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Full text of "The principles of chemistry"

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 16 ih 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 work 2 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 






i <3 






5 ' fi 



& 



. P 









. H . 



' 1 ' fi ' 

6 . 1 - H 



. . > . ,3 . fi . 1 



be ' fl 



bo * 



o o 



PI 







3 * a 3.1.1 



Xlll 



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. 


R0 2 


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. 


R 2 


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. 


R0 2 


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 sulphur 6 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 mass 11 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 tube 15 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- 
bonate 17 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, grams 2 ' 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 w r as 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 A M B" 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. 2 C0 3 ), and in photography (LiBr). 
Boron, B=l 1. As Borax, B 4 Na 2 O 7 , and as boric anhydride, B 2 O 3 . 



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, CrO 3 , and potas- 
sium dichromate, K 2 Cr 2 O 7 . 

As manganese peroxide, Mn0 2 , and po- 
tassium permanganate, MnKO 4 . 

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, As 2 3 . 

A brow T ii volatile liquid ; sodium bromide, 
NaBr. 

In coloured tires (SrN,O 6 ). 

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, BaS0 4 . 

^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;oircTiON 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 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 ,u r as 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 nf: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; = N H ~-^. 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 and 760 mm. ? The pressure of aqueous vapour corresponding 
witli 15"3 C 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 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 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 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 

V J1/ 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 


^ 


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fj S-s "as* ^ 




a 








O 






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I. 


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152 




24 




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152 1,300 80 1 2,609 


II. 


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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 ( Sul P huric 
'DO/. ] ail d 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 collected 7 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 system 10 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 - 5 D . . . 0-99929 At 30 ... G'99577 

., ... 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 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 it is 999'8 kilos. The weight 
of a cubic metre of ice at 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 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 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 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 to 1, i.e., by 1 C., is called the unit of heat or calorie; the specific 
heat of liquid water at 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 
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 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 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 (0'007 J 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 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 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 400 600 D . 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 





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 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 and 760 mm. weighs 

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

1368 

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 

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 (CoH 4 O.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 w r hole 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 H 2 SO 4 , H 2 O ; or NaCl, 10H 2 O ; 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 H 2 SO 4 , or SO 3 + H 2 O, 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 H 2 SO 4 
to H 2 SO 4 + H 2 O (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 H 2 SO 4 ). For 

alcohol C 2 H 6 O, whose aqueous solutions have been more accurately investigated than all 
others, three definite compounds must be acknowledged in its solutions, C 2 H C O -f 12H 2 O, 
C 2 H ( ,O + 3H 2 O, and 3C 2 H 6 O + H 2 O. 

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.- 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 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 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 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 At 20 At 100 



, Oxygen, O 2 
Carbonic anhydride, CO 2 



"^fe ^ _ 

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

, Phenol, C fi HO ! 4'9 5'2 oo 

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

^ Sulphuric acid, H,>SO 4 .... oo OO OO 

( Gypsum, CaSO 4 , 2HoO . j ^ i ^ 

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

Solids - Anhydrous sodium sulphate, NaoSO 4 



Common Salt, NaCl 
Nitre, KN0 3 



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 : 

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 exactly coincides with 
the composition of a definite chemical compound NaXO5,7H 2 O. The experiments 
made by Ditte showed that all saturated solutions between 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, FeSO 4 , 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 
FeSO 4 ,14H 2 O, 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. 



7 k 2 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*017172 2 + 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 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 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 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 Na 2 SO 4 + 10H. 2 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, 
Na 2 S0 4 . 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 NH 3 ), 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 H 2 O), evolve 7,535 units of heat, and there- 
fore the formation of the solution NH 3 + H 2 O 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,S s O 3 ,5H 2 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, C 6 H 6 O, 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 67 D 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 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, CaCl 2 ,6H 2 O, with seven parts of water, the temperature may even 
fall from 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 2 S( ) ,), 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 le borne in mind that the expulsion of the water taken for 
-olution is not accomplished with equal facility throughout, because 
watei 1 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 gas 29 (at 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 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 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 
and 760 mm. multiplied by its density referred to air and by the weight of one volume 
of air at 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 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 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 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 and 7(50 mm.) : 

123 456 7 89 10 

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 


Hydrochloric acid 
gas dissolved in 100 
trnmis of water at 




( 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, H 2 O, 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 + 8H 2 O. 

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 Dalton 33 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 (SO 2 ) 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 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) (vp) = 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 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 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 j 7 ( * 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 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 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 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 
H 2 SO 4 ,H 2 O (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 le 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_17wi, 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_ r un 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 diolved 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/" 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,, ] u hle 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_ r ane>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 Na 2 S0 4 ,10H 2 O 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 (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 Na 2 SO 4 ,7H 2 O, 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 7H 2 O), in the place of the more stable 
deca-hydrated salt (with mol. 10H 2 O), 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 TH 2 O 
are distinguished for their instability ; if they stand in contact not only 
with crystals of Na 2 SO 4 ,10H 2 O, 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 Na 2 SO 4 ,H. 2 O) 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 H 2 O) 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. 2 O,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 Na 2 CO 3 with 10H 2 O, 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 Na 2 SO 4 at 33 the heats 
of the formation of + lOHoO and 7H 2 O 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, KFe 2 (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 (As 2 S-), 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 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 + 10H 2 O (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 data 58 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 + 10H 2 O. At temperatures above 10 common salt separates out in 
anhydrous crystals, and at temperatures near 10, in combination with water of 
crystallisation, NaCl + 2H 2 O, 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 + 2H 2 O 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 + 2H 2 O) 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, 
C 2 H6O + 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/' 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 snlst 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 + 6H 2 O, 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-,H s O-fH 2 O, 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. 2 O, 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 H 2 SO 4 
from H 2 O and SO 3 , and of HC1 + 6H. 2 O (or of aqueous solutions analogous to it) from HC1 
and H 2 0. 

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 t 3 higher than 100 3 . At this tempera- 
ture it is decomposed into two substances, A + H 2 O. 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, H 2 S), 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 + 5H 2 O, 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), FeSO 4 + 7H 2 O, 
be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt, 
( 'uSO.j+7H 2 O, 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 (Iv 3 CO 3 ) 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, BaH 2 O 2r 
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, BaH 2 O. 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 BaH 2 O + 8H 2 O, which separate out, have a tension of 1(10 mm. ; on 
losing water they give BaH 2 O 2 --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 5H 2 O, 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 (C 2 H 3 Na0 2 ,3H.,O) melts 
at .">8 (but resolidifies only on contact with a crystal, otherwise it may 
remain liquid even at ; 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, H 2 SO 4 , with hH parts of water, H 2 0, 
or H 2 SO 4 ,H 2 O, or H 4 SO^) 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 CO 2 + 8HUO (see note 31), according to Wroblewski, only occurs in 
a solid form. Hydrates like H.,S + 1'2H 2 O (De Forcrand and Villard), HBr + H 2 O (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.iSO 4 , itself, which undoubtedly is a definite compound, fumes in a liquid form, 
evolving the anhydride, SO 5 that is, exhibits a very unstable equilibrium. The crystallo- 
hydrates of chlorine, C1 3 + 8H 2 O, of hydrogen sulphide, H 2 S + 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 ^4H 2 O. 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, Na 2 SO 4 + 10H 3 O, or Na 3 SO 4 + 7H 2 O, 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 C 3 H 4 nHoO, which easily splits up into (' .,Il[ 
and HoO, but it also gives the system of alcohol, CoH.^HoO or C,.H G O, which is compara- 
tively stable. Thus oxygen can dissolve in water, and it can combine with it, forming 
peroxide of hydrogen. Turpentine, C 10 H 1(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.H 2 O) 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, Na 2 CO 3 , separates out as crystals, Na 2 CO 3 ,lCH 2 O, 
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, Na 2 CO 3 , 10H. 2 O, 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.,C0 3 , 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, SO 3 , combines with water, forming its hydrate, sulphuric 
acid, H 2 SO,. 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, H 2 O, 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 H 2 SO, ; 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 (HoSO 4 , 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 HoSO 4 is formed from SO 3 ( = 80) and H. 2 O ( = ]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 ' takin s U P the 




t s m - 

determining the relation be- possible to directly show the decomposition 

tween the volumes of hydrogen * . * L 

and oxygen. o f 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 portion 5 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 Na 2 O 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, H 2 O, is formed) 69,000 calories, 
for iron (when the oxide, FeO, is formed) 69,000, and if the oxide FeoO 3 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 it 1 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. r l'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 

,,:. r pi 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 : H 2 O 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 + H 2 O = H + NaHO would evolve about 32,500 calories. We shall after- 
wards learn that hydrogen contains in its smallest isolable particles H 2 and not H, 
and therefore it follows that the reaction should be written thus 2Na + 2H 2 O = H 2 + 
'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 + H 2 O = H 2 + 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 + H 2 O 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 = Fe 5 O4+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 = Fe 3 O 4 + 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 + H 3 SO 4 
H 2 + ZnSO 4 , 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, H 2 S0 4 , 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, S0 5 (forming zinc sulphate, ZnSO 4 ), and 
the affinity of the resultant salt, ZnSO 4 , 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 H 2 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, H 2 ), 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, FeSO 4 ). 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 le 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 tin 1 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"!' _;'! \'inu r 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\,. >iilstaiices 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 

hylr..'_'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 + FeSO 4 ). 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 hydrogen 23 at 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. 


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 D 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 to 100 they expand like air; 
namely, a thousand volumes of a gas measured at 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 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 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 O 3 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 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 ) on the right, 
hand side of the equations. Carbon bisulphide, CSg, = 127'9; 10 = 198'5; 20 = 298*1; 
30 = 431-6; 40 = 617'5; 50 = 857'1. Chlorobenzene, C C H 5 C1, 70 = 97'9; 80 = l-il'8; 
90 = 208'4; 100 = 292'8; 110 = 402-6; 120 = 54t2'.s; 13u- = 71iK). Aniline, C 6 H 7 N, 
150 = 283-7; 160 = 887-0; 170 = 515-6; 180 = 677"2; LS.V =771-5. Methyl sulicylate, 
C 8 H 8 O 5 , 180 = 249-4; 190 = 330'9; 200 = 432'4; 210 C = 557'5; 220 C = 710"2; 224' : =77i".. 
Mercury, Hg, 300 = 246'8 ; 310 = 304'9 ; 320 = 373'7 ; 330 = 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> bO c = 0'4 ; 20 = 0*6; 
-10 = 1; = 1'5; +10 = 2'3; 20 = 3'2; 30=5'3. Ammonia, NH 5 , -40 = 0'7; 
-30 = 1-1; -20 = l-8; -10 = 2'8; = 4'2; + 10 = 6'0; 20 = 8'4. Carbonic anhydride, 
CO 2 , -115 = 0-033 ; -80 = 1; 70 = 2'1; -60 = 3'9 ; -50 = 6'8 ; -40 = 10; -20 = 23; 
= 35; +10 = 46; 20=58. Nitrous oxide, N 2 O, -125 = 0'033 ; -92 = 1; -80 = 1'9; 
-50 = 7-6; -20 = 23-1; = 36'1; +20 = 55'3. Ethylene, CoH 4 , -140 = 0'033; 
-130 = 0-1; -103 = 1; -40 = 13; -1 = 42. Air, -191 = 1; -15~8 = 14; -140 = 39. 
Nitrogen, N 2 , -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 low r 
pressures, gives an easy means for obtaining low temperatures. Thus liquefied carbonic 
anhydride, under the ordinary pressure, reduces the temperature to - 80 3 , 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 is 4'5 mm. A 
sufficiently low temperature may be obtained by forcing fine streams of air through 
common ether, or liquid carbon bisulphide, CS 2 , or methyl chloride, CH 3 C1, 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. 

C 2 H 4 N a O CO., H.,S AsH 3 NH* 3 HC1 CH 3 C1 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 t c 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. 


N 2 - 14f, 


88 


H 2 S + 108 


92 


CO - 140 


39 


C 2 N 2 + 124 62 


O. 2 - 119 


50 


NH 3 + 181 114 


CH 4 - 100 


50 


CH 3 C1 + 141 


78 


NO - 93 
CsHt + 10 


71 
51 


SO 2 + 155 
C 5 H 10 + 192 


79 
84 


CO* + 82 


77 C 4 H 10 O + 193 


40 


N 2 + 53 


75 


CHC1 3 + 268 


55 


C 2 Ho + 87 


68 


CS., + 278 


78 


HCf H 52 


86 


C 6 H 6 + 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 7S 3 . 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 rarefied 3 - 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, 




forliuef 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 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 
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 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, = S -K or S,= S (1-Kf)], or that the volumes 
increase in inverse proportion to the binomial 1 K, that is, V/ = V (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 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, SiCl 4 , 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, V t =-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 compounds 38 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. 2 H, 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 a hundred volumes of water dissolve 1'9 volumes of hydrogen, and 
alcohol 6'9 volumes measured at 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 directly 43 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, H 2 , 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 C 2 H,0, and of 
alcohol C 2 H 6 O. 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-fH 2 =H 2 O + 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, H O. 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 H 2 O 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, SiO 2 , 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 ( = 51xl0 14 ) tons. Consequently there are about 2 x 10 15 
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 
YO 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> 2 7.-Graham's apparatus for the decomposition of air 
by pumping it through india-rubber. 



-o lining. u'ive up the oxygen absorbed that is, lv 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 sulphuric 1 anhydride, or 
sulphur trioxide. S( ) ;j : and this su list a net 1 (which is a solid and volatile, 
and therefore, easily separated from the nitrogen and sulphurous 
anhydride), ly 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. 
Ia< ) 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_ r en from the air. so that the j. r a! 

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.^MnO 4 
+ 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 CrO 5 in 
evolving oxygen give chromium oxide, Cr.,O 3 , 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 art 4 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^SO I ^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. r l'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 (Mn0 2 + H 2 S0 4 = MnS0 4 -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, KC10 3 . 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 KC1O 3 =KC1 + O 3 . 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.)O 7 ) 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 KC1O 5 , 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, CaCl 2 O 2 , 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 CaCl 2 O 2 = 
CaCl 2 + O 2 . In this manner a small quantity of cobalt oxide 17 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 I 1 ' . l>v employing a 
preure above 100 atmospheres. According t () I*ewar, the density <>f 
\y^cn in a critical stale is ()().") (\\-atcr=l ), Inn it. like all (ilici 1 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 tiudei 1 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-- lieinu 1 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 ^ r ases 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 necessary 22 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 heat 23 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 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 galvanic 1 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! tin 1 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 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 th a ti 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-Lussac 1 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 aiiy j;< 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 500 . 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 20 3 . 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 (H 2 O) 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( l uid or solid), or escape by the tube/ into an n 1M Kua- 

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 
point 39 (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 10000 . 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. (',,,! F s , 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 more 1 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 tin 1 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 flam J G ^ 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 pi e ted. 

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 a nd 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 

V el\ 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 

,. M i,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. r J"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, C 2 oH 1(j O 3 , and 
phenolphthale'm, C..> H 14 O 4 , 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 w r e 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, Na 2 C(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 + CO 2 = 
Na 2 CO 3 + 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 .So 4 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. f el 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, Na a SO.,, may be obtained by the replacement of hydrogen 
in sulphuric acid, H. 2 SO 4 , 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, Na 2 SO 4 , sodium is again the metal, but the 
complex group, S0 4 , is the haloid. In sulphate of copper, CuSO 4 , 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, AgNO 3 , a white 
insoluble precipitate of silver chloride, AgCl, is formed, and a new salt, 
sodium nitrate, NaNO 3 , 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* arc 1 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._,O 7 , equal to L'XallSO,, less H 2 O. 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, SO V 
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;_ f eii 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, lut our notion of the immense varietv of the 
chemical >tates in which oxygen can occtii 1 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 
ow, ' 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, lv 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- lcen 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. 2 O 3 ) into arsenic anhydride (As. 2 5 ) <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 + H 2 O + 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, NH 5 , is oxidised by ozone into 
ammonium nitrite (and nitrate), 2NH 5 + O 3 = NH 4 NO^ + H 2 O, 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, PbSO 4 , by ozone. A neutral solution of 
manganese sulphate gives a precipitate of manganese peroxide, and an acid solution may 
be oxidised into permanganic acid, HMnO 4 . With respect to the oxidising action of ozone 
on organic substances, it may be mentioned that with ether, C 4 H 10 O, ozone gives ethyl 
peroxide, which is capable of decomposing with explosion (according to Berthelot), and is 
decomposed by water into alcohol, 2C.>H e O, 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 rcl 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> 7 E 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 H 2 O 2 , and in O 3 , 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 H 2 or O 2 , 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, Cr 2 O 7 , 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 O 2 and gives H 3 O 2 . 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 H 2 O 2 = H 2 O + 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,8H 2 O, 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 + H 2 SO 4 =H 2 O 2 + BaSO 4 . 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 tin 1 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 i 1 . -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 Mxvu f en ( 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..\il.-. 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, MnO 2 , and hydrochloric acid, HC1, and hydrogen peroxide from 
barium peroxide, BaO 2 , 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, H 2 O, 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, A n B OT , A^B r . . . ., 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, CH 4 , and olefiant 
gas, C 2 H 4 , 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, Cu 2 O 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.rti 1 '?''* 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 till 1 ^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 A n B m , 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 A n B m , 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 A M B m . also yield another compound A,.B <; , then by- 
expressing the composition of the first compound by A lir B mr (and this 
is the same composition as A H B m ), and of the second compound by 
A ru B 5n , 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, t B TO . 

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 H 2 S. Sulphur 
dioxide, SO 2 , 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'>L f en 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 4r 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, N 2 O 3 , and on the other hand, 
ammonia, NH 3 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.< + N 2 3 = 3H 2 O -f- N 4 . 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, 
NH 4 C1, 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, + NH 4 C1 == KC1 + 2H 2 O -f N 2 . 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, NH 4 NO 2 , 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, NO 2 ) 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, N 2 O, 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, NH 5 , 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 point 5 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, K 2 CO 3 + 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 moist 8 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;>N 2 , 
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 similai 1 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-. lut. 
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 u f ns 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, NH 3 , 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 NH 3 = 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, NH 4 C1, 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, H 3 , and HC1. 14 Berthelot (1876) showed that under the 
action of a silent discharge many non-nitrogenous organic sub- 
stances (benzene, C G H 6 , cellulose in the form of paper, resin, glucose, 
C 6 H 10 O 5 , 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 S 6 into S 2 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 N 2 
tin 'ii give less complex molecules, N containing one atom. Probably under a silent 
discharge the molecules of oxygen, O 2 , are partly decomposed and the individual atoms 
O combine with O.>, forming ozone, O 3 . 

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 + N 3 
-=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 and 700 mm. 
pressure at Jong. 4S 3 , 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 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, C 6 H 6 O 3 , 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 anhydride 22 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 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 anhydride 29 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, CaCl 2 + 2H 2 O = CaOH 2 O + 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 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 
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 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 and 760 mm. pressure 
is not 1-2930 grams, but this weight multiplied by 1-000156 and by 

7fin ^ a P ressure (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 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 host 1 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 In 1 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 w e 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, NH 4 C1, on the 
passage of a series of electric 
sparks. 1 In ammonium chlo- 
ride, HC1 is combined with 
NH :i , consequently N" with 
H 3 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, NH 4 C1, 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, HgNO 3 . 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 + H 3 ) 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. (NH 5 + nH 2 O) 8'4 (heat of solution); 
(NH 3 ,nH 2 + HCl,nH 2 0)12-8; (N + H 4 + Cl) 90'6 ; (NH 3 + 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, NH 4 C1, and ammonia sulphate, (NH 4 ) 2 SO 4 . They are saline 
substances which are formed because ammonia, NH 3 , combines with all 
acids, HX, forming ammonia salts, NH 4 X. Sal-ammoniac, NH 4 C1, 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, 
NH 4 C1, as from any other ammoniacal salt, by heating it with lime. 
Calcium hydroxide, CaH 2 O 2 , as an alkali takes up the acid and sets 
free the ammonia, forming calcium chloride, according to the equation 
2NH 4 Cl + CaH 2 O 2 = 2H 2 O + CaCl 2 + 2NH 3 . I n 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- 2NH 4 C1 = Pb 2 OCl 2 + H 2 O + 2NH 3 . 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 
tlx 1 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,8NH 3 is formed, and above 
20 the compound 2AgCl,8NH 5 . 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 tin 1 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 inconsiderable 1 . 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 iis/>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 .I 1 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- 0'21p 2 ; 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 solutions approaching NH 3 ,H.,O (48'6 p.c. NH 3 ) 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. NH S is rarely contained 
in solution. Ammoniacal solutions containing a considerable amount of ammonia give 
ice-like crystals at temperatures far below (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, H SO 4 , 
in absorbing ammonia, forms (on evaporating the solution) two salts, 
according to the relative quantities of ammonia and acid. One salt is 
formed from NH 3 + H 2 SO 4 , and consequently has the composition 
NH S S0 4 , and the other is formed from 2NH 3 + H 2 SO 4 , and its composi- 
tion is therefore N 2 H 8 SO 4 . 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, NH 4 C1, be contrasted with table salt, NaCl ; and the 
ammonium hydrogen sulphate, NH 4 HSO 4 , with the sodium hydrogen 
sulphate, NaHSO 4 ; or ammonium nitrate, NH 4 NO 3 , with sodium nitrate, 
NaN0 3 . 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 NH 4 , 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 H 2 + NaCl 

Sodium hydroxide Hydrochloric acid Water Table salt. 

NH 4 HO + HC1 = H 2 + NH 4 C1 

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, NH 4 C1 + HgNa = NaCl + HgNH 4 . 
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 NH 3 K, 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, NH 4 , 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, NH 4 = NH 3 + 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 NH 4 OH 
= NH 3 + H 2 O. The liability to a greater or less decomposition is 
proper, in the same degree, to substances containing NH 3 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,3NH 3 and 2AgCl,3KH 3 , 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 NH 3 + H 2 O, in which these substances are contained in 
caustic ammonia, and perhaps it may be possible at exceedingly low temperatures to 
obtain ammonium hydroxide, NH 4 HO, 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 NH 4 HO. 

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, NR 4 HO, 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 NH 4 HO is replaced by complex groups, E = CH 3 , C 2 H 5 , &c., for 
instance N(CH 3 ) 4 HO. 

18 The fact that ammoniacal salts are decomposed when ignited, and not simply 
sublimed, may be proved by a direct experiment with sal-ammoniac, NH 4 C1, which in a 
state of vapour is decomposed into ammonia, NH 3 , 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, 
CuSO 4 . As we saw in Chapter I., it gives with water blue crystals, 
CuSO 4 ,5H 2 O ; but it also absorbs ammonia in the same molecular 
proportion, forming a blue substance, CuSO 4 ,5NH 3 , 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, NH 2 K (according to the equation 
NH 3 + K = NH 2 K + 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 NH 2 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 practice 21 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 - H 2 0=MX. If aqueous ammonia, NH 4 OH, 
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\x f en 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, H 2 0, may be 
divided in two ways, because it contains 3 atoms : into H and (HO) 
on the one hand, and into H 2 and O on the other. Consequently, being 
given RH, its substitution products will be R(HO) according to the 
first form, and R 2 O according to the second ; being given RH 2 , its 
corresponding substitution products will be RH(OH), R(OH) 2 , RO, 
(RH). 2 O, 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 NH 4 C1 correspond with hydroxides of the alkalis 
Xa(OH) or NH 4 (OH). In hydrocarbons for instance, C 2 H 6 the 
hydrogen is replaceable by chlorine and by hydroxyl. Thus common 
alcohol is C 2 H 6 , in which one atom of H is replaced by (OH) ; that 
is, C 2 H 5 (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 H 2 O 2 . 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 H 2 is also a common 
chemical phenomenon. Thus common alcohol, C. 2 H 6 O, or C 2 H 5 (OH), 
when oxidising in the air, gives, as every one knows, acetic acid, 
C 2 H 4 O 2 , or C 2 H 3 0(OH), in which H 2 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 a 1 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- sult 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, 
NH 3 (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, NH 4 (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 NO 2 (OH), or NHO 3 , 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 NHO 2 , equal to N(OH) 3 - H 2 O, and nitrous anhydride 
N 2 O 3 = 2N(OH) 3 - 3H 2 O. 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, KNO 2 , 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 : 
KNO 2 + KHO + 2SO 2 + 2H 2 O = NH 2 (OH),H 2 SO4 + K 2 SO4. The salts may be separated 
from each other by crystallisation. 

With respect to substances intermediate between NH 3 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 N 2 O 5 , which will afterwards be described, 
but which must be regarded as the highest saline oxide of nitrogen, just as Na 2 O (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 N 2 O7. 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 PH 3 , corresponding 
with ammonia, NH 3 , and forms phosphorous acid, PH 3 O 3 , which is analogous to nitrous 
arid, just as phosphoric acid is to nitric acid ; but phosphoric (or, better, orthophosphoric) 
acid, PH 3 O.j, is able to lose water and give pyro- and meta-phosphoric acids. The latter 
is equal to the ortho-acid minus water = PHO 3 , and therefore nitric acid, NHO 3 , is really 
meta-nitric acid. So also nitrous acid, HNOo, is meta-nitrous (anhydrous) acid, and then 
the ortho-acid is NH 3 O 3 = 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, 
NH0 3 + 8H=NH 3 + 3H. 2 O. 

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 : 

N 2 O, nitrous oxide ; hydrate NHO. 
N 2 O 2 , nitric oxide, NO. 
N 2 O 3 , nitrous anhydride ; hydrate NHO 2 . 
N 2 O 4 , peroxide of nitrogen, NO 2 . 
N 2 O <5 , nitric anhydride ; hydrate NH0 3 . 

Of these compounds, 29 nitrous and nitric oxides, peroxide of nitrogen, 
and nitric acid, NHO 3 , 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 NO 2 form N 2 O 3 , 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 N 2 O 4 gives N 2 O 3 and N 2 O 3 , 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, NHO 3 , 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 KNO 3 is common nitre, and the sodium salt NaNO 3 
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) : 

N 2 N 2 2 N,0 3 N 2 4 N 2 5 

-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.,O 2 , by combining with O.,, gives N.,,O 4 , 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, NHO 3 , were decomposed into N + O 3 + 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, NH 5 , 
from the gases N + H 3 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 sulphuric 1 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 poible 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, HNO 3 , 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(NO 3 )o + 2HCl = PbCl 2 + 2NHO 5 and Pb(NOs). 2 + H.,,SO 4 = PbSO4 + 2NHO 3 . . 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, HNO 3 , 
which corresponds with the salts, NMO 3 , 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, HNO 3 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 HNO 3 ; 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 2HNO 3 ,3H. 2 O, 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, NHO 5 ,2H 2 O ( = 63'6 p.c.). 
Such a hydrate may be expressed by N(HO) 5 , that is as NH 4 (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, NHO S ,5H 2 O (41'2 p.c. HNO 3 ), 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, HNO 3 ,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 
HNO 5 ,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 
HNO 3 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 HNO 3 , corresponding with the ordinary salts, may be termed 
the monohydrated acid, because the anhydride N.,O 5 with water forms this normal nitric- 
acid. In thi> M-HSI; the hydrate HNO 3 ,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( i N() 3 )., + lI 2 O. .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, 
C 6 H 6 , for instance, acts according to the equation C 6 H 6 + HNO 3 
=H 2 O + C 6 H 5 NO 2 . Nitrobenzene is produced. The substance taken, 
C G H G , 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, NH 4 NO 3 , 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, 
NH 4 NO 3 = 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 NH 4 NO 3 ,2NH 3 is 
formed : it is a liquid of sp. gr. 1'50, which loses all its ammonia under the influence of heat. 
At -f 28 NH 4 NO 3 ,NH 5 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, C 6 H 10 O 5 , 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 NO 2 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 C 6 H 7 N 3 O U = C 6 H 10 O 5 -f 8NHO 3 
3HoO. The proportion of the group NO 2 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 NO 2 ; 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 NO 2 , 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(NO 2 )O. 

/Hydrate of lime . . CaH 2 O 2 . 

1 Calcium nitrate . . . Ca(NO 2 ) 2 O 2 . 

(Glycerin C 3 H 5 H 5 O 3 . 

I Nitroglycerin . . . C 3 H 5 (NO 2 ) 3 O 3 . 

(Phenol C 6 H 5 OH. 

I Picric acid .... C e H 2 (NO 2 ) 3 OH, &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 YIK< 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 2NH0 3 =H 2 O + 2NO 2 + O. 
The decomposition is complete at a white heat that is, nitrogen is 
formed, 2NH0 3 =H 2 O + N 2 -f 5 . 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 NO 2 of nitro-compounds in many cases (like all the oxidised compounds of 
nitrogen) passes into the ammonia group or into the ammonia radicle NH 2 . It is evident 
that this requires the action of reducing substances evolving hydrogen : RNO 2 + 6H 
= KNH 5 + 2H. 2 O. Thus Zinin converted nitrobenzene, C 6 H 5 'NO.>, into aniline, C 6 H 5 'NHo, 
by the action of hydrogen sulphide. 

Admitting the existence of the group NO 2 , 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, (NO 2 ) 2 O, 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 NO 3 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 NO 2 . 

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 iixyu r '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 ,i r jd 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 illH 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, SnH 2 O 3 , which is obtained in the form of a white powder, 
Sn + 4NH0 3 =H,SnO 3 + 4NO 2 + H 2 O. Silver is able to take up still 
more oxygen, and to convert a large portion of nitric acid into nitrous 
anhydride, 4Ag + 6HNO 3 =4AgNO 3 + N 2 O ; , + 3H 2 O. 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 + 10NHO 3 =4Zn(NO 3 ). 2 + N 2 O + 5H 2 O. 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(NO 5 ).,, 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.>O 5 , 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, M 2 S, into sulphates, 
M. 2 SO 4 , &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, C 10 H 16 , 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, C 7 H 8 , into benzoic acid, C 7 H 6 O 2 . 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, C 8 H 6 O 4 , is obtained from naphthalene, C 10 H 8 . 
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, N 2 O 5 ; 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 (2AgNO 3 + CL 2 = 2 AgCl + N 2 O 5 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, 
P 2 O 5 , 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, H 2 O,2N 2 O r> 
=N 2 O 5 ,2H]Sr0 3 , whilst the greater part of the nitric acid gives the 
anhydride according to the equation 2NHO 3 + P 2 O 3 =2PHO 3 + N 2 O 5 . 
On heating, and sometimes even spontaneously with explosion, nitric 
anhydride decomposes into nitric peroxide and oxygen, N 2 O 5 
=N 2 O 4 + O. 

Nitrogen peroxide, N 2 O 4 , and nitrogen dioxide, N0 2 , 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, O 3 loses heat in 
passing into O 2 , whilst N 2 O 4 absorbs heat in' forming NO 2 . 

Nitric acid in acting on tin and on many organic substances (for 
example, starch) gives brown vapours, consisting of a mixture of N 2 O 3 
and NO 2 . A purer product is obtained by the decomposition of lead 
nitrate by heat, Pb(NO 3 ) 2 ==2NO 2 + 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 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 N0 2 (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, N 2 O 4 , 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. 2 4 gives 
colourless and transparent vapours, whilst those of NO 2 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 N 2 O 5 (and oxygen) is formed, and this substance remains liquid at 
80, or it may be that the passage from 2NO 2 into N 2 O4 is not so easily accomplished 
as the passage from N 2 O 4 into 2NO 2 . 

Liquid nitrogen peroxide (that is, a mixture of NO 2 and N 2 O 4 ) 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 N 2 O 4 with 21 parts by 
weight of ISTO 2 . 45 It is evident that a decomposition here takes place 
whose peculiarity consists in the fact that the product of decomposition, 
NO 2 , is polymerised (i.e. becomes denser, combines with itself) at a 
lower temperature ; that is, the reaction, 

N 2 O 4 =NO 



is a reversible reaction, and consequently the whole phenomenon repre- 
sents a dissociation in a homogeneous gaseous medium, where the 
original substance, N 2 O 4 , and the resultant, NO 2 , 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 N 2 O 4 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 N 2 O 4 has ceased to exist and NO 2 alone remains. It is 

45 Because if x equal the amount by weight of N. 2 O 4 , its volume will = x/46, and the 
amount of NO 3 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 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, NO 2 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 
N 2 O 4 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, N 2 O 4 , and nitrogen 
dioxide, NO 2 . In the brown liquid boiling at 22 probably a portion 
of the N 2 O, has already passed into NO 2 , 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. N 2 O 4 then acts on water like a mixture of the anhy- 
drides of nitrous and nitric acids. The first, X 2 O 3 , 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 NO 2 , 
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) . N0j . N0 2 ) . NO ) . 

Hj> N0i> N0 2) > N0 2 , r > 

or H 2 ; N 2 O :J ; N 2 O 5 ; N 2 O 4 . 



In fact, nitrogen peroxide at low temperatures gives with water (ice) 
both nitric, HNO 3 , and nitrous, HNO 2 , acids. The latter, as we shall 
afterwards see, splits up into water and the anhydride, N 2 O 3 . If, how- 
ever, warm water act on nitrogen peroxide, only nitric acid and oxide 
of nitrogen are formed : 3NO 2 + H 2 O=NO + 2NHO 3 . 

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, 2NO 2 + 
Hg=HgNO 3 -|-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, N 2 O 4 , at temperatures of from 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 NO 2 must be regarded as a gas dissolved in the volatile 
liquid N 2 O 4 . 

Liquid nitrogen peroxide is said by Geuther to boil at 22-26, and to have a sp. gr. 
at = 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 N 2 O 4 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 C 5 H 10 and C 10 H.,o. 

It may not be superfluous to here mention that the measurement of the specific heat 
of a mixture of the vapours of N 2 O 4 and NO 2 enabled Berthelot to determine that the 
transformation of 2NO 2 into N 2 O 4 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, N 2 O 3 , corresponds 49 with nitrous acid, NHO 2 , 
and with the latter corresponds a series of salts, the nitrites for ex- 
ample, the sodium salt NaNO 2 , the potassium salt KNO. 2 , the ammonium 
salt (NH ,)X( ),, :i " the silver salt AgNO 2 , 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 NHO 2 from its salts, it always gives water and 
the anhydride, whilst the latter, as an intermediate oxide, easily splits 
up into NO + NO 2 . But the salts of nitrous acid are distinguished for 
their great stability. Potassium nitrate, KNO 3 , 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, KNO 3 + Pb=KNO 2 + 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 : 2KN0 2 + H 2 SO 4 =K 2 SO 4 + N 2 O 3 + H 2 O. The 
same gas (N 2 O 3 ) is obtained by passing nitric oxide at 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 NH 4 NO 2 = 
iiHoO + N 2 . 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, AgNO 2 , is obtained as a very slightly soluble substance, as a preci- 
pitate, 011 mixing solutions of silver nitrate, AgNO 5 , and potassium nitrite, KNO 2 . 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, KNO 2 , when strongly heated, especially with metallic 
oxides, evolves N and O, and gives potassium oxide, K 2 O, because nitre is liable to such 
a decomposition, but it has, as yet, been but little investigated. 

53 It is evident that the reaction NoO 5 = NO 2 + NO is reversible, and that it resembles 
the conversion of N 2 O 4 into NO 2 , 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 + NO 2 . 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 HNO 2 -l-NO) acts directly on ammonia, forming 
nitrogen and water, HNO 2 + NH 3 =N 2 + 2H 2 0. 56 

As nitrous anhydride easily splits up into NO 2 + NO, so with warm 
water it, like NO 2 , gives nitric acid and nitric oxide, according to the 
equation 3N 2 O 3 + H 2 O=4NO + 2NHO 3 . 

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, N 2 O 5 , 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, N 2 O 3 boils at +18. According to Geuther, at +3'5, and its 
sp. gr. at = 1-449. 

55 In its oxidising action nitrous anhydride gives nitric oxide, N<jO 5 = 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 O 3 . In a physical aspect the affinity between N. 2 O 3 and O 3 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, C 6 H 5 'NO.,, (Note 37), is converted by nitrous anhy- 
dride into phenol, C 6 H 5 '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 N 2 3 and 
NO 2 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=3H 2 O + 3Cu(NO 3 ) 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, KMnO 4 , on nitrous acid in 
the presence of sulphuric acid is determined by the fact that the higher oxide of man- 
ganese, MnoO 7 , contained in the permanganate is converted into the lower oxide MnO, 
which as a base forms manganese sulphate, MnSO 4 , and the oxygen serves for the oxida- 
tion of the N 2 O 3 into N.,O 3 , 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=N 2 O 3 , 2NO + O 2 =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 + H 2 O + O 3 = 2HNO 3 , 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 HNO 3 , 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 2FeSO 4 . 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, KMnO 4 , 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 NO 2 (OH), in nitric an- 
hydride, N 2 O:s or (NO 2 )._,O, and in ammonium chloride, NH 4 C1. 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 
H 2 , be indicated by X 2 , then the three above-named compounds of 
nitrogen should be considered as compounds of the type or form NX 5 . 
For example, in nitric acid X 5 =O 2 + (OH), where O 2 =X 4 , and 
OH=X ; whilst nitric oxide is a compound of the form NX 2 . 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. NX 2 passes consecutively into NX 3 - namely, into N 2 O 3 and 
NHO 2 , NX 4 (for instance NO 2 ) and NX 5 . 

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, SO 2 , 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, SO 3 , and if it does 
form sulphuric acid with water and the oxygen of the atmosphere, 
SO 2 +H 2 O + O=H 2 SO 4 , 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 



3SO 2 + 2NHO 3 -f 2H 2 O=2H 2 S0 4 + 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 
H 2 SO 4 , 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. 2 SO 4 ) 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 H 2 O) 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 2H 2 SO 4 with water), besides l equivalents (24 parts) for con- 
verting the iron into oxide, Fe 2 O 3 ; 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 H 2 SO 4 , 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 town 1 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. 2 O, 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 =NH 3 , 
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\>H 4 , 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), CHN 2 .COOH + 
2ELO = C. 2 O..,(OH)o + N.,H 4 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, N 2 H 4 HX, and N 2 H 4 H.^X,, 
as, for example, with HC1, HUSO 4 , &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.,H 4 ,H. 2 SO 4 , 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, NoH 4 ,2HCl, crystallises in octahedra, is very 
soluble in water, but not in alcohol ; it fuses at 198, evolving hydrogen chloride, and 
forming the salt N 2 H 4 HC1 ; 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 N 2 H,,2HX give hydrate of amidogen, N 2 H 4 ,H 2 O, 
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,SO 4 + N 2 O. 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, NH 4 N0 3 = 2H 2 O + N 2 O (a mixture of NH 4 C1 and 
KNO 3 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 pump 71 shown in fig. 51. 



metals platinum and silver from their solutions. With mercuric oxide it explodes. It 
reacts directly with the aldehydes HO, forming N 2 R 2 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 
(N 2 H 4 ) 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(NH 2 ), forms hydrazine, (NH 2 )(NH 2 ) that is, the free radicle of ammonia, 
NH 2 , 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 100 volumes of water dissolve 130 volumes of N 2 O, 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 = 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, N 2 O + H 2 =H 2 O + N 2 . 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 (CO 2 + 32, N 2 O + 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, N 2 O 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 ly 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, t v<-.. 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 NO 2 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. N0 2 , 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 NO 2 , 
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 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, NH 4 C1, 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, NH 3 , 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 \vatei 1 , 
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 eju;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 ji 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, NH 3 , reacts directly with hydrochloric acid, HC1, forming 
sal-ammoniac, NH 4 C1, and in this case the NH 3 , or 17 parts by weight 
of ammonia, occupy the same volume as the 36*5 parts by weight of 
hydrochloric acid. 7 Ethylene, C 2 H 4 , combines with chlorine, C1 2 , in 
only one proportion, forming ethylene dichloride, C 2 H 4 C1 2 , and this 
combination proceeds directly and with great facility ; the reacting 
quantities occupying equal volumes. Chlorine reacts with hydrogen in 
only