

ONaHO + 2NaI03 + 4C1 - 4NaCl + Na4I2O9 + 3H,O.

This compound is sparingly soluble in water, but easily dissolves in a very dilute
solution of nitric acid. If silver nitrate be added to this solution a precipitate? is formed
which contains the corresponding compound of silver, Ag4IoO9,3HoO. If this sparin^ly
soluble silver compound be dissolved in hot nitric acid, then, on evaporating, orange
crystals of a salt having the composition AgIO4 separate. This salt is formed from the
preceding by the nitric acid taking up silver oxide. Ag4I2O9 + 2HNO3 = 2AgNO3
+ 2AgIO4 + H2O is decomposed by water, with the re-formation of the preceding salt,
while iodic acid remains in solution —

4 AgIO4 + H2O = Ag4I2O9 + 2HIO4.

The structure of the first of these salts, Na4I2O9,8H2O, presents itself in a simpler
form if the water of crystallisation is regarded as an integral portion of the salt; the
formula is then divided in two, and takes the form of IO(OH)3(ONa)2 — that is, it answers
to the type IOX5, or IX7 (like AgIO4) which is IO3(OAg). The;! composition of all the
salts of periodic acids are expressed by this type IX7. Kimmins (1889) classes all
the salts of periodic acid into four types — the meta-salts of HIO4 (salts of Ag, Cu, Pb),
the meso-salts of H3IO5 (Pb, Ag2,H, Cd, H),the para-salts of H5IO6 (Na.JI-. Na.-.H ,'. and
the di-salts of H4I2O9 (K4, Ag4, Ni2). The three first are direct compounds of the type
IX7, namely, IO3(OH), IO2(OH)3, and IO(OH)5, and the latter are types of diperiodic

THE HALOGENS 503

It crystallises from solutions as a hydrate containing 2H2O (corre-
sponding with HC1O4,2H2O), but as there are salts containing up to 5
atoms of metals, this water must be counted as water of constitution.
Therefore IO(OH)5 = HIO4,2H2O corresponds with the higher form
of halogen compounds, IX7.85 In. decomposing (at 200°) or acting as an
oxidiser, periodic acid first gives iodic acid, but it may also be ultimately
decomposed, with the formation of hydriodic acid.

Thus both bromine and iodine present a great analogy to chlorine
in their behaviour with various substances, but nevertheless a series of
qualitative distinctions characterise each element. The formation of a
compound between chlorine and iodine must be classed among these
distinctions.86 These elements combine directly together with the
evolution of heat, and form iodine monochloride, IC1, or iodine tri-
chloride, IC13.87 As water reacts on these substances, forming iodic

salts, which correspond with the type of the meso-salts, as pyrophosphoric salts corre-
spond with orthophosphoric salts — i.e. 2H2IO5 — H2O = H4IoO9.

85 Periodic acid, discovered by Magnus and Ammermtiller, and whose salts were
afterwards studied by Langlois, Rammelsberg, and many others, presents an example of
hydrates in which it is evident that there is not that distinction between the water of
hydration and of crystallisation which was at first considered to be so clear. In HC1O,2H2O
the water, 2H2O, is not displaced by bases and must be regarded as water of crystallisa-
tion, whilst in HIO4,2H2O it must be regarded as water of hydration. We shall after-
wards see that the system of the elements obliges one to consider the halogens as
substances giving a highest saline type, GX7, if G signify a halogen, and X oxygen
(O = -X"2), OH, and other like elements. The hydrate IO(OH)5 corresponding with many
of the salts of periodic acid (for example, the salts of barium, strontium, mercury) does
not exhaust all the possible forms. It is evident that various other pyro-, meta-, &c., forms
are possible by the loss of water, as will be more fully explained in speaking of phosphoric
acid, and as was pointed out in the preceding note.

86 With respect to hydrogen, oxygen, chlorine, and other elements, bromine occupies
an intermediate position between chlorine and iodine, and therefore there is no particular
need for lingering over the compounds of bromine. This is the great advantage of a natural
grouping of the elements.

87 They were both obtained by Gay-Lussac and many others. Eecent verified data
respecting iodine monochloride, Id, entirely confirm the numerous observations of Trapp
(1854), and even confirm his statement as to the existence of two isomeric (liquid and
crystalline) forms (Stortenbeker). With a small excess of iodine, iodine monochloride
remains liquid, but in the presence of traces of iodine trichloride it easily crystallises.
Schiitzenberger amplified the data concerning the action of water on the chlorides (Note 88)
and Christomanos gave the fullest data regarding the trichloride.

After being kept for some time, the liquid monochloride of iodine yields red deliques-
cent octahedra, having the composition IC14, which are therefore formed from the mono-
chloride with the liberation of free iodine, which dissolves in the remaining quantity of the
monochloride. This substance, however, judging by certain observations, is impure iodine
trichloride. If 1 part of iodine be stirred up in 20 parts of water, and chlorine be passed
through the liquid, then all the iodine is dissolved, and a colourless liquid is ultimately
obtained which contains a certain proportion of chlorine, because this compound gives a
metallic chloride and iodate with alkalis without evolving any free iodine : IC15 + 6KHO
= 5KC1 + KIO3 + 8H2O. The existence of a pentachloride IC15 is, however, denied,
because this substance has not been obtained in a free state.

504 PRINCIPLES OF CHEMISTRY

acid and iodine, they have to be prepared from dry iodine and chlorine.88
Both substances are formed in a number of reactions ; for example, by
the action of aqua regia on iodine, of chlorine on hydriodic acid, of
hydrochloric acid on periodic acid, of iodine on potassium chlorate
(with the aid of heat, &c.). Trapp obtained iodine monochloride, in
beautiful red crystals, by passing a rapid current of chlorine into molten
iodine. The monochloride then distils over and solidifies ; it melts at
27°. By passing chlorine over the crystals of the monochloride, it is
easy to obtain iodine trichloride in orange crystals, which melt at 34°
and volatilise at 47°, but in so doing decompose (into C1.2 and Oil).
The chemical properties of these chlorides entirely correspond with the
properties of chlorine and iodine, which would be expected, because, in
this instance, a combination by similitude took place as in the forma-
tion of solutions or alloys. Thus, for instance, the unsaturated hydro-
carbons (for example, C2H4), which are capable of directly combining
with chlorine and iodine, also directly combine with iodine monochloride.

Stortenbeker (1888) investigated the equilibrium of the system containing the mole-
cules I2, IC1, IC13, and C12, in the same way that Koozeboom (Chapter X. Note 38) examined
the equilibrium of the molecules HC1, HC1,2H2O, and H2O. He found that iodine
monochloride appears in two states, one (the ordinary) is stable and melts at 27'2°, whilst
the other is obtained by rapid cooling, and melts at 18'9°, and easily passes into the
first form. Iodine trichloride melts at 101° only in a closed tube under a pressure of 16
atmospheres.

88 By the action of water on iodine monochloride and trichloride a compound IHCL,
is obtained, which does not seem to be altered by water. Besides this compound, iodine
andiodic acid are always formed, 10IC1 + 3H2O = HIO3 + 5IHCL> + 2L> ; and in this respect
iodine trichloride may be regarded as a mixture IC1 + IC15 = 2IC15, but IC15 + 3H2O =
IHOj + SHCl; hence iodic acid, iodine, the compound IHCL>, and hydrochloric acid are
also formed by the action of water.

505

CHAPTER XII

SODIUM

WHEN common salt is submitted to the action of sulphuric acid, as we
saw in Chapter X., hydrochloric acid is disengaged, and a neutral salt,
sodium sulphate, Na.2SO4, is formed if the mixture be strongly heated
at the end of the reaction. This salt1 forms a colourless saline mass
consisting of fine crystals, soluble in water. It is the product of many
other double decompositions, sometimes produced on a large scale ; for
instance, it is formed when ammonium sulphate is heated with common
salt, in which case the sal-ammoniac is vola.tilised, also when sulphuric
acid acts on sodium nitrate, (fee. A similar decomposition also takes
place when, for instance, a mixture of lead sulphate and common salt
is heated ; this mixture easily fuses, and if the temperature be further
raised heavy vapours of lead chloride appear. When the disengagement
of these vapours ceases, the remaining mass, on being treated with
water, yields a solution of sodium sulphate mixed with a solution of
undecomposed common salt. A considerable quantity, however, of the
lead sulphate remains unchanged during this reaction, PbS04 + 2NaCl
= PbCl., •+• Xa.2SO4, the vapours will contain lead chloride, and the
residue will contain the mixture of the three remaining salts. Here
the decomposition is produced by the lead salt as with sulphuric
acid. The cause and nature of the reaction are just the same as were
pointed out when considering Berthollet's doctrine. And here it may
evidently be shown that the double decomposition is not determined
by any other means than the removal of the substance formed from
the sphere of the action of the remaining substances. This is seen

1 Whilst describing in some detail the properties of sodium chloride, hydrochloric
acid, and sodium sulphate, I wish to impart a conception, by separate examples, of
the properties of saline substances, but the dimensions of this treatise and its purpose
and aim do not permit the possibility of entering into particulars concerning every
salt, acid, or other substance. The fundamental object of this work — an account of
the characteristics of the elements and an acquaintance with the forces acting between
atoms — has nothing to gain from the multiplication of the number of as yet ungeneralised
properties and relations.

from the fact that sodium sulphate, on being dissolved in water and
mixed with a solution of any lead -ah land even with a solution of lead
chloride, although thi- latter i- but -paringlv soluble in water), imme-
diately gives a white precipitate of lead -ulphate. In this case the
lead lake- up the elements of sulphuric acid from the sodium sulphate
in the solutions. ( >n heating, the reverse phenomenon is observed.
The contrary reaction in i he solution i- founded on the insolubility of
the lead sulphate, and the phenomenon which takes place on heating is
founded on the volatility of the lead chloride. Let us cite other ex-
amples of double decompositions, in solutions giving sodium sulphate.
Silver sulphate. Ag.,S<)j. \vhen treated \\ith common salt, gives silver
chloride, because ii is insoluble in water. Ag,S<>, -f- -_'Na('l = Na.,Sn,
-f l'Ag('l. Sodium carbonate, mixed in -olution with sulphates of
iron, copper, manganese, magnesium, iVc.. gives in solution sodium
sulphate, and in the precipitate a carbonate of the corresponding metal,
because these salts of carbonic acid are insoluble in water: for instance,
.MgS<), 4- Na. ,('(),,= Na.,S<), + .Mu'( '<),,. In precisely the same way
sodium hvdro.xide acts on solution- of the majoritv of the salts of
sulphuric acid containing metal-, i he hvdroxides of which are insoluble.
in water — for instance, ('uS<>. + L'Nal 1 <) = Cu( I [<>)., + Na.,S<),.
Sulphates of magnesium, aluminum, iron, iVc.. on being mixed in
solution with common salt, ought to form chlorides of magnesium,
aluminium. iVc., and sodium sulphate, as the above-mentioned chlorides
are more -oluble in water than sodium sulphate. ( >n cooling the
mixture ot stich (concentrated) solution- -odium sulphate is deposited.
This i- made u.-e ot tor preparing it on the large scale in works where1
-ea water is treated. In a similar wav -odium sulphate is prepared bv
the aid ot the magnesium sulphate which is found in large (juantities
in Stas-fiirl in the seams « if common -a.lt. In this case, on cooling,
the reaction L'NaCl -f .MgSOj — Mg('l -f Na.,S(), takes place.

Thus \\here suljihates and -alls « if sodium are in contact, it mav
alway- be expected that sodium -ulphate will be formed and sepa-
rated it' the conditions are favourable; for 'his reason it is not sur-
prising; to often lind -odium -ulphale in the native state. Some of the
springs and sail lakes in the steppes Uvond the Volga, and in the
( 'auca-.il . ' -on ! a i n a considerable <|iia nt it v of sodium std] »hate, and yield
it bv -imple evaporation of the solutions. lleds of t his salt; are also
met with : tint- at a depth of onlv ~> feet, alioiit .">S \crsts to the
east of Tillis, al the foot of the range of t he * Wolf 's mane ' ( Voltchia
gri\a) moii n ' a i us. a deep stratum ot verv pure (daubers sail,
NaS< ).. I n| | ( ». ha., been found.' A aver two metres thick of the

n, ..I. • is known ii

SODIIM 507

same salt lies at the bottom of several lakes (an area of about 10
square kilometres) in the Kouban district near Batalpaschinsk, and
here its working has been commenced (1887). In Spain, near Aran-
goulz and Madrid, mineral sodium sulphate has likewise been found,
and is already being worked.

The methods of obtaining salts by means of double decomposition
from others already prepared are so general, that in describing a given
salt there is no necessity to enumerate the cases hitherto observed of
its being formed through various double decompositions.3 The possi-
bility of this occurrence ought to be foreseen according to Berthollet's
doctrine from the properties of the salt in question. On this account
it is important to know the properties of salts ; all the more so because
up to the present time those very properties (solubility, formation of
crystallo-hydrates, volatility, tfec.) which may be made use of for sepa-
rating them from other salts have not been generalised.4 These pro-
perties as yet remain subjects for investigation, and are rarely to be
foreseen.

Sodium sulphate very easily parts with water, and may be obtained
in an anhydrous state if it be carefully heated until the weight re-
mains constant ; but if heated further, it partly loses the elements of
sulphuric anhydride. It fuses at 861° (red heat), and volatilises in a
small proportion when very strongly heated, in which case it naturally
decomposes. At 0° in 100 parts of water, 5 parts of the anhydrous
salt dissolve, at 10° 9 parts, at 20° 19 -4, at 30° 40, and at 34° 55 parts,
the same being the case in the presence of an excess of crystals of
Na2SO4,10H2O.5 At 34° the latter fuses, and the solubility decreases

salt-cake, in mineralogy tlienardite. Crystalline decahydrated salt is termed in mine-
ralogy mirabilite. On fusing it, the monohydrate NaaSC^BLjO is obtained, together
with a supersaturated solution.

3 The salts may be obtained not only by methods of substitution of various kinds, but
also by many other combinations. Thus sodium sulphate may be formed from sodium
oxide, and sulphuric anhydride by oxidising sodium sulphide, Na2S, sodium sulphite,
Na^SOs, &c. When sodium chloride is heated in a mixture of the vapours of water, air,
and sulphuric anhydride, sodium sulphate is formed.

4 Many observations have been made, but little general information has been extracted
from particular cases. In addition to which, the properties of a given salt are changed
by the presence of other salts. This takes place not only in virtue of mutual decomposi-
tion or formation of double salts capable of separate existence, but is determined by the
influence which some salts exert on others, or by forces similar to those which act during
decomposition. Here, nothing has been generalised to that extent which would render
it possible to foretell uninvestigated examples. Let us state one of these numerous
cases : 100 parts of water at 20° dissolve 34 parts of potassium nitrate, but on the addition
of sodium nitrate the solubility of potassium nitrate increases to 48 parts in 100 of water
(Carnelley and Thomson). In general, in all cases of which there are accurate observa-
tions, it appears that the presence of foreign salts changes the property of any given
salt.

5 The information concerning solubility (Chapter I. p. 72) is given according to the
determinations of Gay-Lussac, Lovell, and Mulder.

508 PRINCIPLES OF CHEMISTRY

at higher temperatures.6 A concentrated solution at 34° has a composi-
tion nearly approaching to Na2SO4 + 14H2O, and the decahydrated
salt, when mixed with 100 parts of water, contains 78*9 of the
anhydrous salt, from whence it is seen that the decahydrated salt
cannot fuse without decomposing,7 similarly to hydrate of chlorine,
C12,8H2O, or sulphurous acid, SO2,7H2O. Not only the fused deca-
hydrated salt, but also the concentrated solution at 34° (not all at once,
but gradually), yields the monohydrated salt Na2SO4,H2O. The hepta-
hydrated salt, Na2SO4,7H2O, also splits up, even at low temperatures,
with the formation of this monohydrated salt, and therefore from 35°
the solubility can be given only for the latter, and it is as follows for
100 parts of water : at 40° 48-8, at 50° 46-7, at 80° 437, at 100° 42'5
parts of the anhydrous salt. If the decahydrated salt be fused, and
the solution be allowed to cool in the presence of the monohydrated
salt, then at 30° 50' 4 parts of anhydrous salt are retained in the solu-
tion, and at 20° 52 '8 parts. Hence, with respect to the anhydrous and
monohydrated salts, the solubility is one and the same, and falls with

6 In Chapter I. Note 24, we have already seen that with many other sulphates the
solubility also decreases after a certain temperature. Gypsum, CaSO4,2H2O, lime, and
many other compounds present such a phenomenon, not as yet, however, thoroughly
investigated. The observation of Tilden and Shenstone (1884) is most instructive, and
shows that on raising the temperature (in closed vessels) above 140° the solubility
of sodium sulphate again begins to increase. At 100°, 100 parts of water dissolve about
43 parts of anhydrous salt, at 140° 42 parts, at 160° 43 parts, at 180° 44 parts, at 280°
46 parts. It is evident that the phenomenon of saturation, determined by the presence of
an excess of the dissolved substance, is very complex, and, therefore, for the theory of solu-
tions considered as liquid indefinite chemical compounds, many useful results can hardly
be given ; more especially as the physico-mechanical side of the transition of the solid
into the liquid condition (or the reverse) is up to the present time less clearly understood
in theory than the transition of a liquid into vapour.

7 As has been referred to in Chapter I. Note 5(5.

The example of sodium sulphate is historically very important for the theory of solu-
tions. Notwithstanding the mass of investigations which have been made, it is still
insufficiently studied, especially from the point of the vapour tension of solutions and
crystallo-hydrates, so that those processes cannot be applied to it which Guldberg,
Roozeboom, Van't Hoff, and others applied to solutions and crystallo-hydrates. It would
also be most important to investigate the influence of pressure on the various phenomena
corresponding with the combinations of water and sodium sulphate, because when crystals
are separated — for instance, in the decahydrated salt — an increase of volume takes place,
as can be seen from the following data : — the sp. gr. of the anhydrous salt is 2'66, that
of the decahydrated salt = 1-4(5, but the sp. gr. of solutions at 15°/4° = 9992 + 90'2p + 0'35jp2
if p represents the percentage of anhydrous salt in the solution, and if the sp. gr. of
water at 4° = 10000. Hence, for solutions containing 20° of anhydrous salt, the sp. gr.
- T193G ; therefore the volume of 100 grams of this solution = 88'8 c.c., and the volume of
anhydrous salt contained in it is equal to 20 2'(56, or — 7'5 c.c., and the volume of water
= 80'1 c.c. Therefore, the solution, on decomposing into anhydrous salt and water,
increases in volume (from 83'8to87'6); but in the same way 83'8c.c. of 20p.c. solution are
formed from (45'4/l'4(> = ) 31'1 c.c. of the decahydrated salt, and 54'G c.c. of water — that is
to say, that during the formation of a solution fromH5'7 c.c., 83'8c.c. are formed.

SODIUM 509

tin1 temperature, whilst with respect to decahydrated salt, the solubility
rises with the temperature. So that if in the presence of a solution of
sodium suljthatc there be only crystals of that heptahydrated salt
(p. r>9) Na2SO.s,7H.2O which is formed from saturated solutions, then
saturation sets in when the solution has the following composition per
100 parts of salt : at 0° 19-6, at 10° 30-5, at 20° 44*7, and at 25°
52'9 parts of anhydrous salt. Above 27° the heptahydrated salt, like
the decahydrated salt at 34°, splits up into the monohydrated salt and
a saturated solution. Thus sodium sulphate has three curves of solu-
bility : one for Na2SO4,7H2O (from 0° to 26°), one for Na2SO4,10H2O
(from 0° to 34°), and one for Na2SO4,H2O (a descending curve begin-
ning at 26°), because there are three of these crystallo-hydrates, whilst
the solubility can only be referred to a definite state of a substance
which is present (or separated) in excess.8

Thus solutions of sodium sulphate give crystallo-hydrates of three
kinds on cooling the saturated solution : the unstable equilibrium of
the heptahydrated salt at temperatures below 26°, the decahydrated
salt forms under ordinary conditions at temperatures below 34°, and
the monohydrated salt at temperatures above 34°. Both the latter
crystallo-hydrates present a stable state of equilibrium, and the hepta-
hydrated salt decomposes into them, probably according to the equa-
tion 3Na2SO4,7H2O = 2NaaSO4,10H2O + Na2SO4,H2O. The ordi-
nary decahydrated salt is called Glauber's salt. All forms of these
crystallo-hydrates entirely lose their water, and give the anhydrous salt
when dried over sulphuric acid.9

Sodium sulphate, Na2SO4, enters into only a few reactions of com-
bination with other salts, and chiefly with salts of the same acid,
forming double sulphates. Thus, for example, if a solution of sodium

8 From this example it is evident that the phenomena of saturation are not able
to contribute much towards the understanding of solutions themselves. The solution
remains the same, but from the contact of a solid it becomes either saturated or super-
saturated, because crystallisation is determined by the attraction to a solid, as the
phenomenon of supersaturation clearly demonstrates.

9 According to the demonstrations of Pickering (1886), the molecular weight in grains
(that is, 142 grams) of anhydrous sodium sulphate, on being dissolved in a large mass of
water, at 0° absorbs (therefore the — sign) —1100 heat units, at 10° — 700, at 15° — 275, at
20° (gives out) + 25°, at 25°, + 800 calories. For the decahydrated salt Na,2SO4,10H.,O,
5° -4225, 10° -4000, 15°- 3570, 20°- 8160, 25° -2775. Hence (just as in Chapter I. Note 56)
the heat of the combination Na^SO^lOEUO at 5°= +8125,10° = +3250,20°= +8200
and 25° = + 8050.

It is evident that the decahydrated salt dissolving in water gives a decrease of tempe-
rature. Solutions in hydrochloric acid give a still greater decrease, because the water of
crystallisation is here taken in a solid state — that is, like ice — and on melting absorbs heat.
A mixture of 15 parts of Na->SO4,10H.,O and 12 parts of strong hydrochloric acid produces
>utiicient cold to freeze water. During the treatment with hydrochloric acid a certain
quantity of sodium chloride is formed.

510 PRINCIPLES OF CHEMISTRY

sulphate be mixed with a solution of aluminium, magnesium, or ferrous
sulphate, it gives crystals of a double salt when evaporated. Sulphuric
acid itself forms a compound with sodium sulphate, which is exactly
like these double salts. It is formed with great ease if sodium sulphate
is dissolved in sulphuric acid and the solution be evaporated. On
evaporation, crystals of the acid salt separate, Na2SO4 + H2SO4
= 2NaHSO4. This separates from hot solutions, whilst th'e crystallo-
hydrate NaHSO4,H2O10 separates from cold solutions. The crystals,
when exposed to damp air, split up into H2SO4, which deliquesces, and
Na2SO4 (Graham, Rose) ; alcohol also extracts sulphuric acid from the
acid salt. This shows the feeble force which holds the sulphuric acid
to the sodium sulphate.11 Both acid sodium sulphate and all mixtures
of the normal salt and sulphuric acid lose water when heated, and are
converted into sodium pyrosulphate, Na2S2O7, at a low red heat. This
anhydrous salt, at a bright red heat, parts with the elements of
sulphuric anhydride, the normal sodium sulphate remaining behind -
Na2S2O- = Na2SO4 + SO3. From this it is seen that the normal salt
is able to combine with water, with other sulphates, and with sulphuric
anhydride or acid, &c.116

Sodium sulphate may, by double decomposition, be converted into
a sodium salt of any other acid, by means of heat and taking advantage
of the volatility, or by means of solution and taking advantage of the
different degree of solubility of the different salts. Thus, for instance,
owing to the insolubility of barium sulphate, sodium hydroxide or
caustic soda may be prepared from sodium sulphate, if barium hydroxide
be added to its solution, Na2SO4 + Ba(HO)2 = BaSO4 + 2NaHO. And
by taking any salt of barium, BaX2, the corresponding salt of sodium

10 The very large and well-formed crystals of this salt resemble the hydrate
H-,SO4,H2O, or SO(OH)4. In general the replacement of hydrogen by sodium modifies
many of the properties of acids less than its replacement by other metals. This most
probably depends on the volumes being nearly equal.

11 In solution (Berthelot) the acid salt in all probability decomposes most in the
greatest mass of water. The specific gravity (according to the determinations of
Marignac) of solutions at 15°/4° = 9992 + 77'92^> + 0'231^2 (see Note 7). From these figures,
and from the specific gravities of sulphuric acid, it is evident that on mixing solutions of this
acid and sodium sulphate expansion will always take place; for instance, H2SOi + 25H2O
with Na2SO4 + 25H2O increases from 483 volumes to 486. In addition to which, in weak
solutions heat is absorbed, as shown in Chapter X. Note 27. Nevertheless, even more
acid salts may be formed. For instance, on cooling a solution of 1 part of sodium
sulphate in 7 parts of sulphuric acid, crystals of the composition NaHSO4,H.>SO,
are separated (Schultz, 1868). This fuses at about 100° ; the ordinary acid salt, NaHS< ) , .
at 149°.

116 In order to demonstrate the weakness of the bond acting in sodium hydrogen
sulphate, NaHSO4, it is useful to remember that on decreasing the pressure this salt dis-
sociates much more easily than at the ordinary pressure ; it loses water and forms the
pyrosulphate, Na^O; ; this reaction is utilised in chemical works.

SODir.M 511

may be obtained, Na2SO4 + BaX., = 1 JaS< > , + - NaX. Barium sulphate
thus formed, being a very sparingly-soluble salt, is obtained as a pre-
cipitate, whilst the sodium hydroxide, or salt, NaX, is obtained in
solution, because all salts of sodium are soluble. Berthollet's doctrine
permits all such cases to be foreseen.

The reactions of decomposition of sodium sulphate are above all
noticeable by the separation of oxygen. Sodium sulphate itself is very
stable, and it is only at a temperature sufficient to melt iron that it is
possible to separate the elements SO3 from it, and then only partially.
However, the oxygen may be separated from sodium sulphate, as from
all other sulphates, by means of many substances which are able to
combine with oxygen, such as charcoal and sulphur, but hydrogen is
not able to produce this action. If sodium sulphate be heated with
charcoal, then carbonic oxide and anhydride are evolved, and there is
produced, according to the circumstances, either the lower oxygen
compound, sodium sulphite, Na2SO3 (for instance, in the formation
of glass) ; or else the decomposition proceeds further and sodium
sulphide, Na2S, is formed, according to the equation Na2SO4 + 20
= 2CO2 + Na2S.

On the basis of this reaction the greater part of the sulphate of
sodium prepared at chemical works is converted into soda ash — that is,
sodium carbonate, Na2CO3, which is used for many purposes. In the
form of carbonates, the metallic oxides behave in many cases just as
they do in the state of oxides or hydroxides, owing to the feeble acid
properties of carbonic acid. However, the majority of the salts of
carbonic acid are insoluble, whilst sodium carbonate is one of the few
soluble salts of this acid, and therefore reacts with facility. Therefore
sodium carbonate is employed for many purposes, in which it acts owing
to its alkaline properties. Thus sodium carbonate, even under the action
of feeble organic acids, immediately parts with its carbonic acid, and
gives a sodium salt of the acid taken. Furthermore, its solutions
already exhibit an alkaline reaction on litmus, and in many cases are
able to act as an alkali. Thus, for instance, sodium carbonate, like the
alkalis, aids the passage of certain organic substances (tar, acids) into
solution, and is therefore used, like alkalis and soap (which also acts
by virtue of the alkali it contains), for the removal of certain organic
substances, especially in bleaching tissues in cotton and similar works.
Besides which, a considerable quantity of sodium carbonate is used
for the preparation of sodium hydroxide or caustic soda, which has
also a very wide application. In large chemical works where sodium
carbonate is manufactured it is usual to first manufacture sulphuric
acid, and then by its aid to convert common salt into sodium sulphate,

512 PRINCIPLES OF CHEMISTRY

and lastly to convert the sodium sulphate thus obtained into carbo-
nate and caustic soda. Hence these works prepare alkaline substances
(soda ash and caustic soda) and acid substances (sulphuric and hydro-
chloric acids), those two forms of chemical products which are dis-
tinguished for the greatest energy of their reactions, and which are
therefore frequently applied to technical purposes. And therefore the
works manufacturing soda are generally called chemical works (alkali
works).

The process of the conversion of sodium sulphate into sodium
carbonate consists in strongly heating a mixture of the sulphate with
charcoal and calcium carbonate. The following reactions then t.-iko
place : the sodium sulphate is first deoxidised by the charcoal, forming
sodium sulphide and carbonic anhydride, Na2SO4 + 2C = ]S"a.2S 4- 2C(X.
The sodium sulphide thus formed then enters into double decomposi-
tion with the calcium carbonate taken and gives calcium sulphide and
sodium carbonate, Na2S + CaCO3 = Na2CO3 + CaS.

Besides which, under the action of the heat, a portion of the excess
of calcium carbonate is decomposed into lime and carbonic anhydride,
CaCO3=CaO + CO2, and the carbonic anhydride with the excess of
charcoal forms carbon monoxide, which towards the end of the opera-
tion shows itself by the appearance of a blue flame. Thus from a mass
containing sodium sulphate we obtain a mass which includes sodium
carbonate, calcium sulphide, and calcium oxide, but none of the sodium
sulphide first formed, or, strictly speaking, only a small portion. The
entire process, which proceeds at a high temperature, may be expressed
by a combination of the three above-mentioned formulae, if it be taken
into consideration that the product contains one equivalent of calcium
oxide to two equivalents of calcium sulphide.12 The sum of the
reactions may then be expressed thus : 2ISra2S04 + 3CaCO3 + 9C
= 2Na2CO3 + CaO,2CaS + 10CO. Indeed, the quantities in which
the substances are mixed together at chemical works approaches to the
proportion required by this equation. The entire process of decompo-
sition is carried on in reverberatory furnaces, into which a mixture of
1000 parts of sodium sulphate, 1040 parts of calcium carbonate (as a

18 Calcium sulphide, CaS, like many metallic sulphides which are soluble in water, is
decomposed by water (page 426), CaS + H2O-CaO + H2S, because hydrogen sulphide is
a very feeble acid. If calcium sulphide be acted on by a large mass of water, linn- may
be precipitated, a state of equilibrium will be entered on, when the system CaO-f '2< 'aS
remains unchanged. Lime, being the product of the action of water on CaS, limits
this action. Therefore, if in black ash the lime was not in excess, a part of the
sulphide compounds would be in solution (actually there is but very little). In this
manner in the manufacture of sodium carbonate the conditions of equilibrium which
enter into double decompositions have been made use of (see above), and the aim is to
form directly the unchangeable product CaO,2CaS. This was first regarded as a special
insoluble compound, but nothing points to its independent existence.

SODIUM

513

somewhat porous limestone), and 500 parts of small coal is charged
from above. This mixture is first heated in the portion of the furnace
which is furthest removed from the fire-grate ; it is then brought to the

FIG. 69. — Kovorberatory furnace for the manufacture of sodium carbonate. F, grate. A, bridge.
M, hearth for the ultimate calcination of the mixture of sodium sulphate, coal, and calcium car-
bonate, which is charged from above into the part of the furnace furthest removed from the fire F.
P, P, doors for stirring and bringing the mass towards the grate F by means of stirrers R. At the
end of the operation the semifused mass is charged into trucks C.

portion nearest to the fire-grate, when it is stirred during heating.
The partially-fused mass obtained at the end of the process is cooled,
and then subjected to methodical lixiviation 1S to extract the sodium

15 Methodical lixiviation is the extraction, by means of water, of a soluble substance
from the mass containing it. It is carried on so as not to obtain weak aqueous solutions,
and in such a way that the residue shall not contain any of the soluble substance. This
problem is practically of great importance in many industries. It is required to extract
from the mass all that is soluble in water. This is easily effected if water be first poured

FIG. 70.— Apparatus for the methodical lixiviation of black ash, &c. Water flows into the tanks
from the pipes r, r, and the saturated liquid is drawn off from c, c.

on the mass ; the strong solution thus obtained is then decanted, then water is again
poured on, time being allowed for it to act ; then again decanted, and so on until the
water does not take up anything. But then finally such weak solutions are obtained
that it would be very disadvantageous to evaporate them. ' This is avoided by pouring
VOL. I. L L

514 PRINCIPLES OF CHEMISTRY

carbonate, the mixture of calcium oxide and sulphide forming the so-
called 'soda waste.'14

the fresh hot water destined for the lixiviation, not on the fresh mass, but upon a mass
already subjected to preparatory lixiviation by means of weak solutions. In this way the
fresh water gives a weak solution. The solution which is obtained flows to those
parts of the apparatus which contain the fresh, as yet unlixiviated, mass, and thus in
the latter parts the weak alkali formed in the other parts of the apparatus becomes
saturated as far as possible with the soluble substance. Generally several intercommuni-
cating vessels are constructed (standing at the same level) into which in turn the fresh
mass is deposited which is intended for lixiviation ; the water is poured in, the alkali
drawn off, and the lixiviated residue cleared out. The illustration represents such an
apparatus, consisting of four communicating vessels. The water poured into one of them
flows through the two nearest and issues from the third. If the fresh mass be placed in
one of these cases or vessels, the stream of water passing through the apparatus is
directed in such a manner as to finally issue from this vessel containing the fresh un-
lixiviated mass. The fresh water is added to the vessel containing the matter which is
almost completely exhausted. The fresh water passing through this vessel is conveyed
by the pipe (syphon passing from below the first case to the top of the second) communi-
cating with the second ; it finally passes (also through a syphon pipe) into the case (the
third) containing the fresh stuff. The water will extract all that is soluble in the first
vessel, leaving only an insoluble mass. This vessel is then destined to be emptied, and
refilled with fresh matter. The level of the liquids in the various vessels will naturally
be different, in consequence of the various strengths of the solutions which they con-
tain.

14 The whole of the sulphur used in the production of the sulphuric acid employed in
decomposing the common salt enters into this residue. This residue carrying off the
sulphur is the great burden and expense of the soda works which use Leblanc's method.
As an instructive example from a chemical point of view it is worth while describing here
one of the various methods of regaining the sulphur from the soda waste.

Kynaston (1885) treats the soda waste with a solution (sp. gr. 1'21) of magnesium
chloride, which disengages sulphuretted hydrogen : CaS + MgCl2 + 2H2O = CaCl2
+ Mg(OH)2 + H2S. Sulphurous anhydride is passed through the residue in order to form
the insoluble calcium sulphite: CaCl2 + Mg(OH)2 + SO2 = CaSO3 + MgCl2 + H2O. The
solution of magnesium chloride obtained is again used, and the washed calcium sulphite
is brought into contact at a low temperature with hydrochloric acid (a weak aqueous
solution) and hydrogen sulphide, the whole of the sulphur then separating :

CaS05 + 2H2S + 2HC1 = CaCl2 + 3H2O + 88.

It is necessary to turn once more to the lixiviation of the sodium carbonate from the
mass formed in the furnace (see also Note 25).

It must not, however, be thought that sodium carbonate alone passes into the solution ;
there is also a good deal of caustic soda with it, formed by the action on the carbonate of
sodium of the lime remaining from the first process of the action of the charcoal, and there
are also certain sodium sulphur compounds with which we shall partly become acquainted
hereafter. The sodium carbonate, therefore, is not obtained in a very pure state. The
solution is subjected to evaporation. The evaporation is conducted at the expense of the
waste heat from the soda furnaces, together with that of the gases given off. The
process in the soda furnaces is only carried on at a high temperature, and therefore the
smoke and gases issuing from the furnaces are inevitably very hot. If the heat they
contain was not made use of there would be a great waste of fuel*; consequently in
immediate proximity to the soda furnaces there is generally a series of pans or evapo-
rating boilers, under which the gases from the furnace pass, and into these the alkali
solution obtained is poured; On evaporating the solution first of all the undecomposed
sodium sulphate separates, then the sodium carbonate or soda crystals. These crystals

SODIt'M 515

The above-mentioned process for making soda was discovered in the
year 1808 by the French doctor Leblanc, and is known as the Leblanc
process. The particulars of this discovery are somewhat remarkable.
Sodium carbonate, having a considerable application in industry, was
for a long time prepared exclusively from the ash of marine plants
(Chapter XI. page 490). Even up to the present time this process is
-carried on in Normandy. In Prance, where for a long time the manu-
facture of large quantities of soap (so-called Marseilles soap) and various
fabrics required a large amount of soda, the quantity prepared at the
-coast was insufficient to meet the demand. For this reason during
the wars at the beginning of the century, when the import of foreign
.goods into France was interdicted, the want of sodium carbonate was felt.
The French Academy offered a prize for the discovery of a profitable
method of preparing it from common salt. Leblanc then proposed the
above-mentioned process, which is remarkable for its great simplicity.15

which separate are raked out and placed on planks, where the liquid flows off. Caustic
.soda remains in the residue, and also any sodium chloride which was not decomposed in
the foregoing process.

Part of the sodium carbonate is crystallised in order to purify it more thoroughly. In
-order to do this a saturated solution is left to crystallise at a temperature below 30° in a
current of air, in order to promote the separation of the vapour of water. Then the large
transparent crystals (efflorescent in air) of Na2CO5,10H.2O are formed which have been
.spoken of already (Chapter I.).

13 Among the drawbacks of the Leblanc process are the accumulation of ' soda waste,'
.and the impossibility at the comparatively low price of sulphur (especially in the form of
pyrites) of finding a suitable employment for it (although this waste can furnish sulphur
and sulphur compounds, for which purposes it is sometimes treated), and also the insuffi-
-cient purity of the sodium carbonate for many purposes. The advantage of the Leblanc
^process, besides its simplicity and cheapness, are that almost all acids having a commercial
value are obtained as bye-products ; chlorine and bleaching powder are produced with
4ihe assistance of the large amount of hydrochloric acid which appears as a bye-product,
-and caustic soda is very easily made, and the demand for it increases every year. In
those places where salt, pyrites, charcoal, and limestone (which are the materials required
.for soda works) are found side by side — as, for instance, in the Ural or Don districts — all
conditions are favourable to the development of the manufacture of sodium carbonate on
.an enormous scale ; and where, as in the Caucasus, sodium sulphate occurs naturally, the
conditions are still more favourable. A large amount, however, of the latter salt, even
from soda works, is used in making glass. The most important soda works, as regards
the quantity of products obtained from them, are the English works.

As an example of the other numerous and various methods of manufacturing soda
from sodium chloride, the following processes may be mentioned : Sodium chloride is
^decomposed by oxide of lead, PbO, forming lead chloride and sodium oxide, which, with car-
bonic anhydride, yields sodium carbonate (Scheele's process). In Cornu's method sodium
chloride is treated with lime, and then exposed to the air, when it yields a small quantity
of sodium carbonate. In E. Kopp's process sodium sulphate (125 parts) is mixed with
oxide of iron (80 parts) and charcoal (55 parts), and the mixture is heated in reverbe-
ratory furnaces. Here a compound, Na6Fe4S3, is formed, which is insoluble in water and
absorbs oxygen and carbonic anhydride, and then forms sodium carbonate and ferrous
sulphide; this when roasted can give sulphurous anhydride, which is indispensable
-for the manufacture of sulphuric acid, and ferric oxide, which is again used in the

L L 2

( >f ;ili other industrial processes foiMiuinuf act tiring sodium carbonate
tin* niiuinnt'ni /./•/«•/ xx is the mo-t worthy of mention. li; In thi- the
vapour- of ammonia, and then an excess of carbonic anhvdride. are
directly int rodm-ed into a ci mcent rat ed solution of sodium chloride in
order TO t'oi-ni the acid ammonium carbonate. N 1 1 , 1 1 ( '< > ,. Then, by
means of the double saline decomposition of this salt, sodium chloride
i- deeompo-ed,, ;md in virtue of its -!i:_;'lit solubility acid sodium
carbonate. Na 1 1 ('<>.. js pivcipit at ed and ammonium chloride, Nl [,('!, is
obtained in solution ( \\"it h a port ion of sod i tin i chloride and acid sodium
carbonate ). 'I lie ammonia is removed from the -i » hit ion bv heat int;' wit h
lime or magnesia, and t he precipitated acid sodium ca rhona lei s converted
into the normal -alt bv heat in if. It is thus obtained in a \ erv pure state.

Sodium carbonate.17 like sodium sulphate, loses all its water on
beino- heated, and when anhydrous fuses at a bright-red heit (SIT).
A small <|uantity of sodium carbonate placed in the lo-p of a platinum
\\ ii'e volatilises in the heat of a u'as flame, and t hei ef< »re in t he furnaces
of ifla.-- wi irks pa rt of the soda is always t ran -formed into the cmidit ion
of \apour. Sodium carbonate, resembles sodium sulphate in its relation
to water.1" Mere also it- ^reatest solubility is at the tem]»erat tire of

<ull.lii.l- - .i:~. •!_:_•,, I ;,,„] .,,,iiiin, .•arbcnatc fonnc.l.

Scdinin carl.diuitc may likt-wi-c 1 l.tainnl (Vein i-rynliti- i]>a.L'i- -Is i i : tlic method

"f treating tlii- will !,.• mentioned under aluminium.

I(i Tlii- pi-oce^s I'H,, X,,te -Jli wa< first jioiiitod ,,nt l.y Tun-k. worked out by
Sdiloexi,,._r. ;,,,,] fnially aj. plied industrially l>y Solvay. The iirsi ilssii 1 a r.^e soda works
crci-ti I! | i'kiiiLr tlii- |.rore« are on the liauks of the Kama at r.ere/niak.

i • n • . • I. ul. inn IT. I'.ut UtisMii. which -till iinjmrts from ahroad a

' , n-yiii- on the Lel.lanc ]iroces-. Such are in course of coiistnic-
rh of i ,!,, t, •. til] i,, emhryo in liu^sia.

ir ' • • ' od,< a li calefied, anhvdi-ous! i-^ rarely |iii)'e ; the ci'vstallised -oda i-

. | t . t further i! he • tol.oil M concentrated solution of

• ' • • : I of tin colled the soda Which settle-,, wa-ll With

I remain in the mother liquor-. A'c.

'• 'i'li. i . i-' Ill in -i ' o uTi-at t hat. noi\\-it h-taiidiiiLT the ditTei'i'iice in t he molecular

. •• i ! f1 •()..,.;, | N, i .('O, ihr \ oliL'ht to he cla-^ed under tile tVJie (NllOl.jR,

v. i en I; -(!,,.(( >. \J;,|P, , ti , r • , ' ,, contain In mol. II <>.

en I iiliiim i';n hoiiate. The specific gravity of the

h. it of the dec, led sail T Mi. '\\\i< \ ariel i,. ; are known of

||,e liepl ' .1!: I. -•/.' ! \Iari-nac. Kaniiiie] her- ,, uhich are for d (o.irether by

....... ,1 , I, -d solution to co,,l under a layer oi alcoh,.! : the one i- |,-^ stahleflike

lhr-fr.cn | -.lid n •/ • ulplut.- :IIM| al H I,., ;, ol uhi 1 it >' of li'J pa I't - I of ;i id ivd 1MI1S sa 1 1 ) ill 1 00

..,,;•,.,', ,„-, talili .1 tin ..lul.iliU 'Jii part- l..f anhy.h-ou- salt . |»er Inn of

...ati M" •' t\ ..I the di-c; \drati I ill l-ill \s ater - at n . 7'll ; at 'Jn . 'Jl'7 : at

:;n . :;V-j I • \- -o : ;,...,;.• ,,nh n;-i . at '.MI i:,-7. at mo

;., ; i • : on • 'I ; . pecil I; ' te ?. of tin olut imi of sodium

• the data o! Cei , ixl Kohllall ch. at IT. 1 i expressed hy

soi)] r.M 517

37° ; both salts, on crystallising at the ordinary temperature, combine
with ten molecules of water, and such crystals of soda, like crystals of
Glauber's salt, fuse at 34°. Sodium carbonate also forms a super-
saturated solution, and, according to the conditions, gives various com-
binations with water of crystallisation (mentioned on page 107), &c.

At a red heat superheated steam liberates carbonic anhydride from
sodium carbonate and forms caustic soda, Na2CO3 + H.2O = 2NaHO4-
CO.2. Here the carbonic anhydride is replaced by water ; this depends
on the feebly acid character of carbonic anhydride. By direct heating,
sodium carbonate is only slightly decomposed into sodium oxide and
carbonic anhydride ; thus, when sodium carbonate is fused, about
1 per cent, of carbonic anhydride is disengaged.19 The carbonates of
many other metals — for instance, of calcium, copper, magnesium, iron,
<fcc. — on being heated lose all their carbonic anhydride. This shows
the considerable basic energy which sodium possesses. With the
soluble salts of most metals, sodium carbonate gives precipitates
either of insoluble carbonates of the metals, or else of the hydroxides
{in this latter case carbonic anhydride is disengaged) ; for in-
stance, with barium salts it precipitates an insoluble barium car-
bonate (BaCls + Na^COg^NaCl + BaCOa) and with the aluminium
salts it precipitates aluminium hydroxide, carbonic anhydride being
disengaged : 3Na,CO3 + A1,(SO4)3 + 3H20 = 3Na2S04 + 2A1(OH)3 +
3C0.2. Sodium carbonate, like all the salts of carbonic acid, evolves
carbonic anhydride on treatment with all acids which are to any extent
•energetic. But if an acid diluted with water be gradually added to a
solution of sodium carbonate, at first such an evolution does not take
place, because the excess of the carbonic anhydride forms acid sodium
carbonate (sodium bicarbonate), NaHCO3. The composition of this
salt, however, may be also represented as a combination of car-
bonic acid, H.2C03, with the normal salt, Na.2CO3, just as the latter
also combines with water. Such a representation is all the more

the formula, s = 9092 + 104-5^ + 0-165^. Weak solutions occupy a volume not only less
than the sum of the volumes of the anhydrous salt and the water, but even less than the
water contained in them. For instance, 1000 grams of 1 p.c. solution occupy (15C) a
volume of 990'4 c.c. (sp. gr. 1'0097), but contain 900 grams of water, occupying at 15° a
volume of 990'8 c.c. A similar case of solubility, which is comparatively rare (it occurs
also with sodium hydroxide), occurs in those dilute solutions for which the factor A is
greater than 100 if the sp. gr. of water at 4° = 100000, and if the sp. gr. of the solution be
expressed by the formula S= S0 + Ap + Bp2, where S0 is the specific gravity of the water
(this is more fully discussed in my treatise, The Investigation of Aqueous Solutions,
1887, § § 94 and 95). For 5 p.c. the sp. gr. 15°/4c = r0520 ; for 10 p.c. T1057 ; for 15 p.c.
1*1603. The changes in the sp. gr. with the temperature are here almost the same as
with solutions of sodium chloride with an equal value of p.

19 According to the observations of Pickering. According to Rose, when solutions of
sodium carbonate are boiled a certain amount of carbonic anhydride is disengaged.

-518 PRINCIPLES OF CHEMISTRY

likely because (1) there exists another salt, Na.2CO3,2NaHCO3,2H.2(>
(sodium sesquicarbonate), obtained by cooling a boiling solution of
sodium bicarbonate, or by mixing this salt with the normal salt ;
but the formula of this salt cannot be derived from that of normal
carbonic acid, as the formula of the bicarbonate can ;20 (2) water
of crystallisation does not enter into the composition of the crystals
of the acid salt, so that on its formation (occurring only at low tem-
peratures, as in the formation of crystalline compounds with water)
the water of crystallisation of the normal salt separates and the water
is, as it were, replaced by the elements of carbonic acid. In any case
the acid sodium carbonate is an unstable salt. Not only when heated
alone, but even on being slightly heated in solution, and also at the
ordinary temperature in damp air, it loses carbonic anhydride and forms
the normal salt. And at the same time it is easy to obtain it in a pure
crystalline form, if a strong solution of sodium carbonate be cooled and
a stream of carbonic anhydride gas passed through it. The acid salt
is less soluble in water than the normal,21 and therefore a strong
solution of the latter gives crystals of the acid salt if carbonic
anhydride be passed through it. The acid salt may be yet more
conveniently formed from effloresced crystals of sodium carbonate,.

20 At the same time the sesqui-salt has all the properties of a definite compound ; it.
crystallises in transparent crystals, has a constant composition, its solubility (at 0° in 100
of water, 12'6 of anhydrous salt) differs from the solubility of the normal and acid salts,
and shows changes in composition ; it is found in nature, and is known by the names of
trona and urao. The observations of Watts and Richards showed (1886) that on pouring
a strong solution of the acid salt into a solution of the normal salt saturated by heating,
crystals of the salt NaHCO3,Na.,CO5,2HoO may be easily obtained, as long as the tempe-
rature is above 85°. The natural urao (Boussingault) has, according to Laurent, the same-
composition. This salt is very stable in air, and may be used for purifying sodium carbo-
nate on the large scale. From the theoretical side such compounds have been little studied,,
yet are particularly interesting because, in all probability, they correspond with ortho-
carbonic acid C(OH)}, and at the same time correspond with double salts like astrachanite
(Chapter XIV.).

n 100 parts of water at 0° dissolves 7 parts of the acid salt, which corresponds with
4 '8 parts of the anhydrous normal salt, but at 0° 100 parts of water dissolves 7 parts of
the latter. The solubility of the acid salt varies with considerable regularity ; 100 parts
of water dissolves at 15° 9 parts of the salt, at 80C 11 parts.

The ammonium, and more especially the calcium, salt, is much more soluble in water-
The ammonia process (see ante, p. 516) is founded upon this. Ammonium bicarbonate
(acid carbonate) at 0° has a solubility of 12 parts in 100 water, at 80° of 27 parts. The
solubility, therefore, increases very rapidly with the temperature. But its saturated solu-
tion is more stable than a solution of sodium bicarbonate. In fact, saturated solutions of
these salts have the vapour tension of a mixture of carbonic anhydride and water — namely
at 15° and at 50°, for the sodium salt 120 and 750 millimetres, for the ammonium salt 120
and 563 millimetres. These data are of great importance in understanding the pheno-
mena connected with the ammonia process. They demonstrate that with an increased
pressure the formation of the sodium salt ought to increase if there be an excess of ammo-
nium salt.

SODIUM 519

which, on being considerably heated, very easily absorb carbonic
anhydride.38 The acid salt crystallises well, but not, however, in such
large crystals as the normal salt ; it has a brackish and not an alkaline
taste like that of the normal salt, its reaction is feebly alkaline,
nearly neutral. At 70° its solution begins to lose carbonic anhydride,
and on boiling the evolution becomes very abundant. From the
preceding remarks it is clear that in most reactions this salt, especially
when heated, acts similarly to the normal salt, but has, naturally, some
distinctions from it. Thus, for example, if a solution of sodium
carbonate be added to a normal magnesium salt a turbidity (precipi-
tate) is formed of magnesium carbonate, MgCO3. No such precipitate
is formed by the acid salt, because magnesium carbonate is soluble in
the presence of an excess of carbonic anhydride.

Sodium carbonate is used for the preparation of caustic soda 23 —
that is, the hydrate of sodium oxide, or the alkali which corresponds
with sodium. For this purpose the action of lime on a solution of
sodium carbonate is generally made use of. The process is as follows :
a weak, generally 10 per cent., solution of sodium carbonate is taken,24
and boiled in a cast-iron, wrought-iron, or silver boiler (sodium
hydroxide does not act on these metals), and lime is added, little by
little, during the boiling. This latter is soluble in water, although
but very slightly. The clear solution becomes turbid on the addition
of the lime because a precipitate is formed ; this precipitate consists
of calcium carbonate, almost insoluble in water, whilst caustic soda
is formed and remains in solution. The decomposition is effected
according to the equation : Na.2CO3 + Ca(HO).2=CaCO3 + 2NaHO. On
cooling the solution the calcium carbonate easily settles as a precipitate,

22 Crystalline sodium carbonate (broken into lumps) also absorbs carbonic anhydride,
but the water contained in the crystals is then disengaged: Na2CO3,10H2O + CO2 =
Na.jCO3, HoCO,-, + 9H..O, and dissolves part of the carbonate; therefore part of the sodium
carbonate passes into solution together with all the impurities. When it is required to
avoid the formation of this solution, a mixture of ignited and crystalline sodium carbonate
is taken. Sodium bicarbonate is prepared chiefly for medicinal use, and is then often
termed carbonate of soda, also, for instance in the so-called soda powders, for preparing
certain artificial mineral waters, for the rapid generation of considerable quantities of
.carbonic anhydride in the domestic preparation of water charged with carbonic acid, for
the manufacture of digestive lozenges like those made at Essentuki, Vichy (pastilles
digestives de Vichy), &c.

23 In chemistry sodium oxide is termed ' soda,' which word must be carefully distin-
guished from the word sodium, meaning the metal.

24 With a small quantity of water, the reaction either does not take place, or even
proceeds in the reverse way — that is, sodium and potassium hydroxides remove carbonic
anhydride from calcium carbonate (Liebig, Watson, Mitscherlich, and others). The in-
fluence of the mass of water is evident. According to Gerberts, strong solutions of
sodium carbonate are, however, decomposed by lime, which is very interesting if confirmed
by further investigation.

520 PRINCIPLES OF CHEMISTRY

and the clear solution or alkali above it contains the easily-soluble
sodium hydroxide formed in the reaction.25 After the necessary
quantity of lime has been added, the solution is allowed to stand, and
is then decanted off and evaporated in cast or wrought iron boilers,
or in silver pans if a perfectly-pure product is required.26 The
evaporation cannot be conducted in china, glass, or similar vessels,
because caustic soda eats into glass and china, although but slightly.
The solution does not crystallise on evaporation, because the solubility
of caustic soda when heated is very great, but crystals containing
water of crystallisation may be obtained by cooling. If the evapora-
tion of the alkali be conducted until the specific gravity reaches 1'38,
and the liquid is then cooled to 0°, transparent crystals appear contain-

25 As long as part of the undecomposed sodium carbonate remains in solution, excess
of acid added to the solution disengages carbonic anhydride, and the solution after dilu-
tion gives a white precipitate with a barium salt soluble in acids, showing the presence of
a carbonate in solution (if there be sulphate present, it also forms a white precipitate,
but this is insoluble in acids). For the decomposition of sodium carbonate, milk of lime
— that is, slaked lime suspended in water — is employed. Formerly pure sodium hydrox-
ide was prepared (according to Berthollet) by dissolving the impure substance in alcohol
(sodium carbonate and sulphate are not soluble), biit now that metallic sodium has become
cheap and is purified by distillation, pure caustic soda is prepared by acting on a small
quantity of water with sodium. By allowing strong solutions to crystallise (in the cold)
completely pure sodium hydroxide may also be obtained (Note 27).

In alkali works where the Leblanc process is used, caustic soda is prepared directly from
the alkali remaining in the mother liquors after the separation of the sodium carbonate
evaporation (Note 14). If excess of lime and charcoal has been used, much sodium
hydroxide may be obtained. After the removal as far as possible of the sodium carbon-
ate, a red liquid (from iron oxide) is left, containing sodium hydroxide mixed with com-
pounds of sulphur and of cyanogen (formed in the Leblanc furnaces, see p. 225, and
Chapter IX.) and also containing iron. This red alkali is evaporated and air is blown
through it, which oxidises the impurities (for this purpose sometimes sodium nitrate is
added, or bleaching powder, &c.) and leaves fused caustic soda. The fused mass is allowed
to settle in order to separate the ferruginous precipitate, and poured into iron barrels, where
the sodium hydroxide solidifies. Such caustic soda contains about 10 p.c. of water in
excess and some saline impurities, but when properly manufactured is almost free from
carbonate and from iron.

26 Lb'wig gave a method of preparing sodium hydroxide from sodium carbonate by
heating it to a dull red heat with an excess of ferric oxide. Carbonic anhydride is given
off, and warm water extracts the caustic soda from the remaining mass. This reaction,
as experiment shows, proceeds very easily, and is an example of contact action similar
to the influence of ferric oxide on the decomposition of potassium chlorate. The.
reason of this may be that a small quantity of the sodium carbonate enters into double
decomposition with the ferric oxide, and the ferric carbonate produced is decomposed into
carbonic anhydride and ferric oxide, the action of which is renewed. Similar explana-
tions expressing the motive of a reaction really add but little to that elementary concep-
tion of contact which, according to my opinion, consists in the change of motion of the
atoms in the molecules under the influence of the substance in contact. In order to
represent this clearly it is sufficient, for instance, to imagine that in the sodium carbonate
the elements CO.> move in a circle round the elements Na.X), but at the points of contact
with Fe2O3 the motion becomes elliptic with a long axis, and at some distance from NaoO
the elements of CO2 are parted, not having the faculty of attaching themselves to Fe^O-.

SODir.M 521

ing 2XaHO,7H.2O, they fuse at +6°.27 If the evaporation be con-
ducted as long as water is disengaged, which requires a considerable
amount of heat, then, on cooling, the hydroxide, XaHO, solidifies in
semi-transparent crystals.28

Caustic soda, free from water, forms a colourless crystalline mass,
which eagerly absorbs moisture and carbonic anhydride from the air.29
Its specific gravity is 2-1 3 ;30 it is easily soluble in water, with
disengagement of a considerable quantity of heat.31 A saturated
solution at the ordinary temperature has a specific gravity of about
1*5, contains about 45 per cent, of sodium hydroxide, and boils at
130° ; at 55° water dissolves an equal weight of it.32 Caustic soda
is not only soluble in water but in alcohol, and even in ether.
Solutions of sodium hydroxide produce the sensation of soaping on the
skin, because the active base of soap consists of caustic soda.33

27 By allowing strong solutions of sodium hydroxide to crystallise in the cold, impuri-
ties— such as, for instance, sodium sulphate — may be separated from them. The fused
crystallo-hydrate 2NaHO,7H,O forms a solution having a specific gravity of T405 (Hermes).
Let us remark that, according to certain determinations, less water — namely, only
NaHO,3H2O — enters into the composition of the crystallo-hydrate. The crystals on dis-
solving in water produce cold.

!B In solid caustic soda there is generally an excess of water beyond that required by
the formula NaHO. The caustic soda used in laboratories is generally cast in sticks,
which are broken into pieces. It must be preserved in carefully closed vessels, because it
absorbs water and carbonic anhydride from the air.

29 By the way it changes in air it is easy to distinguish caustic soda from caustic
potash, which in general resembles it. Both alkalis absorbs water and carbonic adhydride
from the air, but caustic potash forms a deliquescent mass of potassium carbonate, whilst
caustic soda forms a dry powder of efflorescent salt.

50 As the molecular weight of NaHO = 40, the volume of its molecule = 40/2'13 = 18-5,
which very nearly approaches the volume of a molecule of water. The same in general
refers to the compounds of sodium — for instance, its salts have a molecular volume ap-
proaching the volume of the acids from which they are derived.

31 The molecular quantity of sodium hydroxide (40 grams), on being dissolved in a
large mass (200 gram molecules) of water, develops, according to Berthelot 9780, and
according to Thomsen 9940, heat-units, but at 100° about 13000 (Berthelot). Solutions of
NaHO + wH2O, on being mixed with water, evolve heat if they contain less than 6H2O,
but if more they absorb heat.

•>2 The specific gravity of solutions of sodium hydroxide at 15°/4° is given in the short
table below : —

NaHO, p.c. . . 5 10 15 20 80 40

Sp.gr. . . . 1-057 1-113 1-169 T224 1'331 T436

1000 grams of a 5 p.c. solution occupies a volume of 946 c.c. ; that is, less than the water
serving to make the solution (see Note 18).

00 Sodium hydroxide (and other alkalis) is capable of hydrolysing — saponifying, as it
is termed — the compounds of acids with alcohols. If RHO (or R(HO)w) represent the
composition of an alcohol — that is, of the hydroxide of a hydrocarbon radicle — and QHO an
acid, then the compound of the acid with the alcohol or ethereal salt of the given acid
will have the composition RQO. Ethereal salts, therefore, present a similitude to
metallic salts, just as alcohols resemble basic hydroxides. Sodium hydroxide acts on

522 PRINCIPLES OF CHEMISTRY

The chemical reactions of sodium hydroxide serve as a type for those-
of a whole class of alkalis — that is, of soluble basic hydroxides which
may be obtained from water and a metallic oxide, and whose compo-
sition is expressed as a compound of the metal, M, and hydroxyl, OH ;
MOH. The solution of sodium hydroxide is a very caustic liquid —
that is to say, it acts in a destructive way on most substances, for
instance, on most organic tissues — hence caustic soda, like all soluble
alkalis, is a poisonous substance ; acids, for instance, hydrochloric,
forming common salt, serve as antidotes. The action of caustic soda
on bones, fat, starch, and similar vegetable and animal substances
explains its action on organisms. Thus bones, when plunged into a
weak solution of caustic soda, fall to powder,34 and evolve a smell of
ammonia, which is due to the caustic soda changing the gelatinous-
organic substance (it contains carbon, hydrogen, nitrogen, oxygen, and
sulphur, like albumin) of the bones, dissolving it and in part destroy-
ing it, whence ammonia is disengaged. Fats, tallow, and oils become

ethereal salts in the same way that it acts on the majority of metallic salts — namely, it
liberates alcohol, and forms the sodium salt of that acid which was in the ethereal salt.
The reaction takes place in the following way : —

RQO + NaHO = NaQO + RHO
Ethereal Caustic Sodium Alcohol,

salt. soda. salt.

Such a decomposition is termed saponification, because similar reactions were known
very long ago for the ethereal salts corresponding with glycerin, C3H5(OH)3 (Chapter
IX.), found in animals and plants, and composing what are called fats or oils. Caustic
soda, acting on fat and oil, forms glycerin, and sodium salts of those acids which were
in union with the glycerin in the fat, as Chevreul showed at the beginning of this century.
The sodium salts of the fatty acids are known in practice as soaps. That is to say, that
soap is made from fat and caustic soda, glycerin being separated and a sodium salt.
formed. As glycerin is usually found in union with certain acids, so also are the sodium
salts of certain acids found in soap. The greater part of the acids found in conjunction
with glycerin in fats are the solid, palmitic and stearic acids, C16H32O2 and C18H56Oo, and
the liquid, oleic acid, C18H34O2 ; hence soap principally contains a mixture of the sodium
salts of these acids. In preparing soap the fatty substances are mixed with a solution of
caustic soda until an emulsion is formed ; the proper quantity of caustic soda is then
added in order to produce saponification when heated, the soap being separated from
the solution either by means of an excess of caustic soda or else by common salt, which
displaces the soap from the aqueous solution (salt water does not dissolve soap, neither
does it form a lather). The water, acting on the soap, partly decomposes it (because the
acids of the soap are feeble), and the alkali set free acts during the application of soap.
Hence it may be replaced by a very feeble alkali. Strong solutions of alkali corrode the
skin and tissues. They are not formed from soap, because the reaction is reversible,,
and the alkali is only set free by the excess of water. Thus we see how the teaching of
Berthollet renders it possible to understand many phenomena which occur during every-
day experience.

34 On this is founded the process of Henkoff and Engelhardt for treating bones. The-
bones are mixed with ashes, lime, and water : it is true in this case that more
potassium hydroxide than sodium hydroxide is formed, but their action is almost
identical.

SODIUM 523

saponified with a solution of caustic soda — that is to say, they form
with it Koaps soluble in water, or sodium salts of the organic acids
contained in the fats.35 The clearest reactions of sodium hydroxide
are determined by the fact that it saturates all acids forming salts with
them, which are almost all soluble in water, and in this respect caustic
soda is as characteristic amongst the bases as nitric acid is among the
acids. It is impossible to detect sodium by means of the formation of
precipitates of insoluble sodium salts, as may be done with other
metals, many of whose salts are but slightly soluble. The powerful
alkaline properties of caustic soda determine its capacity for com-
bining with even the feeblest acids, its property of disengaging
ammonia from ammonium salts, its faculty of acting on salts whose-
bases are insoluble in water, <fcc. If a solution of the salt of almost
any metal be mixed with caustic soda, then a soluble sodium salt will
be formed, and an insoluble hydroxide of the metal will be separated—
for instance, copper nitrate yields copper hydroxide, Cu(NO3)2
+ L)XaHO=Cu(HO).2 + 2]SraNO3. Even many basic oxides— as, for in-
stance, the oxides of zinc and aluminium — precipitated by caustic soda,
are capable of combining with it and forming soluble compounds, and
therefore caustic soda in the salts of such metals first forms a pre-
cipitate of hydroxide, and then, employed in excess, dissolves this,
precipitate. This phenomenon occurs, for instance, when caustic soda
is added to the salts of aluminium. This shows the property of such
an alkali as caustic soda of combining not only with acids, but also-
with feeble basic oxides. For this reason caustic soda acts on most
elements which are capable of forming acids or oxides similar to-
them ; thus, for instance, the metal aluminium gives hydrogen with
caustic soda in consequence of the formation of alumina, which com-
bines with the caustic soda — that is, in this case, the caustic alkali
acts on the metal just like an acid. If the substance which is mixed
with the caustic soda is capable of combining with the hydrogen
evolved (aluminium does not form such a compound), then such a,
compound will be formed. Thus, for instance, in this way phosphorus
acts on caustic soda, yielding hydrogen phosphide. If the hydrogen
compound disengaged is capable of combining with the alkali, then,
naturally, a salt of the corresponding acid is formed. For instance,
chlorine and sulphur act thus on caustic soda. Chlorine, with the
hydrogen of the caustic soda, forms hydrochloric acid, and the latter
forms common salt with the sodium hydroxide, whilst the other atom
in the molecule of chlorine, C1.2, takes the place of the hydrogen, and

33 As explained in Note 83.

524 PRINCIPLES OF CHEMISTRY

forms the hypochlorite, NaClO. In the same way, by the action of
sodium hydroxide on sulphur, hydrogen sulphide is formed, which acts
on the soda forming sodium sulphide, in addition to which sodium
thiosulphate is formed (see sulphur). In virtue of the possibility of
similar reactions, sodium hydroxide acts on many metals and
non-metals. Such action is often accelerated by the presence of the
oxygen of the air, as by this means the possibility of the formation of
acids and oxides rich in oxygen is increased. Thus many metals and
their lower oxides, in the presence of an alkali, absorb oxygen and
form acids. Even manganese peroxide, when mixed with caustic soda,
is capable of absorbing the oxygen of the air, because sodium manga-
nate is formed. Organic acids, when heated with caustic soda, give up
to it the elements of carbonic anhydride, forming sodium carbonate,
and separating that hydrocarbon group which exists, in combination
with carbonic anhydride, in the organic acid.

Thus sodium hydroxide, like the soluble alkalis in general, ranks
amongst the most active substances in the chemical sense of the term ;
but few substances are capable of resisting it. Even the rocky siliceous
substances, as we shall see further on, are transformed by it, forming
vitreous slags with it, at all events on fusion. Sodium hydroxide, as a
typical example of the basic hydrates, in distinction from many other
basic oxides, easily forms acid salts with acids (for instance, NaHSO4,
NaHCO3), and does not form any basic salts at all ; whilst many
less energetic bases, such as the oxides of copper and lead, easily
form basic salts, but acid salts only with difficulty. This capability
of forming acid salts, particula.rly with polybasic acids, may be ex-
plained by the energetic character of the basic properties of sodium
hydroxide, and the small development of these properties in the
bases which easily form basic salts. These latter bases are even
capable of combining with such bases as sodium hydroxide and
ammonium hydroxide. An energetic base is capable of retaining a
considerable quantity of acid, which a slightly energetic base would
not have the power of doing. Certain feeble bases, particularly inter-
mediate ones (like alumina) are not at all capable of retaining such feeble
acids as carbonic, or, if they form compounds with them, they are very
unstable and basic. The formation of acid salts with such acids as
carbonic, oxalic, sulphuric, phosphoric, <fcc., which contain two or
more atoms of hydrogen, capable of being replaced by metals, is
explained by the fact that the normal salt represents the substi-
tution of sodium for all the atoms of hydrogen. Such monobasic
acids as nitric, hydrochloric, «fec., do not form any stable acid salts
(although they form unstable compounds of the normal salt with the

SODIUM 525

acid), because they contain only one atom of hydrogen capable of
being replaced by the metal. Also, as will be shown in the subse-
quent chapters, sodium belongs to the univalent metals, exchangeable
for hydrogen atom for atom ; amongst metals sodium may, like
chlorine amongst the non-metals, serve as a representative of the
univalent properties. Most of the other elements which are not
capable of forming acid salts are bivalent, from whence it may
be understood that in a bibasic acid — for instance, carbonic, H2CO3,
or sulphuric, H.2SO4 — the hydrogen may be exchanged, atom for atom,
for sodium, and yield an acid salt by means of the first substitution
and a normal salt by means of the second — for instance, NaHSO4,
and Na.2SO4, whilst such bivalent metals as calcium or barium do
not form acid salts because one of their atoms at once takes the place
of both hydrogen atoms, forming, for instance, CaCO3 and CaSO4.
It may be expected, from what has been mentioned above, that
bivalent metals easily form acid salts with acids containing more than
two atoms of hydrogen— for instance, with tribasic acids, such as
phosphoric acid, H3PO4 — and actually such salts do exist ; but all
such relations are complicated by the fact that the character of the
base very often changes and becomes weakened with the increase of
valency and the change of atomic weight, and feebler bases (for
instance, silver oxide), although corresponding with univalent metals,
do not form acid salts, and the feeblest bases (for instance, CuO, PbO)
easily form basic salts, and, notwithstanding their valency, do not form
any acid salts which are in any degree stable — for instance, which are
undecomposable by water. In addition, basic and acid salts ought to be
regarded rather as compounds similar to crystallo-hydrates, because
such acids as sulphuric form with sodium not only an acid and a
normal salt, as might be expected judging from the valency of
sodium, but in addition to these also salts containing a greater
quantity of acid. In sodium sesquicarbonate we saw an example of
such compounds. Taking all this into consideration, we ought to say
that the property of more or less easily forming acid salts more
readily accords with the energy of the base than with the valency,
and it is truest of all to state that the faculty of a base to form acid and
basic salts is its characteristic, just as the faculty of forming com-
pounds with hydrogen is a characteristic property of elements. In
this respect sodium hydroxide distinguishes itself by the facility with
which it forms acid salts and an absence of the faculty of forming
basic salts. This property is also shared with sodium by the bases
formed from the metals potassium and lithium. The metals lead and
copper do not form acid salts, but easily give basic salts. Barium,.

526 PEINCIPLES OF CHEMISTRY

calcium, and silver form acid and basic salts with difficulty, although
they easily form normal ones.

We have seen the transformation of common salt into sodium
sulphate, of this latter into sodium carbonate, and of sodium carbonate
into caustic soda. Lavoisier still regarded sodium hydroxide as an
element, because he was unacquainted with its decomposition with the
elimination of oxygen and its formation by means of metallic sodium,
which separates the hydrogen from water, forming caustic soda. The
preparation of metallic sodium was one of the greatest discoveries in
chemistry, not only because through it the conception of elements became
broader and more regular, but especially because in sodium chemical
properties were observed which were but feebly shown in the other
metals more familiarly known. This discovery was made in 1807 by
the English chemist Davy by means of the galvanic current. By con-
necting with the positive pole (of copper or charcoal) a piece of moist
(in order to obtain electrical conductivity) caustic soda, and boring a hole
in it filled with mercury connected with the negative pole of a strong
Yolta's pile, Davy observed that on passing the current, a peculiar
metal dissolved in the mercury, less volatile than mercury, and capable
•of decomposing water, again forming caustic soda. In this way (by
.analysis and synthesis) Davy demonstrated the complexity of alkalis,
which up to that time had been regarded as undecomposable substances.
•On being decomposed by the galvanic current, caustic soda disengages
(hydrogen and sodium at the negative pole and oxygen at the positive
pole. Davy showed that the metal formed volatilises at a red heat,
which is the most important physical property for extracting sodium,
'because all further methods are founded on the volatility of sodium.
Besides this Davy observed that sodium easily oxidises, that its
^vapours take fire in air, and the latter circumstance was for a long time
.an obstacle to the easy preparation of this metal. The properties of
sodium were later on more thoroughly investigated by Gay-Lussac and
Thenard, who furnished easier means of obtaining sodium, and observed
'that metallic iron at a high temperature was capable of reducing sodium
from caustic soda.36 Brunner latterly discovered that not only iron,

36 Deville supposes that such a decomposition of sodium hydroxide by metallic iron
depends solely on the dissociation of the alkali at a white heat into sodium, hydrogen,
and oxygen. Here the part played by the iron is only that it retains the oxygen formed,
otherwise the decomposed elements would again reunite upon cooling, as in other cases
of dissociation. If it be supposed that the temperature at the commencement of the dis-
MK-iiition of the iron oxides is higher than that of sodium oxide, then the decomposition
may be explained by the hypothesis of Deville. Deville demonstrates his views by the
: following experiment : — An iron bottle, filled with iron shavings, was heated in such a
;way that the upper part became red hot, the lower part remaining cooler; sodium

SODIT.M 527

but also charcoal, has this property, although hydrogen does not reduce
sodium.37 But still the methods of extracting sodium were very
troublesome, and consequently sodium was a great rarity. The principal
obstacle to its production was that they endeavoured to condense the
•easily-oxidising vapours of sodium in vacuum in complicated apparatus.
For this reason, when Donny and Maresca, having thoroughly studied
the matter, constructed a specially simple condenser, the production of
sodium was much facilitated. Furthermore, in practice the most im-
portant epoch in the history of the production of sodium is comprised in
the investigation of Sainte-Claire Deville, who avoided the complex
methods in vogue up to that time, and furnished those simple means by
which the production of sodium is now rendered feasible in chemical
works.

For the production of sodium according to the method of Deville, a
mixture of sodium carbonate (7 parts) free from water, charcoal (two
parts), and lime or chalk (7 parts) is heated. This latter ingredient is
•only added in order that the sodium carbonate, on fusing, shall not
separate from the charcoal.38 The chalk on being heated loses carbonic
-anhydride, leaving infusible lime, which is permeated by the sodium
carbonate and forms a thick mass, in which the charcoal is situated, in
immediate contact with the sodium carbonate. That is to say, the lime
only serves as a mechanical admixture. When the charcoal is heated

hydroxide was introduced into the upper part. The decomposition was then effected —
that is, sodium vapours were produced (this experiment was done really with potassium
hydroxide). On unfastening the bottle it was found that the iron in the upper part
was not oxidised, but only that in the lower part. This may be explained by the decom-
position of the alkali into sodium, hydrogen, and oxygen taking place in the upper part,
whilst the iron in the lower part absorbed the oxygen set free. If the whole bottle be
subjected to the same moderate heat as the lower- extremity, no metallic vapours are
formed. Then, according to the hypothesis, the temperature would be insufficient for
the dissociation of the sodium hydroxide.

57 It has been previously remarked (Chapter II. Note 9) that Beketoff showed
the displacement of sodium by hydrogen, not from sodium hydroxide but from the oxide
Na2O ; then, however, only one half is displaced, with the formation of NaHO.

58 In latter times in England, where the preparation of sodium is at present carried
•on on a large commercial scale (from 1860 to 1870 it was only manufactured in a few
works in France), they have begun to add to Deville's mixture more iron, or iron oxide,
which, with the charcoal, gives metallic and carburetted iron, which still further
facilitates the decomposition. At present a kilogram of sodium may be purchased
for about the same sum (2/-) as a gram cost thirty years ago. The industrial prepa-
ration of sodium in large quantities ought not only to influence the extraction of such
metals as aluminium (metal of clay and alum), but also many other branches of industry.
The method of preparation by means of the action of the galvanic current, proposed and
tried many tunes, still requires further improvement. Deville overcame the practical
difficulties connected with his process, which is that in use at present, and is distinguished
by its simplicity and cheapness; but so far industry has but scantily availed itself of the
possibility of obtaining cheap sodium.

528

PRINCIPLES OF CHEMISTRY

with the sodium carbonate, at a white heat, carbonic oxide and vapours
of sodium are disengaged, according to the equation :

On cooling the vapours and gases disengaged, the vapours condense
into molten metal (in this form sodium does not easily oxidise, whilst in
vapour it burns) and the carbonic oxide remains as gas.

Here are the simple means by which metallic sodium may be obtained.
An iron tube, about a metre long and a decimetre in diameter, is made
out of boiler plate. The pipe is cemented into a furnace having a strong

FIG. 71.— Manufacture of sodium by Deville's process,
soda, charcoal, and chalk.

A C, iron tube containing a mixture of
B, condenser.

draught, capable of giving a high temperature, and the tube is charged
with the mixture required for the preparation of sodium. One end of the
tube is closed with a cast-iron stopper A with clay luting, and the other
with the cast-iron stopper C provided with an aperture. On heating, first
of all the moisture contained in the various substances is given off, then
carbonic anhydride and the products of the dry distillation of the charcoal,
then the latter begins to act on the sodium carbonate, and carbonic oxide
and vapours of sodium appear. It is easy to observe the appearance of
the latter, because on issuing from the aperture in the stopper C they
take fire spontaneously and burn with a very bright yellow flame. The
pipe of the condenser is then introduced into the aperture C, compelling
in this way the vapours and gases formed to pass through the condenser
B. This condenser consists of two square cast-iron trays, A and A',
Fig. 72, with wide edges firmly screwed together. Between these two
trays there is a space in which the condensation of the vapours of
sodium is accomplished, because the thin metallic walls of the condenser

SODIUM 529

are cooled by the air but remain sufficiently heated to preserve the
sodium in a liquid state, and so it does not choke the apparatus, but con-
tinually flows from it. The vapours of sodium, condensing in the cooler,
flow in the shape of liquid metal into a vessel
containing some non- volatile naphtha or hydro-
carbon. This is used in order to prevent the
sodium oxidising as it issues from the conden-
ser at a somewhat high temperature. During
the operation it is necessary (with an iron
rod) to occasionally clear the pipe which con-
ducts the vapours to the condenser, because it HI^HB^^

becomes choked up with solid compounds FIG. 72.— Donny and Maresca's

, . sodium condenser, oonsist-

formed by the sodium. In order to obtain ing of two cast-iron plates

, . f !•.•.• v j.«i screwed together.

sodium of a pure quality it is necessary to distil

it once more, which may even be done in porcelain retorts, but the dis-
tillation must be conducted in a stream of some gas on which sodium
does not act, for instance in a stream of nitrogen ; carbonic anhydride
is not applicable, because sodium partially decomposes it, absorbing the
oxygen from it.

Pure sodium is a lustrous metal, white as silver, soft as wax ; it be-
comes brittle in the cold. In ordinary moist air it quickly tarnishes and
becomes covered with a film of hydroxide, NaHO, formed at the expense
of the water in the air. In completely dry air sodium retains its lustre
for an indefinite time. Its density at the ordinary temperature is
equal to 0'9S, so that it is lighter than water ; it fuses very easily at a
temperature of 95°, and distils at a bright red heat (about 900°). It
forms alloys with most metals, combining with them, heat being some-
times evolved and sometimes absorbed. Thus if sodium (having a clean
surface) be thrown into mercury, especially when heated, there is a flash,
and such a considerable amount of heat is evolved that part of the mer-
cury is transformed into vapour.39 Compounds or solutions of sodium

59 By dissolving sodium amalgams in water and acids, and deducting the heat of
solution of th« sodium, Berthelot found that for each atom of the sodium in amalgams
containing a large amount of mercury (more than 90 p.c.) the amount of heat evolved
increases with ihe quantity of sodium, and also that when a composition is reached which
approaches NaHg5 the heat evolved decreases. In the formation of the latter compound,
about 18500 calories are evolved; when NaHg5 is formed, about 14000; and for NaHg
about 10000 calories. Kraut and Popp regarded the definite crystalline amalgam as
having the composition of NaHg6, but at the present time, in accordance with Grimaldi's
results, it is thought to be NaHg5. A similar amalgam is very easily obtained if a
8 p.c. amalgam be left several days in a solution of sodium hydroxide until a crystalline
mass is formed, from which the mercury may be removed by strongly pressing in
chamois leather. This amalgam with a solution of potassium hydroxide forms a
potassium amalgam, KHg10 (Crookewitt, Grimaldi). It may be mentioned here that

VOL. I. M M

530 PRINCIPLES OF CHEMISTRY

in mercury, or amalgams of sodium, even when containing 2 parts of
sodium to 100 parts of mercury, are solids. Only those amalgams
which are the very poorest in sodium are liquid. Such alloys of sodium
with mercury are often used instead of sodium in chemical investiga-
tions, because, in combination with mercury sodium is not easily acted
on by air, and is a mass heavier than water, and therefore more conve-
nient to handle, whilst at the same time it retains the principal pro-
perties of sodium.40

It is easy to form an alloy of mercury and sodium having a crystal-
line structure, and a definite atomic composition NaHg5. The alloy of
sodium with hydrogen or sodium hydride, Na2H, which has the external
appearance of a metal,41 is a most instructive example of the character-
istics of alloys. At the ordinary temperature sodium does not absorb
hydrogen, but from 300° to 421° the absorption takes place at the
ordinary pressure (and at an increased pressure even at higher tem-
peratures), as shown by Troost and Hautefeuille (1874). One volume
of sodium absorbs as much as 238 vols. of hydrogen. The metal
increases in volume, and when once formed the alloy can be preserved
for some time without change at the ordinary temperature. The
appearance of sodium hydride resembles that of sodium itself ; it is
as soft as this latter, when heated it becomes brittle, and decomposes
above 300°, evolving hydrogen. In this decomposition all the pheno-
mena of dissociation are very clearly shown — that is, the hydrogen gas
evolved has a definite tension.42 which corresponds with each definite
temperature. This confirms the fact, that the formation of substances
capable of dissociation is only accomplished within the limits of the
dissociation. Sodium hydride melts more easily than sodium itself, and

the latent heat of fusion (of atomic quantities) of Hg = 360 (Personne), Na = 730
(Joannis), and K = 610 calories (Joannis).

40 Alloys are so similar to solutions (exhibiting such complete parallelism in proper-
ties) that they are contained in the same class of so-called indefinite compounds (Chap-
ter I.). But in alloys, as substances passing from the liquid to the solid state, it is
easier to discover the formation of definite chemical compounds, and therefore, in order
to arrive at a correct understanding of solutions, it is very important to study alloys.
In addition to this, they are of themselves of considerable interest. The combination of
feodium with hydrogen, where the change of physical properties is so evident and where
the conservation of chemical properties and easy dissociation are so apparent, ought in
this respect to supply much towards an understanding of alloys and also of solutions.
Alloys and solutions are homogeneous ; if the decomposition has commenced or increased
we may not perceive it, but the products of the splitting up of the alloy Na-2H are hetero-
geneous ; they are visible.

41 Potassium forms a similar compound, but lithium, under the same circumstaiu <-s,
does not.

42 Namely, the tension of dissociation of hydrogen, p, in millimetres of mercury, is : —

t = 330° 350° 380° 400° 420° 430°
p = 28 57 150 447 752 900

SODIUM 531

then does not undergo decomposition if it is in an atmosphere of hydrogen.
It oxidises easily in air, but not so easily as potassium hydride. The
•chemical reactions of sodium are retained in its hydride, and, if we may

x press it, they are even increased by the addition of hydrogen. At
all events, in. the properties of sodium hydride43 we see other properties
than in such hydrogen compounds as HC1, H2O, H3N, H4C, or even
in the gaseous metallic hydrides AsH3, TeHe. But platinum, palladium,
nickel, and iron, in absorbing hydrogen form compounds in which
hydrogen is in a similar state. In them, as in sodium hydride, the
hydrogen is compressed, absorbed, occluded (page 141).

The most important property of sodium is its power of easily decom-
posing water and evolving hydrogen from the majority of the hydrogen
compounds, and especially from all acid and hydrate compounds in which
hydroxyl ought to be recognised. This depends on the power of sodium
of combining with the elements which are in combination with the hydro-
gen. We already know that sodium disengages hydrogen, not only from
water, hydrochloric acid,44 and all other acids, but also from ammonia,
although it does not displace hydrogen from the hydrocarbons.45

43 In general, during the formation of alloys the volumes change very slightly, and
therefore from the volume of Na2H some idea may be formed of the volume of
hydrogen in a solid or liquid state. Archimedes even concluded that there was gold
in an alloy of copper and gold by reason of its volume and density. From the fact that
the density of Na2H is equal to 0'959, it may be seen that the volume of 47 grams (the
gram molecule) of this compound = 49'0 c.c. The volume of 46 grams of sodium con-
tained in the Na,2H is equal (the density in the same conditions 0'97) to 47'4 c.c. There-
fore the volume of 1 gram of hydrogen in Na2H is equal to 1*6 c.c., and consequently the
density of metallic hydrogen, or the weight of 1 c.c., approaches 0'6 gram. This density
is also proper to the hydrogen alloyed with potassium and palladium. Judging from the
•scanty information which is at present available, liquid hydrogen near its absolute
boiling-point (Chapter II.) has a far less density.

It ought to be mentioned that sodium hydride, according to the usual equivalency of
H., with O, corresponds, not with the oxide Na^O.but with the suboxide of sodium, Na4O,
and if we judge the atomicity of elements by hydrogen compounds) sodium ought to be
counted us srmivalent. According to the law of substitution, Na ought to be taken as
univalent in all its ordinary combinations : Na^O, NaCl, NaHO, NaHSO4, &c. Therefore
sodium hydride belongs to the series Na.2X, and not NaX.

44 H. A. Schmidt remarked that completely dry hydrogen chloride is decomposed
with great difficulty by sodium, although the decomposition proceeds easily with potas-
sium and with sodium in moist hydrogen chloride. Wanklyn also remarked that sodium
burns with great difficulty in dry chlorine. Probably in relation to this there is a corre-
spondence with other phenomena observed by Dixon, who found that completely dry
carbonic oxide does not explode with oxygen on passing an electric spark.

4& As sodium does not di^phn T hydrogen from the hydrocarbons, it may be preserved

in liquid hydrocarbons. Naphtha is generally used for this purpose, as it consists of a

mixture of various liquid hydrocarbons. However, in naphtha sodium usually becomes

1 with a crust composed of matter produced by the action of the sodium on certain of

tin- substances contained in the mixture composing naphtha. In order that sodium may

• i its lustre in naphtha, secondary octyl alcohol is added. (This alcohol is obtained by

distilling castor oil with caustic potash.) Potassium and sodium keep well in a mixture

•of pure benzene and naphthalene.

M M 2

532 PRINCIPLES OF CHEMISTRY

Sodium burns both in chlorine and in oxygen, evolving much heat.
These properties of sodium are closely connected with its power of
taking up oxygen, chlorine, and similar elements from most of their
compounds. Just as sodium removes the oxygen from the oxides of
nitrogen and from carbonic anhydride, so also does it decompose the
majority of oxides at definite temperatures. Here the essence of the
matter is the same as in the decomposition of water. Thus, for instance,
when acting on magnesium chloride, the sodium displaces the magne-
sium, or when acting on aluminium chloride sodium displaces metallic
aluminium. Sulphur, phosphorus, arsenic, and a whole series of other
elements, also combine with sodium.46

With oxygen sodium unites in three degrees of combination, forming
a suboxide, Na4O, an oxide, Na2O, and a peroxide, NaO. They are
thus termed because Na2O is a basic oxide (with water it forms a basic
hydroxide), but Na4O and NaO do not form corresponding saline
compounds. The suboxide is a grey inflammable substance which
easily decomposes water, disengaging hydrogen ; it is formed by the
slow oxidation of sodium at the ordinary temperature. The peroxide
is a greenish yellow substance, fusing at a bright red heat ; it is produced
by burning sodium in an excess of oxygen, and it yields oxygen when
treated with water :

Suboxide: ]STa4O-f 3H2O=

Oxide: Na2O + H26=2NaH048
Peroxide : NaO + H20==2NaHO + O

46 If sodium does not directly displace the hydrogen in hydrocarbons, still by indirect
means compounds may be obtained which contain sodium and hydrocarbon groups. Some
of these compounds have been produced, although not in a pure state. Thus, for instance
zinc ethyl Zn(C2H5)2, when treated with sodium, loses zinc and forms sodium ethyl, C2H5Na,
but this decomposition is not complete, and the compound formed cannot be separated by
distillation from the remaining zinc ethyl. In this combination the energy of the sodium
clearly appears, for which reason it reacts with substances containing haloids, oxygen,
&c., and directly absorbs carbonic anhydride, forming a salt of a carboxylic acid
(propionic).

47 A compound, Na2Cl, which corresponds with the suboxide, is evidently formed
when a galvanic current is passed through fused common salt ; the sodium liberated dis-
solves in the common salt, and does not separate from the compound either on cool-
ing or on treatment with mercury. It is therefore supposed to be Na2Cl ; the
more so as the mass obtained gives hydrogen when treated with water : Na-jCl + H2O
= H + NaHO + NaCl, that is, it acts like suboxide of sodium. If NaoCl really exists as a
salt, then the corresponding base Na4O, according to the example of other bases of the
composition M4O, ought to be called a quaternary oxide. According to certain evidence,
a suboxide is formed when thin sheets or fine drops of sodium slowly oxidise in moist
air.

48 According to observations easily made, sodium when fused in air oxidises but does
not burn, the combustion only commencing with the formation of vapour — that is, when con-
siderably heated. Davy and Karsten obtained the oxides of potassium, K2O, and of sodium.

SODH'M 533

All three oxides form sodium hydroxide with water, but only the
oxide, Na2O, is directly transformed into a hydrate. The other oxides
either liberate hydrogen or oxygen ; they also present a similar dis-
tinction with reference to many other agents. Thus carbonic anhydride
combines directly with the oxide Na20, which when heated in the gas
burns, forming sodium carbonate, whilst in this case the peroxide 49 also
yields oxygen. When treated with acids, sodium and all its oxides only
form the salts corresponding with sodium oxide — that is, of the formula or
type NaX. Thus the oxide of sodium Na2O is the only salt-forming
oxide of this metal, as water is in the case of hydrogen. Although the
peroxide H2O2 is derived from hydrogen, yet there are no correspond-
ing salts known, and if they are formed they are probably as unstable as
hydrogen peroxide itself. Although carbon forms carbonic oxide, CO,
still it has only one salt-forming oxide — carbonic anhydride, CO2.
Nitrogen and chlorine both give several salt-forming oxides and types of
salts. But of the oxides of nitrogen, NO and NO2 do not form salts,
as do N2O3, N204, and N2O5, but N2O4 does not form special salts,
and N205 corresponds with the highest form of the saline compounds
of nitrogen. Such distinctions between the elements, according to
their power of giving one or several saline forms, is a radical property
•of no less importance than the basic or acid properties of the oxides
produced. Sodium as a typical metal does not form any acid oxides,

, by heating the metals with their hydroxides, whence NaHO + Na = Na^O + H, but
N. N. Beketoff failed to obtain oxides by this means. He prepared them by directly
igniting the metals in dry air, and afterwards heating with the metal in order to destroy
any peroxide. The oxide Na.2O produced, when heated in. an atmosphere of hydrogen,
gave a, mixture of sodium and its hydroxide: Na^O + H = NaHO + Na (see Chapter II.
Note l)j. If both the observations mentioned are accurate, then the reaction is reversible.
Sodium oxide ought to be formed during the decomposition of sodium carbonate by oxide
of iron (see Note 26), and during the decomposition of sodium nitrite. According to
Karsten, its specific gravity is 2'8, according to Beketoff 2'3. The difficulty in obtaining
it is owing to an excess of sodium forming the suboxide, and an excess of oxygen the
peroxide. The grey colour peculiar to the suboxide and oxide perhaps shows that they
contain metallic sodium. In addition to this, in the presence of water it may contain
.sodium hydride.

49 Of the oxides of sodium, that easiest to form is the peroxide NaO or Na,2O2 ; this
is obtained when sodium is burnt in an excess of oxygen. When heated with iodine
vapour, it loses oxygen: Na^Oa + I^NaaOIa + O. The compound Na^OIj is akin to
the compound Cu2OCl3 obtained by oxidising CuCl. This reaction is one of the few
where iodine directly displaces oxygen. The substance Na.2OI2 is soluble in water, and
when acidified gives free iodine and a sodium salt. Carbonic oxide is absorbed by heated

sodium peroxide with formation of sodium carbonate: NaoCO5=Na.2O2 + CO, whilst
carbonic anhydride liberates oxygen from it. With nitrous oxide it reacts thus : Na2O2

+ 2N2O = 2NaNO2 + N2; with nitric oxide it combines directly, forming a nitrite, NaO

+ NO = NaNO2. Sodium peroxide, when treated with water, does not give hydrogen
peroxide, because the latter in the presence of the alkali formed (Na.2O2 + 2H2O = 2NaHO

4- H3O2) decomposes into water and oxygen.

534 PRINCIPLES OF CHEMISTRY

whilst chlorine, being a typical non-metal, does not form bases with
oxygen. Therefore sodium as an element may be thus characterised :
it forms one very stable salt-forming oxide, Na2O, having powerful
basic properties, its salts are of the general formula, NaX, therefore
in saline compounds it is, like hydrogen, a univalent element.50

On comparing sodium and its analogues, which will be described later
with other metallic elements, it will be seen that those properties,
together with the relative lightness of the metal itself and its com-
pounds, and the magnitude of the atomic weight of sodium, comprise
the most essential properties of this element, clearly distinguishing it
from others, and enabling us easily to recognise its analogues.

50 By heating sodium in dry ammonia, Gay-Lussac and Tlu'nard obtained an olive-
green, easily-fusible mass, sodamide, NH2Na, hydrogen being separated. This sub-
stance with water forms sodium hydroxide and ammonia; with carbonic oxide, CO, it
forms sodium cyanide, NaCN, and water H2O ; and with dry hydrogen chloride it forms
sodium and ammonium chlorides. These and other reactions of sodamide show that the
metal in it preserves its energetic properties in reaction, and that this compound of
sodium is but little more stable than the corresponding chlorine amide, although it does
not show its property of spontaneous decomposition, which is evident from the difference
between the properties of metallic sodium and gaseous chlorine. When heated, sodam-
ide, NH2Na, only partially decomposes, with evolution of hydrogen, the principal part of
it giving ammonia and sodium nitride, Na.-^N, according to the equation 8NH2Na = 2NH-
+ NNa5. The latter is an almost black powdery mass, decomposed by water into ammonia
and sodium hydroxide.

585

CHAPTER XIII

POTASSIUM, RUBIDIUM, CESIUM, AND LITHIUM. SPECTRUM ANALYSIS

JUST as the series of halogens, fluorine, bromine, and iodine, correspond
with the chlorine contained in common salt, so there also exists a cor-
responding series of elements : lithium, Li =7, potassium, K=39,
rubidium, Rb=85, and cesium, Cs=133, which are analogous to the
sodium in common salt. These elements bear as great a resemblance
to sodium, Na=23, as fluorine, F=19, bromine, Br=80, and iodine,
1=127, have to chlorine, Cl=35'5. Indeed, in a free state, these
elements are, like sodium, soft metals, which rapidly oxidise in moist
air, and decompose water at the ordinary temperature, forming soluble
hydroxides, having clearly -defined basic properties, and the composi-
tion RHO, like that of caustic soda. The resemblance between these
metals is sometimes seen with striking clearness, especially in com-
pounds such as salts. The corresponding salts of nitric, sulphuric,
carbonic, and nearly all acids with these metals have many points in
common. The metals which resemble sodium so much in their reac-
tions are termed the metals of the alkalis.

Among the metals of the alkalis, the most widely distributed in
nature, after sodium, is potassium. Like sodium, it does not appear
either in a free state or as oxide or hydroxide, but in the form of salts,
which present much in common with the salts of sodium in the manner
of their occurrence. The compounds of potassium and sodium in the
earth's crust occur as mineral compounds of silica. With silica, SiO2,
potassium oxide, like sodium oxide, forms saline mineral substances
like glass. If different other oxides, such as lime, CaO, and alumina,
A12O3, combine with these compounds, there is formed glass, or a
glassy stony mass, which is distinguished by its great stability. It is
such complex silicious compounds as these which contain potash
(potassium oxide), K20, or soda (sodium oxide), Na2O, and some-
times both together, silica, SiO2, lime, CaO, alumina, A12O3, and other
oxides, that form the chief mass of rocks, out of which, judging by
the distribution of the strata, the chief mass of the accessible crust
(envelope) of the earth is made up. The primary rocks, like granite,

536 PRINCIPLES OF CHEMISTRY

porphyry, ifcc.,1 are formed of such crystalline silicious rocks as these.
The oxides entering into the composition of these rocks do not form a
homogeneous amorphous mass like glass, but are distributed in a series
of peculiar, and in the majority of cases crystalline, compounds, into
which the primary rocks may be divided. Thus, as has been already
mentioned, granite contains felspar, quartz, and mica. Potash and
soda, (fee., are generally found in these component parts of rocks. Thus
a felspar (orthoclase) in granite contains from 8 to 15 percent, of potas-
sium, whilst another variety (plagioclase) which also occurs in granite
contains 1-2 to 6 per cent, of potassium, and 6 to 12 per cent, of sodium.
The mica in granite contains 3 to 10 per cent, of potassium. As
has already been mentioned, and will be further explained, the friable,
crumbling, and stratified formations which in our times cover a
large part of the earth's surface have been formed from these primary
rocks by the action of the atmosphere, and of water containing car-
bonic acid. It is evident that in the formation of these friable forma-
tions from the primary rocks by the action of water, the compounds of
potassium, as well as the compounds of sodium, must have been dis-
solved by the water (as they are soluble in water), and, therefore, the
compounds of potassium must be accumulated together with those of
sodium in sea water. And, indeed, compounds of potassium are always,
as we have already pointed out (Chaps. I. and X.) found in sea water.
This forms one of the sources from which they are extracted. After
the evaporation of sea water, there remains a mother liquor, which
contains potassium chloride and a large proportion of magnesium
chloride. On cooling this solution crystals separate out, which contain
chlorides of magnesium and potassium. A double salt of this kind,
called carnallite, KMgCl3,6H2O, occurs at Stassfurt. This carnallite2
is now employed as a material for the extraction of potassium chloride,
and of all the compounds of this element.3 Besides which, potassium

1 The origin of the primary rocks has been mentioned in Chap. X. Note 2.

2 Carnallite belongs to the number of double salts which are directly decomposed by
water, and it only crystallises from solutions which contain an excess of magnesium
chloride. It may be prepared artificially by mixing strong solutions of potassium and
magnesium chlorides, when colourless crystals of sp. gr. 1'60 separate, whilst the Stass-
furt salt is usually of a reddish tint, owing to traces of iron. At the ordinary tempera-
ture sixty-five parts of carnallite are soluble in one-hundred parts of water in the pre-
sence of an excess of the salt. It deliquesces in the air, forming a solution of magne-
sium chloride and leaving potassium chloride.

3 The method of separating sodium chloride from potassium chloride has been
described on p. 72. On evaporation of a mixture of the saturated solutions, sodium chloride
separates ; and then, on cooling, potassium chloride separates, owing to the difference of
the variation of their solubilities with the temperature. The following are the most
trustworthy figures for the solubility of potassium chloride in one hundred parts of
water (for sodium chloride, see Chap. X. JJote 18) : —

10° 20J 40J 60° 100°

32 85 40 40 57

POTASS i TM. Krninit'M. CAMS i r:\r, AND LITIIII \M 537

•chloride itself is sometimes found at Stassfurt as sylvine. By a
method of double saline decomposition, the chloride of potassium
may be converted into all the other potassium salts,4 some of
Avhich are of practical use. The potassium salts have, however,
their greatest importance as an indispensable component of the food
of plants/5

When mixed with solutions of other salts the solubility of potassium chloride naturally
varies, but not to any great extent. The specific gravity of the solid salt is 1'99 — that is,
less than that of sodium chloride. All the salts of sodium are specifically heavier than
the corresponding salts of potassium, as are also their solutions for equal percentage
compositions. If the specific gravity of water at 4° = 10000, then at 15° the specific
gravity of a solution of p p.c. potassium chloride = 9992 + 63'29j? + 0'226 p2, and therefore
for 10 p.c. = 10647, 20 p.c. = 11348, &c.

Potassium chloride combines with iodine trichloride to form a compound KC1 + IC15 =
KIC14, of a yellow colour, which is fusible, loses the iodine trichloride at a red heat, and
.gives potassium iodate and hydrochloric acid with water. It is not only formed by
direct combination, but also by many other methods ; for instance, by passing chlorine
into a solution of potassium iodide so long as the gas is absorbed KI + 2C12 = KCl,ICl.-(.
Potassium iodide, when treated with potassium chlorate and strong hydrochloric acid,
also gives this compound ; another method for the formation of which is shown by the
equation KC1O5 + 1 + OHC1 = KC1,IC13 + 3C1 + 3H2O. This is a kind of a salt correspond-
ing with KIOo (unknown) in which the oxygen is replaced by chlorine. If atomicity is
taken as a starting point in the study of chemical compounds, and if the elements are
considered as having a constant atomicity (number of bonds) — that is, if K, Cl, and I
be counted as univalent elements — then it is impossible to explain the formation of
such a compound, because, according to this view, univalent elements are only able to
form dual compounds with each other ; for example, KC1, C1I, KI, &c., whilst here they
are grouped together in the molecule KIC14.

4 It is possible to directly extract the compounds of potassium from the primary
rocks which are, especially in some localities, so widely distributed over the earth's
surface. From a chemical point of view this problem presents no difficulty; for in-
stance, by fusing powdered orthoclase with lime and fluor spar (Ward's method) and
then extracting the alkali with water (in the fusion the silica gives an insoluble com-
pound with lime), or by treating the orthoclase with hydrofluoric acid (in which case
silicon fluoride is evolved as a gas) it is possible to transfer the alkali of the orthoclase
to an aqueous solution, and to separate it in this manner from the other insoluble oxides.
Thus after treating with hydrofluoric acid, many fluorides are obtained in solution,
chiefly the fluorides of aluminium and potassium. If the solution be evaporated and
sulphuric acid then added, hydrofluoric acid is evolved and the metals are obtained as
sulphates. On adding ammonia to the solution of these salts, the aluminium is precipi-
tated as hydroxide, and the salts of ammonia and potassium remain in solution. The
ammonia salt is then decomposed by igniting, and the potassium sulphate is obtained
alone. However, as yet there is no profit in, nor necessity for, having recourse to this treat-
ment, as there still exist abundant sources for the extraction of potassium compounds
by cheaper methods. Furthermore, the salts of potassium are now in the majority of
chemical reactions replaced by salts of sodium, especially since the preparation of sodium
carbonate has been facilitated by Leblanc's method. The replacement of potassium
compounds by sodium compounds not only presents the advantage that the salts of
sodium are in general cheaper than those of potassium, but also that a less quantity of a
sodium salt is needed for a given reaction than of a potassium salt, because the combin-
ing weight of sodium (23) is less than that of potassium (39).

5 It has been shown by direct experiment on the cultivation of plants in artificial
soils and in solutions that under conditions (physical, chemical, and physiological) other-
wise identical plants are able to thrive and become fully developed in the entire absence
of sodium salts, but that their development is impossible without potassium salts.

'I he primary rucks contain an almo-t euual proportion of potassium
and -odium. l>ut in sea water the compounds ot the latter metal pre
iloininate. It may 1>e a-ked, what hecame of the compounds of potas-
sium in t!n- di-inte^ration of tin- primary rock>. if so -mall a quantity
\vi-nl -M tin- sea water ; They remained \\ it li the other products of the
docomjMoitii.ii < if the jirimarv rock-. \Vhen granite or any other
similar rock formation is di-int Dialed, tin-re are formed, lie.-ide.- the
soluble • distance- — -and ;md tinel v-divided

clav, co . .\ '. alumina, and silica. 'I his clav i- carrie<l awav

li\" th<- water, and i - then depo-u«'d in - ' ra t a . It. and esj)eciallv
it.- a i ii i\ ui'e \\iih v<'u ' ••; i ;;:i!!-. 1'e'ain coi n pounds of jiota»ium

in a u'l'c tlian tho^e of sodium. Tlii- ha- hccii jiro\'ed.

•' ;' ' .* ;'. If a dilute solution of a |)ota--ium comjiound lie
'_''! common nmuld u-ed for Lfrowinu' jilant^. and coii-
: •• -. and the remains oi \t-e;etah!e olecoinjtosit ion, it \\'ill l>e
t • nd to have retained a :-omewlial consideralile percentage of the
mpoMiuK. It' a -:;!' ' >t' potassium \»- taken, then dunni;'
the ti 1 1 ;vi I ion an e pu \ a lent « juant it v of a salt of calcium which is a No
found, as a rule, in soils i- -el free. Such a process of filtration
through 1'melv-divided earth\' substances pi-oceods in nature, and the
compound- of poia-siiim are e\'erv\\ here retained 1»\ the frialile can h
in con-idei'ahlo (juantitv. 'I hi- explains the jire-ence ot -o small an
amount of poia--ium salts in the water of rivers, lakes, streams, and
oceans, w lie re the 1 1 me and -oda ha \ e accumulated. 'I IM- compounds of
potas-ium retained li\- i he frialile ma-s of the earth are ahsorUed as an
aijueous solution l.v the root- of />/>i//f*. Plants, as e\erv one kno\\'s,
\\h<-n liui'iit l'-a\e an a-h. aiid this ash. l)C.sides various other siihst ancos,
without e.xcepiiii ins coi 1 1 j M )ii i ids of potas-ium. Many land

plants contain a very small amount of -odium compounds.'1 whil-t
j iota -sium and :' compounds ncciii1 in all kinds ot \'ei;'etah|e a-h.
AIIIOIIL;' the 'j.'i.i ra 1 1 \ ciilti\"ated plants, u'l'a-s. pot a toe.-, the turnip,
and liiickwheai are part icn larl\' rich in pota-.-mm compounds. 'I he
a-h of plant-, and e-peciallv of herhaceous plants. lmrk\\heat Straw,
iintlowcr and potato lea\e.- are u-ed in jiractice for the extraction of
pot a --i II in coi 1 1 poll I ids. 'I'll ere is Ho dold »t that pot a;-.-] ll In <icc ui's III the
plant-: them el\e- in the t'oi'in • >]' complex compounds, and otten as sails
of organic arid-. In certain ca-e- Mich salts of potas-ium are even
ex t racier) from the juice of p|:i ni -. 'I'h ii . . or re] and or"//* for exam jile,
con i a in in their juice t lie arid oxalat e of potassium, (.V, II K< ),, which is

POTASS I CM, RUBIDIUM, CAESIUM, AND LITHIUM 539

employed for removing ink stains. Grape juice contains the so-called
cream of lartar, which is the acid tartrate of potassium, C4H.5KO6.T
This salt also separates as a sediment from wine. When the plants,
containing one or more of the salts of potassium, are burnt, then the
carbonaceous matter is oxidised, and in consequence the potassium is
obtained in the ash as carbonate, K2CO3, which is generally known
as i><>f<ixhes. Hence potashes occur ready prepared in the ash of
plants, rtnd therefore the ash of land plants is employed as a source
for the extraction of potassium compounds. Potassium carbonate is
extracted by lixiviating the ash with water.8 Potassium carbonate

7 As plants always contain mineral substances and cannot thrive in a sphere which
docs not contain them, especially which is free from the salts of the four basic oxides,
KaO, CaO, MgO, and Fe.2O5, and of the four acid oxides, CO2, N.2O5, PoO5, and SO3, the
question involuntarily arises as to what part these play in the development of plants.
With the present store of chemical data only one answer is possible to this question,
and it is still only a hypothesis. This answer was particularly clearly expressed by
Professor Gustavson of the Petroffsky Agricultural Academy. Starting from the fact
(Chap. XI. Note 55) that a small quantity of aluminium renders possible and facilitates
the reaction of bromine on hydrocarbons at the ordinary temperature, it is easy to arrive
at the conclusion, which is very probable and in accordance with many data respecting
the reactions of organic compounds, that the addition of mineral substances to organic
compounds lowers the temperature of reaction and in general facilitates chemical reac-
tions in plants, and thus aids the conversion of the most simple nourishing substances
into the complex component parts of the plant organism. The province of chemical
reactions proceeding in organic substances in the presence of a small quantity of mineral
substances lias as yet been but little investigated, although there are already several
disconnected data of this kind, and although a great deal is known concerning such reac-
tions among inorganic compounds. The essence of the matter may be expressed thus —
two substances, A and B, do not react on each other of their own accord, but the addi-
tion of a small quantity of a third particularly active substance, C, produces the reac-
tion of A on B, because A combines with C, forms AC, and B reacts on this new com-
pound, which has a different store of chemical energy, forming the compound AB or its
products, and setting C free again.

It may here be remarked that all the mineral substances necessary for plants (those
enumerated at the beginning of the note) are the highest saline compounds of their
elements, that they enter into the plants as salts, that the lower forms of oxidation of the
same elements (for instance, sulphites and phosphites) are harmful to plants (poisonous) y
and that strong solutions of the salts assimilated by plants (their osmotic pressure is
great and contracts the cells, as De Vries showed, see p. 822) not only do not enter into
the plants but kill them (poison them).

Besides which, it will be understood from the preceding paragraph that the salts of
potassium may become exhausted from the soil by long cultivation, and that there may
therefore be cases when the direct fertilisation by salts of potassium may be profitable.
But manure and animal excrements, ashes, and, in general, nearly all refuse which may
serve for fertilising the soil, contain a considerable quantity of potassium salts, and
therefore, as regards the natural salts of potassium (Stassfurt), and especially potassium
sulphate, if they often improve the crops, it is in all probability because they influence
the properties of the soil. The agriculturist cannot therefore be advised to add
potassium salts, without making special experiments showing the advantage of such a
fertiliser on a given kind of soil and plant.

8 The animal body also contains potassium compounds, which is natural, since animals
consume plants. For example, milk, and especially woman's milk, contains a somewhat

540 PRINCIPLES OF CHEMISTRY

may also be obtained from the chloride by a method similar to that by
which sodium carbonate is prepared from sodium chloride. There

• considerable quantity of potassium compounds. However, cow's milk contains but little
potassium salts. Sodium compounds generally predominate in animal bodies. The
excrement of animals, and especially of herbivorous animals, on the contrary, often con-
tains a large proportion of potassium salts. Thus sheep's dung is rich in them, and in
washing sheep's wool salts of potassium pass into the water.

The ash of tree stems, as the already dormant portion of the plant (Chap. VIII.
Note 1), contains little potash. For the extraction of potash, which was formerly (before
the discovery of the Stassfurt salt) carried on extensively in the east of Russia, the ash
of grasses, and the green portions of potatoes, buckwheat, &c., are taken and treated
with water (lixiviated), the solution is evaporated, and the residue ignited in order to
destroy the organic matter present in the extract. The residue thus obtained is com-
posed of raw potash. It is refined by a second dissolution in a small quantity of water,
because the potash itself is very soluble in water, whilst the impurities are sparingly
soluble. The solution thus obtained is again evaporated, and the residue ignited, and
this potash is then calle refined potash, pearlash. This method of treatment cannot
give chemically pure potassium carbonate. A certain amount of impurities remain.
To obtain chemically pure potassium carbonate, some other salt of potassium is gene-
rally taken and purified by crystallisation. Potassium carbonate crystallises with diffi-
culty, and it cannot therefore be purified by this means, whilst other salts, such as the
tartrate, acid carbonate, sulphate, or nitrate, &c., crystallise easily and may thus be
directly purified. The tartrate is most frequently taken, all the more as it is prepared
in large quantities for medicinal use under the name of cream of tartar. When
ignited without the access of air, it leaves a mixture of charcoal and potassium car-
bonate. The charcoal is then obtained in a finely-divided condition (this mixture is
called ' black flux,' and is sometimes used for reducing metals from their oxides, by the
aid of heat). A certain quantity of nitre is added to burn the charcoal formed by
heating the cream of tartar. Potassium carbonate thus prepared is further purified by
converting it into the acid salt, by passing a current of carbonic anhydride through
a strong solution. KHCO3 is then formed, which is less soluble than the normal salt
(as is the case with the corresponding sodium salts), and therefore crystals of the acid
salt separate from the solution on cooling. When ignited, they part with their water
and carbonic anhydride, and pure potassium carbonate remains behind. The physical
properties of potassium carbonate distinguish it with sufficient distinctness from sodium
carbonate ; it is obtained from solutions as a powdery white mass, having an alkaline
taste and reaction, and, as a rule, shows only traces of crystallisation. It also attracts
the moisture of the air with great energy. The crystals do not contain water, but
•absorb it from the air, deliquescing into a saturated solution. It melts at a red heat
(880°), and at a still higher temperature is even converted into vapour, as has been
observed at glass works where it is employed. It is very soluble. At the ordinary
temperature, water dissolves an equal weight of the salt. Crystals containing two
equivalents of water separate from such a saturated solution when strongly cooled.
There is no necessity to describe its reactions, because they are entirely analogous to
those of sodium carbonate. When manufactured sodium carbonate was but little
known, the consumption of potassium carbonate was very considerable, and even now
washing soda is frequently replaced for household purposes by 'ley' — i.e. an aqueous
solution obtained from ashes. It contains potassium carbonate, which acts like the
sodium salt in washing tissues, linen, &c.

A mixture of potassium and sodium carbonate fuses with much greater ease than the
separate salts, and a mixture of their solutions gives salts of a fine crystalline form — for
instance (Marguerite's salt), K2CO;;,6H2O,2Na2CO3,6H2O. Crystallisation also pro-
ceeds in other multiple proportions of K and Na (in the above case 1 : 2, but 1 : 1 and 1 : 3
are known), and always with 6 mol. H2O. This is evidently a combination by analogy, as
in alloys, solutions, &c.

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 541

is no difficulty in obtaining any salt of potassium — for example, the
sulphate,9 bromide, and iodide 10 — by the action of the corresponding

iPofautium fnil/iJnite, K.^SO.!, crystallises from its solutions in an anhydrous condi-
tion, in which respect it differs from the corresponding sodium salt, just as potassium car-
bonate differs from sodium carbonate. In general, it must be observed that the majority
of sodium salts combine more easily with water of crystallisation than the potassium
salts. The solubility of potassium sulphate does not show the same peculiarities as
sodium sulphate, because it does not combine with water of crystallisation ; at the
ordinary temperature 100 parts of water dissolve about 10 parts of the salt, at 0° 8'8'
parts, and at 100° about 2(5 parts. The acid sulphate, KHSO4, obtained easily by
heatiiiLT crystals of the normal salt with sulphuric acid, is frequently employed in
chemical practice. On heating the mixture of acid and salt, fumes of sulphuric acid are
at first given off ; when they cease to be evolved, the acid salt is contained in the residue..
At a higher temperature (of above 600°) the acid salt parts with all the acid contained in
it, the normal salt being re-formed. The definite composition of this acid salt, and the
ease with which it decomposes, render it exceedingly valuable for certain chemical trans-
formations accomplished by means of sulphuric acid at a high temperature, because it is
possible to take, in the form of this salt, a strictly definite quantity of sulphuric acid,
and to act on a given substance at a high temperature, which it is often necessary to do,
more especially in chemical analysis. In this case, the acid salt acts in exactly the
same manner as sulphuric acid itself, but the action of the latter is inconvenient at
temperatures above 400°, because it would all evaporate, while at that temperature the
acid salt still remains in a fused state, and acts with the elements of sulphuric acid on
the substance taken. Hence by its means the boiling-point of sulphuric acid is raised.
Thus the acid potassium sulphate is employed, for example, where for conversion of
certain oxides, such as the oxides of iron, aluminium, and chromium, into salts, a high
temperature is required.

Weber, by heating potassium sulphate with an excess of sulphuric acid at 100°,.
observed the formation of a lower stratum, which was found to contain a definite com-
pound containing eight equivalents of SO3 per equivalent of KoO. The salts of
rubidium, caesium, and thallium give a similar result, but those of sodium and lithium
do not.

10 The bromide and iodide of potassium are used, like the corresponding sodium
compounds, in medicine and photography. Potassium iodide is easily obtained in a pure-
state by saturating a solution of hydriodic acid with caustic potash. In practice, how-
ever, this method is rarely had recourse to, other more simple processes being em-
ployed although they do not give so pure a product. They aim at the direct formation
of hydriodic acid* in the liquid in the presence of potassium hydroxide or carbonate.
Thus iodine is thrown into a solution of pure potash, and hydrogen sulphide passed
through the mixture, the iodine being thus converted into hydriodic acid. Or a solution
is prepared from phosphorus, iodine, and water, containing hydriodic and phosphoric acid ;
lime is then added to this solution, and calcium iodide is obtained in solution, and"
calcium phosphate as a precipitate. The solution of calcium iodide gives, with potassium
carbonate, insoluble calcium carbonate and a solution of potassium iodide. If iodine is
added to a slightly-heated solution of caustic potash (free from carbonate — that is, freshly
prepared), so long as the solution is not coloured from the presence of an excess of iodine,
then there is formed (as in the action of chlorine on a solution of caustic potash) a
mixture of potassium iodide and iodate. On evaporating the solution thus obtained and
igniting the residue, the iodate is destroyed and converted into iodide, the oxygen being
disengaged, and potassium iodide only is left behind. On dissolving the residue in water
and then evaporating, cubical crystals of the anhydrous salt are obtained, which are
soluble in water and alcohol, fuse and give an alkaline reaction, owing to the fact that
when ignited a portion of the salt decomposes, forming potassium oxide. The neutral
salt may be obtained by adding hydriodic acid to this alkaline salt until it gives an
acid reaction. It is best to add some finely-divided charcoal to the mixture of iodate-

542 PRINCIPLES OF CHEMISTRY

acid on the carbonate, whilst the hydroxide, caustic potash^ KHO, which
is in many respects analogous to caustic soda, is easily obtained by
means of lime in exactly the same manner as sodium hydroxide is pre-
pared from sodium carbonate. 1 1 Therefore, in order to complete our
knowledge of the alkali metals, we will only describe two salts of
potassium which are of practical importance, and whose analogues have
not been described in the preceding chapter.

Potassium cyanide, which presents in its chemical relations a certain
analogy with the halogen salts of potassium, is not only formed accord-
ing to the equation, KHO + HCN=H2O + KC]Sr, but also when-
ever a nitrogenous carbon compound — for instance, animal matter — is
heated in the presence of metallic potassium, or of a compound of
potassium, and even when a mixture of potash and carbon is heated in
a stream of nitrogen. Potassium cyanide is obtained from yellow
prussiate (p. 401), which has been already mentioned and whose pre-
paration on a large scale will be described in speaking of iron. If
the yellow prussiate be ground to a powder and dried, so that it loses
its water of crystallisation, then it melts at a red heat, and decomposes
into carbide of iron, nitrogen, and potassium cyanide, FeK4C6N6 =
4KCN + FeC2 + N2. After the decomposition it is found that the
yellow salt has been converted into a white mass of potassium cyanide.
The carbide of iron formed collects at the bottom of the vessel. If

and iodide before igniting it, as it facilitates the evolution of the oxygen from the iodate.
'The iodate may also be converted into iodide by the action of certain reducing agents,
such as zinc amalgam, which, when boiled with a solution containing an iodate, converts
it into iodide. Potassium iodide may also be prepared by mixing a solution of ferrous
iodide (it is best if the solution contains an excess of iodine) and potassium carbonate, in
which case ferrous carbonate, FeCOs, is precipitated (with an excess of iodine the pre-
cipitate is granular, and contains a compound of the suboxide and oxide of iron), while
potassium iodide remains in solution. Ferrous iodide, FeI2, is obtained by the direct
action of iodine on iron in water. Potassium iodide considerably lowers the temperature
(even by 24°), when it dissolves in water, 100 parts of the salt dissolve in 73'5 parts of
water at 12'5°, in 70 parts at 18°, whilst the saturated solution which boils at 120° con-
tains 100 parts of salt per 45 parts of water. Solutions of potassium iodide dissolve a
considerable amount of iodine; strong solutions even dissolving as much or more iodine
than they contain as potassium iodide (see Chap XI. Note 64).

11 Caustic potash is not only formed by the action of lime on dilute solutions of
potassium carbonate (as sodium hydroxide is prepared from sodium carbonate), but also
by igniting potassium nitrate with finely-divided copper (see Note 15), and also by mixing
solutions of potassium sulphate (or even of alum, KA1S2O8) and barium hydroxide,
BaH2O2. It is sometimes purified by dissolving it in alcohol (the impurities, for example,
potassium sulphate and carbonate, are not dissolved) and then evaporating the alcohol.

The specific gravity of potassium hydroxide is 2'04, but that of its solutions (sec
Chap. XII. Note 18) at 15° S = 9992 + 90'4p + 0'28p2 (here p'2 is +, and for sodium
hydroxide it is — ). Strong solutions, when cooled, yield a crystallo-hydrate, KHO,4H2O,
which dissolves in water, producing (like 2NaHO,7H2O) cold, whilst potassium hydros idi:
.in solution develops a considerable amount of heat.

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 543

the mass thus obtained be treated with water, the potassium cyanide
is partially decomposed by the water, but if it be treated with alcohol,
then the cyanide is dissolved, and on cooling separates in a crystalline
form.12 A solution of potassium cyanide has a powerfully alkaline
reaction, a smell like that of bitter almonds, peculiar to prussic acid,
and acts as a most powerful poison. Although ^exceedingly stable
in a fused state, potassium cyanide easily changes when in solution.
Prussic acid is so very feebly energetic that even water decomposes
potassium cyanide. A solution of the salt, even without access of air,
easily turns brown and decomposes, and when heated evolves ammonia
and forms potassium formate ; this is easily comprehensible from the
representation of the cyanogen compounds which was developed in
Chap. IX., KCN + 2H2O=CHKO2 + NH3. Furthermore, as carbonic
anhydride acts on potassium cyanide with evolution of prussic acid, and
as potassium cyanate, which is also unstable, is formed by the action of
air, it will be easily seen that solutions of potassium cyanide are very
unstable. Potassium cyanide, containing as it does carbon and
potassium, is a substance which can act in a very vigorously reducing
manner, especially when fused ; it is therefore used as a powerful
reducing agent at a red heat. A considerable quantity of potassium
cyanide is used in the arts, more particularly for the preparation of
metallic solutions which are decomposed by the action of a galvanic
current ; thus it is very frequently employed in electro-silvering and

12 In this case it is evident that all the cyanogen which was in combination with the
iron is decomposed into nitrogen, which is evolved as gas, and into carbon, which com-
bines with the iron. In order to avoid this, potassium carbonate is added to the yellow
prussiate while it is being fused. A mixture of 8 parts of anhydrous yellow prussiate
and 8 parts of pure potassium carbonate is generally taken. On fusing, double decom-
position takes place, resulting in the formation of ferrous carbonate and potassium
cyanide. But by this method, as by the first, a pure salt is not obtained, (1) because a
portion of the potassium cyanide is oxidised at the expense of the iron carbonate and
forms potassium cyanate, FeCO3 + KCN = CO2 + Fe + KCNO ; (2) a portion of the iron
under the action of the water again passes into solution ; and (3) the potassium cyanide
very easily forms oxide, which acts on the sides of the vessel in which the mixture is
heated (to avoid this iron vessels should be used), &c. By adding one part of charcoal
powder to the mixture of 8 parts of anhydrous yellow prussiate and 8 parts of potassium
carbonate a mass is obtained which is free from cyanate, because the carbon absorbs the
oxygen, but then it is impossible to obtain a colourless potassium cyanide by simple
fusion, although this may be easily done by dissolving it in alcohol. Naturally, pure
potassium cyanide may be easily obtained if hydrocyanic acid be saturated with caustic
potash, and especially if caustic potash be dissolved in alcohol and hydrocyanic acid gas
be passed through this solution; crystals of potassium cyanide then separate directly from
the solution. Potassium cyanide is now prepared in large quantities from yellow prussiate
for gilding and silvering. "When fused in large quantities the action of the oxygen of the
air is limited, and the entire operation may be conducted with great care, and therefore,
on a large scale very pure salt is sometimes obtained. When slowly cooled, the fused
salt separates in cubical crystals like potassium chloride.

544 PKINCIPLES OF CHEMISTRY

gilding. An alkaline solution is prepared,13 which is moderately stable
owing to the fact that potassium cyanide in the form of certain double
salts — that is, combined with other cyanides — is far more stable
than when alone (yellow prussiate, which contains potassium cyanide
in combination with ferrous cyanide, is an example of this). The pro-
perty of potassium cyanide of giving double salts with other cyanides is
very clearly shown by the fact that many metals dissolve in a solution
of potassium cyanide, with the evolution of hydrogen. For example,
iron, copper, and zinc act in this manner. In which case potassium
hydroxide is formed ; for example —

4KCN + 2H2O -i- Zn=K.2ZnC4N4 + 2KHO + H2.

Gold and silver are soluble in potassium cyanide in the presence of
air, in which case the hydrogen, which might be evolved in the reaction,
combines with the oxygen of the air, forming water. Platinum,
mercury, and tin are not dissolved in a solution of potassium cyanide,,
even with the access of air.

Potassium nitrate, or common nitre or saltpetre, KNO3, is chiefly
used as a component part of gunpowder, in which it cannot be replaced
by the sodium salt, because the latter is deliquescent. It is necessary
that the nitre in gunpowder should be perfectly pure, as even small
traces of sodium, magnesium, and calcium salts, especially chlorides,
render the nitre and gunpowder capable of attracting moisture. Nitre
may easily be obtained pure, owing to its great disposition to form
crystals both large and small, which aids its separation from other
salts. The considerable differences between the solubility of nitre at
different temperatures aids this crystallisation. A solution of nitre
saturated at its boiling point (116°) contains 335 parts of nitre to 100
parts of water, whilst at the ordinary temperature — for instance, 20°—
the solution is only able to retain 32 parts of the salt. Therefore, in
the preparation and refining of nitre, its solution, saturated at the
boiling point, is cooled, and nearly all the nitre is obtained in the form
of crystals. If the solution be quietly and slowly cooled in large
quantities then large crystals are formed, but if it be rapidly cooled and
agitated then small crystals are obtained. In this manner, if not all
at all events the majority of the impurities present in small quantities
remain in the mother liquor. If an unsaturated solution of nitre be
rapidly cooled, so as to prevent the formation of large crystals (in whose
crevices the mother liquor, together with the impurities, would remain),
the very minute crystals of nitre known as saltpetre flour are obtained.

13 The electro-deposition of metals generally proceeds more uniformly and gives a
purer deposit from alkaline, than from acid, solutions.

POTASSIUM, KUBIDIUM, CAESIUM, AND LITHIUM 545

Common nitre occurs in nature, but only in small quantities in
admixture with other nitrates, and especially with sodium, magnesium,
and calcium nitrates. Such a mixture of salts of nitric acid is formed
in nature in fertile earth, and in those localities where, as in the soil,
nitrogenous organic remains are decomposed in the presence of alkalis
or alkaline bases with free access of air. This method of the formation
of nitrates requires moisture, besides the free access of air, and takes
place principally during warm weather. Besides which Schloessing and
Miintz, by employing similar methods to Pasteur, showed that the for-
mation of nitre in the decomposition of nitrogenous substances is
accomplished by the aid of peculiar micro-organisms (ferments) without
which the simultaneous action of the other necessary conditions
{alkalis, moisture, a somewhat elevated temperature, air, and nitroge-
nous substances) cannot give nitre. In warm countries, and in tempe-
rate climates during the summer months, fertile soils produce a small
quantity of nitre. In this respect India is especially known as affording
a considerable supply of nitre extracted from the soil. The soil, which
contains nitre, after the rainy season sometimes becomes covered
during the summer with crystals of nitre, formed by the evaporation
of the water in which it was previously dissolved. This soil is col-
lected, subjected to repeated lixiviations, and treated for nitre, as will
be presently described. In temperate climates nitrates are obtained
from the lime rubbish of demolished buildings which have stood for
many years, and especially from those portions which have touched
the ground. The conditions are there very propitious to the forma-
tion of nitre, because the lime used as a cement in buildings contains
the base necessary for the formation of nitrates, while the excrement,
urine, and other animal refuse are sources of nitrogen. By the metho-
dical lixiviation of this kind of rubbish a like solution of nitro-
genous salts is obtained as by the lixiviation of fertile soil. A similar
solution is also obtained by the lixiviation of the so-called nitre plan-
tations. They are composed of manure interlaid with brush-wood, and
strewn over with ashes and other alkaline and lime rubbish. These
nitre plantations are set up in those localities where the manure is not
required for the fertilisation of the soil, as, for example, in the south-
eastern * black earth ' governments of Russia. The same process of
oxidation of nitrogenous matter free to the access of air and mois-
ture during the warm season in the presence of alkalis takes place
in nitre plantations as is accomplished in the fertile soil and in the
walls of buildings. From all these sources there is obtained a solution
containing different salts of nitric acid mixed with soluble organic
anatter. The simplest method of treating this impure solution of nitre

VOL. i.

546 PRINCIPLES OF CHEMISTRY

is to add a solution of potassium carbonate, or to simply treat it with
ashes containing this substance. The potassium carbonate enters into-
double decomposition with the calcium and magnesium salts, forming
insoluble carbonates of these bases and leaving the nitre in solution.
Thus, for instance, K2CO3 + Ca(NO3)2:=2KNO3 + CaCO3. Both cal-
cium and magnesium carbonates are insoluble, and therefore after
treatment with potassium carbonate the solution no longer contains
salts of these metals but only the salts of sodium and potassium together
with organic matter. The latter partially separates on heating in an
insoluble form, and is entirely destroyed by heating the nitre to a
low red heat. The nitre thus obtained is easily purified by repeated
crystallisation. The greater part of the nitre used for making gun-
powder is now obtained from the sodium salt Chili saltpetre or cubic
nitre, which occurs, as has been already mentioned, in nature. The
conversion of this salt into common nitre is also carried on by means
of a double decomposition. This is done either by adding potassium
carbonate (when, on mixing the strong and hot solutions, sodium car-
bonate is directly obtained as a precipitate), or as is now most fre-
quent, potassium chloride. When a mixture of strong solutions of
potassium chloride and sodium nitrate is evaporated, sodium chloride
first separates, because this salt, which is formed by the double decom-
position KCl + NaNO3==KNO3-f NaCl, is almost equally soluble in
hot and cold water ; on cooling, therefore, a large amount of potassium
nitrate separates from the saturated solution, while the sodium chloride
remains dissolved. The nitre is ultimately purified by recrystallisation
and by washing with a saturated solution of nitre, which cannot dis-
solve a further quantity of nitre but only the impurities.

Nitre is a colourless salt having a peculiar cool taste. It easily
crystallises in long striated six-sided rhombic prisms terminating in
similar pyramids. Its crystals (sp. gr. 1*93) do not contain water, but i
their cavities generally contain a certain quantity of the solution from j
which they have crystallised. For this reason in refining nitre, large
crystals are not obtained but saltpetre flour is prepared. At a low
red heat (339°) nitre melts to a colourless liquid.14 Potassium nitrate j
at the ordinary temperature and in a solid form is inactive and stable,

14 Before fusing, the crystals of potassium nitrate change their form, and take the j
same form as sodium nitrate — that is, they change into rhombohedra. Nitre crystal-
lises from hot solutions, and in general under the influence of a rise of temperature, in a
different form from that given at the ordinary or lower temperatures. Fused nitre solidi-
fies- into a radiated crystalline mass ; but it does not exhibit this structure if metallic
chlorides be present, so that this method may be taken advantage of to determine
the degree of purity of nitre. With a small trace of sodium chloride, the mass of nitre
solidified after fusion does not present any signs of crystallisation in the middle.

Carnelley and Thomson (1888) determined the fusing point of mixtures of potassium

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 547

but at a hif/h temperature it acts as a powerful oxidising agent,
because it gives up a considerable amount of oxygen to substances
mixed with it.1"' When thrown on incandescent charcoal it brings
about its rapid combustion, and a mechanical mixture of powdered
charcoal and nitre ignites when brought into contact with a red-hot
substance, and continues to burn by itself. In this action, nitrogen is
evolved, and the oxygen goes to oxidise the charcoal, in consequence of
which potassium carbonate and carbonic anhydride are formed :
4KNO3 + 5C=2K2CO3 + 3CO.2 + 2N2. This phenomenon depends on
the fact that oxygen in combining with carbon evolves more heat than
it does in combining with nitrogen. Hence when once the combustion
has been started at the expense of the nitre, it is able to go on without
requiring the aid of external heat. A like oxidation or combustion at
the expense of the contained oxygen proceeds when nitre is heated
with different combustible substances. If a mixture of sulphur and
nitre be thrown upon a red-hot surface, then the sulphur burns, form-
ing potassium sulphate and sulphurous anhydride. In this case, also,
the nitrogen of the nitre is evolved as gas : 2KNO3 + 2S=K2SO4-fN2
+ SO.2. A similar phenomenon takes place when nitre is heated with

and sodium nitrates. The first salt fuses at 339° and the second at 316°, and if p be
the percentage amount of potassium nitrate, then the result obtained —

#=10 20 30 40 50 60 70 80 90

298° 288° 268° 242° 231° 231° 242° 284° 306°

which confirms Shaffgotsch's observation (1857) that the lowest fusing point (about 231°)
is given by mixing molecular quantities (^? = 54'3) of the salts — that is, in the formation
of the alloy, KNO3,NaNO3.

A somewhat similar result was discovered by the same observers for the solubility of
mixtures of these salts at 20° in 100 parts of water. Thus, if p be the weight of potas-
sium nitrate mixed with 100—^? parts by weight of sodium nitrate taken for solution,
and c be the quantity of the mixed salts which dissolves in 100, the solubility of sodium
nitrate being 85, and of potassium nitrate 34, parts in 100 parts of water, then —

p= 10 20 80 40 50 60 70 80 90

C = 110 13(5 18(5 188 106 81 73 54 41

The maximum solubility proved not to answer to the most fusible mixture, but to one
much richer in sodium nitrate.

Both these phenomena show that in homogeneous liquid mixtures the chemical forces
that act between substances are the same as those that determine the molecular weights
of substances, even when the mixture consists of such analogous substances as potas-
sium and sodium nitrates, between which there is no direct chemical interchange. It is
instructive to note also that the maximum solubility does not correspond with the mini-
mum fusing point, which naturally depends on the fact that a third substance, namely
water, participates in solution, although an attraction between the salts, like that which
exists between sodium and potassium carbonates (Note 8), also partially acts.

15 Fused nitre, with a further rise of temperature, disengages oxygen and then nitro-
gen. The nitrite KNO.^ is first formed and then potassium oxide. The admixture of
certain metals — for example, of finely -divided copper — aids the last decomposition. The
oxygen in this case naturally passes over to the metal.

N N 2

."> IS i'UNVll'l.Ks oF rilKMlSTUY

many metals. Tl xidation of those metals which are able to form

acid oxides \\ith an excess of oxygen is especially remarkable. In
this ca-e they remain in combination with potassium oxide as potassium
salts. Manila ne.se, antimony, arsenic, iron, chromium, A.V., are in-
-t a i ices i if tin- kind. These elements, like ca rb< >n aid sulphur, displace
free nitrogen. The lower oxides of these metals \\ht-n fused with nitre
pa-s into the higher oxides. ( ir^anic substances are also oxidised
when heated with nitre that is, they burn at the expense of the nitre.
It will be n-adilv understood from this that nitre is frequently used in
chemical practice and t he arts as an oxidising auent at hi^h temper-
atures. Its application in //"///">/'•'/,/• is based mi this : gunpowder con-
sists of a mechanical mixture of tinely-^round sulphur, nitre, and char-
coal. The relative proportion of these substances varies according to
the destination of the powder and to the kind of charcoal employed (a
friable, incompletely-burnt charcoal, containing therefore hydrogen and
oxygen, is employed). (Jases are formed in its combustion, chielly
nitron-,.)) and carbonic anhydride, which create a considerable pres-
sure if their escape be in any way impeded. This action of gun-
pouder may be expressed by the equation : '_' K N <>., + :>(' -f S= K ,,S 4-

It i- found by this equation that gunpowder should contain thirty-
six parts of charcoal ( I •">•;> p.c. ). and thirt v-two parts ( 1 1 -(.l p.c. ) of sul-
jihnr. to L'M-J part s ( 7 1 '> ]).c. ) of nitre, which is verv near to its actual
ci imposition. '"

''' In rhaia. win re the nianiifactiire of L.fuiipowder lias loiip; heeii practised, they use

7.V7 ol ' : • . 1 r J of charcoal, and (.l'!l of sulphur. < Irdimiry p, >w der for sporting purposes

-" ; • ,,] nitre. ]-2 of charcoal, and .s ,,( -ulphur. \\hil-t the gunpowder used

v,ln-ii i.eated to i'.un , uheti struck, or l,\ contact with a spark. A compactor finely-

1 • 1 1 ';•.'•! \ . I'o act properly t he irunpou'der must ha\ i' a known
, -. • • n. ~o that the prer-~nre -Imulil increase and not diminish during the

j . •.-. d.-r tin ape < .t c, -ided pri ins u it h holes through them (prismatic

:

I • mi the r.-a on of the dynamical .icti-.n i.f ;j unp< <\\ .1. -r. a nd i -J i a solid residue,

iv-idiie I.IK . ' • nta M p.. i • mm -ulphide. K S. and a wliol.- series ,,f
• . , arlionate and nlphate. Thi- alread\ -hows that tin- comluis-
iiot o imple .1 it appeal to he from the aho\e fiii'iiiula. and hence
• , , due i- al-t greater than mdieated l.ythat formula. According to
27ii part- ol /iiiii nwdi >• 1 1 n pa 1 1 - of r« sidue that is, I dii part - of

,, 7n p. ,-, 'enerally .V.» p.c.i. 'I'd ditTerence depends on the fact that .so
ol the nitivi remain- in the n id ue. and it i- evident that if the residue
i p, i ion of t lie /.' < , , ol\ed li\ I lie powder will \ary also, ami therefore

POTASSir.M, KTBIDIUM, CAKSITM, AND LITHIUM 549

Potassium was obtained like sodium ; first by the action of a gal-
vanic current, then by reduction of the hydroxide by means of metallic
iron, and lastly, by the action of charcoal on the carbonate at a high
temperature. The preparation of metallic potassium differs, however,
from that of sodium in that it easily combines with carbonic oxide,
forming an explosive and inflammable mass.17

However, in its essential points the method of its extraction is
not different from that of sodium, because potassium is quite as volatile
as sodium, if not more so. At the ordinary temperature, potassium
is even softer than sodium, its freshly- cut surfaces present a whiter
colour than sodium, but, like the latter, and with even greater ease, it
oxidises in moist air. It is brittle at low temperatures, but is quite
soft at 25°, and melts at 58°. At a low red heat (720°) it distils with-
out change, forming a green vapour, whose density,18 according to

the entire process will be different in different cases. The difference in the composition
of the gases and residue depends, as the researches of Gay-Lussac, Shishkoff and Bun-
sen, Noble and Abel, Federoff, &c., show, on the conditions under which the combustion
of the powder proceeds. When gunpowder bums in an open space, the gaseous products
which are formed do not remain in contact with the residue, and then a considerable
portion of the charcoal entering into the composition of the powder remains unburnt
1 "realise the charcoal burns after the sulphur at the expense of the oxygen of the nitre.
In this extreme case the commencement of the combustion of the gunpowder may be
expressed by the equation. -JKNO5 + 3C -f S = 2C + K2SO4 + CO., + N2. The residue in a
blank cartridge often consists of a mixture of C, K.2SO4, KoCO3, and K2S2O3. If the
combustion of the gunpowder be impeded — if it take place in a cartridge in the barrel of
a gun — the quantity of potassium sulphate will first be diminished, then the amount
of sulphite, whilst the amount of carbonic anhydride in the gases and the amount of
potassium sulphide in the residue will increase. The quantity of charcoal entering into
the action will then be also increased, and hence the amount in the residue will de-
crease. Under these circumstances the weight of the residue will be less — for example,
4KaCO3 + 4S = KoSO4-(-8K2S-l-4CO.,. Besides which, carbonic oxide has been found in
the gases, and potassium bisulphide, K2S2, in the residue of gunpowder. The amount
of potassium sulphide. K.,,S, increases with the completeness of the combustion, and
is formed in the residue at the expense of the potassium sulphite. In recent times the
knowledge of the action of gunpowder and other explosives has made much progress, and
has developed into a vast province of artillery science.

17 The substances obtained in this case are mentioned in Chap. IX. Note 81.

18 A. Scott (1887) determined the vapour densities of many of the compounds of the
alkali elements in a platinum vessel heated in a furnace and previously filled with nitro-
gen. But these, the first data concerning a subject of great importance, have not yet
been sufficiently full}1 described, nor have they received as much attention as could be
desired. Taking the density of hydrogen as unity, Scott found the vapour densities of
the following substances to be —

Na 12-75 (12-5). KI 92 (84).

K 19 (19-5). RbCl 70 (60).

CsCl 89-5 (84-2). Csl 188 (180),

FeCl5 68 AgCl 80 (71'7).

In brackets are given the densities corresponding with the formulae, according to
Avogadro-Gerhardt's law. This figure is not given for FeClj, because in all probability

550 PRINCIPLES OF CHEMISTRY

A. Scott, is equal to 19 (if that of hydrogen =1). This shows that the
molecule of potassium (like that of sodium, mercury, and zinc) contains
but one atom. This is also the case with other inetals, judging by
recent researches.19 The specific gravity of potassium at 15° is 0'87,
and is therefore less than that of sodium, as is also the case with all
its compounds.20 Potassium decomposes water with great ease at the
ordinary temperature, evolving 45000 heat units per atomic weight.
The heat evolved is sufficient to inflame the hydrogen, the flame being
coloured violet from the presence of particles of potassium.21

With regard to the relation of potassium to hydrogen and oxygen,
it is closely analogous to sodium in this respect. Thus, with hydrogen
it forms potassium hydride, K2H (between 200° and 411°), and with
oxygen it gives a suboxide K4O, oxide K2O, and peroxide, only more
oxygen enters into the composition of the latter than in sodium per-

under these conditions (the temperature at which it was determined) a portion of the
FeCls was decomposed. If it was not decomposed, then a density 81 would correspond
with the formula FeCls, and if the decomposition were Fe.^Clg = '2FeClo + Clo, then the
.density should be 54. With regard to the silver chloride, there is reason to think that
the platinum decomposed this salt.

The majority of Scott's results so closely correspond with the formulae that a better
concord cannot be expected in such determinations.

19 The molecules of non-metals are more complex — for instance, H.,, Or,, CL, &c. But
arsenic, whose superficial appearance recalls that of metals, but whose chemical proper-
ties approach more nearly to the non-metals, has a complex molecule containing As4 .
With respect to the vapour of iodine, see Chap. VII. p. 313.

20 As the atomic weight of potassium is greater than that of sodium, therefore the
volumes of the molecules, or the quotients of the molecular weight by the specific gravity,
for potassium compounds are greater than those of sodium compounds, because both the
denominator and numerator of the fraction augment. We cite for comparison the
volumes of the corresponding compounds —

Na 24 NaHO 18 NaCl 28 NaNO5 37 Na2SO4 54
K 45 KHO 27 KC1 39 KNO3 48 K2SOt <>(>

21 The same precautions must be taken in decomposing water by potassium as have
to be observed with sodium (Chap. II. Note 8).

It must be observed that potassium decomposes carbonic anhydride and carbonic
oxide when heated, the carbon being liberated and the oxygen taken up by the metal,
whilst on the other hand charcoal takes up oxygen from potassium, as is seen from the
preparation of potassium by heating potash with charcoal, hence the reaction K2O + C =
K2 + CO is reversible and the relation is the same in this case as between hydrogen and
zinc. Nothing of the kind could be expected from a comparison of the quantities of
heat evolved in the formation of these compounds, for charcoal in combining with
oxygen to form carbonic oxide evolves (for its molecular weight) only about 30000
heat units (Chap. IX. Note 25), whilst potassium in forming the oxide K2O gives
about 100000 units, and iron in forming ferrous oxide, FeO, evolves about 70000 units.
It is evident that the decomposition of potassium oxide by charcoal is accompanied by
the absorption of a large amount of heat, and the reverse reaction — which, however,
proceeds with greater difficulty — with the development of heat, which again shows
the impossibility of judging the direction in which a reaction will proceed from
thermal data.

POTASS i r.M. KU i;i i HUM, CAESIUM, AND LITHIUM 551

oxide ; potassium peroxide contains KO2, but it is probable that in the
combustion of potassium an oxide KO is also formed. Potassium,
like sodium, is soluble in. mercury.-2 In a word, the relation between
sodium and potassium is as close at that between chlorine and bromine,
or, better still, between fluorine and chlorine, as the atomic weight of
sodium, 23, is as much greater than that of fluorine, 19, as that of
potassium, .'39, is greater than that of chlorine, 35 '5.

The resemblance between potassium and sodium is so great that
(/!>'!/• compounds can only be easily distinguished in the form of certain
of their salts. For instance, the acid potassium tartrate, C4H5KO6
(cream of tartar), is distinguished by its sparing solubility in water and
in alcohol, and in a solution of tartaric acid, whilst the corresponding
sodium salt is easily dissolved. Therefore, if a solution of tartaric acid
be added in considerable excess to the solutions of the majority of
potassium salts, then a precipitate of the sparingly- soluble acid salt is
formed, which does not occur with salts of sodium. The chlorides KC1
and NaCl in solutions easily give double salts K2PtCl6 and Na2PtCl6,
with platinic chloride, PtCl4, and the solubility of these salts is very
different, especially in a mixture of alcohol and ether. The sodium salt
is easily soluble, whilst the potassium salt is insoluble or almost so, and
therefore the reaction with platinic chloride is that most often used
for the separation of potassium from sodium, as is more fully described
in works on analytical chemistry.

It is possible to discover the least traces of these metals in admix-
ture together, by means of their property of imparting different colours
to aflame. The presence of a salt of sodium in a flame is recognised
•by a brilliant yellow coloration, and a pure potassium salt colours a
colourless flame violet. However, in the presence of a sodium salt
the pale violet coloration given by a potassium salt is quite undistin-
guishable, and it is at first sight impossible in this case to discover the
potassium salt in the presence of that of sodium. But by decomposing
the light given by a flame coloured by these metals, or a mixture of
ithem, by means of a prism, they are both easily distinguishable, because

•l The definite crystalline amalgam of potassium contains twice as much mercury,
KH<;.2 (as the potassium peroxide contains twice as much oxygen, Chap. XII. Note 89).

Potassium forms alloys with sodium in all proportions. The alloys containing 1 and
•3 equivalents of potassium to one equivalent of sodium are liquids, like mercury at the
•ordinary temperature. Joannis, by determining the amount of heat developed by these
.alloys in decomposing water, found the evolution for Na^K, NaK, NaK2, and NaK3 to be
44-5, 44-1, 48'8 and 44'4 thousand heat units respectively (for Na 42'6 and for K 45'4).
The formation of the alloy NaK2 is therefore accompanied by the development of heat,
•whilst the other alloys may be regarded as solutions of potassium or sodium in this alloy.
In any case a fall of the temperature of fusion is evident in this instance as in the alloys
of nitre (Note 14).

552

PRINCIPLES OF CHEMISTRY

the yellow light emitted by the sodium salt depends on a group of light
rays having a definite index of refraction which corresponds with the
yellow portion of the solar spectrum, having the index of refraction
of the Frauenhofer line (strictly speaking, group of lines) D, whilst the
salts of potassium give a light in which these rays are entirely wanting,
but which contain rays of a red and violet colour. Therefore, if a
potassium salt occur in a flame, then, on decomposing the light (from

a flame coloured by the
salt) after passing through
a narrow slit by means
of a prism, there will be
obtained red and violet
bands of light situated at a
considerable distance from
each other ; whilst if a so-
dium salt be present a yel-
low line will also appear. If
both metals simultaneously
occur in a flame and emit
light, then the spectrum
lines corresponding to the
potassium and the sodium
will simultaneously appear.
For convenience in car-
Fig. 72A.— Spectroscope. The prism and table are covered with rying Oil this form of test-
an opaque cover. The spectrum obtained from the flame

coloured by a substance introduced on the wire is viewed ing, Spectroscopes (fig. 72A)
through B. A light is placed before the scale D in order to

illuminate the image of the" scale reflected through B by are constructed/*5 COnSlst-
the side of the prism. . ,, ,, ,.

ing or a retracting prism

and three tubes directed on the plane of the refracting angle of the
prism. One of the tubes, C, has a vertical slit at the end, giving access
to the light to be tested, which then passes into the tube (collimator),

23 For accurate measurements and comparative researches more complicated spec-
troscopes are required which give a greater dispersion, and which are furnished for this
purpose with several prisms — for example, in Browning's spectroscope the light passes
through six prisms, and then, having undergone an entire internal reflection, passes
through the upper portion of the same six prisms, and again by an entire internal reflec-
tion passes into the ocular tube. With such a powerful dispersion the relative position
of the spectral lines may be determined with accuracy. For the absolute and exact
determination of the wave lengths it is particularly important that the spectroscope
should be furnished with diffraction gratings. The construction of spectroscopes des-
tined for special purposes (for example, for investigating the light of stars, or for deter-
mining the absorption spectra in microscopic preparations, &c.) is exceedingly varied
Details of the subject must be looked for in works on physics and on spectrum analysis.
Among the latter the best known for their completeness and merit are those of Hoscce*
Kayser, Vogel, and Lecoq de Boisbaudran.

POTASS I I'M. IHT.IIMrM. CAESIUM, AND LITHIUM 553

which gives the rays a parallel direction. The rays of light having
passed through the slit, and having become parallel, are refracted and
dispersed in the prism, and the spectrum formed is observed through
the orifice of the other telescope B. The third tube D contains a hori-
zontal transparent scale (at the exterior end) which is divided into
equal divisions. The light from a source such as a gas burner or
candle placed before this tube, passes through the scale, and its image
is thus reflected on that face of the prism which stands before the tele-
scope B, so that the image of the scale is seen through this telescope
simultaneously with the spectrum given by the rays passing through
the slit of the tube C. In this manner the image of the scale and the
spectrum given by the source of light under investigation are seen
simultaneously. If the sun's rays be directed through the slit of the
tube C, then the observer looking through the orifice of B will see
the solar spectrum, and (if the aperture of the slit be narrow and the
apparatus truly adjusted) the dark Frauenhofer lines in it.24 Small-
sized spectroscopes are usually so adjusted that (looking through B)
the violet portion of the spectrum is seen to the right and the red portion
to the left, and the Frauenhofer line D (in the bright yellow portion of
the spectrum) is situated on the 50th division of the scale.'25 If the
light emitted by an incandescent solid — for example, the Drummond
light — be passed through the spectroscope, then all the colours of the
solar spectrum are seen, but not the Frauenhofer lines. Let us now
consider what will happen if the light from a flame coloured by various
salts be passed through the spectroscope. This is done by placing a
Bunsen gas burner (or the pale flame of hydrogen gas issuing from a
platinum orifice) giving so pale a flame that its spectrum will be invisible
before the slit. If any compound of sodium be placed in the flame of

24 The arrangement of all the parts of the apparatus so as to give the clearest possible
vision and accuracy of observation must evidently precede every kind of spectroscopic
determination. Details concerning the practical use of the spectroscope must be also
looked for in special works on the subject. In tkis treatise the reader is supposed to
have a certain knowledge of the physical data respecting the refraction of light, and its
dispersion and diffraction, and the theory of light, which allows of the determination
of the length of the waves of light in absolute measures on the basis of observations
with diffraction gratings, the distance between whose divisions may be easily measured
in fractions of a millimetre : by such means it is possible to determine the wave lengths
of definite rays of light.

K In order to give an idea of the several sizes of the scale',"we may observe that the
common spectrum extends from the zero of the scale (where the red portion is situated)
to the 170th division (where the end of the visible violet portion of the spectrum is
situated), and that the Frauenhofer line A (the extreme prominent line in the red) cor-
responds with the 17th division of the scale; the Frauenhofer line F (at the beginning of
the blue, near the green colour) is situated on the 90th division, and the line G, which is
clearly seen in the beginning of the violet portion of the spectrum, corresponds with the
127th division of the scale.

554 PRINCIPLES OF CHEMISTRY

the gas burner (for which purpose a platinum wire on whose end sodium
chloride is fused is fixed to the stand), then the flame is dyed a yellow
colour, and on looking through the spectroscope the observer will see a
bright yellow line falling upon the 50th division of the scale, which is
seen together with the spectrum in the telescope. No yellow lines
of other refractive index, nor any rays of any other colour, will be
seen, and, therefore, the spectrum corresponding with sodium com-
pounds consists of yellow rays of that index of refraction which belong
to the Frauenhofer (black) line D of the solar spectrum. If a potassium
salt be introduced into the flame instead of a sodium salt, then two
bands will be seen which are much feebler than the bright sodium band
— namely, one red line near the Frauenhofer line A and another violet
line. Besides which, a pale, almost continuous, spectrum will be
observed in the central portions of the scale. If a mixture of sodium
and potassium salts be now introduced into the flame, three lines will
be simultaneously seen— namely, the red and pale violet lines of
potassium and the yellow line of sodium. In this manner it is possible,
by the aid of the spectroscope, to determine the relation between the
spectra of metals and known portions of the solar spectrum. The con-
tinuity of the latter is interrupted by dark lines (that is, by an absence
of light of a definite index of refraction), termed the Frauenhofer lines
of the solar spectrum. It has been shown by careful observations (by
Frauenhofer, Brewster, Foucault, Angstrom, Kirchhoff, Cornu, Lockyer,
Dewar, and others), that there exists an exact agreement between the
spectra of certain metals and certain of the Frauenhofer lines. Thus the
bright yellow sodium line exactly corresponds with the dark Frauen-
hofer line D of the solar spectrum. A similar agreement is observed
in the case of many other metals. This is not an approximate or chance
correlation. In fact, if a spectroscope having a large number of re-
fracting prisms and a high magnifying power be used, then it is seen
that the dark line D of the solar spectrum consists of an entire system of
closely adjacent but definitely situated fine and wide (sharp, distinct)
dark lines,26 and an exactly similar group of bright lines is obtained
when the yellow sodium line is examined through the same apparatus,
so that each bright sodium line exactly corresponds with a dark line in
the solar spectrum. In the common spectroscopes which are usually
employed in chemical research, one yellow band, which does not split

2ti The two most distinct lines of D, or of sodium, have wave lengths of 589'5 and 588'0
millionths of a millimeter, besides which fainter and fainter lines are seen whose wave
lengths in millionth parts of a millimeter are 588'7 and 588'1, 616'0 and 615'4, 515'5 and
515*2, 498'8 and 498'2, &c., according to Liveing and Dewar. Many strive to find a simple
relation, subject to a law, in the wave lengths, both of these pairs of lines and those
of other elements.

POTASSir.M, IiriJIDlUM, CAESIUM, AND LITHIUM 555

up into thinner lines, is seen instead of the system of sodium lines, owing
to the small dispersive power of the prism and the considerable aper-
ture of the slit of the object tube.

This conformity of the bright lines formed by sodium with the
dark lines of the solar spectrum cannot be accidental. This conclusion
is further confirmed by the fact that the bright lines of other metals
correspond with dark lines of the solar spectrum. Thus, for example,
a series of sparks passing between the iron electrodes of a Ruhmkorff
coil gives 450 very distinct lines characterising this metal. All these
450 bright lines, or the whole spectrum corresponding with iron, is re-
peated, as Kirchhoff showed, in the solar spectrum as dark Frauenhof er
lines which occur in exactly the same situations as the bright lines
occur in the iron spectrum, just as the sodium lines correspond with
the band D in the solar spectrum. Many observers have in this
manner simultaneously studied the solar spectrum and the spectra of
different metals, and discovered in the former lines which correspond
not only with sodium and iron, but also with many other metals.27 The
spectra of such elements as hydrogen, oxygen, nitrogen, and other
gases may be observed in the so-called Geissler's tubes — that is, in
glass tubes filled with rarefied gases, through which the discharge of a
Ruhmkorff Js coil is passed. Thus hydrogen gives a spectrum composed

37 The most accurate investigations made in this respect are carried on with spectra
obtained by diffraction, because in this case the position of the dark and bright lines does
not depend on the index of refraction of the material of the prism, nor on the dispersive
power of the apparatus. The best— that is, the most general and accurate — method of
expressing the results of such determinations consists in determining the lengths of the
waves corresponding to the rays of a definite index of refraction. We will express this
wave length in millionth parts of a millimetre (the ten millionth parts are already
doubtful, and fall within the limits of error). In order to illustrate the relation between
the wave lengths and the positions of the spectrum, we will cite the wave lengths cor-
responding with the chief Frauenhofer lines and colours of the spectrum.

Frauenhofer line ABC D Eb F G H

Wave length . 761'0 687'5 656'6 589'5-588'9 527'8 518'7 486'5 481'0 897'2

Colour . . red orange yellow green blue violet

In the following table are given the wave lengths of the light rays (the longest and
most distinct, see later) for certain elements, those in black type being the most clearly
defined and distinct lines, which are easily obtained either in the flame of a Bunsen's
burner, or in Geissler's tubes, or in general, by an electric discharge. These lines refer
to the elements (the lines of compounds are different, as will be afterwards explained,
but many compounds are decomposed by the flame or by an electric discharge), and
furthermore to the elements in an incandescent and rarefied gaseous state, for
the spectra sometimes vary considerably with a variation of temperature and
pressure.

It may be said that the red colour corresponds with lines having a wave length of
fromJBO (with a greater wave length the lines are hardly visible, and are ultra red) to 650,
the orange from 650 to 590, the yellow from 590 to 520, the green from 520 to 490, the

556

PRINCIPLES OF CHEMISTRY

of three lines— a red line corresponding with the Frauenhofer line-
C, a green line corresponding with the line F, and a violet line corre-

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blue from 490 to 420, and the violet
from 420 to 380 millionth parts of
a millimetre. Beyond 380 the lines are
scarcely visible, and belong to the
ultra-violet.

In this table, which is arranged in
conformity to the image of the spectra
as they are seen (the red lines on the
left hand and the violet on the right
hand side), the figures in black type
correspond with lines which are so
bright and distinctly visible that
they may easily be made use of, both
in determining the relation between
the divisions of the scale and the
wave lengths, and in determining
the admixture of a given element
with another. The brackets join those
lines between which several other
lines are clearly visible if the disper-
sive power of the spectroscope per-
mits distinguishing the neighbouring
lines. In the ordinary laboratory
spectroscopes with one prism, even
with all possible precision of arrange-
ment and with a brilliancy of light
permitting the observations being
made with a very narrow aperture,
the lines whose wave lengths only
differ by 2-3 millionth parts of a mil-
limetre, are blurred together ; and
with a wide aperture a series of lines
differing by even as much as 20*
millionths of a millimetre appear as
one wide line. With a faint light
(that is, with a small quantity of
light entering into the spectroscope)
only the most brilliant lines are*
clearly visible. The length of the
lines does not always correspond
with their brilliancy. According to
Lockyer this length is determined
by placing the carbon electrodes
(between which the incandescent
vapours of the metals are formed), not
horizontally to the slit (as they are
generally placed, to give more light),
but vertically to it. Then certain
lines appear long and others short.
As a rule (Lockyer, Dewar, Cornu),
the longest lines are those with which
it is easiest to obtain reversed.

POTASSlfM. KlT.IDIl'M, CAKSIT.M, AND LITHIUM 557

spending with one of the lines between G and H. Of these rays the red
is the brightest, and therefore the general colour of luminous hydrogen
(with an electric discharge through a Geissler tube) is reddish.

The correlation of the Frauenhofer lines with the spectra of metals
depends on the phenomenon of the so-called reversed spectrum. This
phenomenon consists in that instead of the bright spectrum correspond-
ing with a metal, under certain circumstances a similar dark spectrum
in the form of Frauenhofer lines may be obtained, as will be directly
explained. In order to clearly understand the phenomenon of reversed
spectra, it must be known that in the passage of light through certain
transparent substances these substances retain rays of a certain re-
frangibility. The colour of solutions is a proof of this. Light which
has passed through a yellow solution of a uranium salt contains no
violet rays, and after having passed through a red solution of a per-
manganate, does not contain many rays in the yellow, blue, and green
portions of the spectrum. Solutions of copper salts absorb nearly all
red rays. Sometimes colourless solutions also absorb rays of certain
definite refractive indexes, and give absorption spectra. Thus solu-
tions of salts of didymium absorb rays of a certain refrangibility, and
therefore an impression of black lines is received,28 as shown in fig.

spectra (see later). Consequently, these lines are the most characteristic. Only the
longest and most brilliant are given in our table, which is composed on the basis of a
collection of the data at our disposal for bright spectra of the incandescent and rarefied
vapours of the elements. As the spectra change with great variations of temperature
and vapour density (the faint lines become brilliant whilst the bright lines sometimes
disappear), which is particularly clear from Ciamician's researches on the halogens, there-
fore, until the method of observation and the theory of the subject are enlarged, par-
ticular theoretical importance should not be given to the wave lengths showing the
maximum brilliancy, and which only possess any significance in a practical respect for
the common methods of spectroscopic observations.

!8 The ocular impressions of light (it is essentially the same with all other impressions
received by the senses) are all relative ; in those portions of a spectrum, received through
an absorptive medium, where there appears to be an absence of light, it may be only
rendered fainter, and for absorption spectra this is directly proved to be the case both
by experiment (by employing solutions of different strengths or strata of different thick-
nesses), and by direct measurement by the aid of the spectroscope — for instance, by
Vierordt's apparatus, which is described in works on physics. The relative distinctness
of the dark lines in an absorption spectrum, and of the bright coloured lines in luminous
spectra of vapours and gases, which are self-evident in making observations, offer great
difficulty with regard to precise measurement, just as is the case, for instance, with the
relative brilliancy of the stars.

The method of observing absorption spectra consists in taking a continuous spectrum
(one which does not give either dark lines or particularly bright luminous bands in the
spectrum) of white light— for instance, the light of a candle, lamp, or other source. The
collimator (that is, the tube with the slit) is directed towards this light, and then all
the colours of the spectrum are visible in the ocular tube. A transparent absorptive
medium — for instance, a solution or tube containing a gas— is then placed between the
source of light and the apparatus (or anywhere inside the apparatus itself in the path of
the rays). In this case either the entire spectrum is uniformly fainter, or absorption

558

PRINCIPLES OF CHEMISTRY

73. Many vapours (iodine) and gases (nitric peroxide) give similar
spectra. Light which has passed through a deep layer of aqueous
vapour, oxygen, or nitrogen also gives its absorption spectrum. For
this reason the peculiar (winter) dark lines discovered by Brewster are

FIG. 73. — Absorption spectrum (Lecoq de Boisbaudran) of salts of didymium in concentrated
;uid dilute solutions.

observed in sunlight, especially in the evening and morning, when the
sun's rays pass through the atmosphere (containing these substances)
by a longer path than at mid-day. It is evident that the Frauenhofer

bands appear on the bright field of the continuous spectrum in definite positions along
it. These bands have different lengths and positions, and distinctness and intensity
of absorption, according to the properties of the absorptive medium. Like the luminous
spectra given by incandescent gases and vapours, the absorption spectra of a number
of substances have already been studied, and some with great precision — as. for example,
the spectrum of the brown vapours of nitrogen dioxide by Hasselberg (at Pulkowa), or

PIG. 74.— Absorption spectra of nitrogen dioxide oxide and iodine.

the spectra of colouring matters (Eder and others), and especially those applied to ortho-
chromatic photography, or the spectra of blood, chlorophyll (the green constituent of
leaves), and other similar substances, all the more as by the aid of their spectra the
presence of these substances may be discovered in small quantities (even in microscopical
quantities, by the aid of special appliances or the microscope), and the changes they
undergo investigated.

The absorption spectra, obtained at the ordinary temperature and proper to substances
in all physical states, offer a most vast but as yet little studied field, both for the theory
of all spectroscopy, and for gaining an insight into the structure of substances. The
investigation of colouring matters has already shown that in certain cases a definite
change of composition and structure entails not only a definite change of the colours but
also a displacement of the absorption bands by definite wave lengths. This subject,
which has only begun to be worked out, promises great scientific fruition.

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 559

lines may be ascribed to the absorption of certain rays of light in its
passage from the luminous mass of the sun to the earth. The remark-
able progress made in all spectroscopic research dates from the in-
r>>xti Cations <)»<"/<• !>;/ Kirchhoff (I&5$) on the relation between absorption
spectra and the spectra of luminous incandescent gases. It had already
been long since observed (by Frauenhofer, Foucault, Angstrom) that
the bright spectrum of the sodium flame gives exactly the same two
bright lines which are known as the line D in the solar spectrum, and
expressed by black lines which evidently belong to an absorption spec-
trum. When Kirchhoff caused moderated sunlight to fall upon the slit
of a spectroscope, and placed a sodium flame before it, a perfect super-
position was observed— the bright sodium lines completely covered
the black lines D of the solar spectrum. When later the continuous
spectrum of a Drummond light appeared with the black line D on placing
a sodium flame between it and the slit of the spectroscope — that is, when
the Frauenhofer line of the solar spectrum was artificially produced—
then there was no doubt that it was seen in the solar spectrum because
the light somewhere travelled through incandescent vapours of sodium.
Hence a new theory of reversed spectra2^ arose — that is, respecting the
relation between the waves of light emitted and absorbed by a substance
under given conditions of temperature, which is expressed by Kirchhoff's
law discovered by a careful analysis of the relations between the lumi-
nous rays absorbed and emitted by a substance. This law of the theory
of light may be formulated in an elementary form in the following
manner : At a given temperature the relation between the intensity
of the light emitted (of a definite wave length) and the absorptive
capacity with respect to the same colour (of the same wave length) is
a constant quantity.30 As a black dull surface emits a considerable

29 A number of methods have been invented to demonstrate the reversibility of
spectra; among these methods we will cite two which are most easily carried out. In
Bun sen's method sodium chloride is put into an apparatus for evolving hydrogen (the
spray of the salt is then carried off by iftie hydrogen and colours the flame with the yellow
sodium colour), and the hydrogen is ignited in two burners — in one large one with a wide
flame giving a bright yellow sodium light, and in another with a small fine orifice whose
flame is pale : this flame will throw a dark patch on the large bright flame. In Ladoffsky's
method the front tube (p. 552) is unscrewed from a spectroscope directed towards the light
of a lamp (a continuous spectrum), and the flame of a spirit lamp coloured by a small
quantity of NaCl is placed between the tube and the prism ; a black band corresponding
to sodium will then be seen on looking through the ocular tube. This experiment is
always successful if there be only the requisite relation between the strength of the light
of both lamps.

50 The absorptive capacity is the relation between the intensity of the light (of a
given wave length) falling upon and retained by a substance. Bunsen and Roscoe
showed by direct experiment that this ratio is a constant quantity for every substance.
If A stand for this ratio for a given substance at a given temperature — for instance, for
a flame coloured by sodium — and E be the intensity of the light of the same wave length

560 PRINCIPLES OF CHEMISTRY

quantity of heat rays and also absorbs a considerable quantity,
whilst a polished metallic surface both absorbs but few and emits but
few, so a flame coloured by sodium emits a considerable quantity
•of yellow rays of a definite refrangibility, and has the property of
retaining a considerable quantity of the rays of the same refractive
index. In general, the sphere which evolves definite rays also retains
them.

Thus the bright spectral rays characteristic of a given metal may
be reversed — that is, converted into dark lines — by passing light which
gives a continuous spectrum through a space containing the heated vapours
of the given metal. A similar phenomenon to that thus artificially pro-
duced may be observed naturally in sunlight, which shows dark lines
characteristic of known metals — that is, the Frauenhofer lines form an
absorption spectrum or depend on a reversed spectrum ; it being pre-
.supposed that the sun itself, like all known sources of artificial light,
gives a continuous spectrum without Frauenhofer lines.31 We must
imagine that the sun, owing to the high temperature which is proper to it,
'emits a brilliant light which gives a continuous spectrum, and that this
light, before reaching our eyes, passes through a space full of the vapours
•of different metals and their compounds. As the earth's atmosphere32
•contains no, or very little, metallic vapours, and as they cannot be sup-
posed to exist in the heavenly space, therefore the only place in which
the existence of such vapours can be admitted is in the atmosphere
surrounding the sun itself. As the cause of the sun's luminosity must
be looked for in its high temperature, therefore the existence of an
atmosphere containing metallic vapours is readily understood, because
at its high temperature such metals as sodium, and even iron, are sepa-
rated from their compounds and converted into vapour. The sun must
be imagined as surrounded by an atmosphere of incandescent vaporous

emitted at the same temperature by the same substance, then Kirchhoff s law, the ex-
planation and deduction of which must be looked for in text books of physics, states that
the fraction A/JE is a constant quantity depending on the nature of a substance (as A
depends on it) and determined by the temperature and wave length.

31 Heated metals begin to emit light (only visible in the dark) at about 420° (vary-
ing with the metal). On further heating solids first emit red, then yellow, and lastly
white light. Compressed or heavy gases (see Chap. III. Note 44), when strongly heated,
also emit white light. Heated liquids (for example, molten steel or platinum) also

•give a white compound light. This is readily understood. In a dense mass of matter
the collisions of the molecules and atoms are so frequent that waves of only a few
definite lengths cannot appear ; the reverse is possible in rarefied gases or vapours.

32 Brewster, as is mentioned above, first distinguished the atmospheric, cosmical
Frauenhofer lines from the solar lines. Janssen showed that the spectrum of the atmo-
sphere contains lines which depend on the absorption produced by aqueous vapour.
Egoreff, Olszewski, Janssen, and Liveing and Dewar showed by a series of experiments
that the oxygen of the atmosphere determines certain lines of the solar spectrum,

•especially the line A.

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 561

and gaseous matter,33 including those elements whose reversed spectra
correspond with the Frauenhofer lines — namely, sodium, iron, hydrogen,
lithium, calcium, magnesium, tkc. Thus in spectrum analysis we find
a means of determining the composition of the inaccessible heavenly
luminaries, and much has been done in this respect since Kirchhoff's
theory was formulated. By observations on the spectra of many
heavenly bodies, changes have been discovered going on in them,34 and
certain of the elements known to us have been found with certainty in
them.3"' From this it must be concluded that the same elements which
exist on the earth occur throughout the whole universe, and that at

35 Eruptions, like our volcanic eruptions, but on an incomparably larger scale, are a
frequent occurrence on the sun. They are seen as protuberances visible during a total
eclipse of the sun, in the form of vaporous masses on the edge of the solar disc and
emitting a faint light. These protuberances of the sun are now observed at all times by
means of the spectroscope (Lockyer's method), because they contain luminous vapours
(giving bright lines) of hydrogen and other elements.

54 The great interest and vastness of astro-physical observations concerning the sun,
comets, stars, nebulae, &c., render this new province of natural science very important,
and necessitate referring the reader to special works on the subject. I cannot, how-
ever, but caution the reader against those premature conclusions which many form on a
first acquaintance with the subject. Just as the astromomer may easily arrive at an
arbitrary conclusion about the composition of elements, from observations upon the
spectra of the heavenly bodies, so the chemist may easily fall into error as to the nature
of the phenomena of the heavens by only judging from spectroscopic observations.

The most important astro-physical data since the time of Kellner are those referring
to the displacement of the lines of the spectrum. Just as a musical note changes its
pitch with the approach or withdrawal of the resonant object or the ear, so the pitch of
the luminous note or wave length of the light varies if the luminous vapour and the
earth from which we observe it approach or recede from each other ; this expresses
itself in a visible displacement of the spectral lines. The solar erruptions even give
broken lines in the spectrum, because the rapidly moving eruptive masses of vapour and
gases either travel in the direction of the eye or fall back towards the sun. As the
earth travels with the solar system among the stars, so it is possible to determine the
direction and velocity with which the sun travels in space by the displacement of the
spectral lines and light of the stars.

The changes proceeding on the sun in its mass, which must be pronounced as
vaporous, and in its atmosphere, are now studied by means of the spectroscope. For
this purpose, there now exist many special astro-physical observatories where these
investigations are carried on.

35 Spectrum analysis has proved the indubitable existence in the sun and stars of
a number of elements known in chemistry. Huggins, Secchi, and others have furnished
a large amount of material upon this subject. A compilation of existing information
on this subject has been given by Prof. S. A. Kleiber, in the Journal of the Kussian
Physico-chemical Society for 1885 (vol. xviii. p. 146). Besides which, a peculiar element
called helium has been discovered, which is characterised by a line (whose wave length
is 587'5, situated nearD), which is seen very brightly in the projections (protuberances)
and spots of the sun, but which does not belong to any known element and is not repro-
ducible as a reversed, dark line. This may be a right conclusion — that is to say, it is pos-
sible that an element may be discovered to which the spectrum of helium corresponds —
but it may be that the helium line belongs to one of the known elements, because
spectra vary in the brilliancy and position of their lines with changes of temperature
and pressure. Thus, for instance, Lockyer could only see the line 428, at the very end

VOL. I. 00

562 PRINCIPLES OF CHEMISTRY

that degree of heat which is proper to the sun those simple matters
which we accept as the elements in chemistry are still undeconiposed

of the calcium spectrum, at comparatively low temperatures, whilst the lines 397 and
893 appear at a higher temperature, and at a still higher temperature the line 423-
becomes quite invisible.

Lockyer, to whom spectroscopy is indebted for many excellent spectroscopic observa-
tions, supposes that the elements existing on the earth are decomposed at the tempera-
ture of the sun — for instance, that iron decomposes into two new elements having par-
ticular spectra, because an unequal intensity of all the lines of iron is observed in dif-
ferent portions of the sun (spots, prominences, &c.), and because a displacement (Note
34) of certain of the lines of this metal may be observed in the sun's spots at the same
time that other lines remain undisplaced. Lockyer supposes this to depend on the
movement of one component part of iron with an immobility of the position (in space) of
the other product of the decomposition of iron which remains in a lower stratum. Prof.
Kleiber explains these phenomena by the fact that the visible spectrum of the sun is
determined by the entire thickness of the atmosphere of the sun, by the inequality of the
movement of different strata of the sun's atmosphere, and by the fact that lines of dif-
ferent wave lengths offer a different constant with reference to Kirchhoff's law. It is
enough if the thickness, pressure, and temperature of an incandescent vapour be dif-
ferent in a laboratory experiment and in a given stratum of the solar atmosphere to
obtain a marked difference in the intensity of the light of different bands of the spectrum
of one and the same element. As regards the displacement of only one portion of the
iron lines, Kleiber supplements Liveing and Dewar's observation that the displaced lines
are those emitted by the most rarefied vapours (Note 27), by pointing out that in the
first place one and the same line is sometimes observed on the sun both in a contorted
(broken) and normal position (as Lockyer himself observe!), and in the second place that
the intensity of the light of different lines depends on the different temperatures and den-
sities of the strata of the solar atmosphere, and therefore the lines determined by the
upper strata may, by their movement, be displaced, whilst the other lines determined by
the lower strata may remain unchanged. Besides which I may observe, for my part,
that if, under ordinary conditions, we see the normal spectrum of iron in the sun, Lock-
yer's supposed component parts of iron must occur together in the sun, and therefore it
is difficult to understand how one component part of iron is able to move whilst the
other remains at rest. Furthermore, as the solar spectrum of iron entirely corresponds
with that obtained by experiment at the comparatively low temperatures obtainable in
the laboratory, it is necessary to admit one of the two following propositions — either
the decomposition of iron does not require so high a temperature as that on the sun, and
then it would be easy to prove this supposed complexity of iron by a laboratory experi-
ment, or it must be admitted that the two component parts of iron when combined to-
gether (forming undecomposed iron) do not alter the positions of their spectral lines,
whilst, as we shall see later, the spectra of elements change when they combine together,
and therefore that in the case of iron a proposition must be made contradictory to this
fact. Other data put forward by Lockyer in favour of the decomposability of certain
elements proved when re-investigated (by Liveing and Dewar), by means of spectroscopes
of high dispersive power, to be only founded on the confusion of dissimilar lines. The
arguments of Lockyer concerning the decomposability of elements, which at one time
made a great impression, do not thus appear to support the doctrine of a single common
material (p. 20, Note 26), but must be made use of for the further development of spec-
troscopic science. Besides which I consider it well to turn attention to the facts (1) that the
conception of the elements stands in all respects more firmly than any deductions obtained
by the spectroscope; (2) this comparatively young doctrine of the spectra of elements is
only the fruit of the chemical doctrine of elements; and (3) that as yet no generalisa-
tions, besides Kirchhoff's law, have appeared for spectroscopic phenomena, which could
allow predictions being made, whilst the conception of elements has already arrived at
this stage. When, however, spectroscopic science has been developed to the same extent

ASSIOI, RtT.IDH'M, CAESIUM, AND LITHIUM 563

and ronmin uiifli.-ui^'d. A high temperature forms one of those con-
ditions under which compounds most easily decompose ; and therefore
it' sodium or a similar element were a compound, then in all probability
it would be decomposed into component parts at the high temper-
ature of the sun. This may already be concluded from the fact that
in ordinary spectroscopic experiments the spectra obtained often
belong to the metals and not to the compounds taken ; this depends
011 the decomposition of these compounds in the heat of the flame. If
common salt be introduced into the flame of a gas-burner, a portion of
it is decomposed, first forming, in all probability, with water, hydro-
chloric acid and sodium hydroxide, and the latter is then partially
decomposed by the hydrocarbons, giving metallic sodium, whose incan-
descent vapour emits light of a definite ref rangibility. This conclusion
is arrived at from the following experiment : — If hydrochloric acid gas
l>e introduced into a flame coloured by sodium it is observed that the
sodium spectrum disappears, owing to the fact that metallic sodium
cannot remain in the flame in the presence of an excess of hydrochloric
acid. The same thing takes place on the addition of sal-ammoniac,
which in the heat of the flame gives hydrochloric acid. If a porcelain
tube containing sodium chloride (or sodium hydroxide or carbonate), and
closed at both ends by glass plates, be so powerfully heated that the
salt volatilises, then the sodium spectrum is not observable ; but if the
salt be replaced by sodium, then both the bright line and the absorp-
tion spectra are obtained, according to whether the light emitted by the
incandescent vapour be observed, or only that which passes through the
tube. Thus tlie above spectrum is not given by sodium chloride or
other sodium compound, but is proper to the metal sodium itself. It
is the same with other analogous metals. The chlorides and other
halogen compounds of barium, calcium, copper, &c., give independent
spectra which differ from those of the metals. If barium chloride be
introduced into a flame, it gives a mixed spectrum belonging to metallic
barium and barium chloride. If besides barium chloride, hydrochloric
acid or sal-ammoniac be introduced into the flame, then the spectrum
of the metal disappears, and that of the chloride remains, which differs
distinctly from the spectrum of barium fluoride, barium bromide, or
barium iodide. A certain common resemblance and certain common
lines are observed in the spectra of two different compounds of one and
the same element obtained in the above-described manner, and also in.
the spectrum of the metal, but they all have their peculiarities. The

as chemical learning, then, perhaps, the present ideas will undergo a deep change and
approach perfection. As yet spectroscopic science is still, for want of laws, at the epoch
of the accumulation of facts and not of their possession.

002

564 PRINCIPLES OF CHEMISTRY

independent spectra of the compounds of copper are easily observed
(fig. 75). Thus certain compounds which exist and are luminous at a

i .in- it i inni

3*4 S 6 7 8

\;ilSI^f';illiirii«ai >! 1 1 1

F

311

Fig. 75. — Bright Spectra of Copper Compounds.

high temperature give their independent spectra. In the majority of
cases the spectra of compounds are composed of indistinct luminous lines
and complete bright bands, whilst metallic elements generally give a
few clearly- defined spectral lines.36 There is no reason for thinking

36 Spectroscopic observations are still further complicated by the fact that one and
the same substance gives different spectra at different temperatures. This is especially
the case with gases whose spectra are obtained by an electric discharge in tubes.
Pliicker, Wiillner, Schuster, and others showed that at different temperatures and pres-
sures the spectra of iodine, sulphur, nitrogen, oxygen, &c., are quite different from the
spectra of the same elements at high temperatures and pressures. This may either
depend on the fact that the elements change their molecular structure with a change of
temperature, just as ozone is converted into oxygen, or else because at low temperature
certain rays have a greater relative intensity than those which appear at higher tem-
peratures. If we suppose that the molecules of a gas are in continual movement, with a
velocity dependent on the temperature, then it must be admitted that they often strike
against each other and rebound, and thus communicate peculiar movements to each
other and the supposed ether, which express themselves in luminiferous phenomena. A
rise of the temperature or an increase in the density of a gas must have an influence on
the collision of its molecules and luminiferous movements thus produced, and this may
be the cause of the difference of the spectra under these circumstances. It has been
shown by direct experiment that gases compressed by pressure, when the collision of the
molecules must be frequent and varied, exhibit a more complex spectrum on the passage
of an electric spark than rarefied gases, and that even a continuous spectrum appears.
In order to show the variability of the spectrum according to the circumstances under
which it proceeds, it is enough to say that potassium sulphate fused on a platinum wire
gives, on the passage of a series of sparks, a distinct system of lines, 583-578, whilst
when a series of sparks is passed through a solution of this salt this system of lines is
faint, and when Roscoe and Schuster observed the absorption spectrum of the vapour of
metallic potassium (which is green) they remarked a number of lines of the same inten-
sity as the above system in the red, orange, and yellow portions.

The spectra of solutions are best observed by means of Lecoq de Boisbaudran's
arrangement, shown in fig. 76. A bent capillary tube, D F, inside which a platinum wire,

POTASSIUM, KUBIDJl'.M. CAKSlt'M. AND LlTHir.M

565

that the spectrum of a compound is equal to the sum of the spectra of
its elements — that is, every compound which is not decomposed by heat
Imx its oi<-)t pi'npi /• sjircfrtu/i. This is best proved by absorption spectra,
which are essentially only reversed spectra observed at low temperatures.
If every salt of sodium, lithium, and potassium gives one and the same
spectrum, this must be ascribed to the presence in the flame of the free
metals liberated by the decomposition of their salts. Therefore the phe-
nomena of tlf K/'f-t ni mare determined by molecules, and not by atoms —
that is, the molecules of the metal sodium, and not its atoms, produce
those forms of vibrations which are expressed in the spectrum of a
sodium salt. Where there is no free metallic sodium there is no
sodium spectrum.

Spectr a in ium/i/fiis has not only endowed science with a represent-
ation of the composition of the distant heavenly bodies (of the sun,
stars, nebulae, comets, &c.), but has also given a new method for study-
ing the matter of the earth's surface. Bunsen by its means discovered
two new elements belonging to the group of the alkali metals, and
thallium, indium, and gallium were afterwards discovered by the same
means. The spectroscope is employed in the study of rare metals
(which in solution often give distinct absorption spectra), of dyes, and
in general of many organic substances, &c.37 With respect to the

A a (from 0'3 to 0\") mm. in diameter) is fused, is immersed in a narrow cylinder, C (in,
which it is firmly held by a cork). The projecting end, a, of the
wire is covered by a fine capillary tube, d, which extends 1-2 mm.
beyond the wire. Another straight capillary tube, E, with a plati-
num wire, B Z>, about 1 mm. in diameter (a finer wire soon becomes
hot), is held (by a cork or in a stand) above the end of the tube,
D. If the wire A be now connected with the positive, and the
wire B with the negative, terminal of a Ruhmkorff's coil (if the
wires be connected in the opposite order, the spectrum of air is
obtained), a series of sparks rapidy following each other appeal-
between a and b, and their light may be examined by placing the
apparatus in front of the slit of a spectroscope. The variations to
which a spectrum is liable may easily be observed by increasing
the distance between the wires, altering the direction of the cur-
rent or strength of the solution, &c.

37 The importance of the spectroscope for the purpose of
chemical research was already shown by Gladstone in 1856, but
it did not become an accessory to the laboratory until after the
discoveries of Kirchhoff and Bunsen. It may be hoped that in
time spectroscopic researches will explain certain wants of the
theoretical (philosophical) side of chemistry, but as yet all that
has been done in this respect can only be regarded as attempts
which have not yet led to any trustworthy conclusions. Thus many,
by collating the wave lengths of all the light vibrations excited by a given element,
endeavour to find the law governing their mutual relations ; others (especially Hartley
and Ciamician), by comparing the spectra of analogous elements (for instance, chlorine,
bromine, and iodine), have succeeded in noticing definite features of resemblance in
them, whilst others (Griinwald) search for relations between the spectra of compounds

OB

FIG. 76.— Method of
showing the spec-
trum of substances
ill solution.

566 PRINCIPLES OF CHEMISTRY

metals which are analogous to sodium, they all give such very volatile
salts and such very characteristic spectra that the least traces of them38
are discovered with great ease by means of the spectroscope. For
instance, lithium gives a very brilliant red coloration to a flame and
a very bright red spectral line (wave length, 670 millionths mm.), which
indicates the presence of this metal in admixture with compounds of
other alkali metals.

Lithium, Li = 7, is, like potassium and sodium, somewhat widely
spread in siliceous rocks, but only occurs in small quantities and as
mere traces in considerable masses of potassium and sodium salts. Only
a very few minerals contain more than traces of it.39 The compounds
of lithium are in all respects closely analogous to the corresponding
compounds of sodium and potassium ; but the carbonate is sparingly

and their component elements, &c. ; but — owing to the mutiplicity of the spectral lines
proper to many elements, and (especially in the ultra-red and ultra-violet ends of the
spectrum) the existence of lines which are undistinguishable owing to their faintness,
and also owing to the comparative novelty of spectroscopic research — this subject cannot
be considered as in any way perfected in any of its branches.

58 In order to show the degree of sensitiveness of spectroscopic reactions it will be
enough to cite the following observation of Dr. Bence Jones. If a solution of 3 grains of
a lithium salt be injected under the skin of a guinea-pig, then, after the lapse of four
minutes, lithium may already be discoverved in the bilo and liquids of the eye, and,
after ten minutes, in all parts of the animal.

39 Thus spodumene contains up to 6 p.c. of lithium oxide, and petoljte, and lepidolitc
or lithia mica, about 3 p.c. of lithium oxide. This mica is met with in certain granites
in a somewhat considerable quantity, and is therefore most frequently employed for the
preparation of lithium compounds. The treatment of lepidolite is carried on on a large
scale, because certain salts of lithium are employed in medicine as a remedy for certain
diseases (stone, gouty affections), as they have the power of dissolving the insoluble
uric acid which is then deposited. Lepidolite, which is unacted on by acids in its
natural state, decomposes under the action of strong hydrochloric acid after it has been
fused. After being subjected to the action of the hydrochloric acid for several hours all
the silica is obtained in an insoluble form, whilst the metallic oxides pass into solution
as chlorides. This solution is mixed with nitric acid to convert the ferrous salts into
ferric, and sodium carbonate is then added until the liquid becomes neutral, by which
means a precipitate is formed of the oxides of iron, alumina, magnesia, iV'c., as insoluble
oxides and carbonates. The solution (with an excess of water) then contains the chlo-
rides of the alkaline metals KC1, NaCl, LiCl, which do not give a precipitate with
sodium carbonate in a dilute solution. It is then evaporated, and a strong solution of
sodium carbonate added. This precipitates lithium carbonate, which, although soluble
in water, is much less so than sodium carbonate, and therefore the latter precipitates
lithium from strong solutions as carbonate 2LiCl + Na2CO5 = 2NaCl + Li2CO5. Lith in in
carbonate, which resembles sodium carbonate in many respects, is a substance which is
very slightly soluble in cold water and is only somewhat soluble in boiling water. In
this respect lithium forms a transition between the metals of the alkalis and other
metals, especially the metals of the alkaline earths (magnesium, barium), whose carbo-
nates are only sparingly soluble. Oxide of lithium, Li2O, may be obtained by heating
lithium carbonate with charcoal. Lithium oxide in dissolving gives (per molecule)
26000 heat units ; but the combination of Li2 with O evolves 140000 calories — that is,
more than Na^O (100000 calories) and K2O (97000 calories), as shown by Beketoff
<1887).

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 567

soluble in cold water, which fact is taken advantage of for separating
lithium from potassium and sodium. This salt, Li.2CO3, is easily con-
verted into the other compounds of lithium. Thus, for instance, the
lithium hydroxide, LiHO, is obtained in exactly the same way as caustic
soda, by the action of lime on the carbonate, and it is soluble in water
and crystallises (from its solution in alcohol) as LiHO,H2O. Metallic
lithium is obtained by the action of a galvanic current on fused lithium
chloride ; for this purpose a cast-iron
crucible, furnished with a stout cover,
is filled with lithium chloride, heated
until the latter fuses, and then a strong
galvanic current is passed through the
molten mass. The positive pole (fig. 77)
consists of a dense carbon rod C (sur-
rounded by a porcelain tube P fixed in
an iron tube B B), and the negative pole
of an iron wire, on which the metal is

. FIG. 77.— Preparation of lithium by the

deposited alter the current has passed action of a galvanic current on fused
,. ,, , . , ,. ,, lithium chloride.

the molten mass for a certain length or

time. Chlorine is evolved at the positive pole. When a somewhat
considerable quantity of the metal has accumulated on the wire it is
withdrawn, the metal is collected from it, and the experiment is then
carried on as before. Lithium is the lightest of all metals, its specific
gravity is 0'59, owing to which it even floats on naphtha ; it melts at
180°, but does not volatilise at a red heat. Its appearance recalls that
of sodium, and, like it, it has a yellow tint. At 200° it burns in air
with a very bright flame, forming lithium oxide. In decomposing
water it does not ignite the hydrogen. The characteristic test for
lithium compounds is the red coloration which they impart to a colour-
less flame.40

Bunsen tried to determine by means of the spectroscope whether
any other as yet unknown metals do not occur in different natural
products together with lithium, potassium, and sodium, and he soon
discovered two new alkali metals showing independent spectra. They
are named after the characteristic coloration which they impart to the

40 In determining the presence of lithium in a given compound, it is best to treat the
material under investigation with acid (in the case of mineral silicon compounds hydro-
fluoric acid must be taken), and to treat the residue with sulphuric acid, evaporate" to
dryness, and extract with alcohol, which dissolves a certain amount of the lithium sul-
phate. It is easy to discover lithium in such an alcoholic solution by means of the colo-
ration imparted to the flame on burning it, and in case of doubt by investigating its
light in a spectroscope, because lithium gives a red line, which is very -characteristic and
is found as a dark line in the solar spectrum. Lithium was first discovered in 1817 in
petolite by Arfvedson.

568 PRINCIPLES OF CHEMISTRY

flame. One which gives a red and violet band is named rubidium,
from rubidius (dark red), and the other is called caesium, because it
colours a pale flame sky blue, which depends on its containing bright
blue rays, which appear in the spectrum of caesium as two blue bands.
Both metals accompany sodium, potassium, and lithium, but in small
quantities ; rubidium occurs more frequently than caesium. The amount
of the oxides of caesium and rubidium in lepidolite does not generally
exceed one-half p.c. Rubidium has also been found in the ashes of many
plants, but it does not seem to accompany potassium in sea water.
Rubidium also occurs, although in very small quantities, in the majority
of mineral waters. In a very few cases caesium is not accompanied by
rubidium ; thus, in a certain granite on the Isle of Elba, caesium has
been discovered, but not rubidium. This granite contains a very rare
mineral called pollux, which contains as much as 34 p.c. of caesium
oxide.41 Guided by the spectroscope, and aided by the fact that the
double salts of platinic chloride and rubidium and caesium chlorides
are still less soluble in water than the corresponding potassium salt,
K2PtCl6, Bunsen succeeded in separating both metals from each other
and from potassium, and demonstrated the great resemblance they
bear to each other. The isolated metals,42 rubidium and caesium, have

41 The salts of the majority of metals are precipitated as carbonates on the addition
of ammonium carbonate — for instance, the salts of calcium, iron, etc. The alkalis
whose carbonates are soluble are not, however, precipitated in this case. On evaporat-
ing the resultant solution and igniting the residue (to remove the ammonium salts), we
obtain salts of the alkali metals. They may be separated by adding hydrochloric acid
with a solution of platinic chloride. The chlorides of lithium and sodium give easily-
soluble double salts with platinic chloride, whilst the chlorides of potassium, rubidium
and caesium form double salts which are sparingly soluble. A hundred parts of water
at 0° dissolve 0'74 part of the potassium platinochloride ; the corresponding rubidium
platinochloride is only dissolved to the amount of 0'184 part, and the caesium salt,
0'024 part; at 100° 5'13 parts of potassium platinochloride, K2PtCl6, is dissolved,
0'634 parts of rubidium platinochloride, and 0'177 parts of caesium platinochloride.
From this it is clear how the salts of rubidium and caesium may be isolated. The
separation of caesium from rubidium by this means is very lengthy. It is better
effected by taking advantage of the difference of the solubility of their carbonates in
alcohol ; caesium carbonate, Cs2CO5, is soluble in alcohol, whilst the corresponding salts
of rubidium and potassium are almost insoluble. Setterberg separated these metals ;is
alums, but the best method, that given by Scharples, is founded on the fact that from
a mixture of the chlorides of potassium, sodium, caesium, and rubidium in the pre-
sence of hydrochloric acid stannic chloride precipitates a double salt of caesium, which is
very slightly soluble. The salts of Rb and Cs are closely analogous to those of
potassium.

42 Bunsen obtained rubidium by distilling a mixture of the tartrate with soot, and
Beketoff (1888) by heating the hydroxide with aluminium 2RbHO + Al = Rb A102 +
H2 + Rb. By the action of 85 grams of rubidium on water, 94000 heat units are
evolved. Setterberg obtained caesium (1882) by the electrolysis of a fused mixture of
cyanide of caesium and of barium.

POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIf.M 569

specific gravities of T52 and 1-88 respectively, and melt at 39° and
27°.

Judging by the properties of the free metals, and of their corre-
sponding and even very complex compounds, lithium, sodium, potas-
sium, rubidium, and caesium present an indubitable chemical resem-
blance. The fact that the metals easily decompose water, and that
their hydroxides RHO and carbonates R2CO3 are soluble in water,
whilst the hydroxides and carbonates of nearly all other metals are
insoluble, confirms this, and if the resemblance between the correspond-
ing salts be taken into consideration there is no doubt that the resem-
blance in the chemical character of these metals is very considerable :
therefore they form a natural group of alkali metals. The halogens
and the alkali metals form, by their character, the two extremes of
the elements. Some of the other elements are metals approaching in a
certain degree the alkali metals, both in their capacity of forming salts
and in not forming acid compounds, but are not so energetic as the alkali
metals, and are displaced by the latter from the majority of their com-
pounds ; they also evolve less heat in combining with the halogens, and
form less energetic bases than the alkali metals. Such are the common
metals, silver, iron, copper, <tc. Some other elements, in the character
of their compounds, approach the halogens, and, like them, combine
with hydrogen, but these compounds do not show the energetic property
of the halogen acids ; in a free state they easily combine with metals,
but they do not then form such saline compounds as the halogens do —
in a word, the halogen properties are less sharply defined in them than
in the halogens themselves. Sulphur, phosphorus, arsenic, &c., belong
to this order of elements. Lastly, there is yet another order of elements,
like carbon and nitrogen, in which neither the metallic nor the halogen
properties are clearly defined, and which in this respect occupy an
intermediate position between the two above-mentioned orders of ele-
ments.

The clear distinction of the properties of the halogens and alkali
metals is expressed in the fact that the former give acids and do not
form bases, whilst the latter, on the contrary, only give bases. The
first are true acid elements, the -latter clearly-defined basic or metallic
elements. On combining together, the halogens form, in a chemical
sense, unstable compounds, and the alkali metals alloys in which the
character of the metals remains unaltered, just as in the compound
IC1 the character of the halogens remains undisguised ; thus both classes
of elements on combining with members of their own class form non-
characteristic compounds, which have the properties of their compo-
nents. On the other hand, the halogens on combining with the alkali

570 PRINCIPLES OF CHEMISTRY

metals form compounds which are, in all respects, stable, and in which
the original characters of the halogens and alkali metals have entirely
disappeared. The formation of such compounds is accompanied by
evolution of a large amount of heat, and by an entire change of both
the physical and chemical properties of the substances originally taken.
The alloy of sodium and potassium, although liquid at the ordinary
temperature, is perfectly metallic, like both its components. The
compound of sodium and chlorine has neither the appearance nor the
properties of the original elements ; sodium chloride melts at a higher
temperature, and is more difficultly volatile, than either sodium or
chlorine.

With all these qualitative differences there is, however, an important
quantitative resemblance between the halogens and the alkali metals.
This resemblance is clearly expressed by stating that both orders of
elements belong to those which are univalent with respect to hydrogen.
It is thus possible to express that both the above-named orders of ele-
ments replace hydrogen atom .for atom. Chlorine is able to take the
place of hydrogen by metalepsis, and the alkali metals take the place
of hydrogen in water and acids. As it is possible to consecutively re-
place every equivalent of hydrogen in a hydrocarbon by chlorine, so it
is possible in an acid containing several equivalents of hydrogen to
replace the hydrogen consecutively equivalent after equivalent by an
alkali metal ; hence an atom of these elements is analogous to an
atom of hydrogen, which is taken, in all respects, as the unit for the
comparison of the other elements. In ammonia and in water chlorine
and sodium are able to bring about a direct replacement. According to
the law of substitution, the formation of sodium chloride, NaCl, already
shows the equivalence of the atoms of the alkali metals and the halo-
gens. The halogens and hydrogen and the alkali metals combine with
such elements as oxygen, and it is easily seen that in such compounds
one atom of oxygen is able to retain two atoms of the halogens, of
hydrogen, and of the alkali metals. In this respect it is enough to
compare the compounds KHO, K2O, HC1O, and C12O, with water. It
must not be forgotten, however, that with oxygen the halogens give
higher acid grades of oxidation, besides compounds of the type R2O>
which the alkali metals and hydrogen are not capable of forming. We
shall soon see that these relations are also subject to a special law,
showing the gradual transition of the properties of the elements from
the alkali metals to the halogens.43

45 We may here observe that the halogens, and especially iodine, may play the part
of metals (hence iodine is more easily replaced by metals than the other halogens, and it
approaches nearer to the metals in its physical properties than the other halogens).

POTASSIUM, RUBIDIUM, CAESJt .M. AND LITHIUM 571

The atomic weights of the alkali metals, lithium 7, sodium 23, po-
tassium 39, rubidium 85, and caesium 133, show that here, as in the
halogens, the elements may be arranged according to their atomic
weights in order to compare the properties of the analogous compounds
of the members of this group. Thus, for example, the platinochlorides
of lithium and sodium are soluble in water ; those of potassium,
rubidium, and caesium sparingly soluble, and the greater the atomic
weight of the metal the less soluble is the salt. In other cases the
reverse is observed— the greater the atomic weight the more soluble
are the corresponding salts. The variation of properties with the varia-
tion in atomic weights even shows itself in the metals themselves ; thus
lithium volatilises with difficulty, whilst sodium is obtained by means
of distillation, potassium volatilises more easily than sodium, and rubi-
dium and caesium, as we have seen, are still more volatile.

Schiitzenberger obtained a compound C2H3O(OC1), which he called chlorine acetate, by
acting on acetic anhydride, (C^HjOJoO, with chlorine monoxide, CloO. With iodine this
compound disengages chlorine and forms iodine acetate, C2H3O(OI), which also is formed
by the action of iodine chloride on sodium acetate, C.2H5O(ONa). Such compounds are
very unstable, decompose with an explosion when heated, and are changed by the action
of water and of many other re-agents, which is in accordance with the fact that they con-
tain very closely-allied elements, as CloO itself, or IC1 or KNa. By the action of chlorine
monoxide on a mixture of iodine and acetic anhydride, Schiitzenberger also obtained the
compound ^C^H^O^o, which is analogous to IC^, because the group CgHjOo is, like Cl,
a halogen, forming salts with the metals.

572 PRINCIPLES OF CHEMISTRY

CHAPTER XIV

THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM,
CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM ,

IT is easy by investigating the composition of corresponding com-
pounds, to establish the equivalent weights of the metals — that is, the
quantity which replaces one part by weight of hydrogen. If a metal
directly, decomposes acids, with the evolution of hydrogen, then the
equivalent weight of the metal may be determined by taking a definite
weight of it and measuring the volume of hydrogen evolved by its
action on an excess of acid ; then it is easy to calculate the weight
of the hydrogen from its volume.1 The same result may be arrived at
by determining the composition of the normal salts of the metal ; for
instance, by finding the weight which combines with 35 '5 parts of
chlorine or 80 parts of bromine.2 The equivalent of a metal may be
also learnt by simultaneously (i.e., in one circuit) decomposing an acid
and a fused salt of a given metal by an electric current and determin-
ing the relation between the. amounts of hydrogen and metal separated,
because, according to Faraday's law, electrolytes (conductors of the
second order) are always decomposed in equivalent quantities. The
equivalent of a metal may even be found by simply determining the
relation between the weight of a metal and of its salt giving oxide,
as by this weight we know the weight of the metal which combines
with 8 parts by weight of oxygen, and this will be the weight of an
equivalent, because 8 parts of oxygen combine with 1 part by weight of

1 Under favourable circumstances (by taking all the requisite precautions), the weight
of the equivalent may be accurately determined by this method. Thus Reynolds and
Ramsay (1887) determined the equivalent of zinc to be 32'7 by this method (from the
average of 123 experiments), whilst by other methods it has been fixed (by different
observers) between 82'55 and 33'95.

The difference in their equivalents may be demonstrated by taking equal weights of
different metals, and by collecting the hydrogen evolved by them (under the action of an
acid or alkali).

2 The most accurate determinations of this kind were carried on by Stas, and will be
described in speaking of silver.

THE VALENCY AND SPECIFIC HEAT OF THE METALS 573

hydrogen. One method is verified by another, and all the processes for
the accurate determination of the equivalents require the most accurate
methods to avoid the absorption of moisture, of further 9xidation,
volatility, and other like circumstances which influence exact weigh-
ings. The description of the methods necessary for the attainment of
exact results belongs to the province of analytical chemistry.

For univalent metals, like those of the alkalis, the weight of the
equivalent is equal to the weight of the atom. For bivalent metals
the atomic weight is equal to the weight of two equivalents, for 7i-valent
metals it is equal to the weight of n equivalents. Thus aluminium,
Al=27, is trivalent, that is, its equivalent = 9 ; magnesium, Mg=24,
is bivalent, and its equivalent =12. Therefore, if potassium or sodium,
or in general a univalent metal, M, gives compounds M2O, MHO,
MCI, MNO3, M2S04, &c., and in general MX, then for bivalent
metals like magnesium or calcium the corresponding compounds
will be MgO, Mg(HO)2, MgCl2, Mg(]STO3)2, MgSO4, «fec., or in general
MX2.

By what are we to be guided in ascribing to some metals uni-
valency and to others bi- ter- quadri-. . . n- valency 1 What obliges
us to make this difference 1 Why are not all metals given the same
valency — for instance, why is not magnesium considered as univalent ?
If this be done, taking Mg=12 (and not 24 as now used), not only is
a simplicity of expression of the composition of all the compounds of
magnesium attained, but also we gain the advantage that their com-
position will be the same as those of the corresponding compounds of
sodium and potassium. These compositions were so expressed before,
why has this been changed now ?

These questions could only be answered after the establishment of
the conceptions of multiples of the atomic weights as the minimum
quantities of the elements combining together to form compounds — in
a word, since the time of the establishment of Avogadro-Gerhardt's law
(Chap. VII.). By taking such an element as arsenic, which has many
volatile compounds, it is easy to determine the density of these com-
pounds, and therefore to establish their molecular weights, and hence
to find the indubitable atomic weight, exactly as for oxygen, nitrogen,
chlorine, carbon, &c. It appears that As =7 5, and its compounds cor-
respond, like the compounds of nitrogen, with the forms AsX3 and
AsX5 ; for example, AsH3, AsCl3, AsFl5, As.,O5, etc. It is evident
that we are here dealing with a metal (or, better, element) of two
valencies, and which is furthermore in no way univalent, but tri- or
quinqui-valent. This example alone is sufficient for the recognition of
the existence of polyvalent atoms among the metals. And as antimony

574 PRINCIPLES OF CHEMISTRY

and bismuth are closely analogous to arsenic in all their compounds,
as potassium is analogous to rubidium and caesium ; therefore,
although very few volatile compounds of bismuth are known, it was
necessary to ascribe to them formulae corresponding with those ascribed
to arsenic.

As we shall see in describing them, there are also many analogous
metals among the bivalent elements, some of which also give volatile
compounds. For example, zinc, which is itself volatile, gives several
volatile compounds (for instance, zinc ethyl, ZnC4H10, which boils at
118°, vapour density =6 1-3), and in the molecules of all these com-
pounds there is never less than 65 parts of zinc, which is equivalent to
H2, because 65 parts of zinc displace 2 parts by weight of hydrogen ; so
that zinc is just such an example of the bivalent metals as oxygen,
whose equivalent =8 (because H2 is replaced by O= 16), is a representa-
tive of the bivalent elements, or as arsenic is of the tri- and quinqui-
valent elements. And, as we shall afterwards see, magnesium is in
many respects closely analogous to zinc, which fact obliges us to regard
magnesium as a bivalent metal.

Such metals as mercury and copper, which are able to give not one
but two bases, are of particular importance for distinguishing univalent
and bivalent metals. Thus copper gives the suboxide Cu2O and the
oxide CuO — that is, the compounds CuX corresponding with the sub-
oxide are analogous (in the quantitative relations, by their composition)
to NaX or AgX, and the compounds of the oxide CuX2 to MgX2,
ZnX2, and in general to the bivalent metals. It is clear that in such
examples we must distinguish metals of varying atomicity.

In this manner the valency of many metals may be established
by means of certain and comparatively few volatile metallic com-
pounds, and by the aid of a search into their analogies (concerning
which see Chap. XV.). The law of specific heats discovered by Dulong
and Petit has frequently been applied to the same purpose3 in the

3 The chief means by which we determine the valency of the elements, or what
multiple of the equivalents should be ascribed to the atom, are : (1) The law of Avogadro-
Gerhardt. This method is the most general and trustworthy, and has already been
applied to a great number of elements. (2) The different grades of oxidation and their
isomorphism or analogy in general ; for example, Fe = 56 because the suboxide (ferrous
oxide) is isomorphous with magnesium oxide, &c., and the oxide (ferric oxide) contains
1£ times as much oxygen as the suboxide. Berzelius, Marignac, and others took advan-
tage of this method for determining the composition of the compounds of many elements.
(8) The specific heat, according to Dulong and Petit's law. Regnault, and more especially
Cannizzaro, used this method to distinguish univalent from bivalent metals. (4) The
periodic law (see Chapter XV.) has served as a means for the determination of the
atomic weights of cerium, uranium, yttrium, &c., and more especially of gallium,
scandium, and germanium. The correction of the results of one method by those

THE VALENCY AND SPECIFIC HEAT OF THE METALS 575

history of chemistry, especially since the development given to this law
by the researches of Regnault, and since Cannizzaro (1860) showed the
agreement between the deductions of this law and the consequences
arising from Avogadro-Gerhardt's law.

Dulong and Petit, having determined the specific heat of a number
of solid elementary substances, observed that as the atomic weights of
the elements increase, their specific heats decrease, and that the product
of the specific heat Q into the atomic weight A is an almost constant
<l>Kintity. This means that to bring different elements into a known
thermal state, an equal amount of work is required if atomic quantities
of the elements are taken ; that is, the amounts of heat expended in
heating equal quantities by weight of the elements are far from equal,
but are in inverse proportion to the atomic weights. For thermal
changes the atom is a unit ; all atoms, notwithstanding the difference of
weight and nature, are equal. This is the simplest expression of the
fact discovered by Duloiig and Petit. The specific heat measures that
quantity of heat which is required to raise the temperature of one unit
of weight of a substance by one degree. If the magnitude of the
specific heat of elements be multiplied by the atomic weight, then we
obtain the atomic heat — that is, the amount of heat required to raise
the temperature of the atomic weight of an element by one degree. It
is these products which for the majority of the elements prove to be-
approximately, if not quite, identical. A complete identity cannot be
expected, because the specific heat of one and the same substance varies
with the temperature, with its passage from one state into another, and
frequently with even a simple mechanical change of density (for in-
stance by hammering), not to speak of allotropic changes, £c. We will

of others if? generally had recourse to, and is quite necessary, because phenomena of dis-
sociation, polymerisation, &c., may complicate the individual determinations by each
method.

It will be well to observe that a number of other methods, especially from the province
of those physical properties which are clearly dependent on the magnitude of the atom
(or equivalent) or of the molecule, may lead to the same result. I may point out, for
instance, that even the specific gravity of solutions of the metallic chlorides (Chapter VII.
p. 822) may serve for this purpose. Thus if beryllium be taken as trivalent — that is, if
the composition of its chloride be taken as BeCl3 (or a polymeride of it), then the specific
gravity of solutions of beryllium chloride will not fit into the series of the other metallic
chlorides. But on ascribing to it an atomic weight Be = 7, or taking Be as bivalent, the
composition of its chloride as BeCl2, we arrive at the general rule given on p. 818. Thus
W. G. Burdakoff determined in my laboratory, that the specific gravity at 15°/4° of the
solution BeCl2 + 200H.2O = 1'0138— that is, greater than the corresponding solution
KC1 + 200JL.O (=1-0121), and less than the solution MgCl.^ + 200H2O (= T0203),
as would follow from the magnitude of the molecular weight BeClo = 80, because
KC1 = 74'5 and MgCLj = 95 (see my work Investigation of Aqueous Solutions, 1887,
p. 425).

576 PRINCIPLES OF CHEMISTRY

cite several figures4 proving the truth of the conclusions arrived at
by Dulong and Petit.

Li Xa Mg P

A= 7 23 24 31
Q= 0-9408 0-2934 0-245 0-202
AQ= 6-59 6-75 5-88 6-26

Fe Cu Zn Br

A= 56 63 65 80
Q= 0-112 0-093 0-093 0-0843
AQ= 6-27 5-86 6-04 6-74

Pel Ag Sn I

A= 106 108 118 127
Q^= 0-0592 0-056 0-055 0-541
AQ= 6-28 6-05 6-49 6-87

Pt Au Hg Pb

A= 196 198 200 206

Q= 0-0325 0-0324 0-0333 0-0315

AQ= 6-37 6-41 6-66 6-49

It is seen from this that the product of the specific heat of the
•element into the atomic weight is an almost contant quantity, which
is nearly 6. Therefore the possibility arises of judging the valency
with a sufficient degree of exactitude, by the specific heats of the

4 The specific heats here given refer to different limits of temperature, but in the
majority of cases between 0° and 100° ; only in the case of bromine the specific heat is
taken (for the solid state) at a temperature below — 7°, according to Regnault's deter-
mination. The variation of the specific heat with a change of temperature forms a
very complex phenomenon, the consideration of which I think would here be out of place.
I will only cite certain figures as an example. According to Bystrom, the specific heat of
iron at 0° = 0-1116, at 100° = 0-1114, at 200° = 0'1188, at 300° = 0'1267, and at 1400° =
*0"4081. Between these last limits of temperature a change takes place in iron (a spon-
taneous heating, recalescence), as we shall afterwards see. For quartz SiO2, Pionchon
gives Q = 0-1737 + 394210-6 - 27£210~9 up to 400° ; consequently, as a rule, the specific
'heat varies with the temperature. Still more remarkable are H. E. Weber's observations
on the great variation of the specific heat of charcoal, the diamond and boron :

0° 100° 200° 600° 900°

Wood charcoal 0'15 0'23 0'29 0'44 0'46

Diamond 010 0'19 0-22 0'44 0'45

Boron 0'22 0'29 0'85 —

These determinations (they have been verified by Dewar) are of especial importance as
confirming the universality of Dulong and Petit's law, because the elements mentioned
above form exceptions to the general rule when the mean specific heat for temperatures
between 0° and 100° is taken. Thus in the case of the diamond the product of Ax Q
at 0° = 1-2, and for boron =2'4. But if we take the specific heat towards which there is
^evidently a tendency with a rise of temperature, then we obtain a product approaching

TIIF. VALKNCV AND sl'KCIFIC HEAT OF THE METALS 577

metals. Thus, for instance, the specific heats of lithium, sodium, and
potassium convince one of the fact that their atomic weights are
indeed those which we took, because by multiplying the specific heats
found by experiment by the corresponding atomic weights we obtain
the following figures : Li, 6 -59, Na, 6 '75 and K, 6 '47. Of the
<dkaline earth metals the specific heats have been determined : of mag-
nesium = 0-245 (Regiiault and Kopp), of calcium=0'170 (Bunsen), and
of barium=0'05 (Mendeleeff). If the same composition be ascribed to
the compounds of magnesium as to the corresponding compounds of
potassium, then the equivalent of magnesium will be equal to 12. On
multiplying this atomic weight by the specific heat of magnesium, we
obtain a figure 2 '94, which is half that which is given by the other
elements, and therefore the atomic weight of magnesium must be taken
as equal to 24 and not to 12. Then the atomic heat of magnesium=
24 x 0-245 =5 '9 ; for calcium, giving its compounds a composition
CaX.j— for example CaCl2, CaSO4, CaO (Ca=40) — we obtain an atomic
heat=40x 0-17=6-8, and for barium it is equal to 137x0-05=6-8;
that is, they must be counted as bivalent, or that their atom replaces
H.>, Na2, or K2. This conclusion may be confirmed by a method of
analogy, as we shall afterwards see. A strict application of the prin-
ciple of specific heats to the determination of the magnitudes of the
atomic weights of those metals, the magnitude of whose atomic weights

to 6 as with other elements. Thus with the diamond and charcoal, it is evident that the
specific heat tends towards 0'47, which multiplied by 12 gives 5 '6, the same as for mag-
nesium and aluminium. I may here turn the reader's attention to the fact that for
solid elements having a small atomic weight, the specific heat varies considerably if we
take the average figures for temperatures 0° to 100° :

Li = 7 Be = 9 B =11 C = 12

Q = 0-94 0*42 0'24 0'20

AQ = 6'6 8-8 2-6 2'4

It is therefore clear that the specific heat of beryllium deteTmined at a low temperature
cannot serve for establishing its atomicity. On the other hand, the low atomic heat of
charcoal, graphite, and the diamond, boron, &c., may perhaps depend on the complexity
of the molecules of these elements. The necessity for acknowledging a great complexity
of the molecules of carbon was explained in Chapter VIII. In the case of sulphur the
molecule contains at least S6 and its atomic heat = 82 x 0'168 = 5'22, which is distinctly
below the normal. If a great number of atoms of carbon are gathered together in the
molecule of charcoal, this would to a certain extent account for its comparatively small
atomic heat. With respect to the specific heat of compounds it will not be out of place
to here mention the conclusion arrived at by Kopp, that the molecular heat (that is, the
product of MQ) may be looked on as the sum of the atomic heats of its component
elements ; but as this rule is not a general one, and can only be applied to an approxi-
mate judgment of the specific heats of substances, I do not think it necessary to go into
the details of the conclusions described in Liebig's ' Annalen Supplement-Band,' 1864 ;
which includes a number of determinations made by Kopp.

VOL. I. P P

578 PRINCIPLES OF CHEMISTRY

could not be determined Recording to Avogadro-Gerhardt's law, was
made about 1860 by the Italian professor Carmizzaro.

Exactly the same conclusions respecting the bivalence of magnesium
and its analogues are obtained by comparing the specific heats of their
compounds, especially of the halogen compounds as the most simple, with
the specific heats of the corresponding alkali compounds. Thus, for in-
stance, the specific heat of magnesium and calcium chlorides, MgCl2 and
CaCl2, equal O194 and 0'164, and of sodium and potassium chlorides,
NaCl and KC1, 0*214 andO'172, and therefore their molecular heats (or
the products QM, where M is the weight of the molecule) are 18 '4 and
18*2, 12'5 and 12 '8, and therefore the atomic heats (or the quotient of
QM by the number of atoms) are all nearly 6, as with the elements,
Whilst if, instead of the actual atomic weights Mg=24 and Ca=40,
their equivalents 12 and 20 be taken, then the atomic heats of the
chlorides of magnesium and calcium would be about 4'6, whilst those of
potassium and sodium chlorides are about 6'3.5

As the specific heat or the amount of heat required to raise the
temperature of a unit of weight one degree 6 is a complex quantity —

5 It must be remarked that in the case of oxygen (hydrogen and carbon) compounds
the quotient of MQ/w, where n is the number of atoms in the molecule, is always less
than 6 for solids; for example, in the case of MgO = 5*0, CuO= 5'1, MnOo = 4'6, ice
(Q = 0-504) = 3, Si02 = 3'5, &c. At present it is impossible to say whether this depends
on the smaller specific heat of the atoms of oxygen in its solid compounds (Kopp, Note 4)
or on some other reason ; but, nevertheless, taking into account this decrease, depending
on the presence of oxygen, a reflection of the atomicity of the elements may to a certain
extent be seen in the specific heat of the oxides. Thus in the case of alumina, A12O5
(Q = 0'217), MQ = 22'3, and therefore the quotient MQ/n, = 4'5, which is nearly that
given by magnesium oxide, MgO. But if we ascribe the same composition to alumina as
to magnesia — that is, if aluminium were counted as divalent — we should obtain the figure
3'7, which is much less. In general, in compounds of identical atomic composition
and of analogous chemical properties the molecular heats MQ are nearly equal, as
many investigators have long remarked. For example, ZnS = 11'7 and HgS = 11'8 ~
MgSO4 = 27'0 and ZnSO4 = 28'0, &c.

6 If W be the amount of heat contained in a mass m of a substance at a temperature
t, and dW the amount expended in heating it from t to t + dt, then the specific heat
Q = dWt(m x dt). The specific heat not only varies with the composition and complexity
of the molecules of a substance, but also with the temperature, pressure, and physical
state of a substance. Even for gases the variation of Q with t is to be observed. Thus
it is seen from the experiments of Regnault and Wiedemann that the specific heat of
carbonic anhydride at 0° = 0'19, at 100° = 0'22, and at 200° = 0'24. But the variation of
the specific heat of permanent gases with the temperature is, as far as we know, very in-
considerable. Therefore the specific heat of those permanent gases which contain two
atoms in the molecule (H2, Oo, N2, CO, and NO) may be, as is shown by experiment,
taken as not varying with the temperature. The constancy of the specific heat of perfect
gases forms one of the fundamental propositions of the whole theory of heat, and sup-
ports the determination of temperatures by means of gas-thermometers containing
hydrogen, nitrogen, or air. Le Chatelier (1887), on the basis of existing determinations,

. concludes that the molecular heat — that is, the product MQ — of all gases varies in propor-
tion to the temperature, and tends to become equal ( = 6'8) at the temperature of absolute
zero (that is, at —273°) ; and therefore MQ = 6'8 + a(273 + t), where a is a constant quantity

TIIK VALKNCV AND SPKCIKK1 HEAT OK TIIK M KTA Ls 579

including not only the increase of the energy of a substance with its
rise in temperature, but also the external work of expansion7 and the

which incro uses with the complexity of the gaseous molecule. The magnitude lOOOa for
ammcmia = <>-ll, for chloric anhydride = 7'42, for ethylene, C2H4 = 12-7, for chloroform,
CHC13 = 29'5, \-c. For permanent gases a = 0, and MQ = 6'8— that is, the atomic heat (if
tin- molecule contains two atoms) = 8'4, as it is in reality. As regards liquids (as well as the
rapoun formed by them), the specific heat always rises with the temperature: Thus for
benzene, it equals 0'38 + 0'0014£. R. Schiff (1887) showed that the variation of the specific
heat of many organic liquids is proportional to the change of temperature (as in the case
of gases, according to Le Chatelier), and reduced these variations into dependence with
their composition and absolute boiling point. It is very probable that the theory of
liquids will make use of these simple relations, which recall the simplicity of the varia-
tion of the specific gravity (Chap. II. Note 84), cohesion, and other properties of liquids
with the temperature. They are all expressed by the linear function of the temperature,
<t + bt, with the same degree of proximity as the property of gases is expressed by the
equation pv = Itt.

As regards the relation between the specific heats of liquids (of solids) and of their
vapours, the specific heat of the vapour (and also of the solid) is always less than
that of the liquid. For example, benzene vapour 0"22, liquid 0'38 ; chloroform vapour 0*18,
liquid 0"28 ; steam 0*475, liquid water I'O. But the whole complexity of the relation >
existing in specific heat is seen from the fact that the specific heat of ice = 0'502 is less
than that of liquid water. According to Regnault, in the case of bromine the specific
heat of the vapour = 0-055 (at 150°), of the liquid = 0'107 (at 80°), and of solid bromine
= 0'084 (at —15°). The specific heat of solid benzoic acid (according to experiment and
call illation, BoBfl L888) between 0J and 100° is 0'31, and of liquid benzoic acid 0'50. One
of the problems of the present day is the explanation of those complex relations which
exist between the composition and such properties as specific heat, latent heat, expansion
by heat, compression, internal friction, cohesion, and other like properties. They can
only be connected by a complete theory of liquids, which may now soon be expected,
more especially as many sides of the subject have already been partially explained.

7 According to the above reasons the quantity of heat, Q, required to raise the tem-
perature of one part by weight of a substance by one degree may be expressed by the
sum Q = K + B + D, where K is the heat actually expended in heating the substance, or
that which is termed the absolute specific heat, B the amount of heat expended in the
internal work accomplished with the rise of temperature, and D the amount of heat ex-
pended in external work. In the case of gases the last quantity may be easily deter-
mined, knowing their coefficient of expansion, which is approximately = 0'00368. By
applying to this case the same argument given at the end of Note 11, Chap I., we find
that one cubic metre of a gas heated by 1° produces an external work of 10333 x 0'00868,
or :;.^-ii2 kilngriimmi-uv>, on which 38'02 424 or 0;0897 heat units are expended. This is
the heat expended for the external work produced by one cubic metre of a gas, but the
specific heat refers to units of weight, and therefore it is necessary in order to know D
to 'reduce the above quantity to a unit of weight. One cubic metre of hydrogen at 0°
and 760 mm. pressure weighs 0'0896 kilo, a gas of molecular weight M has a density
M/2, consequently a cubic metre weighs (at 0° and 760 mm.) 0'0448M kilo, and therefore
1 kilogram of the gas occupies a volume 1/0'0448M cubic metres, and hence the external
work D in the heating of 1 kilo, of the given gas by 1° = 0-0896/0'0448M, or D = 2/M.

Taking the magnitude of the internal work B for gases as minute if permanent gases
are taken, and therefore supposing B = 0, we find the specific heat of gases at a constant
pre>-ure Q = K + 2 M. wh.Te K is the specific heat at a constant volume, or the true
specific heat, and M the molecular weight. Hence K = Q — 2/M. The magnitude of the
specific heat Q is given by direct experiment. According to Regnault's experiments, for
oxygen it = 0'2175, for hydrogen 8'405, for nitrogen 0'2488; the molecular weights of these
are 32, 2, and 28, and therefore for hydrogen K = 0'2438 - 0-0714 = 0'1724. These true

pp 2

580 PRINCIPLES OF CHEMISTRY

internal work accomplished in the molecules inciting them to decom-
position according to the rise of temperature 8- — therefore it is irnpos-

specific heats of elements are in inverse proportion to their atomic weights — that is, their
product by the atomic weight is a constant quantity. In reality, for oxygen this product
= 0-155 x 16 = 2'48, for hydrogen 2'40, for nitrogen 0'7724 x 14 = 2'414, and therefore if A
stand for the atomic weight we obtain the expression K x A— a constant, which may be
counted as 2'45; and this is the true expression of Dulong and Petit's law, because K is
the true specific heat and A the weight of the atom. It should be remarked, moreover,
that the product of the observed specific heat Q into A is also a constant quantity (for
oxygen = 3'48, for hydrogen = 3'40), because the external work D is also inversely propor-
tional to the magnitude of the atomic weight.

In the case of gases we distinguish the specific heat at a constant pressure c' (we
designated this quantity above by Q), and at a constant volume c. It is evident that
the relation between both specific heats, Jc, judging from the above, is the ratio of Q
to K, or equal to the ratio of 2'45?i + 2 to 2'45w. When n = 1 this ratio k = 1'8 ; when
n=2 A- = 1-4, when n = 8 A = 1'3, and with an exceedingly large number, n, of atoms in the
molecule, Jc=l. That is, the ratio between the specific heats decreases from 1'8 to I'O as
the number of atoms, n, contained in the molecule increases. This deduction is verified
to a certain extent by direct experiment. For such gases as hydrogen, oxygen, nitrogen,
carbonic oxide, air, and others in which ->i = 2, the magnitude of k is determined by
methods described in physics (for example, by the change of temperature with an altera-
tion of pressure, by the velocity of sound, &c.), and is found in reality to be nearly 1*4,
and for such gases as carbonic anhydride, nitric peroxide, and others it is nearly 1'8.
Kundt and Warburg (1875), by means of the approximate method mentioned on p. 321,
determined k for mercury vapour when n = l, and found it to be = 1'07 — that is, a larger
quantity than for air, as would be expected from the above.

It may be admitted that the true atomic heat of gases =2' 43, only under the condition
that they are distant from a liquid state, and do not undergo a chemical change when
heated — that is, when no internal work is produced in them (B = 0). Therefore this
work may to a certain extent be judged by the observed specific heat. Thus, for instance,
for chlorine (Q = 0'12, Eegnault; A; = 1'33, according to Straker and Martin, and therefore
K = 0'09, MK = 6'4), the atomic heat (3'2) is much greater than for other gases containing
two atoms in a molecule, and one must consider, therefore, that when heated some great
internal work is accomplished of whose nature it is at present impossible to form an
opinion. And as in the case of such gases as ethylene, C2H4 (Q = 0'39), according to
"Wiedemann k = l'2, K = 0'88, MK = 9'2 ; hence the true atomic heat is less than for con-
stant gases =1'5. Therefore the question as to the relation between the specific heats of
gases and the number of atoms and composition cannot be counted as sufficiently
general if we do not consider Le Chatelier's deduction (Note 6) as proved by the asso-
ciation of data. If the latter! be verified, then it will have to be admitted that Dulong
and Petit's law is not applicable to any gases besides those which are permanent and
possess a comparatively low molecular weight. The question might be solved by deter-
mining the specific heat of mercury vapour at different temperatures, but as yet there
are no exact methods of doing this.

All the more remarkable is the adaptability of Dulong and Petit's law to the mass of
the common elements in a solid state. In order to generalise the facts concerning the
specific heat of gases and solids, it appears to me possible to accept the following general
proposition : the atomic heat (that is, AQ or QM/w, where M is the molecular weight
and n the number of molecules) is less (greatest for solids, 6'8 ; for gases, 8'4) the more
complex the molecule (that is, the greater the number (n) of the atoms forming it),
and to a certain extent (with similar physical states of substances) the less the mean
(M/n) weight of the atom.

8 For an example, it is enough to point out the specific heat of nitrogen tetroxide,
NaO4, which, when heated, gradually passes into NOa — that is, chemical work of decom-

Till-: VALENCY AND SPECIFIC HEAT OF THE METALS 581

sible to expect in the magnitude of the specific heat the great
simplicity of relation to composition which we see, for instance, in the
density of gaseous substances. Therefore, although the specific heat
gives one of the important means of judging the atomicity of the
elements, still the mainstay for a true judgment of atomicity is only
given by Avogadro-Gerhardt's law. All other means can only be
accessory or preliminary, until it be possible to have direct recourse to
the determination of the vapour density.

Among the bivalent metals the first place, with respect to their
distribution in nature, is occupied by magnesium and calcium, just as
sodium and potassium stand first amongst the univalent metals. The
relation which exists between the atomic weights of these four metals
confirms the above comparison. In fact, the combining weight of
magnesium is equal to 24, and of calcium 40 ; whilst the combining
weights of sodium and potassium are 23 and 39 — that is, the latter
are one unit less than the former.9 They all belong to the number
of light metals, as they have but a small specific gravity, in which
they differ from the ordinary, generally known heavy, or ore, metals
(for instance, iron, copper, silver, and lead), which are distinguished by
a much greater specific gravity. There is no doubt that their low
specific gravity has a significance, not only as a simple point of dis-
tinction, but also as a property which determines the fundamental
properties of these metals. Indeed, all the light metals have a series
of points of resemblance which approximates them to the metals of
the alkalis ; thus both magnesium and calcium, like the metals of the

position proceeds, which consumes heat. Speaking generally, specific heat is a complex
quantity, in which it is clear that thermal data (for instance., the heat of reaction) alone
cannot give an idea either of chemical or of physical changes individually, but always
depend on an association of the one and the other. If a substance be heated from t0
to t, it cannot but suffer a chemical change (that is, the state of the atoms in the mole-
cules clump-s more or less in one way or another) if dissociation sets in at a temper-
ature /[. Even in the case of the elements whose molecules contain only one atom,
a true chemical change is possible with a rise of temperature, because more heat is
evolved in chemical reactions than that quantity which participates in purely physical
changes. One gram of hydrogen (specific heat = 3'4 at a constant pressure) cooled to the
temperature of absolute zero will evolve altogether about one thousand units of heat, 8
irnuns of oxy^.-n half this amount, whilst in combining together they evolve in the
formation of 9 grams of water more than thirty times as much heat. Hence the store
of chemical energy (that is, of the movement of the atoms, vortex, or other) is much
greater than the physical store proper to the molecules, but it is the change accomplished
by this store that is the cause of chemical transformations. Here we evidently touch on
those limits of existing knowledge beyond which the discipline of science does not yeiz
allow us to pass. A number of new scientific conquests are necessary before this can
become possible.

9 As if NaH = Mg and KH = Ca, which is in accordance with their valency, KH
including two monovalent elements is a bivalent group like Ca.

582 PRINCIPLES OF CHEM1STKY

alkalis, decompose water (without the addition of acids), although not
so easily as the latter metals. The process of the decomposition is
essentially one and the same ; for example, Ca + 2H.2O=CaH2O2 + H2 —
that is, hydrogen is liberated and a hydroxide of the metal formed.
These hydroxides are bases which saturate nearly all acids. How-
ever, the hydroxides RH2O2 of calcium and magnesium are in no
respects so energetic as the hydroxides of the true metals of the
alkalis ; thus when heated they lose water, are not so soluble, develop
less heat with acids, and form various salts, which are less stable
and more easily decomposed by heat than the corresponding salts
of sodium and potassium. Thus calcium and magnesium carbonates
easily part with carbonic anhydride when ignited ; the nitrates are
also very easily decomposed by heat, calcium and magnesium oxides,
CaO and MgO, being left behind. The chlorides of magnesium
and calcium, when heated with water, evolve hydrogen chloride,
forming the corresponding hydroxides, and when ignited the oxide
itself. All these points already evince a weakening of the alkaline
properties.

These metals have been termed the metals of the alkaline earths,
because they, like the alkali metals, form energetic bases. They are
called alkaline earths because they are met with in nature in a state of
combination, forming the insoluble mass of the earth, and because as
oxides, RO, they themselves have an earthy appearance. Not a few
salts are known of these metals which are insoluble in water, whilst
the corresponding salts of the alkali metals are generally soluble — for
example, the carbonates, phosphates, borates, and other salts of the
alkaline earth metals are nearly insoluble. This serves to distinguish
the metals of the alkaline earths from the metals of the alkalis. For
this purpose a solution of ammonium carbonate is added to a mixed
solution of salts of both metals, when by a double decomposition the
insoluble carbonates of the metals of the alkaline earths are formed
and pass into a precipitate, whilst the metals of the alkalis remain in
solution : RX2 4 Na2C03 = RCO3 + 2NaX.

We may here remark that the oxides of the metals of the alkaline
earths are frequently called by special names : MgO is called magnesia
or bitter earth ; CaO, lime ; SrO, strontia ; and BaO, baryta.

In the primary rocks the oxides of calcium and magnesium are
combined with silica, sometimes in variable quantities, so that in some
cases the lime predominates and in other cases the magnesium. Both
oxides, being analogous to each other, replace each other in equivalent
quantities. The various forms of augite, hornblende or amphibole, and
of similar minerals, which enter into the composition of nearly all rocks,

THE YALKNCY AND SPECIFIC HEAT <>K THE METALS 583

contain lime and magnesia and silica. The majority of the primary
rocks also contain alumina, potash, and soda. These rocks, under
the action of water (containing carbonic acid) and air, give up lime
and magnesia to the water, and therefore they are contained in all
kinds of water, and especially sea water. The carbonates, CaCO3 and
MgCO3, frequently met with in nature, are soluble in an excess of
•ii-itt, ,• xnf a rated with carbonic anhydride,10 and therefore many natural
waters contain these salts, and are able to yield them when evaporated.
However, one kilogram of water saturated with carbonic anhydride
does not dissolve more than three grams of calcium carbonate. By
gradually expelling the carbonic anhydride from such water, an in-
soluble precipitate of calcium carbonate separates out. It may confi-
dently be stated that the formation of the very widely distributed
strata of calcium and magnesium carbonates was of this nature, because
these strata are of a sedimentary character — that is, that which would
be exhibited by a gradually accumulating deposit on the bottom of
the sea. Furthermore, the remains of sea organisms, plants, ferns,
<fcc., are frequently found amongst these deposits. These deposits of
calcium and magnesium carbonates are the most important sources of
these metals. Lime generally predominates, because it is present in
rocks and stream water in greater quantity than magnesia, and in this
case these sedimentary rocks are termed limestone. Some common flag-
stones used for paving, &c., and chalk may be taken as examples of this
kind of formation. Those limestones in which a considerable portion
of the calcium is replaced by magnesium are termed dolomites. The
dolomites are distinguished by their hardness, and by their not parting
with the whole of their carbonic anhydride so easily as the limestones
under the action of acids. Dolomites 1 1 sometimes contain an equal
number of molecules of calcium carbonate and magnesium carbonate,
and they also sometimes appear in a crystalline form, which is easily
understood, because calcium carbonate itself is exceedingly common in
this form in nature, and is then known as calc spar, whilst natural

10 Sodium carbonate and other carbonates of the alkalis give acid salts which are less
soluble "than the normal ; here, on the contrary, with an excess of carbonic anhydride, a
salt is formed which is more soluble than the normal, but this acid salt is more unstable
than sodium hydrogen carbonate, NaHCO5.

11 The formation of dolomite may be explained, if only we imagine that a solution of
a magnesium salt acts on calcium carbonate. Magnesium carbonate may be formed by
double decomposition, and it must be supposed that this process is able to cease at a
certain limit (Chapter XII.), when we shall obtain a mixture of the carbonates of calcium
and magnesium. Haitinger heated a mixture of calcium carbonate, CaCO5, with a solu-
tion of an equivalent quantity of magnesium sulphate, MgSO4, in a closed tube at 200°,
and then a portion of the magnesia actually passed into the state of magnesium car-
bonate, MgCO3, and a portion of the lime was converted into gypsum, CaSO4.

f)S4 L'RINCll'LKS 01.' rllKMJSTKY

crvstalline magnesium carbonate is termed magiiesitt3. The formation
of the crystalline varieties of the insoluble carbonates is explained by
the possibility of a slow deposition from solutions containing carbonic
acid. Besides which the calcium and magnesium sulphates art1 obtained
fnnn sea water, and therefore thev are met with both a^ deposits and
in springs. It must be observed that magnesium is held in considerable
quantities in sea water, because the sulphate and chloride of magne-
sium are very soluble in water, whilst calcium sulphate is but little
>oluble; and therefore if the occurrence of considerable deposits of
magnesium sulphate cannot be expected in nature, still, on the other
hand, one would expect (and they do actually occur) large masses of
calcium sulphate or //////>•/////. < 'aS< ) (.'2 1 1 .,< '. (Jypsum sometimes forms
strata of immense si/.e, which extend over many tens of kilometres -
for example, in llu.ssia on the Volga. and in the Donetx and Ualtic
pn >vince>.

Lime and magnesia also, but in much smaller (quantities (often to
the amount of several fractions of a per cent, and rarely more), enter
!/{fn f//>. I'OinjinxltiuH of everv fertile NO/'/, and without these bases the
>oil is tmable to >upport vegetation. Lime is particularly important
in this re>pect, and its ]iresence in a larger quantity generally improves
the harvest, although purelv calcareous soils are generally infertile.
Kor this reason the soil is fertilised both with lime1- itself and with
marl that is. with clay mixed with a certain quantity of calcium carbo
nate. strata of which are found nearlv everywhere.

I'Yom the >oil the lime and magnesia (in a smaller quant itv) pass
into the substance of jilxufs^ where thev occur as salts. Certain of
these -alts separate in the interior of plants in a crystalline form — for
example, calcium oxalate. The lime occurring in plants serves as the
source for the formation of the \ariou-, calcareous secretions which arc
so ei immoii in dii'iniii /N of all classes. The bones of the highest animal
orders, the she] I- < it' mollusca, the covering of the sea-urchin, and similar
solid secretions of sea animals, contain calcium salts; namely, the
shells mainly ••;i]eiiim carbonate. ;md bones mainly calcium phosphate.
(Vrtain lime-tones are almost entirely formed of such deposits.
Odc-sa is situated on a limestone ot' this kind, composed of shells.
Thus magnesium and calcium occur throughout the entire realm of
nat ure.

ihilaUc action ni linn' in increasing tip- I'crtilitv of soil-; — if not in every
il . with ordinary -.oils which have lon^ l.ccn under corn i:-; lia-ed not so
i|iiirenients "I plants for tip' linn' it -< -It .1 - mi those chemical and physical

THE VALENCY AND SPECIFIC HEAT OF THE METALS 585

As lime and magnesia form bases which are in many respects
analogous, they were not distinguished from each other for a long
time. Magnesia was obtained for the first time in the seventeenth
century from Italy, and used as a medicine ; and it was only in the
last century that Black, Bergmann, and others distinguished magnesia
from lime.

Mrtitllic magnesium (and calcium also) is not obtained by heating
magnesium oxide or the carbonate with charcoal, as the alkali metals
are obtained,13 but is liberated by the action of a galvanic current
on fused magnesium chloride (best mixed with potassium chloride) ;
but Davy and Bussy first obtained metallic magnesium by acting OIL
magnesium chloride with the vapours of potassium. At the present
time (Deville's process) magnesium is prepared in rather consider-
able quantities by a similar process, only the potassium is replaced by
sodium. Anhydrous magnesium chloride, together with sodium chloride
and calcium fluoride, is fused in a closed crucible. The latter sub-
stances only serve to facilitate the formation of a fusible mass before
and after the reaction, which is indispensable in order to prevent
the access and action of air. One part of finely-divided sodium to
five parts of magnesium chloride is thrown into the strongly-heated
molten mass, and after stirring the reaction proceeds very quickly, and
magnesium separates, MgCl2-fNa2 = Mg + 2NaCl. In working on a
large scale, the powdery metallic magnesium is then subjected to distil-
lation at a white heat. The distillation of the magnesium is necessary,
because the undistilled metal is not homogeneous 14 and burns unevenly :
the metal is prepared for the purpose of illumination. Magnesium

lc> Sodium and potassium only decompose magnesium oxide at a white heat and very
feebly, probably for two reasons. In the first place, because the reaction Mg + O deve-
lops more heat (about 140 thousand calories) than K2 + O or Nas + O (about 100 thousand
calories) ; and, in the second place, because magnesia is not fusible dt the heat of a
furnace and cannot act on the charcoal, sodium, or potassium — that is, it does not pass
into that mobile state which is necessary for reaction. The first reason alone is not
suttir-ient to explain the absence of the reaction between charcoal and magnesia, because
iron and charcoal in combining with oxygen evolve less heat than sodium or potassium,
yet, nevertheless, they can displace them. With respect to magnesium chloride, it acts
on sodium and potassium, not only because their combination with chlorine evolves more
heat than the combination of chlorine and magnesium (Mg + Cl2 gives 150 and Naj + Clj
about 195 thousand calories), but also because a fusion, both of the magnesium chloride
and of the double salt, takes place under the action of heat. It is probable, however,
that a reverse reaction will take place.

14 Commercial magnesium generally contains a certain amount of magnesium nitride
(Deville and Caron), Mg3N2 — that is, a product of substitution of ammonia which is
directly formed when magnesium is heated in nitrogen. It is yellowish green powder,
which gives ammonia and magnesia with water, and cyanogen when heated with carbonic
anhydride. Perfectly pure magnesium may be obtained by the action of a galvanic
current.

586 PEINCIPLES OF CHEMISTRY

is a white metal, like silver ; it is not soft like the alkali metals, but
is, on the contrary, hard like the majority of the ordinary metals.
This is natural from the fact that it melts at a rather high tempera-
ture— namely, about 500° — and boils at about 1000°. It is malle-
able and ductile, like the generality of metals, so that it can be drawn
into wires and rolled into ribbon ; it is most frequently used for
lighting purposes in the latter form. Unlike the alkali metals, mag-
nesium does not decompose the atmospheric moisture at the ordinary
temperature, so that it is almost unacted on by air ; it is not even
acted on by water at the ordinary temperature, so that it may be
washed to free it from sodium chloride. Magnesium only decomposes
water with the evolution of hydrogen at the boiling point of water,15
and especially at still higher temperatures ; but even this is accom-
plished with difficulty. This is explained by the fact that in decom-
posing water magnesium forms an insoluble hydroxide, MgH2O2, which
covers the metal and hinders the further action of the water. Magne-
sium easily displaces hydrogen from acids, forming magnesium salts.
. When ignited it burns, not only in oxygen but in air (and even in
carbonic anhydride), forming a white powder of magnesium oxide, or
magnesia ; in burning it emits a white and exceedingly brilliant light.
The strength of this light naturally depends on the fact that mag-
nesium (24 parts by weight) in burning evolves about 140 thousand
heat units, and that the product of combustion, MgO, is infusible by
heat ; and therefore the vapour of the burning magnesium will contain
an ignited powder of non-volatile and infusible magnesia, and will
consequently present all the conditions for the production of a brilliant
light. The light emitted by burning magnesium contains many rays
which act chemically, and are situated in the violet and ultra-violet
parts of the spectrum. For this reason burning magnesium may be
employed for taking photographic images.16

Owing to its great affinity for oxygen, magnesium reduces many
metals (zinc, iron, bismuth, antimony, cadmium, tin, lead, copper, silver,
and others) from solutions of their salts at the ordinary temperature,17

i:> Hydrogen peroxide (Welsing) dissolves magnesium. The reaction has not been
investigated.

10 A special form of apparatus is used for burning magnesium. It is a clockwork
arrangement in which a cylinder rotates, round which a ribbon or wire of magnesium is
wound. The wire is subjected to a uniform unwinding and burning as the cylinder
rotates, and in this manner the combustion may continue uniform for a certain time.
The same is attained in special lamps, by causing a mixture of sand and finely-divided
magnesium to fall from a funnel-shaped reservoir on to the flame. In photography it is
best to blow finely-divided magnesium into a colourless (spirit or gas) flame.

17 According to the observations of Maack, Comaille, Bb'ttger, and others. The re-
duction by heat mentioned further on was pointed out by Geuther, Phipson, Parkinson
and Gattermann.

TIIK YALENTY AND SPECIFIC HEAT OF THE METALS 587

and at a red heat finely-divided magnesium takes up the oxygen from
silica, alumina, boric anhydride, Arc. ; so that silicon and similar
elements may be obtained by directly heating a mixture of powdered
silica and magnesium in an infusible glass tube.18 Magnesium acts on
fused potassium and sodium hydroxides, with an energetic evolution of
hydrogen.

The affinity of magnesium for the halogens is much more feeble
than for oxygen,19 as is already evident from the fact that a solution
of iodine reacts feebly on magnesium ; still magnesium burns in the
vapours of iodine, bromine, and chlorine. The character of magnesium
is also determined by the fact that all its salts, especially in the
presence of water, are decomposable at a comparatively moderate
temperature, the elements of the acid being evolved, and the magne-
sium oxide, which is non-volatile and unchangeable by heat, remaining
behind. This naturally refers to those acids which are themselves
volatilised by heat. Even magnesium sulphate is completely decom-
posed at the temperature at which iron melts, oxide of magnesium
remaining behind. This decomposition of magnesium salts by heat
proceeds much more easily than with calcium salts. For example,
magnesium carbonate is totally decomposed at 170°, magnesium oxide
being left behind. This magnesia, or magnesium oxide, is met with
both in an anhydrous and hydrated state in nature (the anhydrous
magnesia as the mineral periclase, MgO, and the hydrated magnesia
as hrucite, MgH2O2). Magnesia is a well-known medicine (calcined
magnesia — magnesia usta). It is a white, extremely fine, and very
voluminous powder, of specific gravity 3*4 ; it is infusible by heat, and
only shrinks or sinters in an oxyhydrogen flame. After long contact
the anhydrous magnesia combines with water, although very slowly,
forming the hydroxide Mg(HO)2, which, however, parts with its
water with great ease when heated even below a red heat, and again
yields anhydrous magnesia. This hydroxide is obtained directly as
& gelatinous amorphous substance when a soluble alkali is mixed with
a solution of any magnesium salt, MgCl2 + 2KHO = Mg(HO)2 + 2KCl.
This decomposition proceeds to the end, and nearly all the magnesium
passes into the precipitate ; and this clearly shows the almost perfect
insolubility of magnesia in water. Water dissolves a scarcely per-

18 This action of metallic magnesium in all probability depends, although only partially
(see Note 18), on its volatility, and on the fact that, in combining with a given quantity of
oxygen, it evolves more heat than aluminium, silicon, potassium, and other elements.

19 Davy, on heating magnesia in chlorine, observed a complete substitution, because
the volume of oxygen was half the volume of chlorine ; it is probable, however, that owing
to the formation of chlorine oxide (Chapter XI. Note 80) the decomposition is not com-
plete and is limited by a reverse reaction if the mass of the oxygen have no effect.

r>SS I'KINCII'LKS OF CIIK.MISTKY

ceptible quantity of magnesium hydroxide namely, one part is dis-
solved by ."),r)000 parts of water. Such a solution. however, has
an alkaline reaction, and gives, with a salt of phosphoric.1 acid, a
precipitate of magnesium phosphate, which is still more insoluble.
Magnesia is not only dissolved bv acids, forming salts, but it also dis-
places certain other bases for example, ammonia from ammonium
salts when boiled: and the hvdroxide also attracts carbonic anhy-
dride from the air. The magnesium salts, like those ofcalcium, potas-
sium, and sodium, are colourless if the\- are formed from colourless
acids. 'I hove \\ liich are soluble have a bitter taste, whence magnesia
ha- been termed li'ttfi-r-i-nrth. In comparison to the alkalis magnesia
is a feeble base, inasmuch as it form- somewhat tin-table salts, easily
U'ives basic salts, forms acid salts with ditliculty. and is able to give
doulile Milts with the salt- of the alkali--, which facts are proper to
t'«-cble bases, as we shall see in becoming acquainted with the ditlerent
metals.

The power of magnesium salts to form double and basic salts is
\ "i-y frequently shown in reactions, and is speciallv marked as re-
gards ammonium salts. If saturated solutions of magnesium and
ammonium -ulphates are mixed together, then a crvstalline double
-alt. .Mgi X 1 1 ,) ,(S< ), ).,,fi| I O,-" I. immediately precipitated. A strong
solution of ordinary ammonium carbonate dissolves magnesium
o\idc or carbonate, and precipitates crystals of a double salt,
M'j' N I T, ),(('<).,).,,! 1 1 .,< ). from which water extracts the ammonium
carbonate. \Vith an excess of an ammonium salt the double salt passes
into solution.-1 and therefore if a solution contain a magnesium salt
and an excess of an ammonium salt for instance, sal-ammoniac
then sodium carbonate \\ill no longer precipit at e magnesium carbonate.
A mixture .if ,-olutions of magnesium and ammonium chlorides, on
evaporation or refrigeration, gives a double salt, M L;( N 1 1 , )( 'I.,,!! II ._,<>.--

K\cn -i -.ilntini] ni' ;iiiiin(iiiiiiiii
It- P. •. !'. I -7-J : lint ji;ir1 ':..'•'

\1 I.Miit 1:;n it I..-,. • ti-r.

"• Tin • iimj.lf «.!' i'(iiiilil.i'iii]ii nii.l ..I tli.' intliicncr i.f mas- ; the .IniiliU- vjilt is

ili-i'i,|ii|.i.-fil liy \\.itcr.liiit ,1 we tiiki' .! .. ' n] llial |>,irt \\lii.li is l'ni'iiic.1 in thtj

• t. .1 ,• all.

It jin ex.'.... ,,[' ainnK.iii.i In' ail.li-il t,, a M.liifi..n r.f ma :ji].-iu in clilnriilf, tlirn only

'!,.• iiia'jnc-iuni i- -«-|iarat»'il in tin- | >\ v,-i | ,il .. t .-. -1 M -( '] , • ~1 \ 1 1 ,.( > 1 1 M:_" < )H K •-

\|_ NH.C] N'If,( ' A nhiVinii (,f aiiiinoiiluiii rliluri.I.- reacts with niau'iH-ia. <'V<>!vin£

. MHI'.IM!) an<l f-.nnii;- a m.luJ if m of t li»- -am.' .ill. M j< ) • :'A 1 1 ,( '1 M •_• N 1 1 ,< M- • I \.,< >

"'. I !

• ,.,1,,iil,l,.-ii1f^t,| iiiiiiiK.iiiuiiiiiinliiM^iii^i tin- j.lio^i.lial.-.M-N I !,!'() ;.r,IIoO,

nnia. Ma'_riif-ia i- vcr\ f i •'•<) n«'iii 1 \ ]>n-f\ pit .1 1 .-i 1 a- this -alt i'min -(ilutions in which
held hi, ainiiiMiiiiiin -alt^. As lin

THE VALENCY AND SPECIFIC HEAT OF THE METALS 589

The salts of potassium, like those of ammonium, are able to enter into
combination with the magnesium salts.-3 For instance, the double
.salt, MgKCl3,6H2O, which is known as carnallite** and occurs in the
salt mines of Stassfurt, may be formed by freezing a saturated solution
of potassium chloride with an excess of magnesium chloride. A satu-
rated solution of magnesium sulphate dissolves potassium sulphate, and
solid magnesium sulphate is soluble in a saturated solution of potassium
sulphate. A double salt, K2Mg(8O4)2,6H2O, which closely resembles
the above-mentioned ammonium salt, crystallises from these solutions.25

ammonia salts, but is then precipitated by sodium carbonate, &c., it is therefore very
easy to separate calcium from magnesium on the basis of these properties.

-•"• In order to see the nature and cause of the formation of double salts, it is sufficient
to regard them from the aspect (although this does not embrace the whole essence of the
matter) that one of the metals of such salts (for instance, potassium) easily gives acid
salts, and the other (in this instance, magnesium) basic salts; the properties of distinctly
basic elements predominate in the former, whilst in the latter these properties are
enfeebled, and the salts formed by them bear the character of acids — for instance, the
salts of aluminium or magnesium act in many cases like acids. By their mutual combi-
nation both these properties of the salts are satisfied.

-4 Carnallite has been mentioned in Chapter X. (Note 4) and in Chapter XIII. These
deposits also contain much kainite, KMgCl(SO4),3H2O (sp. gr., 2'13 ; 100 parts of water
•dissolve 79'6 parts at 18°). This double salt contains two metals and two haloids.

25 The component parts of certain double salts diffuse at different rates, and as the
diffused solution contains a different proportion of the component salts than the
solution taken of the double salt, it shows that such salts are decomposed by water.
According to Riidorff, the double salts, like carnallite, MgK2(SO4).>,6H2O, and the alums,
all belong to this order (1888). But such salts as tartar emetic, the double oxalates, and
double cyanides are not separated by diffusion, which in all probability depends both on
the relative rate of the diffusion of the component salts and on the measure of affinity
acting between them. Those complex states of equilibrium which exist between water,
the individual salts MX and NY, and the double salt MNXY, have been already partially
analysed (as will be shown hereafter) in that case when the system is heterogeneous
(that is, when something separates out in a solid state from the liquid solution), but in
the case of equilibria in a homogeneous liquid medium (in a solution) the phenomenon is
not so clear, because it concerns that very theory of solution which cannot yet be
considered as established (Chapter I. Note U, and others). As regards the heterogeneous
decomposition of double salts, it has long been known that such salts as carnallite and
K2Mg(SO4).,> give up the more soluble salt of magnesium if an insufficient quantity of
water for their complete solution be taken. The complete saturation of 100 parts of water
requires at 0° 14*1, at 20° 25, and at 60° 50'2 parts of the latter double salt (anhydrous),
while 100 parts of water dissolve 27 parts of magnesium sulphate at 0°, 86 parts at 20°,
and 55 parts at 60°, of the anhydrous salt taken.

Of all the states of equilibrium exhibited by double salts the most fully investigated
as yet is the system containing water, sodium sulphate, magnesium sulphate, and their
double salt, Na^Mg^O.^, which crystallises with 4 and 6 mol. OH2. The first crystallo-
hydrate, MgNa2(SO4)2,4H2O, occurs at Stassfurt, and as a sedimentary deposit in many
of the salt lakes near Astrakhan, and is therefore called astrakhanite. The specific
gravity of the monoclinic prisms of this salt is 2'22. If this salt, in a finely-divided state,
be mixed with the necessary (according to the equation) quantity of water, then the
mixture solidifies like plaster of Paris into a homogeneous mass if the temperature be
below 22° (Van't Hoff und Van Deventer, 1886 ; Bakhius Roozeboom, 1887) ; but if the
temperature be above this transition-point then the water and double salt do not react

590 PRINCIPLES OF CHEMISTRY

The nearest analogues of magnesium are able to give exactly similar
double salts, both in crystalline form (monoclinic system) and com-
position ; they, like this salt (see Chapter XV.), are easily able to
(140°) part with all their water of crystallisation, and correspond with
the salts of sulphuric acid, whose type may be taken as magnesium
•sulphate, MgSO4.26 It occurs at Stassfurt as kieserite, MgS04,H.2Or

on each other : MgNa2(SO4).,,4H2O + 13H2O = Xa2SO4,10H.p + MgS04,7H,O ; that is, they
do not solidify or give a mixture of sodium and magnesium sulphates. If a mixture (in
equivalent quantities) of solutions of these salts be evaporated, and crystals of astra-
khanite and of the individual salts capable of proceeding from it be added to the con-
centrated solution to avoid the possibility of a supersaturated solution, then at tempera-
tures above 22° astrakhanite is exclusively formed (this is the method of its production),,
but at lower temperatures the individual salts are alone produced. If equivalent amounts
of Glauber's salt and magnesium sulphate be mixed together in a solid state, then there is
no change at temperatures below 22°, but at higher temperatures astrakhanite and water
are formed. The volume corresponding with X»XoSO4,10H2O in grams =8'22.T46 = 220'5
cubic centimetres, and of MgSO4,7H3O = 246, T68 = 14(V4 ; hence their mixture in equivalent
quantities occupies a volume of 366'9 c.c. The volume of astrakhanite =334/2'22 = 150-5,.
and the volume of 18H2O = 234, hence their sum =380'5 c.c., and therefore it is easy to
follow the formation of the astrakhanite in a suitable apparatus (a kind of thermometer
containing oil and a powdered mixture of sodium and magnesium sulphates), and to see
by the variation in volume that before 22° it remains unchanged, and at higher tempera-
tures proceeds more quickly the higher the temperature. At the transition temperature
the solubility of astrakhanite and of the mixture of the component salts is one and the
same, whilst at higher temperatures a solution which is saturated for a mixture of the
individual salts would be supersaturated for astrakhanite, and at lower temperatures the
solution of astrakhanite will be supersaturated for the component salts, as has been
shown with especial detail by Karsten, Diacon, and others. Roozeboom showed that
there are two limits to the composition of the solutions which can exist for a double salt ;
these limits are respectively obtained by dissolving a mixture of the double salt with
each of its component simple salts. Van't Hoff demonstrated, besides this, that the
tendency towards the formation of double salts has a distinct influence on the progress
of double decomposition, because at temperatures above 31° the mixture 2MgSO4,7H2O
+ 2NaCl passes into MgNa2(SO4)2,4H2O + MgCL,6H2O 4- 4H2O, whilst below 31° there is
not this double decomposition, but it proceeds in the opposite direction, as may be
demonstrated by the above-described methods.

From these examples on double salts we see that there is as close a dependence
between the temperature and the formation of substances as there is between the tem-
perature and a change of state. It is a case of the conceptions of Deville concerning
dissociation, but extended in the direction of the passage of a solid into a liquid. On
the other hand, we here see how essential a rule water plays in the formation of com-
pounds, and how the affinity for water of crystallisation is essentially analogous to
the affinity between salts, and hence also to the affinity of acids for bases, because
the formation of double salts does not in any essential point (except the measure of
affinity — that is, from a quantitative aspect) differ from the formation of salts them-
selves. When sodium hydroxide with nitric acid gives sodium nitrate and water the
phenomenon is essentially the same as in the formation of astrakhanite from the salt*
Na2SO4,10H2O and MgSO4,7H2O. Water is disengaged in both cases, and he-m-r the
volumes are altered.

26 This salt, and especially its crystallo-hydrate with 7H.,O, is generally known M
Epsom salts. It has long been used as a purgative. It is easily obtained from magnesia
and sulphuric acid, and it separates on the evaporation of sea water and of many saline
springs. When carbonic anhydride is obtained by the action of sulphuric acid on

THE VALENCY AND SPECIFIC HEAT OF Till- .MKTALS 591

and grnonilly separates from solutions as a heptahydrated salt,.
MgSO4,7H.,O, and from supersaturated solutions as a hexahydrated
salt, MgSO4,6H.,O ; at temperatures below 0° it crystallises out as a
dodecahydrated salt, MgSO4,12H2O, and a solution of the com-
position MgSO4,2H./) completely solidifies at 5°.27 Thus between
water and magnesium sulphate there may exist several definite and
more or less stable degrees of equilibrium, and the double salt
MgSO4K2SO4,6H.2O may be regarded as one of these equilibrated
systems, all the more as it contains 6H2O, whilst MgSO4 is able to

, magnesium sulphate remains in solution. When dolomite — that is, amixture
of magnesium and calcium carbonates — is subjected to the action of a solution of hydro-
chloric acid until about half of the salt remains, then the calcium carbonate is chiefly
dissolved and magnesium carbonate is left, and, by treatment with sulphuric acid, gives
a solution of magnesium sulphate.

-7 The anhydrous salt, MgSO4 (sp. gr., 2'61), attracts moisture (7 mol. H.,0) from
moist air; when heated in steam or hydrogen chloride it gives sulphuric acid, and
when heated with carbon is decomposed, according to the equation 2MgSO4 + C = 2SO2
+ CO2 + 2MgO. The monohydrated salt (kieserite), MgSO4,HoO (sp. gr., 2'56), dissolves
in water with difficulty, and remains when the other crystallo-hydrates are heated to 135°.
The hexahydrated salt is dimorphous. If a solution, saturated at the boiling-point, be pre-
1 >ared, and cooled without access of crystals of the heptahydrated salt, then MgSO4,6H2O
erystallises out in monochnic prisms (Loewel, Marignac), which are quite as un-
stable as the salt, NaSO4,7H2O (p. 509) ; but if prismatic crystals of the cubic system
of the copper-nickel salts of the composition MSO4,6H2O be added, then crystals of
MgSO4,6H2O are deposited on them as prisms of the cubic system (Lecoq de Boisbaudran)..
The common crystallo-hydrate, MgSO4,7H2O, Epsom salts, belongs to the rhombic
system, and is obtained by crystallisation below 30°. Its specific gravity is 1*69. In a
vacuum, or at 100°, it loses 5H2O, at 132° 6H2O, and at 210° all the 7H.,O (Graham). If
crystals of ferrous or cobaltic sulphate be placed in a saturated solution, then hexagonal
crystals of the heptahydrated salt are formed (Lecoq de Boisbaudran) ; they present
an unstable state of equilibrium, and soon become cloudy, probably owing to their trans-
formation into the more stable common form. Fritzsche, by cooling saturated solutions
below 0°, obtained a mixture of crystals of ice and of a dodecahydrated salt, which easily
split up at temperatures above 0°. Guthrie showed that dilute solutions of magnesium
sulphate, when refrigerated, separate ice until the solution attains a composition
MgSO4,24HoO, which will completely freeze into a crystallo-hydrate at — 5'3° (p. 97).
According to Coppet and Riidorff, the temperature of the formation, of ice falls by 0'078°
for every part by weight of the heptahydrated salt per 100 of water. This figure gives
(Chapter I. Note 40) i = l for both the heptahydrated and the anhydrous salt, from which
it is clearly seen that it is impossible to judge the state of combination in which a dis-
solved substance occurs by the temperature of the formation of ice.

The solubility of the different crystallo-hydrates of magnesium sulphate, according to
Loewel, also varies, like those of sodium sulphate or carbonate (see Chapter XII. Notes
7 and 18). At 0° 100 parts of water dissolves 40'75 MgSO4 in the presence of the hexa-
hydrated salt, :-:4T.7 M^SO4 in the presence of the hexagonal heptahydrated salt, and
only 26 parts of MgSO4 in the presence of the ordinary heptahydrated salt — that is,
solutions giving the remaining crystallo-hydrates will be supersaturated for the ordinary
heptahydnited salt.

All this shows how many diverse aspects of more or less stable equilibria may exist
between water and a substance dissolved in it ; this has already been enlarged on in
Chapter I.

Carefully purified magnesium sulphate in its aqueous solution gives, according to-

combine into a most stable -system with 711.,*': and tin- double salt
IUHV ' >e eonsit lered as this crystallo-hydrate. in which one molecule of
watri1 i.s replaced bv the molecule K ,S< »,. s

'Ih' /""'•'./' "/ /"/'///'//'/ /HIS/I- sii/f* IN ;i verv remarkable peculiarity
• 'f magnesia and other leeble bases, and especially of those corre-
-pondini;' with polwalent nietal.s. The very powerful bases corre-
-pondinu' \vitli unixalent metals —like potassium ami sodium do not
font i basic salts, and, indeed, are more prune in ^ive acid salts, Avliil^t
inau'iiesiitin easilv and frequently forms basic >alts, especially Avith
feeble acids, although tliere are oxides as, for example, copper and
load oxides which Mill more frequently u'ive lia>ic sails. If a cold
solution <»f mau'ne-ium sulphate be mixed \\ith a .solution of sodium
carbonate there is formed a gelatinous precipitate of a basic salt,
.Mir- I l< > i ,. j Mu-< '< ».;,(,il L< ) : but all the magnesia is not precipitated in
thi- ca>e. as a portion nf it remains in solution as an acid double salt.
If sodium carbonate be added to a boiling solution of magnesium
sulphate there is formed a precipitate of a Mill more h;»ic salt,
1 M .-si >_, + -! N*n,C< )..(+ HI J ' = i Xa,S< ) , + ( '< )., + .M -< oil ),.:>,.M «r(."( ).„
'ij I ,* '. This basu.' salt forms the ordinary medicine, t/>>"//c.^t>t (nt<t</-
tt'-xiit </Hi<t). in the toi'm ot li^'ht porous lumps, <)ihei' basic salts are
fiirmeil under certain modifications of temperature and conditions of
decomposition. l>ut //"- normal >•"//. I\I ^C( ).,. \\'hich occurs in nature
;is inau'iiesite in the form of rhoinl )ohedra of s]>. u'r. .')•(».'»(), cannot bt^
obiained bv sueli a method of pi-ecipitation. In fact the formation of
the dillerent basic salt> slio\\sthe power of \vater to decompose the
noi'inal -ah. It is possible, however, to obiain this salt by artificial
mean-, both in an anhydrous and hvd rated state. .V solution of
mau'iiesmm carbonate in water containing carbonic acid is taken for
this purpo-e. 'I lie j'eason (it this is readily understood carbonic
anhx'di'ide is one (jf the products of the decomposition of magnesium

TIIK YALF.NCY .VXD SPECIFIC HEAT OF THE 3IKTALS 598

carbonate in the presence of water. If this solution be left to
evaporate spontaneously the normal salt separates in a hydrated form,
but in the evaporation of a heated solution, through which a stream of
carbonic anhydride is passed, the anhydrous salt is formed as a
crystalline mass, which remains unaltered in the air, like the natural
mineral.29 The decomposing influence of water on the salts of mag-
nesium, which is directly dependent on the feeble basic properties of
magnesia,30 is most clearly seen in magnesium chloride, MgCl2. This
salt is contained,31 as we have already seen (Chapter X.), in sea water,
and remains in the last mother liquors of its evaporation. On cooling
a sufficiently concentrated solution, the crystallo-hydrate, MgCl2,6H2O,
separates;32 but if it be further heated (above 106°) to remove the
water, then hydrochloric acid passes off together with the latter, so
that there ultimately remains magnesia with a small quantity of mag-
nesium chloride.33 From what has been said it is evident that anhy-
drous magnesium chloride cannot be obtained by simple evaporation.
But if sal-ammoniac or sodium chloride be added to a solution of
magnesium chloride, then the evolution of hydrochloric acid does not
take place, and after complete evaporation the remaining mass is
entirely soluble in water. This renders it possible to obtain anhydrous

39 The crystalline form of the anhydrous salt obtained in this manner is not the same
as that of the natural salt. The former gives rhombohedra, like those in which calcium
carbonate appears as calc spar, whilst the natural salt appears as rhombic prisms, like those
sometimes presented by the same carbonate, as aragonite, which will soon be described.

50 Magnesium sulphate enters into certain reactions which are proper to sulphuric
acid itself. Thus, for instance, if a carefully-prepared mixture of equivalent quantities
of hydrous magnesium sulphate and sodium chloride be heated to a red heat, the evolu-
tion of hydrochloric acid is observed just as in the action of sulphuric acid on common
salt, MgSO4 + 2NaCl + H.,O = Na.2SO4 + MgO + 2HC1. Magnesium sulphate acts in a
similar manner on nitrates, with the evolution of nitric acid. A mixture of it with
common salt and manganese peroxide gives chlorine. Sulphuric acid is sometimes
replaced by magnesium sulphate in galvanic batteries — for example, in the well-known
Mridinger battery. In the above-mentioned reactions we see a striking example of how
alike the reactions of acids and salts are, especially of salts which are formed by such
feeble bases as magnesia.

•"' As sea water contains many salts, MCI and MgX._>, it follows, according to Ber-
thollet's teaching, that MgCL is also present.

•"- As the crystallo-hydrates of the salts of sodium often contain 10H.2O, so many of
the salts of magnesium contain 6H.>O.

"• This decomposition is most simply defined as the result of the two reverse reactionp
MgCl3 + H.30 = MgO + 2HC1 and MgO + 2HC1 = MgCl2 + H2O, or as a distribution
between O and C12 on the one hand and H2 and Mg on the other. It is then clear that
according to Berthollet's doctrine, the mass of the hydrochloric acid converts the mag-
nesium oxide into chloride, and the mass of the water converts the magnesium chloride
into oxide. The crystallo-hydrate, MgCl2,6HoO, forms the limit of the reversibility. But
an intermediate state of equilibrium may exist in the form of basic salts. On mixing
ignited magnesia with a solution of magnesium chloride of specific gravity about 1*2, a
solid mass is obtained which is scarcely decomposed by water at the ordinary tempera-
ture (see Zinc).

VOL. I. Q Q

594 PRINCII'LKS OF CIIKMISTKY

magnesium chloride from its aqueous solution. Indeed the mixture
with sal-ammoniac (in excess) may be dried (the residue consists of an
anhydrous double salt, Mg( '1.,,1'X 1 1 ,( '] ) and then ignited (4G()°), when
the sal-ammoniac is converted into vapour and a fused mass of
anhydrous magnesium chloride remains behind. The anhydrous chloride
evolves a very considerable amount of heat on the addition of water,
which shows the great atlinity the salt has for water/" Anhydrous
magnesium chloride is not only obtained bv the above method, but is
also formed by the direct combination of chlorine and magnesium, and
by the action of chlorine on magnesium oxide, oxvgen being evolved :
this proceeds still more easily In/ In'itfni<j HKI</H' *n> />•/?// <•//(//'<•<>(// ni n
*f, -,<//,, ,,/' !•/,/<>,'! /f. when the charcoal serves to take up the oxygen.
Tin-- latter method is also employed for the preparation of chlorides
which an1 formed in an anhydrous form with --till greater difficulty
than magnesium chloride. Anhydrous magnesium chloride forms a
eolotirles>, transparent mass, composed of flexible crystalline plates
of a pearlv lustre. It fuses at low red heat (7<>S'r) into a colourless
lifjiiid. remains unchanged in a dry state, but under the action of
moisture is partially decomposed even at the ordinary temperature,
with formation of hydrochloric acid.

('</!'• i a in compounds in many respects present a great nesem-
blaiice to magnesium compounds, but are also clearly distinguished
from them bv manv properties.3'1 In general calcium stands to
magnesium in the same relation as potassium occupies in respect
to -.odium. I)avv obtained metallic calcium, like potassium, as an
amalgam hv the action of a galvanic current : but neither charcoal nor
iron decomposes calcium oxide, and even sodium decomposes calcium
chloride3'' \\ith difficulty. Hut a galvanic current easily decomposes
calcium chloride, and metallic sodium somewhat easily decomposes
calcium iodide when healed. As in the case of hydrogen, potassium,

n of M'j( '] . u itli '.I I O evolves I'.IiOOO nilories,
heat units.

i I1, it iii'j ca lei n in t'rolll ma;j iie-inm lioides that
t Will In- >llf'licient to nielli ioll the l.eha violir

ivdvated //'//// ! exceedingly *<ilnl,li- in an
-ia is little sollllile. All the lilile IllilY lie e\-
lln ill lure of i ixides lluis olitained. and
Ciirlionie anhvdride precipitates calcium carhonate

' , l-.la r. (ri the lime I1SI-<1 fill' l.llildill-

•a e- t he liindimj |n i\\ er of the innrtar, as I ha\ e my-elf t mi IK I.
In Ka>! India, -laiian i 1 he addil ion ,,| • u-,ir |o cement has loir_r

THE VALENCY AND SPECIFIC HEAT OF TIIF. MKTALS 595

and magnesium, the affinity of iodine for calcium is feebler than that of
chlorine (and oxygen), and therefore it is not astonishing that calcium
iodide may be subjected to that decomposition, into which the chloride
and oxide enter with difficulty.37 Metallic calcium is of a yellow
colour, and has a considerable lustre, which it preserves in dry air. Its
specific gravity is 1 '58. Calcium is distinguished by its great ductility ;
it melts at a red heat and then burns in the air with a very brilliant
flame, which is easily understood from the fact that finely-divided
infusible calcium oxide is formed. Judging from the fact that in
burning calcium gives a very large flame, it is probable that this metal
is volatile. Calcium decomposes water at the ordinary temperature,
and is oxidised in moist air, but not so rapidly as sodium. In burning,
calcium gives its oxide or lime, CaO, a substance which is familiar to
•every one, and of which we have already frequently had occasion to
speak. This oxide is not met with in nature in a free state, because it
is an energetic base which everywhere encounters acid substances, with
which it forms salts. Generally it is combined with silica, or occurs
as calcium carbonate or sulphate. The calcium carbonate and nitrate
are decomposed, at a red heat, with the formation of lime. As a rule,
the carbonate, which is so frequently met with in nature, serves as the
source of the calcium oxide, both commercial and pure. When heated
calcium carbonate dissociates: CaC03=CaO + CO2. In practice the
decomposition is conducted at a bright red heat, in the presence of
steam, or a current of a foreign gas, in heaps or in special kilns.38

37 Calcium iodide may be prepared by saturating lime with hydriodic acid. It is a very
soluble salt (at 20° one part of the salt requires 0'49 parts and at 48° 0'35 parts of water
for solution), is deliquescent in the air, and resembles calcium chloride in many respects.
It changes but little when evaporated, and, like calcium chloride, fuses when heated, and
therefore all the water may be driven off by heat. If anhydrous calcium iodide be heated
with an equivalent quantity of sodium in a closely-covered iron crucible, sodium iodide
and metallic calcium are formed (Lies-Bodart). Dumas advises carrying on this re-
action in a closed space under pressure.

38 Kilns which act either intermittently or continuously are built for this purpose. The
kilns of the first kind are filled with alternate layers of fuel and limestone ; the fuel is
lighted, and the heat developer! by its combustion serves for decomposing the limestone.
When the process is completed the kiln is allowed to cool somewhat, the lime raked out,
and the same process repeated. In the continuously-acting furnaces, constructed like
that shown in fig. 78, the kiln itself only contains limestone, and there are lateral hearths
for burning the fuel, whose flame passes through the limestone and serves for its decom-
position. Such furnaces are able to work continuously because the unburnt limestone
may be charged from above, and the burnt lime raked out from below.

It is not every limestone that is suitable for the preparation of lime, because many
contain impurities, especially clay, dolomite, and sand. Such limestones when burnt
either partially fuse or give an impure lime, called poor lime in distinction from that
pure lime which is obtained from purer limestone, and which is called rich lime. Tlu-
latter quality is characterised by its disintegrating into a fine powder when treated witk
water, and is suitable for the majority of uses to which lime is applied, and for which

Q Q 2

I'KINCII'LKS (>]•' CilKMlsTKY
('alrhiui oxide- th;it i>. <|uieklinif is ;i substance (s}>. UT. .'>•

the p. .. •:..>•; '.1:1,-- i pi ;;,•;,::'.:. 1 i -..•,•. i ,"!:•«• \'. ,

. .1 mass under \vtitrr.

In order tn obtain quite purr limr it. i- nere--ary to t;ikr thr pun's! possible nut
trri;il>. In tin- lal.oratory. niarl.lr r.v >hrll> aiv u-.-d h.r tin- purpose a> a purr funn nf

calriuin ravl H rnatr. 'I'hr\' arc lir-t I'Un.'i iii a luriiarr, ninl thru put in a crucililr ami

1 1 mi •. ,.,1 tl i n in 1 1 i|iiantil\ ill \\alii-.and llini a;_rain tnni-jh i'_n ited, 1>\ \\liiri,

mi i pun nliiained. I 'un • re ra pidl\ pn-pa n .1 l.\ taking

• :e. ( 'aN.jt >,.. u In'i • ' • o!\ iii-j liuir .|,,nr iii nitric arid.

'I'l • il • i i:y i.l linn- in nrili r to | reciji late tin-

nliil.li A.ilrr. 'I In ,,\idr- of in.n. ill iniiiin. A r.. are

(',,(» :! ' , M ( i ,

I • i ' ' • . • | 1 1 ' i \ < • I (lit- linn]

n i ln-hl<,

THE VALENCY AND SPECIFIC HEAT OF THE METALS 597

which is unaffected by heat,39 and may therefore serve as a fire-
resisting material, and was employed by Deville for the construction
of furnaces in which platinum was melted, and silver volatilised by the
action of the heat evolved by the combustion of detonating gas. The
hydrated lime, slaked lime, or calcium hydroxide, CaH2O2 (sp. gr.
'2 '07) is a most common alkaline substance, which is employed largely
in building for making mortars or cements, in which case its bind-
ing property is mainly due to the absorption of carbonic anhydride.40
Lime, like other alkalis, acts on many animal and vegetable sub-
stances, and for this reason has many practical uses — for example,
for removing fats, and in agriculture for accelerating the decom-
position of organic substances in the so-called composts or accumu-
lations of vegetable and animal remains used for fertilising land.
Calcium hydroxide easily loses its water at a moderate heat (530°),
but it does not part with water at 100°. When mixed with water
lime forms a pasty mass, which is known as slaked lime, and in a
more dilute form as milk of lime, because when shaken up in water

burnt and unaltered by air. It attracts moisture from the air and then disintegrates into
a powder ; if left long exposed in the air it also attracts carbonic anhydride and
increases in volume ; it does not entirely pass into carbonate, but forms a compound of
the latter with caustic lime.

59 Lime, when heated to a white heat in the vapour of potassium, gives calcium, and
in chlorine it disengages oxygen. Sulphur, phosphorus, &c., when heated with lime, are
absorbed by it.

40 The greater quantity of lime is used in making mortar for binding bricks or stones
together, in the form of lime or cement, or the so-called slaked lime. For this purpose
the lime is mixed with water and sand, which serves to separate the particles of lime
from each other. If only lime paste were put between two bricks they would not hold
firmly together, because after the water had evaporated the lime would occupy a smaller
space than before, and therefore cracks and powder would form in its mass, so that it would
not at all produce that complete cementation of the bricks which it is desired to attain.
Pieces of stone — that is, sand — mixed with the lime hinder this process of disintegration,
because the lime binds together the individual grains of sand mixed with it, and forms
one concrete mass, in consequence of a process which proceeds after the desiccation or
removal of the water. The process of the solidification of lime, taken as slaked lime,
consists first in the direct evaporation of the water and crystallisation of the hydrate, so
that the lime binds the stones and sand mixed with it, just as glue binds two pieces of
wood. But this preliminary binding action of lime is feeble (as is seen by direct experi-
ment) unless there be further alteration of the lime leading to the formation of carbonates,
silicates, and other salts of calcium which are distinguished by their great cohesion. With
the progress of time the cement is partially subjected to the action of the carbonic anhydride
in the air, owing to which calcium carbonate is formed, but not more than half the lime
is thus converted into carbonate. Besides which, the lime partially acts on the silica of
the bricks, and it is owing to these new combinations simultaneously forming in the
cement that it gradually becomes stronger and stronger. Hence the binding action of
the lime becomes stronger with the progress of time. This is the reason (and not, as is
sometimes said, because the ancients knew how to build stronger than we) why build-
ings which have stood for centuries possess a very strongly binding cement. Hydraulic
cements will be described in the chapter on silicon.

,V.)S

it remain-- suspended in it for a lon^ time, and ^ives it the appearance
of a milkv liijuid. lint, besides this, lime i^ direi-tlv soluble in water,
not to any considerable extent, but still in such a <|tiantitv that I'nnc
n-iit- r is precipitated by carbonic anhydride, and has clearly elis-
tinguishable alkaline properties. One part of lime requires at the
ordinary temperature about ^00 parts of water for solution. At IGU
it require- about 1~>OU parts of water, and therefore lime-water
becomes i.-loudv when boiled. If lime-water be evaporated in a
vacuum, then calcium hydroxide separates in six-sided crystals." If
lime-water be mixed with hvdro^en peroxide then minute crystals of
C'il<'iniii j" /•"./•/'/', ( 'aO.,.s 1 1 .,( )? separate ; this compound is verv un-
stable1 and. like barium peroxide, is decomposed by heal. Lime, as a
powerful base, combines with all acids, and in this re-pee-t presents a
transition from the true alkalis to magnesia. .Manv of the salts of
calcium (the carbonate, phosphate, borate, and oxalate) are insoluble
in water ; besides which the sulphate is onlv sparingly soluble in
water. A- a more energetic base than mau'iu'sia, lime forms salts.
('aX.,. which are distinguished by their constancy in comparison with,
the salt-. Mu'X., : neither does lime so easily form basic and double
salt s as ina u'i iesia.

• Ills' as '.xyo-eii does not combine with hvdroijvn unless aided by a
rise of temperature, so also /////< <l<» x m,f iih*<>rl> <//'// <'(trt>onic a ,/]/ //'/ r'i</'-
n/ t'ii' <>/•/} i n<i i'i/ t> in i» rut a r< . 'This was already known bv Scheele, and
1'rof. Sd uiiachenko showed that there is no absorption even at .">GO~.
Ii onlv proceeds at a red heat.'1' and then the absorption only leads to

(ili . urcd the I n-paivnt l.ri-'hl crystals of calcium hydroxide

n liy.lr, ilii- I'ortlandi cenifiit.

'•- T; ' - ' ' ' In Ml in- l>nirj- t In- -iili-tan.-f into l hat state of internal moxcinent which
; n I ion. One would think" 1 hat liy the act of healing not <>nly is till!

• • i ' . - I' tin- nn ili-eiile--, altered ._•• an -ra 1 1 \ diminished I, not

i • e i nee,

. • r- i-liMiiie anhyilriili a il il... ninl.-r tin- action of In-at. l-'or thecom-

I r, In oi < i'finieal jiheiinmeiia it i eM-ee<lin;jly n -efnl to dearly see tlii-, parallelism.

!',-,•:.• lion-ol calcium chloride

. .] |.-i i.l .nit.- I'l.im dilute, and of calc -par t'r IrmiL;, solutions,

I, • 1 from (hi point »l \ i. v, . \ , ara lite is always Inrnu-il from ]i,,t,

Ih..)', . dilnti ,vitli water act like heal. Tin- following experiment of

K ; I ieiilarh ' t nil'! ; \ i- in 1 1 ' . • . \nh Vilnius i perfect ly <h'\ ' harinni

.,-. :ded • ' t \viLh monohydrated -nlplniric n.-id. H :S( )., (coiitainin- neither freo

' ..,!i-'.|e. SO-i. llnl diredlv an incandeM-ent olijecl or a moist siiK-tance is

r i : t.ict \\ ;th tin- me.tiir.- a \ iol.-nl i-. act ion immediately begins i it is essen-

TIIK 7ALENCY AND SPECIFIC HEAT OF THE METALS 599

the formation of a mixture of calcium oxide and carbonate (Rose).
But if the lime be slaked or taken in solution, then the absorption of
carbonic anhydride proceeds rapidly and to the end. These phenomena
are connected with the dissociation of calcium carbonate, studied by
Debray (1867) under the influence of the conceptions of dissociation
introduced into science by Henri Sainte-Claire Deville. Just as there is
no vapour tension for 11011- volatile substances, so there is no dissociation
tension of carbonic anhydride for calcium carbonate at the ordinary
temperature. Just as every volatile substance has a maximum possible
\,ipour tension for every temperature, so also calcium carbonate has
its corresponding dissociation tension ; thus at 770° (the boiling point
of cadmium) about 85 mm. (of the mercury column), and at 930° (the
boiling point of Zii) it is about 520 mm. As, if the tension be greater,
there will be no evaporation, so also there will be no decomposition.
Debray took crystals of calc spar, and could not observe the least change
in them at the boiling point of zinc (930°) in an atmosphere of carbonic
anhydride taken at the atmospheric pressure (760 mm.), whilst, on the
other hand, calcium carbonate may be completely decomposed at a
much lower temperature if the tension of the carbonic anhydride be
less than the dissociation tension, which may be arrived at either by
directly pumping away the gas with an air-pump, or by mixing it with
some other gas — that is, by diminishing the partial pressure of the
carbonic anhydride,43 just as an object may be dried at the ordinary
temperature by removing the aqueous vapour or by carrying it off in
a stream of another gas. Thus it is possible to obtain calcium carbo-
nate from lime and carbonic anhydride at a certain temperature above
that at which dissociation begins, and conversely to decompose calcium
carbonate at the same temperature into lime and carbonic anhydride.44

following experiment. Lime, or barium oxide, is placed in a flask or retort having an
upper orifice and connected with a tube immersed in mercury. A funnel furnished with
a stopcock and filled with water is fixed into the upper orifice of the retort, which is then
filled with dry carbonic anhydride. There is no absorption. When the temperature of
equilibrium is arrived at, the unslaked oxide is made to absorb all the carbonic anhydride
by carefully letting in water. A vacuum is formed, as is seen by the mercury rising
up the neck of the retort. With water the absorption goes on to the end, whilst under
the action of heat there remains the dissociating tension of the carbonic anhydride.
Furthermore, we here see that, with a certain resemblance, there is also a distinction,
depending on the fact that at low temperatures calcium carbonate does not dissociate ;
this determines the complete absorption of the carbonic anhydride in the aqueous
solution.

43 Experience has long shown that by moistening partially-burnt lime with water
and re-heating it, it is easy to drive off the last traces of carbonic anhydride from it, and
that, in general, by blowing air or steam through the lime, and even by using moist fuel,
it is possible to accelerate the decomposition of the calcium carbonate. The partial pres-
sure is decreased by these means.

44 Before Deville's conception of dissociation, the modus operandi of decompo-

' M certain t . -mperat nre. t hat it i- accelerated hv a rise of temperature, Imt it was

\< • 'I pos-.il, ]c that eonil. ination could proceed at the ,ame temperature as that

h decomposition ifoes on. iSertholId and Deville introduced the conception of

eipiil hriiim int. i chemical science, and elucidated the .pi.- t;,,n of r.-versihle reactions-.

11 V the llh Jed is yet far fr. nil heill- clear the qllest i, n is of the rate and cinnplete-

i ' ' : reaet ion. ol contact , ,Vc.. still int rude themselv es — 1ml an important step has heen
made in chemical mechanic-,, and it has heen made in ihi, diredimi thanks to the
\'- >rk uj out of many data in the sense indicated hv the dodriiies.it' I'.ert holli-i and
I'eville. l;,-.de, Deville h i in s,-l 1 . 1 1 1 ,- French diemi-l- Dehrav, 'I'roost, Leinome.
Hautdeiiille, Le ( ' 1 ia I el ier, and others have done much in w orkinj out the prohlenis

In order t.. sh.iw the exact reseml. lance l.dweeii the pin uonieiia »l evaporation ami

' ' I ' IN he calculated ace.. I'd ill- to the lav, ol the Variation . .f d i ssoc ia t loll -

e. ill exadlv the same manner as it is possihle to calculate the latent heat of
ol the econd law of the mechanical theorv ol heat. This ilepeiuleilce

in..;, he expre ed l.v the equation: I-2IL '!'('/ ~~ |',Y,' ' wl"'lv '• '* !l"' hltl'nt
heat of evaporation. PJ I the niechaiiic.il equivalent of heat. T the ahsolute temper-
ature ,,| transforination, T '21'.} I. i/ the weight of a cnhic measiir.' of the sul. stance
lidorc. and I) alter, the transformation into vapour; am' then-fore 1 ,1 \ D is the
- "/' n, imm proceeding in the t raiisforniiit ion, and lastlv ,1 i//i DI/I/I is the
' "i the p.. . lire hv the temperature, or the vaiiation "t tension (expressed
in n ' ol wei III and Irn-tlii divided l»v the variation of teinperat nre. In the case
1 lea ii. taking (he metre and k i I. >•_> ra in as units, we lind. accordimj I., the data Ljiven
'i ' hapl.-r I., that when / llld - that i--, when T ">7:'> the chanje of volume is
he. ails.- one cnhic metre .>f steam at 1 MO weighs f>r,o;, kilo., whilst a ciihic
; •' . '•: water at I MO weighs '.ii'iO kilo. i. The magnitude of the product for steam at
i"11 found from tin- I'ai 1 thai vapour lension of steam at KM) varies l.y '21 mm. of

Hi in. rciirv column for a variation of 1 temperature. Hence the magnitude of the
'H "1 pre uie in kilogram per si|iiare metre ll'ii-JT I:;;.:M; :;r,",. Hence the

iatii.li i :,:'.i; l:'l ii-J7^r,J. Thai i -. l...th part-, are e,|iial within the
• nl i»l the data, and I herelme t he .piai.tit v ol heat. I .. expended in evapo

. , , it on. \ MM i .ampl. we will cite the data ohtaiiied l.\ Tro,. I and I I a u Ldeii 1 1 le
; ! . i dridi n t p..!, i nin and nd iu m. / I leiu^- the temperat lire, and A the disso-
( I ioi ori • me c. ,ri e |i.i"din w il h .1 in milliuiet res of I he merciirv coliinin

in

II. lie. ' hat III 1 lie c.l c of K II the i i ,e ol pre Ill'e pel' lie :n e at llliollt :',-Ml

: •' mi. ol ! h,- iiiei,-iir\ column, . r l lie product '21 kil... p.-r
Ih, i,, , nitnde ol (he viiriul t volnmi , /. .//// I'mm the lad that the

: . a'jonoi s,,luim i nearlv '-' 'i ". A / :;:'(l . Mierd. >re T 1'i'j:;. and t herefore we

mtiideo! L Iroin the equation t,, he nearlv IHIIIMI .-alone that is, jud^in^

ti-oiiitln variatmn o| the di ociali.Hl pre lire ol pot a mil hv d ride, t here is (lev eloped

THE VALENCY AND SPECIFIC HEAT OF THE METALS 001

r;u mot take place, and thus all the most important phenomena with re-
spect to the behaviour of lime towards carbonic anhydride are explained
by starting from one common basis.45

Calcium carbonate, CaC03 — which occurs, as has already been
mentioned, in limestone, marble, &c. — is sometimes met with in nature
in a crystalline form, and it forms an example of the phenomenon
termed ft tin lu-jihixm— that is, it appears in two crystalline forms.
\Vhen it exhibits combinations of forms belonging to the hexagonal
system (six-sided prisms, rhombohedra, Arc.) it is called calc spar.
Calc spar has a specific gravity of 2'7, and is further characterised by
;i distinct cleavage along the planes of the fundamental rhombohedron
having an angle of 105°. Perfectly transparent Iceland spar presents
a clear example of double refraction (for which reason it is frequently

not been verified by experiment, nor has the calculated figure of 13 calories for the for-
mation of sodium hydride, Na^H, at 300 been verified ; but the figure of 4'1 calories
calculated ('20°) by Moutier from Troost's data for the dissociation pressure of palladium
hydride, entirely agrees with the direct results of Favre, which showed that each gram of
hydrogen which combines with palladium at the ordinary temperature develops 4'174
calories, and therefore one and the same conception of the mechanical theory of heat
ix (tji/ilicdble to dissociation and evaporation. The heat evolved in the absorption of
hydrogen depends, naturally, not only on the physical process of the condensation of the
pis, but also on the formation of a new chemical compound, as is seen from the fact that
different metals in absorbing a gram of hydrogen develop a different amount of heat —
namely, palladium 4 calories, potassium 10 calories, sodium 13 calories, platinum 20
calories — and therefore in the concordance of calculation with fact we should see,
not an identity, but only a resemblance between the phenomena of dissociation and
evaporation.

45 But still the question as to the formation of a basic calcium carbonate remains
undecided. According to certain data one would think that, with a rise of temperature,
not only calcium carbonate but also a basic salt may be formed, which, however, is
denied by certain investigators. Probably the presence of water complicates all the re-
lations between lime and carbonic anhydride, all the more as the existence of an attrac-
tion between calcium carbonate and water is seen from its being able to give a crystallo-
hydrate, CaCO3,5H2O (Pelouze, Salm-Horstmar), which crystallises in rhombic prisms of
sp. gr. about 1'77 and loses its water at 20°. These crystals are obtained when a solution
of lime in sugar and water is left long exposed to the air and slowly attracts carbonic
•in hydride from it, and also by the evaporation of such a solution at a temperature of
about 8°. On the other hand, it is probable that an acid salt CaH2(COr,)2 is formed in
u n aqueous solution, not only because water containing carbonic acid dissolves calcium
carbonate, but more especially judging by the researches of Schloesing (1872), which showed
that at lf>° a litre of water in an atmosphere of earbonic anhydride (pressure 0'984 atmo-
sphere) dissolves 1'08(> grams of calcium carbonate and 1*778 grams of carbonic anhydride,
which corresponds with the formation of calcium hydrogen carbonate, and the solution
of carbonic anhydride in the remaining water. Caro showed that a litre of water is able
to dissolve as muoh as 3 grams of calcium carbonate if the pressure be increased to 4 and
more atmospheres. The calcium carbonate is precipitated when the carbonic anhydride
passes off hi the air or in a current of another gas ; this also takes place in many natural
springs. Tufa, stalactites, and other like formations from waters containing calcium
carbonate and carbonic acid in solution, are formed in this manner. The solubility of
calcium carbonate itself at the ordinary temperature does not exceed 13 milligrams per
litre of water.

602 PRINCIPLES OF CHEMISTRY

employed in physical apparatus). The other form of calcium carbonate
occurs in crystals belonging to the rhombic system, and it is then
called aragonite ; its specific gravity is 3*0. If calcium carbonate is
artificially produced by slow crystallisation at the ordinary temperature,
then it appears in the rhombohedral form, but if the crystallisation be
aided by heat then it appears as aragonite. One may suppose, there-
fore, that calc spar presents the form corresponding with a low temper-
ature, and aragonite with a higher temperature during crystallisation.46
Calcium sulphate in combination with two equivalents of water,
CaSO4,2H20, is very widely distributed in nature, and is known as
yypsum. Gypsum loses its two equivalents of water at a moderate
temperature,47 and anhydrous or burnt gypsum is then obtained, which
is also known as plaster of Paris, and is employed in large quantities
for modelling.48 This use depends on the fact that burnt and finely-
divided and sifted gypsum forms a paste when mixed with water ;.
after a certain time this paste becomes slightly heated and solidifies,
owing to the fact that the anhydrous calcium sulphate, CaSO4, again
combines with water. When the plaster of Paris and water are first
made into a paste they form a mechanical mixture, but when the mass

46 When calcium carbonate separates out from solutions, it at first has a gelatinous
appearance, which leads one to think that this salt appears in a colloidal state. It only
crystallises with the progress of time. The colloidal state of calcium carbonate is parti-
cularly clear from the following observations made by Prof. Fammtsin, who showed that
when it separates from solutions it is obtained under certain conditions in the form of
grains having the peculiar paste-like structure proper to starch, which fact has not only
an independent interest, but presents an example of a mineral substance being obtained
in a form until then only known in the organic substances elaborated in plants. This
shows that the forms (cells, vessels, &c.) in which vegetable and animal substances-
occur in organisms do not present in themselves anything peculiar to organisms,
but are only the result of those particular conditions in which these substances are
formed. Traube and afterwards Monnier and Vogt (1882) obtained formations which,
under the microscope, were in every respect identical in appearance with vegetable cells,
by means of a similar slow formation of precipitates (by reacting on sulphates of
different metals with sodium silicate or carbonate). Owing to its insolubility in water,,
calcium carbonate may be easily obtained from any other soluble calcium salt by the
addition of a solution of an alkali carbonate ; for example, ammonium carbonate.

"7 According to Le Chatelier (1888), l£H2O is lost at 120°— that is, H2O,2CaSO4
is formed, but at 194° all the water is expelled. According to Shenstone and Cundali
(1888) gypsum begins to lose water at 70° in dry air. The semi-hydrated compound,,
HoO,2CaSO4, is also formed when gypsum is heated with water in a closed vessel at 150°
(Hoppe-Seyler).

48 For stucco-work it is usual to add lime and sand, as the mass is then harder and
does not solidify so quickly. For imitating marble, glue is added to the plaster, and the
mass is polished when thoroughly dry. Keburnt gypsum cannot be used over again, as
that which has once solidified is, like the natural anhydride, not able to re-combine with
water. It is evident that the structure of the molecules in the crystallised mass, or in
general in any dense mass, exerts an influence on the chemical action, as is particularly
clearly seen in metals in their different forms (powder, crystalline, rolled, &c.).

THE VALENCY AND SPECIFIC HEAT OF THE METALS 603

solidifies, then a compound of the calcium sulphate with two molecules
of water is produced ; and this may be regarded as derived from S(OH)6
by the substitution of two atoms of hydrogen by one atom of bivalent
calcium. Natural gypsum sometimes appears as perfectly colourless,
or variegated marble-like, masses, and sometimes in perfectly colourless
crystals, selenite, of sp. gr. 2 '33. The semi-transparent gypsum, or
alabaster, is often carved into small statues. Besides which an anhy-
drous calcium sulphate, CaSO4, called anhydrite (sp. gr. 2'97), occurs
in nature. It sometimes occurs along with gypsum. It is no longer
capable of combining directly with water, and differs in this respect
from the anhydrous salt obtained by gently igniting gypsum. If
gypsum be very strongly heated it shrinks and loses its power of
combining with water. Gypsum also passes into anhydrite if it be
heated in water, if the temperature be as high as 150°. One part
of calcium sulphate requires at 0° 525 parts of water for solution,
at 38° 466 parts, and at 100° 517 parts of water. The maximum
solubility, which is at about 36°, is nearly the same as that of sodium
sulphate.49

As lime is a more energetic base than magnesia, so calcium chloride,
CaCl.,, is not so easily decomposed by water, and its solutions only
disengage a small quantity of hydrochloric acid when evaporated, and
when the evaporation is conducted in a stream of hydrochloric acid it.
easily gives an anhydrous salt which fuses at 719° ; otherwise an
aqueous solution yields a crystallo-hydrate, CaCL2,6H20, which melts
at 28°.50

49 As Marignac showed, gypsum especially desiccated at 120° easily gives supersatu-
rated solutions with respect to CaSO4,2H2O, which contain as much as 1 part of CaSC>4
to 110 parts of water. Boiling dilute hydrochloric acid dissolves gypsum, forming cal-
cium chloride. The behaviour of gypsum towards the alkali carbonates has been de-
scribed in Chap. X. Alcohol precipitates gypsum from its aqueous solutions, because,
like the sulphates in general, it is sparingly soluble in alcohol. Gypsum, like all the sul-
phates, when heated with charcoal, gives up its oxygen, forming the sulphide, CaS.

Calcium sulphate, like magnesium sulphate, is capable of forming double salts, but
with difficulty, and they are chemically less stable. They contain, as is always the case
with double salts, less water of crystallisation than the component salts. Rose, Phillips,
Schott, Stchefarovitch, Struve, Ditte, and others obtained the salt CaK2(SO4)2,H2O ;
a mixture of gypsum with an equivalent amount of potassium sulphate and water
solidifies into a homogeneous mass. Fritzsche obtained the corresponding sodium salt
in a hydrated and anhydrous state, by heating a mixture of gypsum with a saturated
solution of sodium sulphate. The anhydrous salt occurs in nature as glanberite.
Fritzsche also obtained gaylussite, Na-jCa^Os^SHjjO, by pouring a saturated solution
of sodium carbonate over freshly-precipitated calcium carbonate.

50 Calcium chloride has a specific gravity 2'20, or, when fused, 2'12, and the sp. gr. of
the crystallised salt CaCl2,6H2O is T69. If the volume of the crystals at 0°=1, then at
29° it is r020, and the volume of the fused mass at the same temperature is 1'118 (Kopp)
(specific gravity of solutions, see Note 27). The solution containing 50 p.c. CaCl2 boils
at 180, 70 p.c. at 158°. Superheated steam decomposes calcium chloride with more dim-

604 PEINCIPLES OF CHEMISTRY

As for potassium, K=39 (and sodium, Na = 23), there are the
near analogues, lib = 85 and Cs = 133, and also another, Li = 7, so
in exactly the same manner for calcium, Ca = 40 (and magnesium,
Mg = 24), there is another analogue of lighter atomic weight, or
beryllium, Be = 9, besides the near analogues, strontium, Sr = 87, and
barium, Ba=137. As rubidium and caesium are more rarely met
with in nature than potassium, so also strontium and barium are rarer

culty than magnesium chloride and with greater ease than barium chloride (Kuhnheim).
Sodium does not decompose fused calcium chloride even with further heating (Lies-
Bodart), but an alloy of sodium with zinc, lead, or bismuth decomposes it, forming an
alloy of calcium with one of the above-named metals (Caron). The zinc alloy may be
obtained with as much as 15 p.c. of calcium. Calcium chloride is soluble in alcohol and
absorbs ammonia.

A molecular gram weight of calcium chloride in dissolving in an excess of water
evolves 18723 calories, and in dissolving in alcohol 17555 units of heat, according to
Pickering.

Roozeboom made detailed researches on the crystallo-hydrates of calcium chloride
(1889), and found that CaCl2,6H2O melts at 30'2°, and is formed at low temperatures from
solutions containing not more than 103 parts of calcium chloride per 100 parts of water ; if
the amount of salt (always to 100 parts of water) reaches 120 parts, then tabular crystals
of CaCl2,4H2O/3 are formed, which at temperatures above 38'4° are converted into the
crystallo-hydrates CaCl2,2H2O, whilst at temperatures below 18° the variety j8 passes into
the more stable CaCl2,4H2Oa, which process is aided by mechanical friction. Hence, as
is the case with magnesium sulphate (Note 27), one and the same crystallo-hydrate ap-
pears in two forms — the #, which easily forms, but does not keep and is unstable, and the
other, a, which is stable. The solubility of the above-mentioned hydrates of chloride of
calcium, or amount of calcium chloride per 100 parts of water, is as follows : —
0° 20° 30° 40° 60°

CaCl2,6H2O 60 75 100 (102'8)

CaCl2,4H2Oo 90 101 117,

CaCl2,4H20)8 104 114

CaCl2,2H2O (308-3) 128 137

The amount of calcium chloride to 100 parts of water in the crystallo-hydrate
given in brackets. The point of intersection of the curves of solubility lies at about
80° for the first two salts and about 45° for the salts with 4H2O and 2H2O. The crystals
CaCl2,2H2O may, however, be obtained (Ditte) at the ordinary temperature from solu-
tions containing hydrochloric acid. The vapour tension of this crystallo-hydrate equals
the atmospheric at 165°, and therefore the crystals may be dried in an atmosphere of
steam and obtained without a mother liquor, whose vapour tension is greater. This
crystallo-hydrate decomposes at about 175° into CaCl2,H2O and a solution ; this is easily
brought about in a closed vessel when the pressure is greater than the atmosphere.
This crystallo-hydrate is destroyed at temperatures above 260°, anhydrous calcium
chloride being formed.

On the other hand, Hammerl showed that solutions of calcium chloride, when frozen,
deposit ice if they contain less than 48 parts of salt per 100 of water, and if more the
crystallo-hydrate CaCl2,6H2O separates, and that a solution of the above composition
(CaCl2,14H2O requires 44'0 parts calcium chloride per 100 of water) solidifies as a cryo-
hydrate at about — 55°. Thus the solubility of calcium chloride is better known than
that of any other salt.

Neglecting the unstable equilibrium CaCl2,4H2O)3, we will cite the temperatures t at
which the passage of one hydrate into another takes place and at which the solution
CaCl2 + wH2O, the two solids A and B and aqueous vapour, whose tension is given ae p

Till-: VALKNVY AND SPECIFIC HEAT OF THE METALS 605

than calcium (as also bromine and iodine are rarer than chlorine).
Whilst exhibit inu' many points of resemblance with calcium, strontium
and barium may be characterised after a very short acquaintance with
their chief compounds ; this shows the important advantages gained by
distributing the elements according to their natural groups, to which
matter we shall turn our attention in the next chapter.

Among the compounds of barium met with in nature the commonest
is the sHlplmti', P>aSO4, which forms anhydrous crystals of the rhombic
system, which are identical in their crystalline form with anhydrite,
and which generally occur as transparent and semi-transparent masses
of tabular crystals having a high specific gravity, namely 4'45, for which
reason this salt bears the name of heavy spar or barytes. Analogous to
it is celestine, SrSO4, which is, however, more rarely met with ; heavy
spar frequently forms the gangue separated on dressing metallic ores
from the vein stuff. This mineral is the source of all other barium
compounds, because the carbonate, although more easily transformed
into the other compounds (because acids act directly on it, evolving
carbonic anhydride), is, however, a comparatively rare mineral (BaCO3
forms the mineral witherite ; SrCO3, strontianite ; both are rare, the
latter is found at Etna). The treatment of barium sulphate is
rendered difficult from the fact that it is insoluble both in water and
acids, and has therefore to be conducted by a method of reduction.51
Like sodium sulphate and calcium sulphate, heavy spar when heated
with charcoal parts with its oxygen and forms barium sulphide, BaS.
For this purpose a pasty mixture of powdered heavy spar, charcoal,
and tar is subjected to the action of a strong heat, in which case
BaSO4 + 40 = BaS + 4CO. The residue is then treated with water, in
which the barium sulphide is soluble.52 When boiled with hydrochloric

in millimetres, are able to exist together in stable equilibrium, according to Roozeboom's

determinations —

t

n

A

B

P

-55°

14-5

ice

CaCl2,6H2O

0

+ 29-8°

6'1

CaCl2,6H20

CaCl2,4H2O

6-8

45-8°

4-7

CaCl2,4H2O

CaCl2,2H2O

11-8

176-5

2-1

CaCL,2H20

CaClo,H20

842

260°

L-8

CaCl2,H20

CaCl2

Several atmospheres.

Solutions of calcium chloride may serve as a convenient example for the study of the
supersaturated state, which easily comes about in this case, because different hydrates
are formed. Thus at 25° solutions containing more than 88 parts of anhydrous calcium
chloride per 100 of water will be supersaturated for the hydrate CaCl2,6H2O.

51 The action of barium sulphate on sodium and potassium carbonates is given on
p. 427.

52 Barium sulphide is decomposed by water, BaS + 2H2O = H2S + Ba(OH)2 (the reac-
tion is reversible), but both substances are soluble in water, and their separation is com-
plicated by the fact that barium sulphide absorbs oxygen and gives insoluble barium

arid, barium chloride, I'aCl.,. i- obtained in solution, and the sulphur is
disengaged as gaseous snip] in ret ted hydrogen, Ba8 + -HC1= 15alMv -f H.,S.

In tins manner barium sulphate is converted into barium chloride,'1''*
and tin- latter by double decomposition with strong nitric acid or nitre
o;ives the less soluMe l>arium nitr.-ite, T>a( X( ).,).,.•'' or with sodium

-' i ' pliulc. The hydro_;i-ii sulphide is sometimes removed from the solution by boiling
with the oxides of copper or /.hie-. Ifsuirarbe added to a solution of l.ariiun sulphide,
then on heating barium saccharate is precipitated : it i- decomposed liy carbonic anhy-
dride, so that liarium carbonate i- I'ornied. An ei|nivalent mixture of sodium sulphate
ui;h barium or strontium sulphates when ignited with charcoal Drives a mixture of
-odium sulphide and bariui tiuin sul])hide. and if this mixture be di-solved in

water and the solution evaporated, then barium or strontium hydroxide crystallises out
on cooling, and sodium hydrosulphide, NallS. is obtained in solution. The hydroxides
I la 11 .< I . and Sri I., O., are prepared on u lar-e scale, bein^ applied to many reactions ; for
example, strontium hydroxide i- pi-epared for sUi,rar works for extract in:.' crystallisable
-ii'j'ar fri on UK ila ssi •-.

We ma\' remark' that 1 '>oii-sinc_:'ault . by iu'nit inu' barium sulphate in h\ drochloric acid
_,i-. obtained a complete decomposition, with the formation of liarium chloride. Attention
-hould also be turned to the fact that (Irouven by lieatinjjf a mixture of charcoal and
strontium -ulphate with maLnie-ium and potassium sulphates sho\vt-d the easy decom-
posability depending on the formation ,,f double salts, such as SrS.K ,S. \\ hich are easily
-olnble in water, and j^ive a precipitate of strontium carlionate with carbonic- anhydride.
In such examples as these we see that the force which binds double salts may play a part
i directing the course of react ion-, and the number of double salt- of silica on the earth's
-nrface shows that nature take- advantage of these force- 'n her chemical processes.

manner: finely-ground barium sulphate is heated with coal and manganese chloride,
forming the residue from the manufacture of chlorine. The mass becomes senii-liijuid,
and when it evolves carbonic oxide the healing is -topped. The following double decom-
po-Mtions proceed durini: this operation : lir-t the carbon takes up the oxygen from the
I- irium sulphate, and -he- sulphide. BaS, which enters into double decomposition with
the chloride of manganese. MnCl ,. forming manganese sulphide. ]\1 nS, which is insoluble
,.,'ter. and -olnMe barium chloride. This solution is easily obtained pure because
many !'oi''':_ n impurities, -uch a- iron, remain in the insolnl.le port ion with the manganese.
The suhiti < III ride obtained in thi- manner i- chielly used for the prepara-

tinii of liarium -idpliate. which is pi'ecipit ated by sulphuric acid, liy which means Imi-iinn
* ,i 1 ,,1,'it r \~ re formed a- a po\\der. 'I'll is salt i- chai'actei'ised b\- the fact thai it is unacted
,,n b\ thei ' nl chemical iva;_-ent -. is insoluble in water, and i- not dissolved by

cid-. but is -!'/i tl\ - led . • icon- -olulion by the prolonged action of alkalis

,1 boili )!•_'. the sohtl ion -i\ es liarinm eai'bonaie. Chapter \.l. OWI'IILT to this, artificial
barium siilpha.te form- a p. nn, HP-!!' uhite painl \vhich i- il-.-d instead of white lead.

; , uhite i|n. '• • I • ' '!< ' tile ill I io)l of Sll 1 ph U I'et t ed h \ d 1'o-e II , W 1 1 icll

IT, • v ; ife ;. i.l. Heue, it I I.. , i i, rmed ' I.I inc lixc ' or • permanenl white.'

The -olntion ot ,,ne part of caleium chloi-ide al I'll requires !•:;<; parts of water, the

p.Tature. and the ,,],„;,,:, ,,| I ,., ri n i n . 1 1 lo,-',|,. L"ss; ] .arts of water. The .olnbihf y of the

;oi-m Iia('l ,.iiU .( > and Sr< '! ..I'd 1 ,( ) l!a latter resembles the salts of calcium and 111:1^.'

•'i 'I i .••.-'<< ' M ive a cr\ lallo hydrate containing

1 | ! ( i . ,. . M I ;,, '.' i . i \ i | -i! '•: ater thai llie\ i-| ia rate in rat lie|-

Till-: VALENCY AND SI'KCIFIC HEAT OF THE METALS 607

varbonate a precipitate of barium carbonate, BaCO3. Both these salts
are able to give barium oxide, or baryta, BaO, and the hydroxide,
Ba(HO).,, which differs from lime by its great solubility in water,5'"'
and by the ease with which it forms a crystallo-hydrate, BaH2O2,8H2O,
from its solutions. Owing to its solubility, baryta is frequently
employed in the manufactures and in chemical practice as an alkali
which has the very important property that it may be always entirely
removed from solution by the addition of sulphuric acid, which
entirely separates the baryta as the insoluble sulphate, BaS04. It
may also be removed so long as it remains in an alkaline state (for
example, the excess which may remain when it is taken for saturating
acids) by means of carbonic anhydride, which also entirely precipitates
barium oxide as a sparingly soluble, colourless, and powdery carbonate.
Both these reactions show that baryta has such properties as would
render its use in practice most widely extended were its compounds
as widely distributed as those of sodium and calcium, and if its soluble
compounds were not poisonous. Barium nitrate is directly decomposed
by the action of heat, barium oxide being left behind. The same takes
place with barium carbonate, especially that precipitated from solutions
and when mixed with charcoal or ignited in an atmosphere of steam.

Barium oxide combines with water with the development of a large
amount of heat, and the resultant hydroxide is so stable in its retention
of the water that it will not part with it under the action of heat
alone, although it entirely dissociates when strongly ignited in a current
of hydrogen or other gas, and especially of air. With oxygen the

or strontium chlorides. They are obtained by the action of nitric acid on the carbonates
•or oxides. They may also be obtained by the action of nitric acid on solutions of the
chlorides, all the more as they are comparatively little soluble, especially in water con-
taining nitric acid— 100 parts of water at 15° dissolve 6'5 parts strontium nitrate and
N'2 parts of barium nitrate, whilst more than 800 parts of calcium nitrate are soluble
at the same temperature. Strontium nitrate communicates a crimson coloration to the
flame of burning substances, and is therefore frequently used for Bengal fire, fireworks,
and signal lights, for which purpose the salts of lithium are still better fitted. Calcium
nitrate is exceedingly hygroscopic. The barium nitrate, on the contrary, does not show
this property in the least degree, and in this respect it resembles potassium nitrate, and
is therefore used instead of the latter for the preparation of a gunpowder which is called
' saxifragin powder ' (76 parts of barium nitrate, 2 parts of nitre, and 22 parts of charcoal).
55 The dissociation of the crystallo-hydrate of baryta is given in Chapter I. Note 65.
100 parts of water dissolve

0° 20° 40° 60° 80°

BaO 1-5 8'5 7-4 18'8 90'8

SrO 0-8 0-7 1-4 8 9

Supersaturated solutions are easily formed.

The anhydrous oxide BaO fuses in the oxyhydrogen flame. When ignited in the
vapour of potassium, the latter takes up the oxygen ; whilst in chlorine the oxygen is
separated and barium chloride formed.

608 PRINCIPLES OF CHEMISTRY

anhydrous oxide gives, as has already been mentioned in Chapters Til..
and IV., a peroxide, BaO2.56 Neither calcium nor strontium oxides
are able to give such a peroxide directly, but they form peroxides under
the action of hydrogen peroxide.

Barium oxide is decomposed when heated with potassium ; and fused
barium chloride is decomposed, as Davy showed, by the action of a
galvanic current, forming metallic barium ; and Crookes (1862) obtained
an amalgam of barium from which the mercury could easily be driven
off, by heating sodium amalgam in a saturated solution of barium
chloride. The same occurs with strontium. Both metals are soluble in.
mercury, and seem to be non-volatile or at least very slightly volatile,
They are both heavier than water ; the sp. gr. of barium is 3 -6, and of
strontium 2-5. They both decompose water at the ordinary temperature,,
like the metals of the alkalis.

Barium and strontium as saline elements are characterised by their
powerful basic properties, so that they form acid salts with difficulty,
and scarcely form basic salts. On comparing them together and with
calcium, it is evident that the alkaline properties in this group (as in
the group potassium, rubidium, caesium) increase with the atomic
weight, and this succession clearly shows itself in many of their corre-
sponding compounds. Thus, for instance, the solubility of the
hydroxides RH2O2 and the specific gravity 57 rise in passing from
calcium to strontium and barium, while the solubility of the sulphates
decreases,^8 and therefore in the case of magnesium and beryllium as
metals whose atomic weights are still less, we should expect the solu-
bility of the sulphates to be greater, and this is the case in reality.

5t> The property of barium oxide of absorbing oxygen when heated, and giving the
peroxide BaOo, is very characteristic for this oxide. It is only proper to the anhydrous
oxide. The hydroxide does not absorb oxygen. Peroxides of calcium and strontium may
be obtained by means of hydrogen peroxide. Barium peroxide is insoluble in water, but
is able to form a hydrate with it, and also to combine with hydrogen peroxide, forming a,
very unstable compound having the composition BaH2O4 (obtained by Professor Schone),
and which in the course of time evolves oxygen (Chapter IV. Note 21).

57 Even in solutions a gradual progression in the increase of the specific gravity shows
itself, not only for equivalent solutions (for instance, KC12 + 200H2O), but even with an
equal percentage composition, as is seen from the curves giving the specific gravity
(water 4° = 10000) M 15° (for barium chloride, according to Bourdiakoff's determina-

tions) :

BeCl2 : S - 9992 + 6TZlp + O'lllp2

CaCl2 : S = 9992 + 80'24p + 0'476p*
SrCl2 : 8 = 9992 + 85-572? +0-733^2
BaClo :

58 One part of calcium sulphate at the ordinary temperature requires about 500 parts
of water for solution, strontium sulphate about 7000 parts, barium sulphate about 400000
parts, whilst beryllium sulphate is easily soluble in water.

TIH-: VALENCY AND SIM-CII'IC HI-AT <>[' T[[|-: MF.TALS ('><)!)

As in the series of the alkali metals we saw the metals p
rubidium, and caesium approaching near to each other in their proper-
ties, and besides them two metals having smaller combining weights -
namely, sodium, and the lightest of all, lithium, which already exhibited
< rrtain particular characteristic properties — so also in the case of the
metals of the alkaline earths we find, besides calcium, barium, and
strontium, the metal magnesium and also beryllium or ylucinum. In
respect to the magnitude of its atomic weight, it occupies the saim-
position in the series of the metals of the alkaline earths as lithium does
in the series of the alkali metals, because the combining weight of
beryllium, Be or Gl=9. This combining weight is greater than the com-
bining weight of lithium (7), as the combining weight of magnesium (24)
is greater than that of sodium (23), or of calcium (40) is greater than
that of potassium (39), &c.59 Beryllium was so named because it occurs
in the mineral beryl. The metal is also called glucinum (from the
Greek word yXvKvs, ' sweet '), because its salts have a sweet taste. It
occurs in beryl, aquamarine, the emerald, and other minerals, which
are generally of a green colour, and sometimes occur in considerable
masses, but which are as a rule comparatively rare and, as trans-
parent crystals, form precious stones. The composition of beryl, the
emerald, and smaragd is as follows : Al2O3,3BeO,6SiO.2. The Siberian
and Brazilian beryls are the best known. The sp. gr. of beryl is
about 2*7. Beryllium oxide, from the feebleness of its basic properties,
presents an analogy to aluminium oxide in the same degree as lithium
oxide is analogous to magnesium oxide.60 Owing to its rare occur-

59 \Ve refer beryllium to the class of the bivalent metals of the alkaline earths — that
is, we ascribe to its oxide the formula BeO, and do not count it as trivalent (Be = 13T>,
p. 318), as has been proposed and argued by many. The true atomic composition of beryl-
lium oxide was first given by the Russian chemist, Avdeeff (1819), in his researches on the
compounds of this metal. He compared the compounds of beryllium to those of magne-
sium, and set aside the then reigning opinion of the resemblance between the oxides of
beryllium and aluminium, by proving that beryllium sulphate presents a greater rest in
blance to magnesium sulphate than to aluminium sulphate. It was especially noticed
that the analogues of alumina give alums, whilst beryllium oxide, although it is a feeble
base, easily giving, like magnesia, basic and double salts, does not form true alums. The
establishment of the periodic system of the elements (1869), which is considered in the
following chapter, immediately showed that Avdeeffs view corresponded with the truth
—that is, that beryllium is bivalent, and therefore necessitated the refutation of the
trivalency of beryllium. This scientific controversy resulted in a vast series of re-
searches (1870-80) concerning this element, and ended in Nilson and Pettersson — two
of the chief advocates of the trivalency of beryllium — determining the vapour density
of BeCl2 ( — 40, p. 818), which gave an undoubted proof of the bivalency of beryllium.

60 Beryllium oxide, like aluminium oxide, is precipitated from solutions of its salts
by alkalis as a gelatinous hydroxide BeHoO2, which, like alnmiua, is soluble in an excess
of caustic potash or soda. This reaction may be taken advantage of for distinguishing
and separating beryllium from aluminium, because wlieii the alkaline solution is diluted
with water and boiled beryllium, hydroxide is precipitated, whilst the alumina remains

VOL. I. R R

610 PRINCIPLES OF CHEMISTRY

rence in nature, to the absence of any especially distinct individual
properties, and to the possibility of foretelling them to a certain
extent on the basis of the periodic system of the elements given in
the following chapter, and owing to the brevity of this treatise, we
will not linger long over the compounds of beryllium, and will only
observe that the individuality of the compounds of beryllium was
pointed out in 1798 by Vauquelin, and that metallic beryllium was
obtained by Wohler and Bussy. "VVohler obtained metallic beryllium
(like magnesium) by acting on beryllium chloride, BeCL2, with potassium.
Metallic beryllium has a sp. gr. 1-64 (Nillson and Pettersson). It is
very infusible, melting at nearly the same temperature as silver, which
it resembles in its white colour and lustre. It is characterised by the
fact that it is very difficultly oxidised, and even in the oxidising flame
of a blow-pipe is only superficially covered by a coating of oxide ; it
does not burn in pure oxygen, does not decompose water at the
ordinary temperature or at a red heat, but gaseous hydrochloric acid
is decomposed by it when slightly heated, with evolution of hydrogen
and development of a considerable amount of heat. Even dilute
hydrochloric acid acts in the same manner at the ordinary temperature.
Beryllium also acts easily on sulphuric acid, but it is remarkable that
neither dilute nor strong nitric acid acts on beryllium, which seems to

in solution. The solubility of the beryllium oxide already clearly indicates its feeble
Imsic: properties, and, as it were, separates this oxide from the class of the alkaline earths.
But on arranging the oxides of the above-described metals of the alkaline earths accord-
ing to their decreasing atomic weights we have the series

BaO, SrO, CaO, MgO, BeO,

in which the basic properties and solubility of the oxides consecutively and distinctly
decrease until we reach a point when, had we not known of the existence of the beryllium
oxide, we should expect to find in its place an oxide insoluble in water and of feeble basic-
properties. So also in the series of the metals of the alkalis the basicity of lithium oxide
is distinctly more feeble than sodium and potassium oxides, and lithium carbonate is
insoluble in water.

Another characteristic of the salts of beryllium is that they give a gelatinous pre-
cipitate with aqueous ammonia, which is soluble in an excess of ammonium carbonate,
like the precipitate of magnesia ; in this beryllium oxide differs from the oxide of
aluminium. Beryllium oxide easily forms a carbonate which is insoluble in water, and
resembles magnesium carbonate in many respects. Beryllium sulphate is distinguished
by its considerable solubility in water — thus, at the ordinary temperature it dissolves
in an equal weight of water ; it crystallises out from its solutions in well-formed crystals,
which do not change in the air, and contain BeSO4,4H.X). When ignited it leaves
beryllium oxide, but this oxide, after prolonged ignition, is re-dissolved by sulphuric acid,
whilst aluminium sulphate, after a similar treatment, leaves aluminium oxide, which is
no longer soluble in acids. With a few exceptions, the salts of beryllium crystallise with
great difficulty, and to a considerable extent resemble the salts of magnesium ; thus, for
instance, beryllium chloride is analogous to magnesium chloride. It is volatile in an
anhydrous state, and in a hydrated state it decomposes, with the evolution of hydro-
chloric acid.

TIIF. YAI.KNC'Y AM* SPECIFIC II MAT OF THK METALS 611

resist oxidising agents with particular ease. Potassium hydroxide acts
on beryllium as on aluminium, hydrogen being disengaged and the
metal dissolved, but ammonia has no action on it. These properties of
metallic beryllium seem to isolate it from the series of the other metals
described in this chapter, but if we compare the properties of calcium,
magnesium, and beryllium we shall see that magnesium occupies an
intermediate position between the other two. Whilst calcium decom-
poses water with great ease, magnesium does so with difficulty, and
beryllium not at all. The peculiarities of beryllium among the metals
of the alkaline earths recalls the fact that in the series of the halogens
we saw that fluorine differed from the other halogens in many of its
properties and has the smallest atomic weight. Just the same is the
case with beryllium among the other metals of the alkaline earths.

END OF THE FIRST VOLUME

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