UNIVERSITY OF CALIFORNIA
SAN FRANCISCO LIBRARY

INORGANIC CHEMISTRY

WORKS by G. S. NEWTH, F.I.C., F.C.S.

DEMONSTRATOR IN THE ROYAL COLLEGE
OF SCIENCE, LONDON.

CHEMICAL LECTURE EXPERIMENTS.
With 230 Illustrations. Crown 8vo, $2.00.

CHEMICAL ANALYSIS, QUALITATIVE
AND QUANTITATIVE.

With 100 Illustrations. Crown 8vo, $1.75.

SMALLER CHEMICAL ANALYSIS.
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A TEXT-BOOK OF INORGANIC
CHEMISTRY.

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With Appendix of Questions, $2.00. Appendix

separately, 25 cents.

ELEMENTARY INORGANIC CHEMISTRY.

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LONGMANS, GREEN, AND CO.

NEW YORK, LONDON, BOMBAY, AND CALCUTTA.

A TEXT-BOOK

OF

INORGANIC CHEMISTRY

G. S. NEVVTH, F.I.C., RC.S.
^^

SENIOR DEMONSTRATOR-'IN THE ROYAL COLLEGE OF SCIENCE (IMPERIAL

COLLEGE OK SCIENCE AND TECHNOLOGY 1 , LONDON. ASSISTANT-

EXAMINER IN CHEMISTRY, BOARD OF EDUCATION,

SOUTH KENSINGTON

California College of Pharmacy

NEW EDITION

LONGMANS, GREEN AND CO.

91 AND 93 FIFTH AVENUE, NEW YORK
LONDON, BOMBAY, AND CALCUTTA

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PREFACE

IN drawing up a systematic course of elementary chemical
instruction based upon the periodic classification of the ele-
ments, whether it be as a course of lectures, or as a text-book,
a number of serious difficulties are at once encountered.
These possibly are sufficient to account for the fact, that
although twenty-five years have elapsed since Mendelejeff
published this natural system of classification, the method has
not been generally adopted as the basis of English elementary
text-books.

I have endeavoured to obviate many of these difficulties,
while still making the periodic system the foundation upon
which this little book is based, by dividing the book into
three parts. Part I. contains a brief sketch of the funda-
mental principles and theories upon which the science of
modern chemistry is built. Into this portion of the book I
have introduced, necessarily in briefest outlines, some of the
more recent developments of the science in a physico-chemical
direction, of which it is desirable that the student should gain
some knowledge, even early in his career.

Part II. consists of the study of the four typical elements,
hydrogen, oxygen, nitrogen, and carbon, and of their more
important compounds. By dissociating these four elements
from their position in the periodic system, and treating them
separately, the student is early brought into contact with many
of the simpler and more familiar portions of the science. Such

vi Preface

subjects as water, the atmosphere, and combustion, to which it
is desirable that he should be introduced at an early stage in
his studies, are thus brought much more forward than would
otherwise be the case.

In Part III. the elements are treated systematically, accord-
ing to the periodic classification. In this manner, while
avoiding a sharp separation of the elements into the two arbi-
trary classes of metals and non-metals, it has been possible to
so far conform to the prevailing methods of instruction, that
all those elements which are usually regarded as non-metals
(with the two exceptions of boron and silicon) are treated in
the earlier portion of the book.

The science of chemistry has of recent years developed and
become extended to such a degree, that the difficulty of giving
a fairly balanced treatment of the subject, within the limits of
a small text-book, is an ever-increasing one, and it necessarily
resolves itself into a question of the judicious selection of
matter. In making such a selection, I have endeavoured, as
far as possible, to keep in view the requirements of students
at the present time, without, however, following any examina-
tion syllabus.

Acting upon this principle, I have omitted all detailed
description of the rare elements and their compounds, con-
fining myself merely to a short mention of them in a few
general remarks at the commencement of the various chapters.

Although from a purely scientific standpoint many of these
rare substances are of the greatest interest and importance,
it must be admitted that they stand quite outside the range
of all the customary courses of chemical instruction ; and so
far as the wants of the ordinary student are concerned, the
space which would be occupied by an account of these
elements is more advantageously devoted to such matters

Preface vii

as are discussed in the Introductory Outlines. Moreover, it
is a matter of common observation that text-books, even
upon the shelves of reference libraries, and which bear un-
mistakable evidence of much use, are frequently uncut in those
portions which treat of these elements.

Details of metallurgical processes, also, are out of place
in a text-book of chemistry, and must be sought in metal-
lurgical text-books. Only such condensed outlines, therefore,
have been given as are sufficient to explain the chemicaj
changes that are involved in these operations.

The great importance to the student, of himself perform-
ing experiments illustrating the preparation and properties of
many of the substances treated of in his text-book, cannot
well be over-estimated. If he be in attendance upon a course
of chemical lectures, opportunity should be given to him for
repeating the simpler experiments he may see performed
upon the lecture table : if he be not attending lectures, the
necessity for this practical work on his part is greater still.
Instead of burdening this text-book with specific directions
for carrying out such elementary experiments, frequent refer-
ences have been made to my "Chemical Lecture Experi-
ments," where minute directions are given for carrying out
a large number of experiments, many of which may be easily
performed, and with the very simplest of apparatus.

Several of the woodcuts have been borrowed from existing
modern works, such as Thorpe's " Dictionary of Applied
Chemistry," Mendelejeffs "Principles of Chemistry," Ost-
wald's " Solutions," and others. Care has been taken, how-
ever, to exclude all antiquated cuts, and a large number of
the illustrations are from original drawings and photographs.

G. S. N.
SOUTH KENSINGTON.

HINTS TO STUDENTS

FOR the help of students who may use this book at the
commencement of their chemical studies, and especially for
those who may not be working under the immediate guidance
of a teacher, the following hints are given :

Begin by carefully reading the first four chapters (pages
1-24). Then pass on to Part II. (page 171), and begin
the study of the four typical elements, hydrogen, oxygen,
nitrogen, and carbon, and their compounds, in the order in
which they are treated. Accompany your reading by per-
forming as many of the experiments referred to as possible,
in order that you may become practically familiar with the
substances you are studying.

During the time occupied in the study of these four
elements and their compounds, again read Chapters I. to IV.,
and slowly and carefully continue reading Part I., so that
by the time Part III. is reached, you may have fairly mastered
at least the first thirteen chapters of the Introductory Out-
lines.

The order in which the elements are treated in Part III.
is based upon the periodic classification, therefore read the
short introductory remarks at the commencement of the
various chapters, in the light of the table on page 118.

Throughout the book temperatures are given in degrees
of the Centigrade thermometer. i Centigrade equals 1.8

x Hints to Students

Fahrenheit, and as the zero of the latter scale is 32 below
that of the Centigrade, temperatures given in degrees of one
scale are readily translated into degrees of the other, by
the simple formula

The abbreviation mm. stands for millimetre; the TTr Vo- P art
of a metre (i 1116^6 = 39.370113 inches; or roughly, 25
mm. = i inch). The abbrevation c.c. signifies cubic centi-
metre; the Y^IT part of a cubic decimetre, or litre (i
litre = 1.76077 pints).

i gramme (the weight of i c.c. of distilled water, taken at
its point of maximum density) = 15.43235 English grains.

TABLE OF CONTENTS

PART I
INTRODUCTORY OUTLINES

CHAP. PAGE

I. Chemical Change The Constitution of Matter Molecules

Atoms I

II. Elements and Compounds Mixtures Chemical Affinity

Modes of Chemical Action .6

III. Chemical Nomenclature .15

IV. Chemical Symbols . . . . . . . .21

V. The Atomic Theory Laws of Chemical Action ... 25

VI. Atomic Weights Modes of Determining Atomic Weights . 34

VII. Quantitative Chemical Notation ...... 53

VIII. Valency of the Elements 59

IX. General Properties of Gases Relation to Heat and Pressure

Liquefaction Diffusion The Kinetic Theory . . 69

X. Dissociation Reversible or Balanced Actions ... 88

XI. Electrolysis Electrolytic Dissociation The Ionic Theory . 96

XII. Classification of the Elements The Periodic System . . 112

XIII. General Properties of Liquids Evaporation, Boiling, Vapour

Pressure of Solutions The Passage of Liquids into Solids

Freezing Point of Solutions Raoult's Method . .126

XIV. Solution Gases in Liquids Liquids in Liquids Solids in

Liquids Osmotic Pressure Crystalline Forms . . 142
XV. Thermo-chemistry 163

PART II

THE STUDY OF FOUR TYPICAL ELEMENTS
Hydrogen Oxygen Nitrogen Carbon,

AND THEIR MORE IMPORTANT COMPOUNDS.

I. Hydrogen Hydrogenium 171

II. Oxygen Allotropy Ozone 181

III. Compounds of Hydrogen with Oxygen 203

xii Contents

CHAP. PAGE

IV. Nitrogen 229

V. Oxides and Oxy-acids of Nitrogen 234

VI. The Atmosphere and the Argon Group of Elements . . 252
VII. Compounds of Nitrogen and Hydrogen Hydroxylamine

Ammon-sulphonates ; Halogen Compounds of Nitrogen . 272

VIII. Carbon 285

IX. Carbon Monoxide Carbon Dioxide Carbonates . . . 295
X. Compounds of Carbon with Hydrogen Methane Ethylene

Acetylene 312

XI. Combustion Heat of Combustion Ignition Point Flame
Structure of Flame Cause of Luminosity of Flames

The Bunsen Flame 321

PART III

THE SYSTEMATIC STUDY OF THE ELEMENTS, BASED
UPON THE PERIODIC CLASSIFICATION

I. ELEMENTS OF GROUP VII. (FAMILY B.)

Fluorine: Hydrofluoric Acid. Chlorine: Hydrochloric
Acid Oxides and Oxyacids of Chlorine. Bromine:
Hydrobromic Acid Oxyacids of Bromine. Iodine:
Hydriodic Acid Oxyacids of Iodine Periodates . . 345
II. ELEMENTS OF GROUP VI. (FAMILY B.)

Sulphur: Compounds of Sulphur with Hydrogen Com-
pounds with Chlorine Oxides and Oxyacids of Sulphur
Oxychlorides Carbon Disulphide. Selenium Tellu-

III. ELEMENTS OF GROUP V. (FAMILY B.)

Phosphorus: Compounds with Hydrogen Compounds with
the Halogens Oxides and Oxyacids. Arsenic : Arsenu-
retted Hydrogen Halogen Compounds Oxides and
Oxyacids Sulphides. Antimony: Antimony Hydride
Halogen Compounds Oxides and Acids Sulphides.
Bismuth : Bismuth and Halogens Oxides Sulphides . 450

IV. ELEMENTS OF GROUP I. (FAMILY A.)

Potassium Soditim Lithium Rubidium Ammonium
Salts .......... 505

V. ELEMENTS OF GROUP I. (FAMILY B.)

Copper Silver Gold . . , , .... . 549

Contents xiii

CHAP. PAGE

VI. ELEMENTS OF GROUP II. (FAMILY A.)

Beryllium Magnesiit in C ale in >n Strontium Barium 5 70

VII. ELEMENTS OF GROUP II. (FAMILY B.)

Zinc Cadmium Mercury 590

VIII. ELEMENTS OF GROUP III.

FAMI LY A. : Scandium ) 'tlrium Lanthanum Ytter-
bium.
FAMILY B. : Boron Aluminium Gallium Indium

Thallium 606

IX. ELEMENTS OF GROUP IV.

FAMILY A. : Titanium Zirconium Cerium Thorium.
FAMILY B. : Silicon Germanium Tin Lead . . 627

X. ELEMENTS OF GROUP V. (FAMILY A.)

Vanadium Niobium Tantalum 655

XL ELEMENTS OF GROUP VI. (FAMILY A.)

Chromium Molybdenum Tungsten Uranium . . 657

XII. ELEMENTS OF GROUP VII. (FAMILY A.)

Manganese 666

XIII. TRANSITIONAL ELEMENTS OF THE FIRST LONG PERIOD.

Iron Cobalt Nickel 671

XIV. TRANSITIONAL ELEMENTS OF THE SECOND AND FOURTH

LONG PERIOD.

Ruthenium Rhodium Palladiiim Osmium Iridium
Platinum Argon Helittm 690

APPENDIX : RADIUM, AND RADIOACTIVE ELEMENTS . 697

INDEX 75

INORGANIC CHEMISTRY

PAET I
INTRODUCTORY OUTLINES

CHAPTER I
CONSTITUTION OF MATTER

THE science of chemistry may be described as the study of a
certain class of changes which matter is capable of undergoing.
Matter is susceptible of a variety of changes, some of which are
regarded as physical and others as chemical. Thus, when a steel
knitting-needle is rubbed upon a magnet, the needle undergoes a
change, by virtue of which it becomes endowed with the power
of attracting to itself iron filings or nails ; and when an ordinary
lucifer match is rubbed upon a match-box the match undergoes a
change, resulting in the production of flame. In the first case the
change is said to be a physical one, while the ignition and com-
bustion of the match is a chemical change.

When a fragment of ice is gently warmed, it is changed from a
hard, brittle solid to a mobile, transparent liquid ; and when white
of egg is gently heated, it changes from a transparent, colourless
liquid to an opaque white solid. These changes, which appear at
first sight to be of a similar order, are in reality essentially different
in their nature : the transformation of solid ice into liquid water
is a physical change, the coagulation of albumen is a chemical
change.

Again, when certain substances (such as the materials which
constitute the so-called luminous paint} are exposed to a bright
light, they undergo a change whereby they become invested with

A

2 Introductory Outlines

the power to emit a feeble light when seen in the dark. A stick of
phosphorus also emits a very similar light when seen in the dark.
The glowing of these materials under these circumstances might
readily be regarded as the result of the same kind of change in
both cases ; but in reality the luminosity of the phosphorus is due
to a chemical change taking place upon the surface of that sub-
stance, while the emission of light from the luminous paint is a
purely physical phenomenon.

The two sciences, chemistry and physics, are so closely related
and interdependent upon each other, that no sharp distinction or
line of separation between them is possible. Every chemical
change that takes place is attended by some physical change, and
it often happens that this accompanying physical change forms
the only indication of the chemical change that has taken place.
In certain important points, however, a chemical change is very
different from one that is purely physical : in the latter case no
material alteration in the essential nature of the substance takes
place. This will be seen in the examples quoted. The steel
needle remains unaltered in its essence, although by magnetisation
it has acquired a new property a property which it again loses,
and which can be again and again imparted to it. The match, on
the other hand, when ignited has undergone a material and per-
manent change : the combustible substance is now no longer
combustible, neither will it ever return to its original state. The
solid water, in being transformed to liquid water, has not under-
gone any vital change ; in essence it is the same substance merely
endowed with a new property of liquidity, a property which it loses
again when cooled, and which can be again and again imparted to
it. On the other hand, the coagulated albumen has undergone a
complete and lasting change, and never returns to its original
condition.

In the same way, the luminous paint gradually ceases to emit
light, and returns to its original state ; it may be exposed to the
influence of light, when it once more acquires the property of
phosphorescence, and this change may be brought about indefi-
nitely, without altering the intrinsic nature of the substance. The
glowing phosphorus, on the other hand, is gradually changed into
a white substance, which escapes from it as a smoke or fume ; in
the act of glowing the phosphorus is undergoing a process of slow
burning, and if allowed to remain will continue glowing and burn-
ing until the whole of it has disappeared in the form of smoke.

Molecules 3

The Constitution of Matter. Molecules. Matter is regarded
by the chemist and physicist as being composed of aggregations
of minute particles ; every substance, whether it be solid, liquid,
or gaseous, presents the appearance to his mind of a vast number
of extremely minute particles. To these particles the name mole-
cules (" little masses ") has been given. The particles or molecules
of any particular substance are all alike : thus in sulphur the
molecules are all of one kind, while in water they are all of another
kind ; the chemical properties associated with sulphur are the pro-
perties of the individual sulphur molecules, while those belonging
to water are the properties of the molecules of that substance. All
matter, therefore, is to be conceived as having what may be called a
grained structure. The actual sizes of molecules is a matter which
has not yet been determined with exactness ; they are orders of
magnitude which are as difficult for the mind to grasp on account
of their minuteness, as many astronomical measurements are by
reason of their vastness. It is certain that their size is less than
half a single wave-length of light,* and that therefore they are
beyond the visual limits of the microscope. Some general idea
of their order of magnitude may be gathered from Lord Kelvin's
calculation, that if a single drop of water were magnified to the size
of the earth, each molecule being proportionately enlarged, the
grained appearance which the mass would present would probably
be finer than that of a heap of cricket-balls, but coarser than a
heap of small shot.

It will be evident, therefore^ that in the strictest sense matter is
not homogeneous^ since it consists of aggregations of molecules,
between which there exist certain interspaces.

The forces which similar molecules exert upon each other are
regarded as physical, in contradistinction to chemical. These
forces are either attractive in their nature, or repellent. The
attractive forces tend to draw the molecules closer together, and
thus to cause the substance to assume the solid state ; while
repellent forces, on the other hand, tend to separate the molecules
and to make the substance pass into the gaseous condition.
Changes which matter undergoes by the action of these forces are
physical changes ; they do not affect the chemical nature and
properties of the substance, which properties, as already stated,
reside in the molecules themselves.

* The wave-length of the blue ray (G) = 0.0004311 millimetre, or
0.0000169 inch.

4 Introductory Outlines

In each of the three states of matter, viz. solid, liquid, or gaseous,
the molecules are conceived as being in a state of motion ; they
are regarded as executing some vibratory movement within the
spaces that divide them. In the solid state this movement is
usually the most restricted, for the reason that in this case the
intermolecular spaces are as a rule the smallest. In the gaseous
condition, however, the attractive force between the molecules
has been almost entirely overcome by the operation of the
repellent forces. The molecules are therefore widely sepa-
rated, and consequently permit of a much greater freedom of
movement.

Such changes in matter, which are merely the result of
alterations in the motions of the molecules, are likewise purely
physical changes.

MoWi|]pq may HP defined as the smallest weight ofinatter
in which the original properties of the matter are retained.

Atoms. It is the beliet ot chemists that most molecules are
possessed of a structure. That is to say, they are not simple,
single, indivisible masses, but themselves consist of aggregations
of still smaller particles, which are held together by the opera-
tions of some other force. These particles of which molecules
are composed are termed atoms, and the force which holds them
together is called chemical affinity, or chemical attraction. To
the mind of the chemist, such molecules are little systems, con-
sisting of a number of atoms which are attracted to each other
by this particular force ; in the ordinary movements of the mole-
cule, the system moves about as a whole. In this respect it bears
some analogy, on an infinitely minute scale, to a solar system.
The atoms of a molecule are regarded as in a state of motion as
respects one another, possibly revolving about one another, while
the entire system, or molecule, at the same time performs its in-
dependent movements, just as in a solar system the various
members perform various movements towards each other, while
at the same time the whole system travels upon its prescribed
orbit. In the case of the heavenly bodies, the force which regulates
the movements of the individual members of the system amongst
themselves is the same force that controls the motion of the united
system, namely, gravitation. What is the precise relation, or
difference, if any, between the forces which control the move-
ments of molecules, and those which operate between the atoms
of the molecule, is not known ; but as the effects produced are

Molecules and Atoms 5

different, the latter force is distinguished by the name of chemical
affinity.

Any change which matter undergoes, in which the integrity of
the molecules is not destroyed, is regarded as a physical change ;
while any change which arises from an alteration in the structure
of the molecule is a chemical change. For example, the molecules
of water consist of three separate atoms, one of oxygen and two
of hydrogen ; any change which water can be made to undergo,
in which these three atoms still remain associated together as the
molecule, is a physical change. The water may be converted into
ice, or it may be changed into steam ; but these alterations still
leave the molecules intact the three atoms still remain united as
an unbroken system, and so long as this is the case chemical
change has not taken place.

Suppose now the molecules of water are heated to a much
higher temperature than that which is necessary to convert the
water into steam, by passing electric sparks through the steam.
It will then be found that a very different kind of change has come
over the substance. The steam, after being so heated, no longer
condenses to water again when cooled ; it has been changed into
a gas which can be bubbled through water and collected in an
inverted vessel filled with water standing in a pneumatic trough,
and if a flame be applied to this gas a sharp explosion takes place.
The change in this case is a chemical change, for the integrity
of the molecules of water has been destroyed. The two atoms
of hydrogen have become detached from the oxygen atom, and
the original triune structure of the system is destroyed.

Atoms are therefore defined as the smallest particles
'which can take part in a chernical

* The study of the phenomena of radioactivity has led to the belief that
atoms are not indivisible particles of matter, but that they are themselves
systems, which under certain circumstances are capable of undergoing change
by ejecting from themselves relatively minute portions of the system called
electrons. (See p. 104; also Appendix.) The precise nature of these electrons
still belongs to the realm of speculation, and the changes resulting from their
movements do not belong to the category of " chemical change" as the term
is here employed.

CHAPTER II
ELEMENTS AND COMPOUNDS

THERE are certain molecules in which all the atoms present are
of the same kind, and there are other molecules which are com-
posed of atoms which differ from one another. Thus, in the
substance sulphur, all the atoms composing the molecules are
alike ; while in water, as already mentioned, there are two distinct
kinds of atoms in the molecule. Matter, therefore, is divided into
two classes, according as to whether its molecules are composed of
similar or of dissimilar atoms. Molecules consisting of atoms of
the same kind are termed elementary molecules, and substances
whose molecules are so constituted are known as elements ; mole-
cules, on the other hand, which contain dissimilar elements are
called compound molecules, and substances whose molecules are
thus composed are distinguished as compounds.

Sulphur, therefore, is an element, and water is a compound. It
will be evident that in the case of elementary molecules, whatever
processes they may be subjected to, only one kind of matter can
be obtained from them ; while in the case of compounds, the
molecules consisting of dissimilar atoms, as many different kinds
of matter can be obtained as there are different atoms present.
By appropriate means the atoms of hydrogen and oxygen in water
molecules can be separated, and two totally different kinds of
matter, namely, hydrogen and oxygen, can be obtained from this
compound.

At the present time there are about seventy substances known to
chemists which are believed to be elements. In the history of the
science it has frequently happened that substances which were
considered to be elements have proved, when subjected to new
methods of investigation, to be in reality compound bodies : thus,
prior to the year 1783, water was thought to be an elementary
substance ; it was indeed regarded as the very type of an element,
until Cavendish and Lavoisier proved that it was composed of
two entirely different kinds of matter. In the year 1807 Sir

Elements and Compounds 7

Humphry Davy showed that the substances known as potash
and soda, which were believed to be elements, were in reality
compound bodies, and he succeeded in separating the constituent
atoms in the molecules of these substances, and in obtaining from
them two essentially different kinds of matter. It is therefore
quite possible, perhaps even probable, that some at least of the
forms of matter which are now held to be elements may yet prove
to be compound bodies. On the other hand, the list is from time
to time extended by the discovery of new elements. Thus during
the last few years at least five new members have been added to
the number.

The number of compounds is practically infinite.

The elements are very unequally distributed in nature, and are
of very different degrees of importance to mankind. Some are
absolutely essential to life as it is constituted, while others might
be blotted out of creation without, so far as is known, their absence
being appreciated. The following thirty elements include all the
most important (for the complete list see page 22) :

Aluminium. Gold. Oxygen.

Antimony. Hydrogen. Phosphorus.

Arsenic. Iodine. Platinum.

Bismuth. Iron. Potassium.

Bromine. Lead. Silicon.

Calcium. Magnesium. Silver.

Carbon. Manganese. Sodium.

Chlorine. Mercury. Sulphur.

Copper. Nickel. Tin.

Fluorine. Nitrogen. Zinc.

On account of certain properties common to a large number of
the elements, and more or less absent in others, properties which
are for the most part physical in character, the elements are
divided into two classes, known as metals and non-metals. The
metals generally are opaque, and their smoothed surfaces reflect
light to a high degree, thus giving them the appearance known as
metallic lustre. They also conduct heat and electricity. Gold,
silver, copper, iron, are metals ; sulphur, bromine, oxygen, phos-
phorus, are non-metals. These two classes, however, gradually
merge into one another, and certain elements are sometimes
placed in one division and sometimes in the other, depending
upon whether the distinction is based more upon their physical

8 Introductory Outlines

or their chemical properties : thus, the element arsenic possesses
many of the physical properties of a metal, but in its chemical
relations it is more allied to the non-metals ; such elements as
these are often distinguished by the name metalloids. The follow-
ing list embraces all those elements which by common consent
are regarded as non-metals and metalloids, including the recently
discovered elements of the argon group, which are here printed in
italics :

Arsenic.

Fluorine.

Phosphorus.

Helium.

Boron.

Hydrogen.

Selenium.

Neon.

Bromine.

Iodine.

Silicon.

Argon.

Carbon.

Nitrogen.

Sulphur.

Krypton.

Chlorine.

Oxygen.

Tellurium.

Xenon.

The number of atoms which compose the various elementary
molecules is not the same in all cases : thus in the elements
sodium, potassium, cadmium, mercury, and zinc, the molecules,
when the elements are in a state of vapour, consist of only one
atom. The same is true also of the newly discovered elements in
the last column. The molecules of all these substances are single
particles of matter. The terms molecule and atom, therefore, as
applied to these elements, are synonymous. Such molecules as
these are called mono-atomic molecules. In many cases elemen-
tary molecules consist of two atoms ; such is the case with the
elements hydrogen, bromine, chlorine, oxygen, nitrogen, and
others. Elementary molecules of this twin or dual nature are
known as di-atomic molecules. Only one instance is known in
which an elementary molecule consists of a trio of atoms, namely,
the molecule of ozone, which is an aggregation of three oxygen
atoms. This molecule is said to be tri-atomic. In two cases,
namely, arsenic and phosphorus, the molecules are composed of
a quartette of atoms, and these elements, therefore, are said to
form tetr-atomic molecules. In a large number of instances the
atomic constitution of the molecule of the elements is not known.
These terms, mono-atomic, di-atomic, &c., are applied exclu-
sively to molecules of elements, and are not used in reference
to compounds, where the molecules are composed of dissimilar
atoms.

Mechanical Mixtures. When molecules of different kinds of
matter are brought together, one of two results may follow : either
they will merely mingle together without losing their identity, that

Mechanical Mixtures 9

is to say, the atoms composing the individual molecules will still
remain associated together as before, or the atoms in the molecules
of one kind will attach themselves to certain atoms present in
molecules of another kind to form still different molecules ; in other
words, there will be a redistribution of the atoms, whereby diffe-
rent systems or molecules are produced.

In the first case the result is said to be a simple or mechani-
cal mixture, in the second it is the formation of a chemical
compound.

In a simple mixture the ingredients can be again separated by
purely mechanical methods ; and as the properties of a substance
are the properties of the molecules of that substance, it follows that
if the integrity of the molecules is not broken, the properties of a
mechanical mixture will be those of the ingredients. For example,
oxygen is a colourless gas without taste or smell ; hydrogen also is
a colourless gas without taste or smell : when these two gases are
mixed together, the mixture is gaseous, is colourless, and tasteless,
and, being- only a mixture, the molecules of one gas can be readily
sifted away from the other.

Again, charcoal is a black solid, insoluble in water ; sulphur is a
yellow solid, also insoluble in water ; nitre is a white solid, readily
dissolved by water : when these three substances are finely
powdered and mixed together, the result is a mechanical mixture,
which is solid, and which is dark grey or nearly black in colour.
If this mixture be placed in water, the nitre is dissolved away and
the charcoal and sulphur are left.

When, however, the integrity of the molecules is disturbed, and a
rearrangement of the atoms takes place, resulting in the formation
of new molecules, then it is said that chemical action has taken
place.

Chemical action, therefore, always results in the formation of
new molecules new molecules which are endowed with their
own special properties, differing often in the most remarkable and
quite inexplicable manner from those of the original molecules.
One or two examples may be quoted in order to illustrate this
extraordinary modifying effect of chemical action. The two
colourless gases, oxygen and hydrogen, when simply mixed to-
gether, give rise, as already mentioned, to a colourless, gaseous
mixture, in which the dual molecules of hydrogen and the simi-
larly constituted oxygen molecules move about freely amongst

IO Introductory Outlines

each other. By suitable means chemical action may be made
to take place between these two elements, whereby a complete
rearrangement of the atoms takes place, resulting in the formation
of molecules of water molecules in which, as has been already
mentioned, one atom of oxygen is associated with two atoms of
hydrogen. The product of the chemical action is therefore water,
while both the forms of matter of which it is composed are
gaseous.

The air we breathe, and which is necessary to life, consists of
a simple mixture of two colourless gases, viz., oxygen and nitrogen.
When chemical action takes place between these substances, a
brown-coloured gas is produced in which no animal or vegetable
life could exist for many minutes, on account of its suffocating
nature.

Common salt, which is a white solid substance, and not only
harmless, but even a necessary article of food, contains two atoms
in its molecules one an atom of chlorine, and the other an atom
of sodium. Chlorine is a yellow gas, intensely suffocating and
poisonous ; and sodium is a soft, silver-like metal, which takes
fire in contact with water.

Why it is that a molecule, consisting of an atom of chlorine
and an atom of sodium held together by chemical affinity,
should be endowed with properties so totally different from
those of the contained elements, is altogether unknown ; and
similarly, it is quite impossible to predicate from the properties
of any compound what are the particular elements of which it is
composed. Thus, sugar is a white crystalline solid, soluble in
water, and possessing a sweet taste ; but no one would have
ventured to predict that the molecules of this substance were com-
posed of atoms of carbon, a black, tasteless, insoluble solid ;
hydrogen, a colourless, tasteless gas ; and oxygen, another colour-
less, tasteless gas.

Chemical Affinity. When molecules, consisting of two atoms,
say A B, come in contact with molecules consisting of other two
atoms, C D, and a chemical change takes place resulting in the
formation of new molecules, A C and B D, the question naturally
arises, Why does the atom A leave the atom B and attach itself to
C ? In other words, what determines the rearrangement of the
atoms into new molecules ?

At present no exact answer can be given to this question.
Chemists express the fact by saying that the chemical affinity

Chemical Affinity 1 1

existing between A and C is greater than that exerted by B upon
A. This remarkable selective power possessed by the atoms of
different elements lies at the root of all chemical phenomena, and
it differs between the various elements to an extraordinary degree.
For example, the atom of chlorine possesses a very powerful
chemical affinity for the atom of hydrogen : when hydrogen mole-
cules, which consist of two atoms, are mixed with chlorine mole-
cules, which are also aggregations of two atoms, at first a simple
mechanical mixture is obtained, the two different kinds of mole-
cules move amongst each other without undergoing change. On
very small provocation, however, the affinity of the hydrogen atoms
for the chlorine atoms can be caused to exert itself; by merely
momentarily exposing the mixture to sunlight a complete redistri-
bution of the atoms suddenly takes place with explosive violence
and new molecules are formed, each containing one atom of
hydrogen and one atom of chlorine.

Again, an atom of nitrogen is capable of associating itself in
chemical union with three atoms of the element chlorine, forming
a compound whose molecules therefore contain four atoms. The
chemical affinity between the atoms of chlorine and nitrogen is
so feeble, the system is, so to speak, in a state of such unstable
equilibrium, that the very slightest causes are sufficient to instantly
separate the atoms in the most violently explosive manner, and
so break up the compound molecules into separate molecules of
chlorine and nitrogen. In this case the affinity between one
chlorine atom and another chlorine atom is greater than that
between chlorine and nitrogen, consequently the redistribution
that results is of the opposite order to that of the former
example.

As a rule, those elements which the more closely resemble each
other in their chemical habits have the least affinity for each other,
while the greatest affinity usually exists between those which are
most dissimilar.

Chemical Action. The actual process of redistribution of the
atoms that takes place when molecules of different kinds of matter
are brought together is called chemical action. In many cases
chemical action takes place when the substances are merely
brought together, while in others it is necessary to expose the
bodies to the influence of some external energy : thus chemical
action is brought about in a great number of instances by the
application of heat to the substances. In some cases the influence

12 Introductory Outlines

of light has the effect of causing chemical action to take place ;
for example, when the gases chlorine and hydrogen are mingled
together, no chemical action takes place in the dark, but on
exposing the mixture to light the hydrogen and chlorine combine,
and form the compound hydrochloric acid. It is upon the effect
of light in causing chemical action to take place that the art
of photography depends.

Chemical action may sometimes be induced by the influence of
pressure ; thus, when the two gases, hydrochloric acid and phos-
phoretted hydrogen, are subjected to increased pressure they
combine together to form a crystalline solid compound known as
phosphonium chloride. In the same way, by very great mechanical
pressure, a mixture of powdered lead and sulphur can be caused
to combine together, when they form the compound, lead sulphide.
There are also a number of chemical actions that are only able
to proceed in the presence of small quantities (often extremely
small) of a third substance, which itself remains unchanged at the
conclusion of the action. These cases are generally included
under the name of catalytic actions : in some of them the modus
operandi of the third substance can be traced (see Oxygen, Modes
of Formation ; also Chlorine, Deacon's Process), while in others
it is not understood. Thus it is found that a number of chemical
actions are quite unable to take place if the materials are abso-
lutely dry ; for example, the element chlorine has a powerful
affinity for the metal sodium, and when these substances are
brought together under ordinary conditions, chemical action in-
stantly takes place, and the compound known as sodium chloride
(common salt) is produced. If, however, every trace of moisture
be perfectly removed from both the sodium and the chlorine, no
action between these elements takes place when they are brought
together, and so long as they remain in this state of perfect dryness
no chemical change takes place. The admission into the mixture
of the minutest trace of the vapour of water, however, at once
induces chemical action between the chlorine and the sodium, but
the exact part that the trace of moisture plays in producing this
effect is not known with certainty. (See also foot-note, page 89.)

A few interesting cases are known in which chemical action is
brought about by the vibration caused by a loud sound or note ;
for example, the molecules of the gas acetylene consist of two
atoms of carbon associated with two of hydrogen. When a quantity
of this gas is exposed to the report produced by the detonation of

Chemical Action 13

mercury fulminate, the mere shock of the explosion causes a re-
distribution of the atoms whereby solid carbon is deposited and
hydrogen set free. We may suppose that the particular vibration
produced by the detonation of the fulminate exercises a disturbing
effect upon the motions of the atoms constituting the molecules of
acetylene, and thereby causes them to swing beyond the sphere of
their mutual attractions, and thus the system undergoes disruption
and rearrangement.

All known instances of chemical action can be referred to one
of three modes, in which the rearrangement of the atoms can take
place.

(i.) By the direct union of two molecules to form a more
complex: molecule. Thus, if CO and C1C1 represent two mole-
cules between which chemical action takes place according to
this mode, they unite to form a molecule containing the four
atoms COC1C1.

(2.) By an exchange of atoms taking place between different
molecules. In its simplest form this is illustrated in the action
of one element upon another to form a compound. Thus, if HH
and C1C1 stand for two elementary molecules between which
chemical action takes place, the result is the formation of the two
molecules HC1 HC1. Such a process as this, in which a com-
pound substance is produced directly from the elements which
compose it, is termed synthesis.

The same mode of chemical action may also be exemplified by
the exact opposite to this process, namely, the resolution of a
compound into its constituent elements. Thus, if OHH OHH
represent two molecules of the same compound, when chemical
action takes place it will result in the formation of the three
elementary molecules OO, HH, and HH. Such a process as
this, in which a compound is resolved into its elements, is known
as analysis.*

(3.) By a rearrangement of the atoms contained in a molecule.
There are a number of instances of chemical change, in which the
molecules of the substance do not undergo any alteration in their
composition that is to say, no atoms leave the molecule, nor are
any added to it. The molecule still consists of the same atoms
after the change as it did before, but the chemical action has

* It will be seen that in each of the examples here given, the process of
rearrangement involves first the decomposition of one or both of the reacting
molecules, and then the combination of the atoms to form different molecules.

14 Introductory Outlines

caused them to assume new relative positions, or different relative
motions with respect to each other. For example, the substances
known to chemists as ammonium cyanate and urea are two totally
different and distinct kinds of matter. These molecules, however,
each contain the same atoms and in the same number ; they each
consist of aggregations of one atom of carbon, one atom of oxygen,
two atoms of nitrogen, and four atoms of hydrogen. When am-
monium cyanate is gently warmed, the eight atoms composing
the molecules undergo this process of rearrangement, and the
substance is changed into urea.

When chemical action takes place between two substances, say A and B,
in ordinary language we say that A acts upon B. Such a statement, however,
must not be understood to imply that A takes the initiative, so to speak, and
that B is in any way less responsible for the action. It is equally true to say
that B acts upon A. For instance, we commonly say nitric acid acts upon
copper, hydrochloric acid acts upon zinc, nitric acid has no action upon gold,
and so on ; but it is equally true to say copper acts upon nitric acid, zinc
acts upon hydrochloric acid, gold has no action upon nitric acid. A more
strictly scientific expression would be A and B react, or do not react, as the
case may be. Thus, nitric acid and copper react, gold and nitric acid do not
react.

CHAPTER III
CHEMICAL NOMENCLATURE

THE names which have been given to the various elementary forms
of matter are not based upon any scientific system. The names of
some have their origin in mythology. Others have received names
which are indicative of some characteristic property, while those of
several bear reference to some special circumstance connected with
their discovery. It has been the custom in modern times to dis-
tinguish metals from non-metals by applying to the former names
ending in the letters um, and consequently such metals as are of
more recent discovery all have names with this termination. The
common metals, however, which have been known since earlier
times, such as gold, silver, tin, and copper, keep their old names.
The two elements selenium and tellurium were at the time of their
discovery thought to be metals, and they consequently received
names with the terminal um ; these substances strongly resemble
metals in many of their physical properties, but in their chemical
relations they are so closely similar to the non-metal sulphur, that
they are by general consent classed among the non-metals ; they are
examples of those elements which are distinguished as metalloids.
On this account selenium is by some chemists termed selenion.

In naming chemical compounds, the chemist endeavours that
the names employed shall not only serve to identify the sub-
stances, but shall as far as possible indicate their composition.
The simplest chemical compounds are those composed of only
two different elements ; such are spoken of as binary compounds*
and their names are made up of the names of the two elements
composing them, thus

* This expression is now sometimes used in a somewhat modified sense.
Thus in the language of the ionic theory (p. 107) the term binary compound is
used to denote a substance which dissociates into two ions, quite irrespective
of the number of elements it may contain. It is to be regretted that under
these circumstances a new word was not coined to denote the newer idea.

1 6 Introductory Outlines

The compound formed by the chemical union of

Hydrogen with sulphur is called hydrogen sulphide.
Sodium ,, chlorine ,, sodium chloride.
Copper ,, oxygen ,, copper oxide.
Calcium ,, fluorine ,, calcium fluoride.
Potassium ,, iodine ,, potassium iodide.

It continually happens, however, that the same two elements
combine together in more than one proportion, giving rise to as
many different compounds, in which case it becomes necessary to
so modify the names that each of the compounds may be dis-
tinguished. This is accomplished by the use of certain terminal
letters or of certain prefixes ; for example, the element phos-
phorus combines with chlorine in two proportions, forming two
different compounds in one the molecules contain one atom of
phosphorus united to three atoms of chlorine, in the other the
molecules consist of one atom of phosphorus associated with five
of chlorine. These two compounds may be distinguished in the
following ways :

i atom of phosphorus with 3 atoms of chlorine forms phosphorous chloride.
i , , , , , , 5 , , , , , , phosphorzV chloride.

or

i atom of phosphorus with 3 atoms of chlorine forms phosphorus /rzchloride.
i ,, ,, ,, 5 ,, ,, ,, phosphorus /tachloride.

The latter method of distinction is the more general, thus

i atom of sulphur with 2 atoms of oxygen forms sulphur dioxide,
i ,, ,, ,, 3 ,, ,, ,, sulphur trioxide.
i atom of carbon with i atom of oxygen forms carbon monoxide,
i ,, ,, ,, 2 atoms ,, ,, carbon dioxide.

Occasionally the prefixes sub &&&proto are employed to denote
these differences of composition, but their use is more limited, and
is becoming out of vogue. When more than two compounds are
formed by the union of the same two elements, the additional
prefixes hypo, under, and^ter, over, are sometimes used.

In a considerable number of instances the systematic names of
familiar compounds give way to the vulgar or common names by
which they are known, thus

{Ammonia . . . Hydrogen nitride ^

Hydrochloric acid . Hydrogen chloride I Systematic

Sulphuretted hydrogen . Hydrogen sulphide j names.

Water .... Hydrogen monoxide '

Chemical Nomenclature 17

Binary compounds consisting of elements united with oxygen
are called the oxides of those elements. Certain of these oxides
are capable of reacting with water, giving rise to substances known
as acids; such oxides are distinguished as acid-forming oxides, or
acidic oxides. They are also sometimes termed anhydrides. All
the non-metallic elements, except hydrogen and the members of
the argon group, form oxides of this order, and the acids derived
from them are known as the oxy-acids or hydroxy-acids.

Certain other oxides also unite with water, but give rise to com-
pounds known as hydroxides. When such oxides, which are all
derived from the metallic elements, are brought into contact with
acids, chemical action takes place, and a compound termed a salt is
formed, together with water. Such oxides are distinguished as salt-
forming or basic oxides. There are also oxides which are neither
acidic nor basic. The names of oxy-acids are derived from the
name of the particular oxide from which they are formed, thus

Carbon dioxide gives carbonic acid.
Silicon dioxide silicic acid.

When the same element forms two acid-forming oxides, the
terminals ic and ous are applied to the acids to denote respectively
the one with the greater and the less proportion of oxygen, thus

Sulphur /rzbxide gives sulphur/V acid.
Sulphur tfVoxide gives sulphurous acid.
Nitrogen /<??;/oxide gives nitmr acid.
Nitrogen /r/oxide gives nitrous acid.

When more than two such acids are known, the additional
prefixes hypo or per are made use of. Thus jztersulphuric acid
denotes an acid containing the highest quantity of oxygen, while
fiyflonitrous acid stands for an acid containing less oxygen than is
present in nitrous acid.

There is a class of binary compounds formed by the combination
of a large number of the elements with sulphur ; these are known as
sulphides. Certain of these sulphides are also capable of -forming
acids which are analogous in their constitution to oxy-acids, but in
which the oxygen atoms are substituted by atoms of sulphur.
These acids are known as thio acids (sometimes sulpho acids),
and the same system of nomenclature is adopted to distinguish
these : thus we have thio-arseni0.y acid, thio-arsenzV acid, denoting

B

1 8 Introductory Outlines

respectively the acid with the smaller and the larger proportion of
sulphur.

It was at one time believed that all acids contained oxygen, that
indeed this element was essential to an acid. The name oxygen
indicates this belief, the word signifying "the acid-producer."
This view is now seen to have been incorrect, for many acids are
known in which oxygen is not one of the constituents. Thus the
elements fluorine, chlorine, bromine, and iodine, which constitute
the so-called Halogen group of elements, each combines with
hydrogen, giving rise respectively to. hydrofluoric, hydrochloric,
hydrobromic, and hydriodic acids.

All known acids contain hydrogen as one of their constituents.

As already stated, when chemical action takes place between an
acid and a base * a salt is formed. Oxy-acids in this way give rise
to oxy-salts, thio-acids to thio-salts, and halogen acids to haloid
salts.

The latter salts being binary compounds, their names are given
according to the system already explained, such, for example, as
calcium fluoride, sodium chloride, potassium bromide, silver iodide.

In the case of the oxy-salts and thio-salts, the names are made
up from the names of the acid and of the metal contained in the
base, with the addition of certain distinctive suffixes : thus if the
acid be one whose name carries the terminal ous, its salts will be

* The word base is unfortunately employed by different chemists in different
senses, so that it is scarcely possible to give a precise definition of it. Originally,
no doubt, the term was employed simply to denote the idea of foundation, and
was applied to the metal or the oxide of the metal entering into the composition
of a salt ; which being the more tangible constituent was thus regarded as the
more important one, or the basis of the salt. At the present day the word
base is used in INORGANIC chemistry chiefly to denote that class of compounds
described on page. 17 as hydroxides, while the oxides from which these
hydroxides are derived are spoken of as basic oxides. Besides this class, it
includes ammonia and a few other compounds which like ammonia are not
derived from metallic oxides. The ORGANIC chemist, on the other hand, regards
ammonia as the true type of a base ; and all organic compounds which can be
regarded as " derivatives " of ammonia are called bases. Not only so, but the
term is even extended so as to include similar " derivatives " of the phosphorus,
arsenic and antimony analogues of ammonia, thus giving rise to the expressions
nitrogen bases, phosphorus bases, &c.

Again, in the language of the modern theory of ionic dissociation, a base is
defined as a compound in which the only negative ions are the hydroxide ions
(page 107). This definition includes the class of hydroxides above mentioned,
but does not include ammonia gas.

Chemical Nomenclature 19

distinguished by the suffix ite, while the names of the salts derived
from acids whose names end in ic are terminated by the letters
ate.

Nitrous acid and potassium oxide give potassium nitrz'/<?.
Sulphurous add ,. sulphite.

Nitric acid ,, nitrate.

SulphurzV acid ,, sulphate.

The formation of a salt by the action of an acid upon a base is
due to the redistribution of the atoms composing the molecules of
the two compounds, in such a manner that some or all of the
hydrogen atoms in the acid molecules exchange places with certain
metallic atoms from the molecules of the base. Acids which con-
tain only one atom of hydrogen so capable of becoming exchanged
for a metal are termed mono-basic acids ; those with two, three, or
four such hydrogen atoms are distinguished respectively as di-basic,
tri-basic, and tetra-basic acids.

If the whole of the displaceable hydrogen in an acid becomes
replaced by the base, the salt formed is known as a normal salt.
On the other hand, when only a portion of the hydrogen atoms
is displaced by the base, the salt is distinguished as an acia
salt Thus sulphuric acid contains two atoms of hydrogen in its
molecule (associated with one of sulphur and four of oxygen) ; if
both the hydrogen atoms are exchanged for potassium, the salt
obtained is normal potassium sulphate, and when only one is so
replaced the salt is known as acid potassium sulphate. By the
term acid salt, therefore, must be understood not a substance
having the familiar properties of an acid, such as a sour taste and
the power to redden litmus, but a salt in which one or more of the
hydrogen atoms of the original acid are still left in the molecule.*
It is quite true that some of the salts of this class do possess acid
qualities and will redden litmus, but this is due to what may be
regarded as merely the accidental circumstance of the acidic
portion of the molecule being derived from a strong acid. Many
substances belonging to the class of acid salts are perfectly neutral
in their behaviour towards litmus, while, on the other hand, some
are strongly alkaline. For example, acid potassium sulphate is acid

* Some chemists prefer to regard the acids themselves as the hydrogen salts ;
accordingly they apply to nitric acid, sulphuric acid, nitrous acid, sulphurous
acid, &c. , the names hydrogen nitrate, hydrogen sulphate, hydrogen nitrite,
hydrogen sulphite, &c. , respectively.

2o Introductory Outlines

to test paper, add calcium carbonate is neutral, while acid sodium
carbonate is alkaline.

A third class of salts is formed by the association of one or
more molecules of normal salt, with one or more additional mole-
cules of the base : these are known as basic salts. Thus, carbonic
acid and the base lead hydroxide form such a salt known as basic
lead carbonate.

CHAPTER IV
CHEMICAL SYMBOLS

CHEMISTS are agreed in adopting certain symbols to denote the
atoms of the various elementary forms of matter. The table on
page 22 contains the names of the elements at present recognised,
and in the second column are given the symbols which are em-
ployed to represent their atoms. The names of the rare elements
are printed in italics.

In a number of instances the atomic symbol is the initial letter
of the ordinary name of the element : thus Boron, B ; Carbon, C ;
Fluorine, F ; Hydrogen, H ; Oxygen, O ; Sulphur, S.

When more than one element has the same initial, either the
first two letters of the name, or the first and another that is pro-
minently heard in pronouncing the word are employed, as Bromine,
Br ; Cobalt, Co ; Chlorine, Cl ; Platinum, Pt. In some cases
letters taken from the Latin names for the elements are used, such
as Antimony (Stibium}^ Sb ; Gold (Aurum\ Au ; Silver (Argentuni),
Ag ; Lead (Plumbum\ Pb ; and Iron (Ferrum), Fe.

These symbols are not intended to be employed as mere short-
hand signs, to be substituted as abbreviations for the full names
of the elements, but in every case they denote one atom of the
element. The symbol H stands for one atom of hydrogen, the
symbol O stands for one atom of oxygen ; Cl means one atom of
chlorine, and Ag represents one atom of silver. No other use of
these symbols is legitimate.

It has been already mentioned (page 8) that the molecules of
the different elements are composed of different numbers of atoms ;
for example, the molecule of hydrogen consists of two atoms, and
ordinary oxygen also forms diatomic molecules. These facts are
expressed in chemical notation by the use of small numerals placed
immediately after the symbol of the atom, thus H 2 denotes a mole-
cule of hydrogen, O 2 a molecule of oxygen. The molecule of ozone
consists of an aggregation of three atoms of oxygen, and is

22

Introductory Outlines

Atomic Weights.
234

Atomic Weights.
2 3

1

V

rt

_|1

1

<u

ill

E

J 8

i|x

g

i ! .

.2'cDp^

Name.

OT

X 3
O T3

Name.

C/3

c

'E

P

I'M

o

's

!>

\\i

o

<^

HH B "2

o

<^

^C Q VO

4jj

< II

^

< tt

Aluminium

Al

27

27.1

Molybdenum .

Mo

96

96

Antimony (Sh'dinm)

Sb

120

120.2

Neodymium .

Nd

144

144-3

Argon ....

A

40

39-9

Neon ....

Ne

20

20.JL,

Arsenic ....

As

75

75-

Nickel ....

Ni

59

58.7

Barium ....

Ba

137

137-4

Nitrogen . .

N

14

I4.OI

Beryllium (Glucinum)

Be

9

9.1

Osmium . .

Os

191

19*?

Bismuth . .

Bi

208

208.0

Oxygen ....

O

16

16.00

Boron ....

B

11

ii

Palladium . . .

Pd

106

106.7

Bromine

Br

80

79.92

Phosphorus

P

31

31.0*

Cadmium . . .

Cd

112

112.4

Platinum . . .

Pt

195

1 95.X

Caesium
Calcium . .

Cs
Ca

133
40

13**

40.9*

Potassium (Kal- )
ium] . . (

K

39

39.10

Carbon . .

C

12

I2.OO

Praseodymium .

Pr

140-5

I40.fr

Cerium ....

Ce

140

140.25

Radium . . .

Ra

226-4

226.4

Chlorine . . .
Chromium .

Cl
Cr

35-5
52

35-4fc
52.1

i Rhodium .
Rubidium .

Rh
Rb

103
85

103
8 5-45

Cobalt ....

Co

59

5 8 -97

Ruthenium

Ru

101-7

101.7

Columbium )

(Niobium] . )

Cb

93-5

93-5

Samarium
Scandium .

Sm
Sc

150
44

150.4
44.1

Copper (Cuprum]
Erbium . .

Cu
Er

63-5
166

63.6

Selenium . . .
Silicon ....

Se
Si

79
28

79-2
28.3

Fluorine . . .

F

19

Z 9~

Silver (Argentum]

Ag

108

107.88

Gallium . ,

Ga

70

76

Sodium (Natrium

Na

23

23.00

Germanium .

Ge

72

72^5

Strontium .

Sr

87-6

87.6

Gold (Aurum]
Helium ....
Hydrogen . . .

Au
He
H

197
4
1

197(2

4
1.008

Sulphur
Tantalum .
Tellurium .

S
Ta
Te

32

181
125 ?

32-0^
127.6

Indium ....

In

115

115

Thallium . . .

Tl

204

204.0

Iodine ....

I

127

126.92

Thorium . . .

Th

232

232.4

Iridium ....

Ir

193

I 93-/

Tin (Stannum] .

Sn

118

119

Iron (Ferrum]

Fe

56

55-85

Titanium . . .

Ti

48

48.1

Krypton . . .

Kr

81-5

sjty.

, Tungsten . . .

W

184

184

Lanthanum .

La

139

'39

Uranium .

U

238-5

238.5

Lead (Plumbum]

Pb

207

207.10

Vanadium . .

V

51-2

Lithium

Li

7

7.0

X

128

7^ft <3^

Magnesium

Mg

24 .

24.32

Ytterbium .

Yb

172

172

Manganese . .

Mn

55 X

54-93

Yttrium . . .

Y

89

89

Mercury (Hydr- \
argyrum . j

Hg

200

200. fo

Zinc

Zirconium

Zn
Zr

65
90-7

65.37

90.6

represented by the symbol O 3 , while the tetr-atomic character of
the phosphorus molecule is expressed in the symbol P 4 . The
composition of compound molecules is expressed by placing the

Chemical Symbols 23

symbols of the atoms which compose such molecules in juxta-
position : thus a molecule consisting of one atom of sodium (symbol
Na) and one atom of chlorine (symbol Cl) is represented by the
united symbols of these two elements, NaCl ; a compound con-
sisting of one atom of carbon and one atom of oxygen by the
symbols of these two atoms, CO. Such arrangements of symbols
representing molecules are termed molecular formula, or, simply,
formula.

When the molecule contains more than one atom of any parti-
cular element, this fact is indicated by the use of numerals placed
immediately after the symbol to be multiplied : thus, a molecule of
water consists of two atoms of hydrogen and one atom of oxygen ;
\\\Q formula for water is therefore H 2 O. One molecule of ammonia,
consisting of an atom of nitrogen with three atoms of hydrogen,
is represented by the formula NH 3 ; and a molecule of sulphuric
acid, which is an aggregation of two atoms of hydrogen, one
atom of sulphur, and four atoms of oxygen, has the formula
H 2 SO 4 .

It is sometimes necessary to represent the presence in a mole-
cule of certain groups of atoms, groups which seem to hold together
and often to function as a single atom. This is accomplished by
the use of brackets : thus (NH 4 ) 2 SO 4 is the formula for a molecule
containing one atom of sulphur, four atoms of oxygen, eight atoms
of hydrogen, and two atoms of nitrogen ; the nitrogen and hydrogen
atoms being present as two groups, in each of which one nitrogen
atom is associated with four hydrogen atoms. Such groups of
atoms are termed compound radicals.

When it is required to indicate more than one molecule of the
same substance, numerals are placed immediately in front of the
formula : thus 2H 2 O signifies two molecules of water, and 3NH 3
expresses three molecules of ammonia.

By means of these symbols and formulae, chemists are enabled
to represent, in a concise manner, the various chemical changes
which it is the province of chemistry to examine. Such changes
are usually termed chemical reactions, and they are represented
in the form of equations in which the symbols and formulae of
the reacting substances as they are before the change are placed
on the left, and those of the substances which result from the
change upon the right, thus

H 2 + C1 2 = 2HC1

24 Introductory Outlines

The sign + has a different significance as used on the left side
of the equation to that which it bears upon the right. On the
left hand it implies tfrat chemical action takes place between the
substances, while on the opposite side it has the simple algebraic
meaning. Thus, the second of the above equations is understood
to mean, that when the compounds, mercuric chloride and potassium
iodide, are brought together in such a way that chemical action
results, a redistribution of the atoms will take place, resulting in
the formation of mercury iodide and also potassium chloride.

As further illustrations of the use of chemical symbols, the
following three examples may be given as exemplifying the three
modes of chemical action mentioned on page 13 :

(1) NH 3 + HC1 = NH 4 C1.

Ammonia combines with hydrochloric acid, and gives ammonium
chloride.

(2) H 2 SO 4 + Na 2 CO 3 = Na 2 SO 4 + CO 2 + H 2 O.

Sulphuric acid reacts with normal sodium carbonate, and yields
normal sodium sulphate, carbon dioxide, and water.

(3) (CN)0(NH 4 ) = (NH 2 ) 2 CO.
Ammonium cyanate is converted into urea.

In all cases where the nature of the chemical change is under-
stood, it is capable of expression by such equations, and as matter
is indestructible, every atom present in the interacting molecules
upon the left of the expression reappears on the right-hand side
in some fresh association of atoms.*

* See also Chemical Notation, chapter vii.

CHAPTER V

THE ATOMIC THEORY

THE atomic view as to the constitution of matter, briefly sketched
out in Chapter I., forms a part of what is to-day known as the
atomic theory.

When chemical changes were carefully studied from a quantita-
tive standpoint, four laws were discovered in obedience to which
chemical action takes place. These laws are distinguished as
the laws of chemical combination. Three of these generalisations
refer to quantitative relations as respects weight ; while one ex-
presses quantitative relations with regard to volume, and only
relates to matter in the gaseous state.

I. Law of Constant Proportion. The same compound always
contains the same elements combined together in the same proportion
by weight; or expressed in other words, The weights of the con-
stituent elements of every compound bear an unalterable ratio to
each other, and to the weight of the compound formed.

II. Law of Multiple Proportions. When the same tiun.
elements confine f^gtithflf to form, more thnn nn^. Compound, tJie^
different weights of one of the elements which unite with a constant
weight of the other bear a simple ratio to one another: or this law
may be stated thus : \Vhen one element unites with another in
two or more different proportions by weight, these proportions are
simple multiples of a common factor.

III. Law of Reciprocal Proportions, or Law of Equivalent
Proportions. The weights of different elements which combine
separately with one and the same weight of another element, are
either the same as, or are simple multiples of, the weights of these
different elements which combine with each other; or in other
words, The relative proportions by weight in which the elements,
A, B, C, D, &*c., combine with a constant weight of another
element, X, are the same for their combinations with any other
element, Y.

26 Introductory Outlines

IV. Law of Gaseous Volumes, or The Law of Gay-Lussae.

When chemical action takes place between gases, either elements
or compounds, the volume of the gaseous product bears a simple
relation to the volumes of the reacting gases.

These four laws are the foundations upon which the whole
superstructure of modern chemistry rests.

(i.) The Law of Constant Proportions. When two sub-
stances are mingled together, and remain as a mere mechanical
mixture, they may obviously be present in any proportion, and it
was at one time thought that when two substances entered into
chemical combination with each other, they could do so also in
any proportion, and that the composition of the resulting com-
pound would vary from this cause. This belief was finally
disproved, and the law of constant proportions definitely estab-
lished by Proust in the year 1806. The same compound, therefore,
however made, and from whatever source obtained, is always
found to contain the same elements united together in the same
proportion by weight. Thus, common salt, or, to adopt its
systematic name, sodium chloride, which is a compound of the
two elements sodium and chlorine, may be made by bringing the
metal sodium into contact with chlorine gas, when the two
elements unite and form this compound. It can also be made
by the action of hydrochloric acid upon the metal sodium, or by
adding hydrochloric acid to sodium carbonate, and by a variety
of other chemical reactions. When the sodium chloride obtained
by any or all of these processes is analysed, it is invariably found
to contain the elements chlorine and sodium in the proportion by
weight of I 10.6479, or, expressed centesimally

Sodium . . 39.32
Chlorine . . 60.68

100.00

and when this is compared with the sodium chloride as found in
nature, obtained either from the salt-mines of Cheshire, or the
celebrated mines in Galicia, or by evaporating sea-water, it is
found that the composition of the compound in all cases is exactly
the same. In the same way the compound water, consisting of
the two elements hydrogen and oxygen, whether it be prepared
synthetically by causing the two elements to unite directly, or
obtained from any natural source, as rain, or spring, or river, is

The Atomic Theory 27

found to contain its constituent elements hydrogen and oxygen in
the ratio by weight of i : 8, or,

Hydrogen . . 11.12
Oxygen . . 88.88

IOO.OO

If in the formation of sodium chloride by the direct combination
of its constituent elements, an excess of either one or other be
present beyond the proportions 39.32 per cent, of sodium and 60.68
per cent, of chlorine, that excess will simply remain unacted upon.
If eight parts by weight of hydrogen and eight parts by weight
of oxygen be brought together under conditions that will cause
chemical action, the eight parts of oxygen will unite with one part
of hydrogen, and the other seven parts of hydrogen merely remain
unchanged. This fact, that elements are only capable of uniting
with each other in certain definite proportions, marks one of the
most characteristic differences between chemical affinity and those
other forces, such as gravitation, that are usually distinguished as
physical forces ; for although there are many instances known in
which the extent to which a chemical action may proceed (that is,
the particular proportion of the reacting bodies which will undergo
the permutation that results in the formation of different mole-
cules) is influenced by the mass of the acting substances, it never
governs the proportion in which the elements combine in these
compounds.

It follows from the law of constant composition that the sum of
the weights of the products of a chemical action will be equal to
that of the interacting bodies ; and upon the validity of this law
depend all processes of quantitative analyses.

(2.) The Law of Multiple Proportions was first recognised
by Dalton, who investigated certain cases where the same two
elements combine together in different proportions, giving rise to
as many totally distinct compounds. These proportions, however,
were always found to be constant for each compound so produced,
so that this law formed no contradiction to the law of constant
composition. The simple numerical relation existing between the
numbers representing the composition of such compounds will be
evident from the following examples. The two' 3 *' compounds of

* In Dalton's day these two substances were the only known compounds of
carbon with hydrogen.

28 Introductory Outlines

carbon with hydrogen, known as marsh gas and ethylene, are
found to contain these elements in the proportions

Marsh gas . . i part by weight of hydrogen with 3 parts of carbon.
Ethylene i ,, 6 ,, ,,

The two compounds of carbon with oxygen contain these ele-
ments in the proportion

Carbon monoxide . i part of carbon with 1.334 parts of oxygen by weight.
Carbon dioxide i ,, ,, 2.667 >

The elements nitrogen and oxygen form as many as five different
compounds, in which the two elements are present in the propor-
tions-
Nitrous oxide . . i part of nitrogen with o. 571 parts of oxygen by weight.
Nitric oxide. . . ,, ,, 1.143 ,.

Nitrogen trioxide . ,, ,, i-7 T 4

Nitrogen peroxide ,, ,, 2.286 ,, ,, ,,

Nitrogen pentoxide ,, ,, 2.857

The relative proportions of carbon combining with a constant
weight of hydrogen in the two first compounds are as i : 2.

Those of oxygen uniting with a constant weight of carbon in the
second example are also as i : 2, while in the nitrogen series the
relative proportions of oxygen in combination with a constant
weight of nitrogen are as i : 2 : 3 : 4 : 5.

(3.) Law of Reciprocal Proportions. Known also as the law
of proportionality, or the law of equivalent proportions. When
the weights of various elements, which were capable of uniting
separately with a given mass of another element, were compared
together, it was seen that these weights bore a simple relation to
the proportions in which these elements combined amongst them-
selves. For example, the elements chlorine and hydrogen each
.separately combine with the same weight of phosphorus, the pro-
portions being

Phosphorus : chlorine = i
Phosphorus : hydrogen = i

The elements chlorine and hydrogen can combine together, and
they do so in the proportion

Chlorine : hydrogen = 35.5 : i

but 35 : i = 3-43 : 0.097

Therefore the proportions by weight in which chlorine and

The Atomic Theory 29

hydrogen separately combine with phosphorus is a measure of the
proportion in which they will unite together.

Again, the two elements carbon and sulphur each separately
combine with the same weight of oxygen, the proportion being

Oxygen : carbon = I '.0.375
Oxygen : sulphur = i : i

But the elements carbon and sulphur themselves unite together,
and in the proportion

Carbon : sulphur = 0.1875 : r
but 0.1875 : : = -375 : 2

Therefore the proportion by weight in which carbon and sulphur
separately unite with the same mass of oxygen is a simple multiple
of that in which these two elements combine together. These
remarkable numerical relations will be rendered still more evident
by comparing the proportions in which the members of a series of
elements combine with a constant weight of various other elements :
thus

Hydrogen. Sodium. Potassium. Silver. Mercury. Chlorine.

0.02817 0.6479 1.02 3.04 2.816 unite separately with i part.

It will be seen that the proportion in which these numbers stand
to each other is as

i : 23 : 39 : 107 : 100 : 35.5

Let us now compare these proportions with those in which the
same elements unite with a constant weight of the element
bromine

Hydrogen. Sodium. Potassium. Silver. Mercury. Bromine.

0.0125 0.2875 0.4875 1.34 1.25 unite with i part,

or as

i : 23 : 39 : 107 : 100 : 80

Each of these five elements in like manner combines with
oxygen, and the weights which are found to unite with a constant
mass of oxygen are

Hydrogea Sodium. Potassium. Silver. Mercury. Oxygen.

0.125 2.875 4- 8 75 I 3-3 8 I2 -5 unite with i part,

again as

i : 23 : 39 : 107 : 100 : 8

30 Introductory Outlines

./"

The same relation will appear in the case of the combination of
these five elements with a constant weight of sulphur

Hydrogen. Sodium. Potassium. Silver. Mercury. Sulphur.

0.0625 I -437S 2.4375 6.69 6.25 unite with i part.

or as

i : 23 : 39 107 : 100 : 16

It is thus evident that the proportions in which the members of
such a series combine with a constant weight of one element is the
same as that in which they unite with a constant mass of another
element. One part by weight of hydrogen combines with 35.5
parts of chlorine, 80 parts of bromine, 8 parts of oxygen, and 16
parts of sulphur that is to say, these proportions of these four
elements satisfy the chemical affinity of I part of hydrogen ; they
are therefore said to be equivalent. Twenty-three parts of sodium
is likewise equivalent to 35.5 parts of chlorine, 80 parts of bromine,
8 parts of oxygen, and 16 parts of sulphur, and by the same
reasoning it is also equivalent to i part of hydrogen, 39 parts of
potassium, 107 parts of silver, and loo parts of mercury. These
numbers, therefore, are known as the equivalent 'weights or the
equivalents of the elements, or their combining proportions, and the
combining weight of an element may therefore be defined as the
smallest weight of that element which will combine with i part by
weight of hvdrogen.

This law of proportionality, or reciprocal proportion, was dis-
covered by Richter, but it was left for Dalton to trace the connec-
tion between these three generalisations. Dalton adopted and
adapted an ancient theory concerning the ultimate constitution of
matter which was expounded by certain of the early Greek philo-
sophers. The exponents of this theory held that matter is built up
of vast numbers of minute indivisible particles, in opposition to the
antagonistic theory believed by others, namely, that matter was
absolutely homogeneous and capable of infinite subdivision.

Dalton embraced the ancient doctrine of atoms, and extended it
into the scientific theory which is to-day known as Dalton's atomic
theory, and is accepted as a fundamental creed by modern chemists.

According to this theory, matter consists of aggregations of minute
particles, or atoms, which are chemically indivisible. Dalton
conceived that chemical combination takes place between atoms
that is to say, when chemical action takes place between two
elements, it is due to the union of their atoms ; the atoms, coming
into juxtaposition with each other under the influence of chemical

The Atomic Theory 31

affinity, are held together by the operation of this force. He further
assumed that the atoms of the various elements possessed different
relative weights, and that the relations existing between these
weights was the same as that between the weights in which experi-
ment had shown the elements to be capable of combining together.
In other words, he said that the numbers representing the combin-
ing proportion of the elements expressed also the relative weights
of the atoms.

Let us now see how this theory satisfies and explains the first
three laws of chemical combination.

(i.) The Law of Constant Composition. It has already been
shown (p. 26) that the compound sodium chloride, wheresoever and
howsoever obtained, contains the elements chlorine and sodium
in the proportion

Chlorine : sodium = I : 0.6479.

These numbers have been shown on p. 29 to represent the com-
bining proportions

Chlorine : sodium = 35.5 : 23.

Now the atomic theory states, that sodium chloride is formed by
the union of atoms of chlorine with atoms of sodium, and that the
relative weights of these atoms is expressed by the combining
weights of the elements, namely, 35.5 and 23. If therefore, sodium
is to combine with chlorine, since atoms are indivisible masses, it
follows that the compound produced by the union of one atom of
each of these two elements must always have the same composi-
tion.

(2.) The Law of Multiple Proportions. The ratio in which
oxygen combines with hydrogen to form the compound water is
seen on p. 27 to be as 8 : i. This number 8, therefore, we will
for the present argument regard as the relative weight of the atom
of oxygen.*

Oxygen combines with carbon as already mentioned, forming
two different compounds ; in the first, the elements are present in
the proportion

Carbon : oxygen = I : 1.334 = 6:8,

* For reasons which will be explained later, chemists now regard the number
16 as representing (in round numbers) the relative weight of the atom of
oxygen.

32 Introductory Outlines

that is to say, in the proportion of one atom of carbon to one atom
of oxygen. According to the theory, if the atom of carbon unites
with more oxygen than one atom, it must at least be with two
atoms. It may be with three or with four, but as the compound
must be formed by the accretion of these indivisible atoms, the
increment of oxygen must take place by multiples of 8. When the
second compound is examined it is found to contain its constituent
elements in the proportion

Carbon : oxygen =i : 2.667 = 6 : 16,

that is to say, in the proportion of one atom of carbon to two
atoms of oxygen. This information respecting the composition of
these two compounds is conveyed both in their names and their
formulas. The first is termed carbon monoxide, and its formula is
expressed by the symbol CO ; while the second is distinguished as
carbon dioxide, and has the formula CO 2 .

The difference in the composition of the five compounds that
nitrogen forms by union with oxygen will be made evident by the
aid of this theory. The proportion of nitrogen to oxygen in these
compounds is

(i.; Nitrogen : oxygen = i : 0.571 = 14 : 8
(2.) Nitrogen : oxygen = i : 1.143 = 14 : 16
(3.) Nitrogen : oxygen = i : 1.714 = 14 : 24
(4.) Nitrogen : oxygen = i : 2.268 = 14 : 32
(5.) Nitrogen : oxygen = i : 2.857 = 14 : 40

And it will be seen that the increase in the proportion of oxygen in
the compounds takes place by the regular addition of a weight of
that element equal to 8, which at the present stage of the argument
we are regarding as representing the relative weight of the atom
of oxygen.

(3.) The Law of Reciprocal Proportions. If the illustrations
given on p. 28 of the operation of this law be examined in the light
of the atomic theory, their explanation will be evident : thus, the
relative proportions in which hydrogen and chlorine separately
combine with phosphorus is 0.097 : 3.43, and the ratio between these
numbers is as I : 35.5, which is the proportion in which these two
elements are known to unite together to form hydrochloric acid.
These numbers, however, represent the relative weights of the
atoms of these elements, therefore hydrochloric acid may be sup-
posed to be formed by the union of one atom of hydrogen with
one atom of chlorine.

The Atomic Theory 33

Again, the relative weights of carbon and sulphur which sepa-
rately combine with a constant weight of oxygen are carbon, 0.375 ;
sulphur, i ; and the ratio between these numbers is as 6 : 16.

Carbon and sulphur, however, unite together in the relative
proportion

Carbon : sulphur = 0.1875 : I = 6 : 32.

Therefore the compound they produce may be supposed to consist
of one atom of carbon, having the relative weight 6, and two atoms
of sulphur, each with the relative weight 16.

CHAPTER VI
ATOMIC WEIGHTS

IN the third column of the table on page 22, the numbers are
given which are at the present time generally accepted by chemists
as representing the approximate atomic weights of the elements.
These numbers depart, in many instances, from those arrived at
by Dalton's methods : thus, the relative weights of carbon, oxygen,
nitrogen, and sulphur, which were found to be equivalent to one
part of hydrogen, are carbon = 6,* oxygen = 8, nitrogen = 4.66,
sulphur = 1 6 ; while the figures given as the approximate atomic
weights of these elements in the table are carbon = 12, oxygen
= 1 6, nitrogen = 14, sulphur = 32. We must now discuss some
of the chief reasons for these departures. In the two compounds
of carbon and hydrogen known to Dalton, namely, marsh gas and
ethylene, the proportions of carbon to hydrogen are

In ethylene . . . Carbon : hydrogen = 6 : I.
In marsh gas . . Carbon : hydrogen = 6:2.

Dalton therefore concluded that ethylene was a compound con-
taining i atom of carbon united with i atom of hydrogen, and to
which, therefore, he gave the formula CH ; and that marsh gas
consisted of I atom of carbon combined with 2 atoms of hydrogen,
and which he accordingly represented by the formula CH 2 .

There was, however, nothing to prove that the weight of carbon
was constant in the two compounds, for it will be obvious that the
same ratio between the weight of carbon and hydrogen will still
be maintained by assuming that the hydrogen is constant, and
that the carbon varies, thus

In marsh gas . . Hydrogen : carbon : : I : 3.
In ethylene . . Hydrogen : carbon : : i 13x2.

* These are the numbers which Dalton ought to have obtained had his
methods of determination been more exact. The figures he actually found for
the combining weights of these four elements were respectively, 5, 7, 5, 13.

34

Atomic Weights 35

That is to say, the ratios are not disturbed by the assumption
that in marsh gas we have I atom of hydrogen combined with i
atom of carbon, having the relative combining weight of 3, and in
ethylene I atom of hydrogen united with 2 atoms of carbon.

It will be evident, however, that if we could gain any exact
information as to the actual number of atoms which are present
in these various molecules, this difficulty would no longer exist.

For example, suppose it were possible to ascertain that in the
molecule of marsh gas there were 4 atoms of hydrogen, then as
the relative weights of hydrogen and carbon in this compound are
as I : 3, the weight of the carbon atom would obviously have to
be raised from 3 to 12 ; and if it could be determined that in the
ethylene molecule there were also 4 atoms of hydrogen, then
seeing that the ratio of hydrogen to carbon in this substance is
as i : 6, we should conclude that it contained 2 atoms of carbon,
of the relative weight not less than 12, and the composition of the
two compounds would be expressed by the formulae, marsh gas
CH 4 , ethylene C 2 H 4 .

Again, the relative weights of hydrogen and oxygen in water
are as i : 8. If the molecule of water contains only i atom of
hydrogen, then we conclude that 8 represents the relative weight
of the oxygen atom, and the formula for water will be HO. But
suppose it to be discovered that there are two atoms of hydrogen in
a molecule of this compound, then it becomes necessary, in order
to retain the ratio between the weight of these constituents (a
ratio ascertained by analysis), to double the number assigned to
the oxygen atom and to regard its weight as 16, as compared with
i atom of hydrogen, and the formula for water in this case would
be H 2 O.

The compound ammonia contains the elements hydrogen and
nitrogen in the ratio

Hydrogen : nitrogen : : i : 4.66.

If the molecule of ammonia contains only i atom of hydrogen,
then 4.66 represents the relative weight of the nitrogen atom, and
the formula will be NH ; but if it should be found that there are
3 atoms of hydrogen in this molecule, then again the relative
weight assigned to the nitrogen must be trebled in order to pre-
serve the ratio, and it will have to be raised from 4.66 to 14 (in
round numbers), and the formula for ammonia will be NH 3 .

From these considerations it will be evident, that it is of the

36 Introductory Outlines

highest importance to gain accurate knowledge as to the actual
number of atoms which are contained in the molecules of matter
in other words, to learn the true atomic composition and structure
of molecules ; and it may be said that this problem has occupied
the minds of chemists from the time that Dalton published his
atomic weights, in the year 1808, down to the present time. There
is no single method of general application, by means of which
chemists are able to determine the atomic weight of an element ;
but they are guided by a number of independent considerations,
some of which are chemical in their character, while others are of
a physical nature ; and that particular number which is in accord
with the most of these considerations, or with what are judged to
be the most important of them, is accepted as the true atomic
weight.

The chief methods employed for determining atomic weights
may be arranged under the following four heads :

1. Purely chemical methods.

2. Methods based upon volumetric relations.

3. Methods based upon the specific heats of the elements.

4. Method based upon the isomorphism of compounds.

I. As an illustration of the chemical processes from which
atomic weights may be deduced, the following examples may be
given, namely, the case of the two elements oxygen and carbon.

Oxygen combines, as already stated, with hydrogen in the
proportion

Hydrogen : oxygen = i : 8.

When water is acted upon by the element sodium, the compound
is decomposed and hydrogen is evolved ; and it is found that if
1 8 grammes of water are so acted on, I gramme of hydrogen is
evolved, and 40 grammes of a compound are formed, which
contains sodium, together with all the oxygen originally in the
18 grammes of water, and some hydrogen. This compound, under
suitable conditions, can be acted upon by metallic zinc, and when
these 40 grammes are so acted on, i gramme of hydrogen is again
evolved, and 72.5 grammes are obtained of a compound containing
no hydrogen, but sodium and zinc combined with all the oxygen
originally contained in the 18 grammes of water.

It will be evident, therefore, that the hydrogen contained in
water can be expelled in two equal moieties ; there must, therefore,
be two atoms of hydrogen in this compound. By no known

Atomic Weights 37

process can the oxygen be withdrawn from water in two stages :
thus, if 1 8 grammes of water are acted upon by chlorine, under the
conditions in which chemical action can take place, 73 grammes of
a compound containing only chlorine and hydrogen are formed, and
the whole of the oxygen is thrown out of combination and evolved
as gas. It is therefore concluded that water contains in its mole-
cule 2 atoms of hydrogen and I atom of oxygen, and as they are
combined in the relative proportion of I : 8, the atomic weight of
oxygen cannot be less than 16.

No compounds have been found in which a smaller weight of
oxygen, relative to one atom of hydrogen, than is represented by
the number 16 (approximately), is known to take part in a chemical
change.

The compound marsh gas contains hydrogen and carbon in
the proportion by weight of 1:3. By acting on this compound
with chlorine, it is possible to remove the hydrogen from it in
four separate portions.

By the first action of chlorine upon 16 grammes of marsh gas,
i gramme of hydrogen is removed in combination with 35.5
grammes of chlorine, and a compound containing carbon, hydrogen,
and chlorine, in the ratio 12 : 3 : 35.5, is formed.

By the successive action of chlorine, three other moieties of
hydrogen can be thus withdrawn, each being in combination with
its equivalent (35.5 parts) of chlorine. The second and third com-
pounds that are formed contain carbon, hydrogen, and chlorine in
the ratios 12:2: (35.5 x 2) and 12:1: (35.5 x 3).

The compound produced by the fourth action of chlorine, which
withdraws the fourth portion of hydrogen, contains only carbon
and chlorine, in the ratio 12 :(35-5 x 4). From the fact that the
hydrogen contained in marsh gas can thus be removed in four
separate portions, the molecule must contain four hydrogen atoms,
and therefore the atomic weight of carbon must be at least 12. No
compounds of carbon are known in which a smaller weight of
carbon, relative to one atom of hydrogen, than is represented by
the number 12, takes part in a chemical change.

The definition of atomic weight, furnished by considerations
of a chemical nature, may be thus stated : the atomic weight of an
element, is the number which represents how many times heavier
the smallest mass of that element capable of taking part in a
chemical change is, than the smallest weight of hydrogen which
can so function.

38 Introductory Outlines

The choice of hydrogen as the unit of atomic weights is a purely arbitrary
selection ; but since atomic weight values can only be determined relatively, it
becomes necessary to select some one element and to assign to its atom some
particular number to serve as a standard. As hydrogen is the lightest of all
elements, Dalton originally adopted it, and arbitrarily fixed unity as the
number which should stand for its atomic weight. The disadvantages of this
particular unit are twofold : in the first place the number of elements that form
hydrogen compounds that are suitable for atomic weight determinations is very
small, whereas nearly all the elements form convenient oxygen compounds, or
compounds with elements whose atomic weights with reference to oxygen are
accurately known, and in actual practice such compounds are almost always
made use of for such determinations. In the second place, the exact ratio of
the weights of an atom of hydrogen and oxygen is not known with certainty, so
that in calculating atomic weights that are determined with reference to oxygen,
possible errors may arise. The ratio Hydrogen : Oxygen is not exactly i : 16.
Various values have been obtained by different experimenters, and at the present
time i : 15.88 is accepted as more nearly the truth.

On account of the extreme difficulty of exactly determining this ratio,
chemists are now generally agreed in adopting as the unit in all exact determi-
nations of atomic weights a number which is -^th the weight of the atom of
oxygen : that is to say, the atomic weight of oxygen is in reality the standard,
and is fixed as 16, and the unit, instead of being the weight of one atom of
hydrogen, is T ^th of this number.

The effect of this change is only of importance in cases of chemical investiga-
tion where a high degree of exactitude is required ; for purposes of ordinary
analyses and chemical calculations the difference that it makes is practically nil.
Fixing the atomic weight of oxygen at 16 merely raises the atomic weight of
hydrogen from i to 1.008. As the use of small decimal fractions introduces
unnecessary complications which tend to obscure simple processes of reasoning,
the approximate atomic weights given in the third column of page 22 will be
employed for the most part in the following Introductory chapters.

The student will frequently meet with slight discrepancies between the
numbers given as the atomic weights of various elements by different writers.
Such discrepancies are often due to the fact that in some cases H = i is used
as the standard, and in others O = 16. For example, the atomic weight of
gold will be 195.7 in the first case, and 197.2 in the second; while with the
lighter metal aluminium the numbers will be 26.9 as against 27.1.

The discrepancy may also arise from the fact that the determination of
atomic weights by different experimenters often vary very considerably. With
a view to arrive at some uniformity, a conference of representative chemists
was held to consider the subject, and the atomic weights finally decided upon
by them were published under the title of International Atomic Weights. A
revised list of these weights is published annually in the Berichte, and in the
fourth column of the table on p. 22 will be found the latest values (1907).

2. Determination of Atomic Weights from Considerations
based upon Volumetric Relations. The Law of Gaseous
Volumes. In the year 1805 the fact was discovered by Gay-
Lussac and Humboldt, that when i litre of oxygen combines with

Atomic Weights 39

2 litres of hydrogen the vapour of water (or steam) which was
produced occupied 2 litres, the volumes in all cases being measured
under the same conditions of temperature and pressure.* This
fact led to the discovery of the simple relation existing between
the volumes of other reacting gases and the volume of the products :
thus it was found that

I vol. of hydrogen unites with I vol. of chlorine, and gives
2 vols. of hydrochloric acid.

1 vol. of hydrogen unites with I vol. of bromine vapour, and

gives 2 vols. of hydrobromic acid.

2 vols. of hydrogen unite with I vol. of oxygen, and give

2 vols. of steam.

2 vols. of carbon monoxide unite with I vol. of oxygen, and
give 2 vols. of carbon dioxide.

1 vol. of carbon monoxide unites with i vol. of chlorine, and

gives i vol. of phosgene gas.

In the same way with compounds that cannot be obtained by
the direct union of their constituent elements, it is found that on
being subjected to processes of decomposition similar simple
volumetric relations exist : thus by suitable methods of decom-
position

2 vols. of ammonia gas yield i vol. of nitrogen and 3 vols. of

hydrogen.
2 vols. of nitrous oxide yield 2 vols. of nitrogen and i vol. of

oxygen.
2 vols. of nitric oxide yield i vol. of nitrogen and I vol. of

oxygen.
I vol. of marsh gas yields 2 vols. of hydrogen and some solid

carbon, which cannot be volatilised, and therefore its

vapour volume is unknown.
i vol. of ethylene yields 2 vols. of hydrogen and solid carbon

as in the preceding.

The observations of these and similar facts gave rise to the law
of Gay-Lussac, and it will be seen that there is evidently a close
connection between the simple 'volumetric relations and those
existing between the multiple proportions by weight, in which one

* For the relations of gaseous volumes to temperature and pressure the
student is referred to chapter ix. , on the general properties of gases.

40 Introductory Outlines

element unites with another. For example, in the two oxides of
nitrogen the ratios of the two elements by weight are

Nitrous oxide . . . Nitrogen : oxygen = 28 : 16.
Nitric oxide . . . Nitrogen :oxygen=i4 : 16,

while the volumetric relation in which the two constituents are
present is

Nitrous oxide . . . Nitrogen : oxygen = 2 : I.

Nitric oxide . . . Nitrogen : oxygen =i : i.

In other words, there is twice as much nitrogen by weight in the
one compound as in the other, and there is twice as much nitrogen
by volume in the one as compared to the other. Moreover, if 14
and 1 6 respectively represent the relative weights of atoms of nitro-
gen and oxygen, then the numbers representing the relative
volumes in which these elements unite will also express the number
of atoms of each in the molecule.

The connection existing between the proportions in which
elements unite by weight, and by volume, was first explained by
the Italian physicist and chemist Avogadro, who in the year
1811 advanced the theory now recognised as a fundamental prin-
ciple, and known as Avogadro's hypothesis. This theory may be
thus stated : Equal volumes of all gases or vapours, under the
same conditions of temperature and pressure, contain an equal
number of molecules. If this be true, if there are the same
number of molecules in equal volumes of all gases, it must follow
that the ratio between the weights of equal volumes of any two
gases will be the same as that between the single molecules of the
particular gases. If a litre of oxygen be found to weigh sixteen
times as much as a litre of hydrogen (under like conditions of tem-
perature and pressure), inasmuch as there are the same number
of molecules in each, the oxygen molecule must be sixteen times
heavier than that of hydrogen ; and therefore by the comparatively
simple method of weighing equal volumes of different gases, it
becomes possible to arrive at the relative weights of their molecules.

The relative weights of equal volumes of gases and vapours, in
terms of a given unit, are known as their densities or specific
gravities. Sometimes densities are referred to air as the unit, but
more often hydrogen, as being the lightest gas, is taken as the
standard. Taking hydrogen as the unit, the density or specific
gravity of a gas is the weight of a given volume of it, as compared

Atomic Weights 41

with the weight of the same volume of hydrogen or in other
words, the ratio between the weight of a molecule of that gas and
a molecule of hydrogen. The ratio that exists between the weight
of a gaseous molecule and half the weight of a molecule of 'hydrogen,
chemists term the molecular weight of that gas ; hence it will be
obvious that the number which represents the molecular weight of
a gas is double that of its density or specific gravity.

If i litre of hydrogen and I litre of chlorine be caused to combine,
2 litres of gaseous hydrochloric acid are formed. As equal volumes
of all gases (under like conditions) contain the same number of
molecules, in the 2 litres of hydrochloric acid there must be twice
as many molecules of that compound as there were of hydrogen
molecules in the I litre, or of chlorine molecules in the other.
But each molecule of hydrochloric acid is composed of chlorine
and hydrogen (from other considerations one atom of each element),
therefore there must have been at least twice as many atoms
of hydrogen in the litre of that gas as there were molecules ;
and by the same reasoning, twice as many chlorine atoms in the
litre of chlorine as there were molecules : in other words, both
hydrogen and chlorine molecules consist of two atoms. The
molecular weight of hydrogen therefore is 2 ; that is, its molecule
is twice as heavy as its atom. The atom of hydrogen is the unit
to which molecular weights are referred, while the weight of the
molecule of hydrogen is taken as the standard of densities or
specific gravities.

In order, therefore, to find the molecular weight of any gas or
vapour, it is necessary to learn its density that is, to ascertain
how many times a given volume of it is heavier than the same
volume of hydrogen,* and to double the number so obtained.t

The following table gives the densities or specific gravities of all
the elements whose vapour densities have been determined. The
list includes all those elements which are gases at the ordinary
temperature, and those that can be vaporised under conditions

* Certain exceptions to this rule are discussed under the subject of Dissocia-
tion, chap. x. p. 88.

f The specific gravity of hydrogen, as compared with air taken as unity,
is 0.0695, or air is 14.3875 times heavier than hydrogen. If, therefore, it be
desired to find the molecular weight of a given gas, whose density as compared
with air is known, it is only necessary to multiply its density (air=i) by the
number 14.3875, which gives its density as compared with hydrogen, and then
to double the number so obtained.

42 Introductory Outlines

which render such determinations experimentally possible. (Hy-
drogen being taken as unity, the other numbers are the approxi-
mate values, which for purposes of discussion are more suitable
than figures that run to two or three decimal places.)

Hydrogen I

Helium ... 2

Neon . . . .10

Nitrogen . . .14

Oxygen . . .16

Fluorine . . 19

Argon . . .20

Sulphur . . -32

Chlorine . . . 35.5

Krypton . . -41

Selenium ... 79

Bromine ... 80

Iodine . . . 127

Sodium . . . 11.5

Potassium . . . 19.5

Zinc .... 32.5

Cadmium ... 56

Mercury . . . 100

Phosphorus . . 62
Arsenic . . .150

Xenon . . . 64.0

Let us now consider how the "knowledge of the relative weights
of gaseous molecules is utilised in assigning a particular number
as the atomic weight of an element.

The molecular weight of chlorine is 71. It has been shown that
the molecule certainly contains more than I atom, and probably 2,
in which case 35.5 would represent the relative weight of the
atom.

The compound hydrochloric acid has the molecular weight 36.5.
It has been already proved that this compound contains I atom of
hydrogen, therefore 36.5 I =35.5.

The compound carbon tetrachloride gives a molecular weight
154. Analysis shows that this compound contains 12 parts of
carbon in 154 parts, therefore 15412=142 = 35.5x4.

In these three molecules the weights of chlorine relative to the
weight of i atom of hydrogen are 142, 35.5, and 71, the greatest
common divisor of which is 35.5. This number, therefore, is
selected as the atomic weight of chlorine.

Again, it has been shown that by the action of metals upon
water, the hydrogen contained in the water could be expelled in two
separate portions, thus proving that there must be 2 atoms of
hydrogen in the molecule of that compound.

The molecular weight of water is found to be 1 8 ; deducting from
this the weight of the two hydrogen atoms we get 18 2 = 16.

The molecular weight of carbon monoxide is 28 ; 28 parts of
this compound contain 12 parts of carbon, therefore 28-12 = 16.

Atomic Weights 43

The molecular weight of carbon dioxide is 44 ; 44 parts of this
compound also contain 12 parts of carbon, therefore 44-12 = 32.

When i litre of oxygen combines with two litres of hydrogen,
2 litres of water vapour are formed ; there are therefore twice the
number of water molecules produced as there are oxygen mole-
cules (since by Avogadro's hypothesis 2 litres contain twice as many
molecules as i litre). But each water molecule contains certainly
i atom of oxygen, therefore the original oxygen molecules must
have consisted of not less than 2 atoms. When the density of
oxygen is determined it is found to be 16, its molecular weight
therefore is 32.

In these four various molecules the weights of oxygen relative to
the weight of I atom of hydrogen are 16, 16, 32, 32, the greatest
common divisor of which is 16. This number, therefore, is selected
as the atomic weight of oxygen.

Again, it has already been shown that in the compound ammonia,
the hydrogen can be removed in three separate moieties, proving
that there must be three atoms of that element in the molecule.
The molecular weight of ammonia is found to be 17, therefore
17 3 = 14, which is the weight of the nitrogen.

The molecular weight of nitrous oxide is 44 ; 44 parts of this
compound are found to contain 16 parts of oxygen and 28 parts of
nitrogen.

The molecular weight of nitric oxide is 30 ; 30 parts of this
compound contain 16 parts of oxygen and 14 parts of nitrogen.

The molecular weight of nitrogen is found to be 28.

In these four different molecules the weights of nitrogen relative
to the weight of i atom of hydrogen are 14, 28, 14, 28, the
greatest common divisor of which is 14. The atomic weight of
nitrogen, therefore, is regarded as 14.

These three examples, namely, chlorine, oxygen, and nitrogen
are instances of elements which are gaseous at ordinary tempera-
tures ; but the same methods are applicable in the case of the non-
volatile elements, such as carbon, provided they furnish a number
of compounds that are readily volatile.

On comparing the numbers in the foregoing table (p. 42),
representing the densities of various elements, with the atomic
weights of those elements as given on p. 22, it will be seen
that in several cases the numbers given are approximately
the same. This agreement is merely because the molecules
of these elements consist of two atoms. The molecules of

44 Introductory Outlines

helium, neon, argon, krypton, xenon, sodium, potassium, zinc,
cadmium, and mercury consist of only one atom ; their atomic
weights, therefore, will be the same as their molecular weights, that
is, twice their densities. The elements arsenic and phosphorus, on
the other hand, contain in their molecules four atoms that is to
say, the number which represents the smallest weight of phosphorus
and of arsenic, capable of taking part in a chemical change, is only
half the density, and therefore a fourth of the molecular weight.

The definition of atomic weight that is furnished by the con-
sideration of volumetric relations may be thus stated. The atomic
weight is the smallest weight of an element that is ever found in a
volume of any gas or vapour equal to the volume occupied by one
molecule of hydrogen at the same temperature and pressure.

The volume occupied by one molecule of hydrogen is regarded
as the standard molecular volume, while that occupied by an atom
of hydrogen or, in other words, the atomic volume of hydrogen is
called the unit volume. The standard molecular volume, therefore,
is said to be two unit volumes; and as, from Avogadro's law, all
gaseous molecules have the same volume, it follows that the mole-
cules of all gases and vapours occupy two unit volumes. Atomic
weight may therefore be defined as the smallest weight of an
element ever found in two unit volumes of any gas or vapour.

The molecular volume of a gas is its molecular weight divided
by its relative density, a ratio which in all cases will obviously
equal 2, that is, two unit volumes.

The atomic volume of an element in the state of vapour is its
atomic weight divided by its relative density. In the case of such
elements as chlorine, nitrogen, oxygen, &c., whose molecules are
diatomic, the quotient will be i that is to say, the atomic volume
of these elements is equal to I unit volume. In the case of mer-

atomic weight = 200

cury vapour, however, we have = : ' =2.

density =100

The atomic volume of mercury vapour, therefore, is equal to 2
unit volumes, and is identical with its molecular volume.

On the other hand, with the element phosphorus the atomic

atomic weight = 31
volume is j . = -5j o r one-half the unit volume,

and therefore one-fourth the molecular volume ; consequently, four
atoms exist in this molecule.

The method of determining atomic weights based upon volu-
metric relations, when taken by itself, is not an absolutely certain

Atomic Weights 45

criterion, for although the atomic weight of an element cannot be
greater than the smallest mass that enters into the composition of
the molecules of any of its known compounds, it might be less than
this, as there is always the possibility of a new compound being
discovered, in which the relative weight of an element is such as to
make it necessary to halve the previously accepted atomic weight.
3. Determination of Atomic Weight from the Specific
Heat of Elements in the Solid State. When equal weights of
different substances are heated through the same range of tempera-
ture, it is found that they absorb very different quantities of heat,
and on again cooling to the original temperature, they consequently
give out different amounts of heat. Thus, if I kilogramme of water,
and I kilogramme of mercury be each heated to a temperature of
100, and then each be poured into a separate kilogramme of water
at o, in the first case the resultant mixture will have a temperature
of 50, while in the second it will only reach the temperature of 3.2;
that is to say, while the water in cooling through 50 has raised the
temperature of an equal weight of water from o to 50, the amount
of heat in I kilogramme of mercury at 100 has only raised the
temperature of an equal weight of water from o to 3.2, and in so
doing has itself become lowered in temperature 100 - 3.2 = 96.8. The
amount of heat contained, therefore, in equal weights of water and of
mercury at the same temperature, as shown by these figures, is as

52. M.-J. i .
50 "96.8" '^'

therefore it requires 30 times as much heat to raise a given weight
of water through a given number of degrees as to raise an equal
weight of mercury through the same interval of temperature, or
the thermal capacity of mercury is 3*0 th that of water.

The specific heat of a substance is the ratio of its thermal
capacity to that of an equal weight of water ; or, the ratio between
the amount of heat necessary to raise a unit weight of the sub-
stance from o to i, and that required to raise the same weight
of water from o to i ; thus, the specific heat of mercury is 3^, or
0.033. Water is chosen as the standard of comparison because it
possesses the highest thermal capacity of all known substances ;
the numbers, therefore, which express the specific heats of other
substances are all less than unity.

Dulong and Petit were the first to draw attention (1819) to a
remarkable relation which exists between the specific heats and
the atomic weights of various solid elements, whose specific heats

46 Introductory Outlines

they themselves had determined. They found that the specific
heats of the solid elements were inversely as their atomic weights ;
that is to say, the capacity for heat of masses of the elements pro-
portional to their atomic weight was equal. This law, known as
the law of Dulong and Petit, may be thus stated : The thermal
capacities of atoms of all elements in the solid state are equal.

The thermal capacity of an atom is termed its atomic heat;
hence the law may be more briefly stated, all elements in the
solid state have the same atomic heat. This important constant
is the product of the atomic weight into the specific heat. From
the following table it will be seen that the number expressing
the atomic heat is not perfectly constant : the departures from the
mean . 6.4 are, as a rule, only slight, and may be attributed to
the fact that the determinations are not always made upon the
elements under conditions that are strictly comparable. At the
end of the table, however, there are certain elements which appear
to present marked exceptions to the law.

Element.

Specific
Heat.

Atomic
Weight.

/

]

Ltomic
Seat.

Lithium .

0.94

X

7

=

6.6

Sodium

0.29

X

23

=

6.7

Potassium

o.i 66

X

39

=

6. 5

Manganese

O.I 22

X

55

=

6. 7

112

X

56

_

6.3

Silver

0.057

X

J

1 08

=

J

6.1

Gold

0.032

X

196

=

6.2

Mercury (solid)

0.032

X

200

=

6.4

Lead

0.031

X

206.4

=

6.5

/'Beryllium

0.41

X

9-i

=

3-7

1 Boron (cryst.) .

0.25

X

ii

**

2.75

j Carbon (diamond) .

0.147

X

12

=

1.76

vSilicon (cryst).

0.177

X

28

=

4-95

It will be seen that, relatively speaking, the four elements
which show a considerable departure from the law of Dulong are
elements with low atomic weights. Low atomic weight, however,
is not always accompanied by such deviation, as is shown in the
case of lithium and sodium.

When the different allotropes of carbon are experimented upon,
it is found that the departure is not the same for each modification
of the element, thus

Atomic Weights 47

CM Specific Atomic Atomic

Heat. Weight. Heat.

Diamond . . . 0.147 x 12 = 1.76
Graphite . . . 0.200 x 12 = 2.40
Charcoal . . . 0.241 x 12 = 2.90

It has been observed that, as a general rule, the specific heat of
an element is slightly higher at higher temperatures ; but in the
case of the four elements showing abnormal atomic heats, this
increase rises rapidly with increased temperature, until a certain
point is reached, when it remains practically constant, and repre-
sents an atomic heat which closely approximates to the normal
value ; thus in the case of diamond, the specific heat at increasing
temperatures is

Specific Atomic Atomic
Heat. Weight. Heat.

Diamond at 10.7 . . . 0.1128 x 12 = 1.35

45 . . . 0.1470 x 12 = 1.76

206 . . . 0.2733 x 12 = 3.28

607 . . . 0.4408 x 12 = 5.30

806 . . . 0.4489 x 12 = 5.4

985 . . . 04589 X 12 = 5.5

The same result is seen in the case of graphite, and it is also to
be remarked, that while at low temperatures there exists a wide
difference between the specific heats of these two modifications of
carbon, this difference vanishes at a temperature of about 600.

Specific Atomic Atomic
Heat. Weight. Heat.

Graphite at 10.8 . . . 0.1604 x 12 = 1.93

61.3 . . . 0.1990 X 12 2.39

642 . . . 04454 x 12 = 5.35

978 . . . 04670 X 12 = 5.50

Both the elements boron and silicon are found to follow the
same rule, and at moderate temperatures their atomic heats nearly
approximate the normal constant.

The case of the somewhat rare element beryllium is of special
interest from another point of view, which will be referred to when
treating of the natural classification of the elements : from the
following numbers* it will be seen that its atomic heat very
rapidly rises with moderate increase of temperature.
* Humpidge.

48 Introductory Outlines

Specific Atomic Atomic
Heat. Weight. Heat.

Beryllium at 100 . . . 0.4702 x 9.1 = 4.28

200 . . . 0.5420 x 9.1 = 4.93

4o 0.6172 x 9.1 = 5.61

500 . . . 0.6206 x 9. i = 5.65

The relation between atomic weight and specific heat, established
by Dulong and Petit, is of service in the determination of atomic
weights, not as a method of ascertaining the exact value with any
degree of refinement, but rather as a means of deciding between
two numbers which are multiples of a common factor.

If specific heat x atomic weight = atomic heat, it will be obvious
that, if we experimentally determine the specific heat, and divide
that value into the constant atomic heat, 6.4, we obtain the
approximate atomic weight.

The two following examples will serve to illustrate the applica-
tion of the method.

The element indium combines with chlorine in the proportion

Indium : chlorine = 37.8 : 35.5.

If InCl is the formula, then 37.8 is the atomic weight of indium ;
but from the chemical similarity between indium and zinc (whose
chloride has the formula ZnCl 2 ), it was believed that the formula
for indium chloride was InCl 2 , in which case, in order to preserve
the ratio between the two elements, the atomic weight would have
to be 37.8 x 2 = 75.6.

When the specific heat of indium was determined,* it was found
to be 0.057.

Therefore the atomic weight must be raised by one-half, from
75.6 to 113.4, and the formula for the chloride will be InCl 3 .

The element thallium combines with chlorine in the proportion

Thallium : chlorine = 203.6 : 35.5.

In some of its compounds thallium exhibits a strong resemblance
to potassium, the chloride of which has the formula KC1. If the
formula for the thallium chloride is T1C1, the atomic weight of the
metal must be 203.6.

In many respects thallium exhibits a striking analogy with lead,
* Bunsen, 1870.

Atomic Weights 49

the chloride of which has the formula, PbCl 2 . If thallium chloride
has a corresponding formula, T1C1 2 , then the atomic weight of
thallium must be raised to 407.2.

When the specific heat of thallium was ascertained,* it was found
to be 0.0335.

6.4

This result shows that the number 203.6 and not 407.2 is the
atomic weight of thallium, and that the chloride has the formula
T1C1.

Molecular Heat of Compounds. The capacity for heat of an
atom undergoes no alteration when the atom enters into combina-
tion with different atoms in other words, the atomic heat of an
element is the same in its compounds. The molecular heat of a
compound (that is, the product of the molecular weight into the
specific heat) will therefore be the sum of the atomic heats of its
constituent elements. Hence it is possible to calculate what will
be the atomic heat of an element which does not exist as a solid
under ordinary conditions ; and therefore the atomic weight of
such an element, as deduced from other considerations, is capable
of verification, by determinations of the molecular heat of various
of its compounds : thus

The specific heat of silver chloride, AgCl, is 0.089 :

Specific Molecular Molecular

Heat. Weight. Heat.

0.089 x 143.5 = I2 '77-

The atomic heat of silver = 6.1, therefore, as deduced from this
compound, the atomic heat of chlorine is 12.77 6.1 = 6.6.

Again, the specific heat of stannous chloride, SnCl 2 , is 0.1016:

Specific Molecular Molecular

Heat. Weight. Heat.

0.1016 x 189 = 19.2.

The atomic heat of tin is 6.6, therefore the atomic heat of two
atoms of chlorine, as deduced from this compound, is 19.2 6.6 =
12.6, giving 6.3 as the atomic heat of chlorine.

The differences that appear in the value, as deduced from

various compounds, are lessened, because the errors of the

method are more equally distributed, if we divide the molecular

beat by the number of atoms in the molecule. Thus, in the

* Regnault.

50 Introductory Outlines

two examples quoted, silver chloride consists of two atoms, while
the molecule of stannous chloride contains three ; if, therefore, the
molecular heats of these two compounds are divided respectively
by 2 and by 3 we get

as the value representing the atomic heat of chlorine.

The element calcium combines with chlorine in the pioportion

Calcium : chlorine = 20 : 35.5.

If the atomic weight of calcium is 20, the formula will be CaCl,
whereas if 40 is the atomic weight of the metal, the compound
must be represented by the formula CaCl 2 .

The molecular weight of CaCl would be 55.5, that of CaCU 1 1 i.o.

When the specific heat of the compound was determined, it
was found to be 0.1642. In order, therefore, to decide between
the two values for the atomic weight of calcium, we calculate the
molecular heat from both of the molecular weights, and divide the
result by the number of atoms in the molecule in each case.

On the supposition that Ca = 2o, and that CaCl represents the
chloride :

Cad. .

Or, if Ca = 4o, and CaCl 2 is the formula for the chloride, then

~ ,-,, o.i642X 1 1 i.o

CaCl. . . - 3 - _

The number 6.07, which nearly agrees with the constant 6.4,
decides the value 40 as the atomic weight of calcium. The
element calcium is one of those metals which it is very difficult to
isolate and obtain in a state of purity, but when in recent years
the specific heat of this metal was experimentally determined,*
it was found to be 0.1704 :

0.1704x40 = 6.8.

Thus affording direct confirmation of the value 40 for the atomic
weight of calcium, which had been deduced from the molecular
heat of its compounds.

* Bun sen.

Atomic Weights 51

Deductions based upon molecular heats of compounds are only
trustworthy in the case of the most simply constituted compounds.

4. Determination of Atomic Weight from Considerations
based on Isomorphism. It was early observed that certain rela-
tions existed between the crystalline forms of compounds and their
chemical composition. Mitscherlich found that certain substances
having an analogous chemical composition, as, for example, sodium
phosphate and sodium arsenate, crystallised in the same geometric
form. In the year 1821 he stated his law of isomorphism as follows :
" The same number of atoms, combined in the same way, give rise
to the same crystalline form, which is independent of the chemical
nature of the atoms, being influenced only by their number and
mode of arrangement." Subsequent investigations, however, have
shown that this statement is too general.

In its broad sense as signifying the same crystalline form,
isomorphism is found to exist

1. Between compounds containing the same number of atoms
similarly combined, and which bear close chemical analogies to
each other.

c ( Zinc sulphate .... ZnSO 4 ,7H 2 O.

Isomorphous < . , , , , _ * __.f _

I Magnesium sulphate . . . MgSO 4 ,7H. 2 O.

, ( Hydrogen disodium phosphate . HNa 2 PO 4 ,12H 2 O.
13 ( Hydrogen disodium arsenate . HNa 2 AsO 4 ,12H 2 O.

/ Rubidium alum . . . . Rb 2 SO 4 ,Al 2 (SO 4 ) 3> 24H 2 O.

Isomorphous < Potass ! um chrome alum '. ' K 2 SO 4 ,Cr 2 (SO 4 ) 3 ,24H 2 O.
) Potassium aluminium selenium )

\ alum )

2. Between compounds containing a different number of atoms,
but which also bear close chemical analogies to one another.

, ( Ammonium chloride . . . NH 4 C1.
Isomorphous <

I Potassium chloride . . . KC1.

Isomorphous { Ammonium sulphate . . (NH 4 ) 2 SO 4 .
( Potassium sulphate . . . K 2 SO 4 .

3. Between compounds containing either the same or a different
number of atoms, and which exhibit little or no chemical analogies.

T , ( Sodium nitrate .... NaNO 3 .

Isomorphous < _ . . _ ^_ J

(. Calcium carbonate . . . CaCO 3 .

Isomorphous { Sodium sulphate (anhydrous) . Na 2 SO 4 .
I Barium permanganate . . BaMn 2 O fl .

52 Introductory Outlines

Isomorphism of this order, where little or no chemical relations
exist between the compounds, is sometimes distinguished as
isogonism. It must not be supposed, that because two chemically
analogous compounds contain the same number of atoms, they will
necessarily crystallise in the same form : there are indeed a large
number of similarly constituted analogous compounds that do not
exhibit isomorphism.

No simple definition of isomorphism is possible, but the following
test is generally accepted as a criterion, namely, the power to form
either mixed crystals or layer crystals. Thus, when two substances
are mixed in a state of liquidity, and allowed to crystallise, if the
crystals are perfectly homogeneous, they are known as mixed
crystals, and the substances are regarded as isomorphous.

Or when a crystal of one compound is placed in a solution of
another compound, and the crystal continues to grow regularly
in the liquid, the compounds are isomorphous. Thus, if a crystal
of potassium alum (white) be placed in a solution of manganese
alum, the crystal continues to grow without change of form, and
a layer of amethyst-coloured manganese alum is deposited upon it.

In making use of the law of isomorphism in the determination of
atomic weights, it is assumed that the weights of different atoms
that can mutually replace each other without altering the crystal-
line form are proportional to their atomic weights.*

Thus, if we suppose that, in the case of the sulphates of zinc
and magnesium, the atomic weight of zinc is known, viz., 65, and
that of magnesium is doubtful ; from the fact of the isomorphism
of the sulphates it may be premised that the elements are present in
proportions relative to their atomic weights. Analysis shows that
the proportion is 24 of magnesium to 65 of zinc, therefore 24 is pre-
sumably the atomic weight of magnesium.

In this way Berzelius corrected many of the atomic weights
which in his day had been assigned to the elements.

* The group (NH 4 ) may be regarded as an atom, having the relative weight 18.

CHAPTER VII
QUANTITATIVE CHEMICAL NOTATION

THE use of chemical symbols and formulae, as a convenient means
of representing concisely the qualitative nature of chemical changes,
has been explained in chapter iv. We are now in a position to
read into these symbols a quantitative significance, which at that
stage it would have been premature to explain.

The symbol of an element stands for an atom ; but, as we have
now learnt, the atoms of the various elements have different relative
weights, hence these symbols represent relative weights of matter.
The symbol Na signifies 23 relative parts by weight of sodium, O
stands for 16 relative parts by weight of oxygen, H for i part of
hydrogen ; in other words, the weight of sodium represented by
the symbol Na is 23 times as heavy as that which is conveyed
by a symbol H. A chemical equation, therefore, is a strictly
quantitative expression, in which certain definite weights of matter
are present in the form of the reacting substances, and which
reappear without loss or gain in the compounds resulting from the
change. In this sense a chemical equation is a mathematical
expression. Thus, the equation

Na + Cl = NaCl,

not only means that an atom of sodium combines with an atom of
chlorine and forms i molecule of sodium chloride, but it also means

23 + 35-5 = 58.5
Na Cl NaCl.

In other words, that sodium and chlorine unite in the relative pro-
portion of 23 parts of the former and 35.5 parts of chlorine, and
produce 58.5 parts of sodium chloride.

In the same way, into the equation which expresses the action of

53

54 Introductory Outlines

sulphuric acid upon sodium carbonate, we read the quantitative
meaning of the symbols

H 2 SO 4 + Na 2 CO 3 = Na 2 SO 4 + CO 2 + H 2 O.
2 46 46

32 12 32 12 2

64 48 64 32 16

98* + 106 = 142 + 44 + 1 8

That is to say, 98 parts by weight of sulphuric acid act upon
106 parts of sodium carbonate, producing 142 parts of sodium
sulphate, 44 parts of carbon dioxide, and 18 parts of water. It will
be evident that it becomes a matter of the simplest arithmetic to
calculate the weight of any product that can be obtained from a
given weight of the reacting substances ; or vice versa, to find
the weight of any reacting substance which would be required to
produce a given weight of the product of the action.

Not only is information respecting the quantitative relations
by weight embodied in a chemical equation, but when gaseous
substances are reacting, the equation also represents the volu-
metric relation between the gases. In order that the volumetric
relations may be more manifest, the equations expressing the re-
actions are written in such a manner as to represent the molecules
of the substances.

H + C1 = HC1

is an atomic equation, but as the molecule is the smallest particle
which can exist alone, a more exact statement of the chemical
change is made, by representing the action as taking place between
molecules, thus

H 2 + C1 2 = 2HCI.

From such an equation we see that i molecule of hydrogen, or
2 unit volumes, unites with i molecule or 2 unit volumes of chlorine,
and forms 2 molecules or 4 unit volumes of hydrochloric acid :
or again

2 + 2H 2 = 2H 2 0.

One molecule, or 2 unit volumes of oxygen, unite with 2 mole-
cules, or 4 unit volumes of hydrogen, and produce 2 molecules of

* The number obtained by adding together the weights of the atoms in a
formula is known as a " formula weight," thus 98 is the formula weight of
sulphuric acid.

Quantitative Notanon 55

water, which when vaporised, and measured under the same con-
ditions of temperature and pres