CHAPTER I_The Principles of Chemistry, Volume I

Note: Project Gutenberg also has an HTML version of this

file which includes the original illustrations.

See 51326-h.htm or 51326-h.zip:

(https://www.gutenberg.org/cache/epub/51326/pg51326-images.html)

(https://www.gutenberg.org/files/51326/51326-h.zip)

Images of the original pages are available through

Internet Archive. See

https://archive.org/details/cu31924012367441

Transcriber's note:

Text enclosed by underscores is in italics (_italics_).

A carat character is used to denote superscription. A

single character following the carat is superscripted

(example: 0·01717_t_^2). Multiple superscripted characters are

enclosed by curly brackets (example: V_{_0_}(1-K_t_)^{-1}).

The ligature oe has been transcribed as [oe].

The dagger sign has been transcribed as [+].

The infinity sign has been transcribed as [oo].

THE PRINCIPLES OF CHEMISTRY

D. MENDELÉEFF

Translated from the Russian (Sixth Edition) by George Kamensky, A.R.S.M.

of the Imperial Mint, St Petersburg: Member of the Russian

Physico-Chemical Society

Edited by T. A. Lawson, B.Sc. PH.D.

Examiner in Coal-Tar Products to the City and Guilds of London

Institute Fellow of the Institute of Chemistry

In Two Volumes

VOLUME I.

Longmans, Green, and Co

39 Paternoster Row, London

New York and Bombay

1897

PREFACE

TO THE

ENGLISH TRANSLATION

The first English edition of this work was published in 1891, and that a

second edition is now called for is, we think, a sufficient proof that

the enthusiasm of the author for his science, and the philosophical

method of his teaching, have been duly appreciated by English chemists.

In the scientific work to which Professor Mendeléeff's life has been

devoted, his continual endeavour has been to bring the scattered facts

of chemistry within the domain of law, and accordingly in his teaching

he endeavours to impress upon the student the _principles_ of the

science, the generalisations, so far as they have been discovered, under

which the facts naturally group themselves.

Of those generalisations the periodic law is perhaps the most important

that has been put forward since the establishment of the atomic theory.

It is therefore interesting to note that Professor Mendeléeff was led to

its discovery in preparing the first Russian edition of this book.

It is natural, too, that the further application and development of

that generalisation should be the principal feature of this, the latest

edition.

There are special difficulties in rendering the Russian language into

good English, and we are conscious that these have not been entirely

overcome. Doubtless also there are errors of statement which have

escaped correction, but we believe that the present edition will be

found better in both respects than its predecessor. We have thought

it our duty as translators to give as far as possible a faithful

reproduction of Professor Mendeléeff's work--the sixth Russian

edition--without amplifying or modifying his statements, and in this we

have the author's approval.

Although other duties have prevented Mr. Greenaway from undertaking the

care of the present edition, he has been kind enough to give us the

benefit of his suggestions on several points. We also wish to thank the

Managers of the Royal Institution for permission to reprint the lecture

delivered at the Royal Institution by Professor Mendeléeff (Appendix

I.), and to the Council of the Chemical Society for permission to

reprint the Faraday lecture which forms Appendix II.

In conclusion, we are indebted to Mr. F. Evershed, who has given us much

valuable assistance in revising the sheets for the press.

G. K.

T. A. L.

_August 1897_

AUTHOR'S PREFACE

THE SIXTH RUSSIAN EDITION

This work was written during the years 1868-1870, its object being to

acquaint the student not only with the methods of observation, the

experimental facts, and the laws of chemistry, but also with the insight

given by this science into the unchangeable substratum underlying the

varying forms of matter.

If statements of fact themselves depend upon the person who observes

them, how much more distinct is the reflection of the personality of him

who gives an account of methods and of philosophical speculations which

form the essence of science! For this reason there will inevitably be

much that is subjective in every objective exposition of science. And as

an individual production is only significant in virtue of that which has

preceded and that which is contemporary with it, it resembles a mirror

which in reflecting exaggerates the size and clearness of neighbouring

objects, and causes a person near it to see reflected most plainly those

objects which are on the side to which it is directed. Although I have

endeavoured to make my book a true mirror directed towards the whole

domain of chemical changes, yet involuntarily those influences near to

me have been the most clearly reflected, the most brightly illuminated,

and have tinted the entire work with their colouring. In this way the

chief peculiarity of the book has been determined. Experimental and

practical data occupy their place, but the philosophical principles of

our science form the chief theme of the work. In former times sciences,

like bridges, could only be built up by supporting them on a few broad

buttresses and long girders. In addition to the exposition of the

principles of chemistry, it has been my desire to show how science has

now been built up like a suspension bridge, supported by the united

strength of a number of slender, but firmly-fixed, chains, which

individually are of little strength, and has thus been carried over

difficulties which before appeared insuperable. In comparing the science

of the past, the present, and the future, in placing the particulars

of its restricted experiments side by side with its aspirations after

unbounded and infinite truth, and in restraining myself from yielding to

a bias towards the most attractive path, I have endeavoured to incite

in the reader a spirit of inquiry, which, dissatisfied with speculative

reasonings alone, should subject every idea to experiment, encourage the

habit of stubborn work, and excite a search for fresh chains of evidence

to complete the bridge over the bottomless unknown. History proves that

it is possible by this means to avoid two equally pernicious extremes,

the Utopian--a visionary contemplation which proceeds from a current of

thought only--and the stagnant realism which is content with bare facts.

Sciences like chemistry, which deal with ideas as well as with material

substances, and create a possibility of immediately verifying that

which has been or may be discovered or assumed, demonstrate at every

step that the work of the past has availed much, and that without it it

would be impossible to advance into the ocean of the unknown. They also

show the possibility of becoming acquainted with fresh portions of this

unknown, and compel us, while duly respecting the teachings of history,

to cast aside classical illusions, and to engage in a work which not

only gives mental satisfaction but is also practically useful to all our

fellow-creatures.[1]

[1] Chemistry, like every other science, is at once a means and an

end. It is a means of attaining certain practical results. Thus,

by its assistance, the obtaining of matter in its various forms

is facilitated; it shows new possibilities of availing ourselves

of the forces of nature, indicates the methods of preparing

many substances, points out their properties, &c. In this sense

chemistry is closely connected with the work of the manufacturer

and the artisan, its sphere is active, and is a means of promoting

general welfare. Besides this honourable vocation, chemistry

has another. With it, as with every other elaborated science,

there are many lofty aspirations, the contemplation of which

serves to inspire its workers and adherents. This contemplation

comprises not only the principal data of the science, but also the

generally-accepted deductions, and also hypotheses which refer

to phenomena as yet but imperfectly known. In this latter sense

scientific contemplation varies much with times and persons, it

bears the stamp of creative power, and embraces the highest forms

of scientific progress. In that pure enjoyment experienced on

approaching to the ideal, in that eagerness to draw aside the

veil from the hidden truth, and even in that discord which exists

between the various workers, we ought to see the surest pledges of

further scientific progress. Science thus advances, discovering

new truths, and at the same time obtaining practical results. The

edifice of science not only requires material, but also a plan,

and necessitates the work of preparing the materials, putting them

together, working out the plans and the symmetrical proportions of

the various parts. To conceive, understand, and grasp the whole

symmetry of the scientific edifice, including its unfinished

portions, is equivalent to tasting that enjoyment only conveyed by

the highest forms of beauty and truth. Without the material, the

plan alone is but a castle in the air--a mere possibility; whilst

the material without a plan is but useless matter. All depends

on the concordance of the materials with the plan and execution,

and the general harmony thereby attained. In the work of science,

the artisan, architect, and creator are very often one and the

same individual; but sometimes, as in other walks of life, there

is a difference between them; sometimes the plan is preconceived,

sometimes it follows the preparation and accumulation of the

raw material. Free access to the edifice of science is not only

allowed to those who devised the plan, worked out the detailed

drawings, prepared the materials, or piled up the brickwork, but

also to all those who are desirous of making a close acquaintance

with the plan, and wish to avoid dwelling in the vaults or in the

garrets where the useless lumber is stored.

Knowing how contented, free, and joyful is life in the realm of

science, one fervently wishes that many would enter its portals.

On this account many pages of this treatise are unwittingly

stamped with the earnest desire that the habits of chemical

contemplation which I have endeavoured to instil into the minds

of my readers will incite them to the further study of science.

Science will then flourish in them and by them, on a fuller

acquaintance not only with that little which is enclosed within

the narrow limits of my work, but with the further learning which

they must imbibe in order to make themselves masters of our

science and partakers in its further advancement.

Those who enlist in the cause of science have no reason to fear

when they remember the urgent need for practical workers in the

spheres of agriculture, arts, and manufacture. By summoning

adherents to the work of theoretical chemistry, I am confident

that I call them to a most useful labour, to the habit of dealing

correctly with nature and its laws, and to the possibility of

becoming truly practical men. In order to become actual chemists,

it is necessary for beginners to be well and closely acquainted

with three important branches of chemistry--analytical, organic,

and theoretical. That part of chemistry which is dealt with in

this treatise is only the groundwork of the edifice. For the

learning and development of chemistry in its truest and fullest

sense, beginners ought, in the first place, to turn their

attention to the practical work of analytical chemistry; in the

second place, to practical and theoretical acquaintance with some

special chemical question, studying the original treatises of the

investigators of the subject (at first, under the direction of

experienced teachers), because in working out particular facts the

faculty of judgment and of correct criticism becomes sharpened; in

the third place, to a knowledge of current scientific questions

through the special chemical journals and papers, and by

intercourse with other chemists. The time has come to turn aside

from visionary contemplation, from platonic aspirations, and from

classical verbosity, and to enter the regions of actual labour for

the common weal, to prove that the study of science is not only

air excellent education for youth, but that it instils the virtues

of industry and veracity, and creates solid national wealth,

material and mental, which without it would be unattainable.

Science, which deals with the infinite, is itself without bounds.

Thus the desire to direct those thirsting for truth to the pure source

of the science of the forces acting throughout nature forms the

first and most important aim of this book. The time has arrived when

a knowledge of physics and chemistry forms as important a part of

education as that of the classics did two centuries ago. In those days

the nations which excelled in classical learning stood foremost, just

as now the most advanced are those which are superior in the knowledge

of the natural sciences, for they form the strength and characteristic

of our times. In following the above and chief aim, I set myself a

second object: to furnish a text-book for an elementary knowledge of

chemistry and so satisfy a want which undoubtedly exists among students

and those who have recourse to chemistry either as a source of truth or

welfare.[2] Hence, although the fundamental object of this work was to

express and embrace the general chemical teaching of the present day

from a personal point of view, I have nevertheless striven throughout

to maintain such a level as would render the 'Principles of Chemistry'

accessible to the beginner. Many aspects of this work are determined

by this combination of requirements which frequently differ widely. An

issue was only possible under one condition, _i.e._ not to be carried

away by what appears to be a plausible theory in explaining individual

facts and to always endeavour to transmit the simple truth of a given

fact, extracting it from the vast store of the literature of the

subject and from tried personal experience. In publishing a new edition

of this work I have striven to add any facts of importance recently

discovered[3] and to revise the former edition in the above spirit. With

this object I have entirely gone over this edition, and a comparison

of it with the former one will show that the additions and alterations

have cost as much labour as many chapters of the work. I also wished

to show in an elementary treatise on chemistry the striking advantages

gained by the application of the periodic law, which I first saw in

its entirety in the year 1869 when I was engaged in writing the first

edition of this book, in which, indeed, the law was first enunciated.

At that time, however, this law was not established so firmly as now,

when so many of its consequences have been verified by the researches

of numerous chemists, and especially by Roscoe, Lecoq de Boisbaudran,

Nilson, Brauner, Thorpe, Carnelley, Laurie, Winkler, and others. The,

to me, unexpectedly rapid success with which the teaching of the

periodicity of the elements has spread in our science, and perhaps also,

the perseverance with which I collected in this work, and upon a new

plan, the most important data respecting the elements and their mutual

relations, explained sufficiently the fact that the former (5th, 1889)

edition of my work has been translated into English[4] and German[5] and

is being translated into French.[6] Deeply touched by the favourable

opinions expressed by English men of science upon my book, I ascribe

them chiefly to the periodic law placed at the basis of my treatise and

especially of the second part of the book, which contains a large amount

of data having a special and sometimes quite unexpected, bearing from

the point of view of this law. As the entire scheme of this work is

subordinated to the law of periodicity, which may be illustrated in a

tabular form by placing the elements in series, groups, and periods, two

such tables are given at the end of this preface.

[2] I recommend those who are commencing the study of chemistry with my

book _to first read only what is printed in the large type_,

because in that part I have endeavoured to concentrate all the

fundamental, indispensable knowledge required for that study.

In the footnotes, printed in small type (which should be read

only after the large text has been mastered), certain details

are discussed; they are either further examples, or debatable

questions on existing ideas which I thought useful to lay

before those entering into the sphere of science, or certain

historical and technical details which might be withdrawn from the

fundamental portion of the book. Without intending to attain in my

treatise to the completeness of a work of reference, I have still

endeavoured to express the principal developments of science as

they concern the chemical elements viewed in that aspect in which

they appeared to me after long continued study of the subject and

participation in the contemporary advance of knowledge.

I have also placed my personal views, suppositions, and arguments

in the footnotes, which are chiefly designed for details and

references. But I have endeavoured to avoid here, as in the

text, not only all that I consider doubtful, but also those

details which belong either to special branches of chemistry

(for instance, to analytical, organic, physical, theoretical,

physiological, agricultural, or technical chemistry), or to

different branches of natural science which are more and more

coming into closer and closer contact with chemistry. Chemistry, I

am convinced, must occupy a place among the natural sciences side

by side with mechanics; for mechanics treats of matter as a system

of ponderable points having scarcely any individuality and only

standing in a certain state of mobile equilibrium. For chemistry,

matter is an entire world of life, with an infinite variety of

individuality both in the elements and in their combinations.

In studying the general uniformity from a mechanical point of

view, I think that the highest point of knowledge of nature

cannot be attained without taking into account the individuality

of things in which chemistry is set to seek for general higher

laws. Mechanics may be likened to the science of statesmanship,

chemistry to the sciences of jurisprudence and sociology. The

general universe could not be built up without the particular

individual universe, and would be a dry abstract were it not

enlivened by the real variety of the individual world. Mechanics

forms the classical basis of natural philosophy, while chemistry,

as a comparatively new and still young science, already strives

to--and will, in the future introduce a new, living aspect into

the philosophy of nature; all the more as chemistry alone is never

at rest or anywhere dead--its vital action has universal sway, and

inevitably determines the general aspect of the universe. Just

as the microscope and telescope enlarge the scope of vision, and

discover life in seeming immobility, so chemistry, in discovering

and striving to discern the life of the invisible world of atoms

and molecules and their ultimate limit of divisibility, will

clearly introduce new and important problems into our conception

of nature. And I think that its _rôle_, which is now considerable,

will increase more and more in the future; that is, I think that

in its further development it will occupy a place side by side

with mechanics for the comprehension of the secrets of nature. But

here we require some second Newton; and I have no doubt that he

will soon appear.

[3] I was much helped in gathering data from the various chemical

journals of the last five years by the abstracts made for me by

Mr. Y. V. Kouriloff, to whom I tender my best thanks.

[4] The English translation was made by G. Kamensky, and edited by

A. J. Greenaway; published by Longmans, Green & Co.

[5] The German translation was made by L. Jawein and A. Thillot;

published by Ricker (St. Petersburg).

[6] The French translation has been commenced by E. Achkinasi and

H. Carrion from the fifth edition, and is published by Tignol

(Paris).

In this the sixth edition I have not altered any essential feature

of the original work, and have retained those alterations which were

introduced into the fifth edition.[7] I have, however, added many

newly discovered facts, and in this respect it is necessary to say a

few words. Although all aspects of the simplest chemical relations are

as far as possible equally developed in this book, yet on looking back

I see that I have, nevertheless, given most attention to the so-called

indefinite compounds examples of which may be seen in solutions. I recur

repeatedly to them, and to all the latest data respecting them, for

in them I see a starting point for the future progress of our science

and to them I affiliate numerous instances of definite compounds,

beginning with alloys and silicates and ending with complex acids.

There are two reasons for this. In the first place, this subject has

deeply interested me from my youth; I have devoted a portion of my

own researches to it, and therefore it occupied an important position

even in the first edition of my book; besides which all that has been

subsequently accomplished in our science, especially during the last

five or six years, shows that at the present day an interest in these

questions plays an important part in the minds of a large circle of

contemporary workers in chemistry. This personal attachment, if I may

so call it, to the question of solutions and such indefinite compounds,

must involuntarily have impressed itself upon my work, and in the later

editions I have even had to strive not to give this subject a greater

development than previously, so great was the material accumulated,

which however does not yet give us the right to consider even the most

elementary questions respecting solutions as solved. Thus, we cannot

yet say what a solution really is. My own view is that a solution is

a homogeneous liquid system of unstable dissociating compounds of

the solvent with the substance dissolved. But although such a theory

explains much to me, I cannot consider my opinion as proved, and

therefore give it with some reserve as one of several hypotheses.[8] As

a subject yet far from solved, I might naturally have ignored it, or

only mentioned it cursorily, but such a treatment of solutions, although

usual in elementary treatises on chemistry, would not have answered my

views upon the subject of our science, and I wished that the reader

might find in my book beyond everything an expression of all that a

study of the subject built up for me. If in solutions I see and can

frequently prove distinct evidences of the existence of those definite

compounds which form the more generalised province of chemical data, I

could not refrain from going into certain details respecting solutions;

otherwise, there would have remained no trace of that general idea, that

in them we have only a certain instance of ordinary definite or atomic

compounds, subject to Dalton's laws. Having long had this idea, I wished

to impress it upon the reader of my book, and it is this desire which

forms the second of those chief reasons why I recur so frequently to

solutions in this work. At present, my ideas respecting solutions are

shared by few, but I trust that by degrees the instances I give will

pave the way for their general recognition, and it is my hope that they

may find adherents among those of my readers who are in a position to

work out by experiment this difficult but highly interesting problem.

[7] The fifth edition was not only considerably enlarged, compared with

the preceding, but also the foundations of the periodic system

of the elements were placed far more firmly in it than in the

former editions. The subject-matter was also divided into

text and footnotes, which contained details unnecessary for a

first acquaintance with chemistry. The fifth edition sold out

sooner than I expected, so that instead of issuing supplements

(containing the latest discoveries in chemistry), as I had

proposed, I was obliged to publish the present entirely new

edition of the work.

[8] This hypothesis is not only mentioned in different parts of this

book, but is partly (from the aspect of the specific gravity of

solutions) developed in my work, _The Investigation of Solutions

from their Specific Gravity_, 1887.

In conclusion, I desire to record my thanks to V. D. Sapogenikoff, who

has corrected the proofs of the whole of this edition and compiled the

indexes which greatly facilitate the search for those details which are

scattered throughout the work.

D. MENDELÉEFF.

TABLE I

_Distribution of the Elements in Groups and Series_

+--------+-------+----------+----------+----------+----------+

| Group | I. | II. | III. | IV. | V. |

+--------+-------+----------+----------+----------+----------+

|Series 1| H | -- | -- | -- | -- |

| | | | | | |

| " 2| Li | Be | B | C | N |

| | | | | | |

| " 3| Na | Mg | Al | Si | P |

| | | | | | |

| " 4| K | Ca | Sc | Ti | V |

| | | | | | |

| " 5| (Cu)| Zn | Ga | Ge | As |

| | | | | | |

| " 6| Rb | Sr | Y | Zr | Nb |

| | | | | | |

| " 7| (Ag)| Cd | In | Sn | Sb |

| | | | | | |

| " 8| Cs | Ba | La | Ce | Di? |

| | | | | | |

| " 9| -- | -- | -- | -- | -- |

| | | | | | |

| " 10| -- | -- | Yb | -- | Ta |

| | | | | | |

| " 11| (Au)| Hg | Tl | Pb | Bi |

| | | | | | |

| " 12| -- | -- | -- | Th | -- |

| | | | | | |

+--------+-------+----------+----------+----------+----------+

| |R_{2}O |R_{2}O_{2}|R_{2}O_{3}|R_{2}O_{4}|R_{2}O_{5}|

| | | | | | |

| | -- |RO | -- |RO_{2} | -- |

| | | | | | |

| | -- | -- | -- |RH_{4} |RH_{3} |

| | | | | | |

+--------+-------+----------+----------+----------+----------+

+---------+----------+----------+--------------------+

| Group | VI. | VII. | VIII. |

+---------+----------+----------+--------------------+

| Series 1| -- | -- | |

| | | | |

| " 2| O | F | |

| | | | |

| " 3| S | Cl | |

| | | | |

| " 4| Cr | Mn | Fe Co Ni Cu |

| | | | |

| " 5| Se | Br | |

| | | | |

| " 6| Mo | -- | Ru Rh Pd Ag |

| | | | |

| " 7| Te | I | |

| | | | |

| " 8| -- | -- | -- -- -- -- |

| | | | |

| " 9| -- | -- | |

| | | | |

| " 10| W | -- | Os Ir Pt Au |

| | | | |

| " 11| -- | -- | |

| | | | |

| " 12| U | -- | |

| | | | |

+---------+----------+----------+--------------------+

| |R_{2}O_{6}|R_{2}O_{7}| Higher oxides |

| | | | |

| |RO_{3} | -- | RO_{4} |

| | | | |

| |RH_{2} | RH | Hydrogen compounds |

+---------+----------+----------+--------------------+

TABLE II

_Periodic System and Atomic Weights of the Elements_

(_Giving the pages on which they are described_)

+---------------------+--------------+--------------+--------------+

| | Typical | 4th | 6th |

+-------+-------------+--------------+--------------+--------------+

| I. | | Li 7 | K 39 | Rb 86 |

| | | | ==== | |

| | | | | |

| II. | | Be 9 | Ca 40 | Sr 88 |

| | | | ----- | |

| | | | | |

| III. | | B 11 | Sc 44 | Y 89 |

| | | ---- | | |

| | | | | |

| IV. | | C 12 | Ti 48 | Zr 91 |

| | | ==== | | |

| | | | | |

| V. | | N 14 | V 51 | Nb 94 |

| | | ==== | | |

| | | | | |

| VI. | | O 16 | Cr 52 | Mo 96 |

| | | ---- | ----- | |

| | | | | |

| VII. | | F 19 | Mn 55 | ? 99 |

| | | | ===== | |

| | | | | |

| | | | Fe 56 | Ru 102 |

| | | | ===== | |

| | | | | |

| VIII. | | | Co 59 | Rh 103 |

| | | | | |

| | | | Ni 59·5 | Pd 106 |

| | | | | |

| | | | | |

| I. | H 1 | Na 23 | Cu 64 | Ag 108 |

| | ---- | | | |

| | | | | |

| II. | | Mg 24 | Zn 65 | Cd 112 |

| | | | | |

| III. | | Al 27 | Ga 70 | In 114 |

| | | | | |

| IV. | | Si 28 | Ge 72 | Sn 119 |

| | | | | |

| V. | | P 31 | As 75 | Sb 120 |

| | | | ----- | |

| | | | | |

| VI. | | S 32 | Se 79 | Te 125 |

| | | | | |

| VII. | | Cl 35·5 | Br 80 | I 127 |

| | | | ----- | |

+-------+-------------+--------------+--------------+--------------+

+-------+-------------+--------------+--------------+

| | | | |

| | 8th | 10th | 12th |

+---- --+-------------+--------------+--------------+

| I. | Cs 133 | -- | -- |

| | | | |

| II. | Ba 137 | -- | -- |

| | ------ | | |

| | | | |

| III. | La 138 | Yb 173 | -- |

| | | | |

| IV. | Ce 140 | ? 178 | Th 232 |

| | | | |

| V. | ? Di 142 | Ta 183 | -- |

| | | | |

| VI. | -- | W 184 | U 239 |

| | | | |

| VII. | -- | -- | -- |

| | | | |

| | -- | Os 192 | |

| | | | |

| VIII. | -- | Ir 193 | |

| | | | |

| | -- | Pt 196 | |

| | | ------ | |

| | | | |

| | 9th | 11th | |

| | | | |

| I. | -- | Au 197 | |

| | | | |

| II. | -- | Hg 200 | |

| | | | |

| III. | -- | Tl 204 | |

| | | | |

| IV. | -- | Pb 207 | |

| | | | |

| V. | -- | Bi 209 | |

| | | | |

| VI. | -- | -- | |

| | | | |

| VII. | -- | -- | |

+-------+-------------+--------------+--------------+

_Note._--Two lines under the elements indicate those which are very

widely distributed in nature; one line indicates those which, although

not so frequently met with, are of general use in the arts and

manufactures.

CONTENTS

THE FIRST VOLUME

PAGE

TRANSLATORS' PREFACE v

AUTHOR'S PREFACE TO THE SIXTH RUSSIAN EDITION vii

TABLE OF THE DISTRIBUTION OF THE ELEMENTS IN GROUPS AND SERIES xv

TABLE OF THE PERIODIC SYSTEM AND ATOMIC WEIGHTS OF THE ELEMENTS xvi

INTRODUCTION 1

CHAP.

I. ON WATER AND ITS COMPOUNDS 40

II. THE COMPOSITION OF WATER. HYDROGEN 113

III. OXYGEN AND THE CHIEF ASPECTS OF ITS SALINE COMBINATIONS 152

IV. OZONE AND HYDROGEN PEROXIDE. DALTON'S LAW 198

V. NITROGEN AND AIR 223

VI. THE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN 246

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

AVOGADRO-GERHARDT 299

VIII. CARBON AND THE HYDROCARBONS 338

IX. COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 379

X. SODIUM CHLORIDE. BERTHOLLET'S LAWS. HYDROCHLORIC ACID 417

XI. THE HALOGENS: CHLORINE, BROMINE, IODINE AND FLUORINE 459

XII. SODIUM 513

XIII. POTASSIUM, RUBIDIUM, CÆSIUM AND LITHIUM. SPECTRUM ANALYSIS 543

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

CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM 581

PRINCIPLES OF CHEMISTRY

INTRODUCTION

The study of natural science, whose rapid development dates from

the days of Galileo ([+]1642) and Newton ([+]1727), and its closer

application to the external universe[1] led to the separation of

Chemistry as a particular branch of natural philosophy, not only owing

to the increasing store of observations and experiments relating to the

mutual transformations of substances, but also, and more especially,

because in addition to gravity, cohesion, heat, light and electricity

it became necessary to recognise the existence of particular internal

forces in the ultimate parts of all substances, forces which make

themselves manifest in the transformations of substances into one

another, but remain hidden (latent) under ordinary circumstances, and

whose existence cannot therefore be directly apprehended, and so for

a long time remained unrecognised. The primary object of chemistry is

the study of the homogeneous substances[2] of which all the objects of

the universe are made up, with the transformations of these substances

into each other, and with the phenomena[3] which accompany such

transformations. Every chemical change or reaction,[4] as it is called,

can only take place under a condition of most intimate and close contact

of the re-acting substances,[5] and is determined by the forces proper

to the smallest invisible particles (molecules) of matter. We must

distinguish three chief classes of chemical transformations.

[1] The investigation of a substance or a natural phenomenon consists

(_a_) in determining the relation of the object under examination

to that which is already known, either from previous researches,

or from experiment, or from the knowledge of the common

surroundings of life--that is, in determining and expressing the

quality of the unknown by the aid of that which is known; (_b_)

in measuring all that which can be subjected to measurement,

and thereby denoting the quantitative relation of that under

investigation to that already known and its relation to the

categories of time, space, temperature, mass, &c.; (_c_) in

determining the position held by the object under investigation

in the system of known objects guided by both qualitative and

quantitative data; (_d_) in determining, from the quantities which

have been measured, the empirical (visible) dependence (function,

or 'law,' as it is sometimes termed) of variable factors--for

instance, the dependence of the composition of the substance on

its properties, of temperature on time, of time on locality, &c.;

(_e_) in framing hypotheses or propositions as to the actual cause

and true nature of the relation between that studied (measured

or observed) and that which is known or the categories of time,

space, &c.; (_f_) in verifying the logical consequences of the

hypotheses by experiment; and (_g_) in advancing a theory which

shall account for the nature of the properties of that studied in

its relations with things already known and with those conditions

or categories among which it exists. It is certain that it is

only possible to carry out these investigations when we have

taken as a basis some incontestable fact which is self-evident

to our understanding; as, for instance, number, time, space,

motion, or mass. The determination of such primary or fundamental

conceptions, although not excluded from the possibility of

investigation, frequently does not subject itself to our present

mode of scientific generalisation. Hence it follows that in the

investigation of anything, there always remains something which is

accepted without investigation, or admitted as a known factor. The

axioms of geometry may be taken as an example. Thus in the science

of biology it is necessary to admit the faculty of organisms for

multiplying themselves, as a conception whose meaning is as yet

unknown. In the study of chemistry, too, the notion of elements

must be accepted almost without any further analysis. However, by

first investigating that which is visible and subject to direct

observation by the organs of the senses, we may hope that in

the first place hypotheses will be arrived at, and afterwards

theories of that which has now to be placed at the basis of our

investigations. The minds of the ancients strove to seize at once

the very fundamental categories of investigation, whilst all the

successes of recent knowledge are based on the above-cited method

of investigation without the determination of 'the beginning of

all beginnings.' By following this inductive method, the _exact

sciences_ have already succeeded in becoming accurately acquainted

with much of the invisible world, which directly is imperceptible

to the organs of sense (for example, the molecular motion of all

bodies, the composition of the heavenly luminaries, the paths

of their motion, the necessity for the existence of substances

which cannot be subjected to experiment, &c.), and have verified

the knowledge thus obtained, and employed it for increasing the

interests of humanity. It may therefore be safely said that _the

inductive method of investigation_ is a more perfect mode of

acquiring knowledge than the deductive method alone (starting from

a little of the unknown accepted as incontestable to arrive at the

much which is visible and observable) by which the ancients strove

to embrace the universe. By investigating the universe by an

inductive method (endeavouring from the much which is observable

to arrive at a little which may be verified and is indubitable)

the new science refuses to recognise dogma as truth, but through

_reason_, by a slow and laborious method of investigation, strives

for and attains to true deductions.

[2] A substance or material is that which occupies space and has

weight; that is, which presents a mass attracted by the earth and

by other masses of material, and of which the _objects_ of nature

are composed, and by means of which the motions and _phenomena_

of nature are accomplished. It is easy to discover by examining

and investigating, by various methods, the objects met with

in nature and in the arts, that some of them are homogeneous,

whilst others are composed of a mixture of several homogeneous

substances. This is most clearly apparent in solid substances.

The metals used in the arts (for example, gold, iron, copper)

must be homogeneous, otherwise they are brittle and unfit for

many purposes. Homogeneous matter exhibits similar properties in

all its parts. By breaking up a homogeneous substance we obtain

parts which, although different in form, resemble each other in

their properties. Glass, pure sugar, marble, &c., are examples of

homogeneous substances. Examples of non-homogeneous substances

are, however, much more frequent in nature and the arts. Thus

the majority of the rocks are not homogeneous. In porphyries

bright pieces of a mineral called 'orthoclase' are often seen

interspersed amongst the dark mass of the rock. In ordinary red

granite it is easy to distinguish large pieces of orthoclase mixed

with dark semi-transparent quartz and flexible laminæ of mica.

Similarly, plants and animals are non-homogeneous. Thus, leaves

are composed of a skin, fibre, pulp, sap, and a green colouring

matter. As an example of those non-homogeneous substances which

are produced artificially, gunpowder may be cited, which is

prepared by mixing together known proportions of sulphur, nitre,

and charcoal. Many liquids, also, are not homogeneous, as may be

observed by the aid of the microscope, when drops of blood are

seen to consist of a colourless liquid in which red corpuscles,

invisible to the naked eye owing to their small size, are floating

about. It is these corpuscles which give blood its peculiar

colour. Milk is also a transparent liquid, in which microscopical

drops of fat are floating, which rise to the top when milk is

left at rest, forming cream. It is possible to extract from every

non-homogeneous substance those homogeneous substances of which

it is made up. Thus orthoclase may he separated from porphyry by

breaking it off. So also gold is extracted from auriferous sand by

washing away the mixture of clay and sand. Chemistry deals only

with the homogeneous substances met with in nature, or extracted

from natural or artificial non-homogeneous substances. The various

mixtures found in nature form the subjects of other natural

sciences--as geognosy, botany, zoology, anatomy, &c.

[3] All those events which are accomplished by substances in time are

termed 'phenomena.' Phenomena in themselves form the fundamental

subject of the study of physics. Motion is the primary and

most generally understood form of phenomenon, and therefore we

endeavour to reason about other phenomena as clearly as when

dealing with motion. For this reason mechanics, which treats of

motion, forms the fundamental science of natural philosophy, and

all other sciences endeavour to reduce the phenomena with which

they are concerned to mechanical principles. Astronomy was the

first to take to this path of reasoning, and succeeded in many

cases in reducing astronomical to purely mechanical phenomena.

Chemistry and physics, physiology and biology are proceeding in

the same direction. One of the most important questions of all

natural science, and one which has been handed down from the

philosophers of classic times, is, whether the comprehension of

all that is visible can be reduced to motion? Its participation

in all, from the 'fixed' stars to the most minute parts of the

coldest bodies (Dewar, in 1894 showed that many substances cooled

to -180° fluoresce more strongly than at the ordinary temperature;

_i.e._ that there is a motion in them which produces light) must

now be recognised as undoubtable from direct experiment and

observation, but it does not follow from this that by motion alone

can all be explained. This follows, however, from the fact that

we cannot apprehend motion otherwise than by recognising matter

in a state of motion. If light and electricity be understood as

particular forms of motion, then we must inevitably recognise

the existence of a peculiar luminiferous (universal) ether as

a material, transmitting this form of motion. And so, under

the present state of knowledge, it is inevitably necessary to

recognise the particular categories, motion and matter, and as

chemistry is more closely concerned with the various forms of the

latter, it should, together with mechanics or the study of motion,

lie at the basis of natural science.

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

[5] If a phenomenon proceeds at visible or measurable distances (as,

for instance, magnetic attraction or gravity), it cannot be

described as chemical, since these phenomena only take place at

distances immeasurably small and undistinguishable to the eye or

the microscope; that is to say, they are purely molecular.

1. _Combination_ is a reaction in which the union of two substances

yields a new one, or in general terms, from a given number of

substances, a lesser number is obtained. Thus, by heating a mixture of

iron and sulphur[6] a single new substance is produced, iron sulphide,

in which the constituent substances cannot be distinguished even by

the highest magnifying power. Before the reaction, the iron could be

separated from the mixture by a magnet, and the sulphur by dissolving it

in certain oily liquids;[7] in general, before combination they might

be mechanically separated from each other, but after combination both

substances penetrate into each other, and are then neither mechanically

separable nor individually distinguishable. As a rule, reactions of

direct combination are accompanied by an evolution of heat, and the

common case of combustion, evolving heat, consists in the combination

of combustible substances with a portion (oxygen) of the atmosphere,

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

combination.

2. Reactions of _decomposition_ are cases the reverse of those of

combination, that is, in which one substance gives two--or, in general,

a given number of substances a greater number. Thus, by heating wood

(and also coal and many animal or vegetable substances) without access

to air, a combustible gas, a watery liquid, tar, and carbon are

obtained. It is in this way that tar, illuminating gas, and charcoal are

prepared on a large scale.[8] All limestones, for example, flagstones,

chalk, or marble, are decomposed by heating to redness into lime and a

peculiar gas called carbonic anhydride. A similar decomposition, taking

place, however, at a much lower temperature, proceeds with the green

copper carbonate which is contained in natural malachite. This example

will be studied more in detail presently. Whilst heat is evolved in the

ordinary reactions of combination, it is, on the contrary, absorbed in

the reactions of decomposition.

3. The third class of chemical reactions--where the number of re-acting

substances is equal to the number of substances formed--may be

considered as a simultaneous decomposition and combination. If, for

instance, two compounds A and B are taken and they react on each other

to form the substances C and D, then supposing that A is decomposed

into D and E, and that E combines with B to form C, we have a reaction

in which two substances A, or D E, and B were taken and two others

C, or E B, and D were produced. Such reactions ought to be placed

under the general term of reactions of '_rearrangement_,' and the

particular case where two substances give two fresh ones, reactions of

'_substitution_.'[9] Thus, if a piece of iron be immersed in a solution

of blue vitriol (copper sulphate), copper is formed--or, rather,

separated out, and green vitriol (iron sulphate, which only differs

from the blue vitriol in that the iron has replaced the copper) is

obtained in solution. In this manner iron may be coated with copper, so

also copper with silver; such reactions are frequently made use of in

practice.

[6] For this purpose a piece of iron may be made red hot in a forge,

and then placed in contact with a lump of sulphur, when iron

sulphide will be obtained as a molten liquid, the combination

being accompanied by a visible increase in the glow of the iron.

Or else iron filings are mixed with powdered sulphur in the

proportion of 5 parts of iron to 3 parts of sulphur, and the

mixture placed in a glass tube, which is then heated in one place.

Combination does not commence without the aid of external heat,

but when once started in any portion of the mixture it extends

throughout the entire mass, because the portion first heated

evolves sufficient heat in forming iron sulphide to raise the

adjacent parts of the mixture to the temperature required for

starting the reaction. The rise in temperature thus produced is so

high as to soften the glass tube.

[7] Sulphur is slightly soluble in many thin oils; it is very soluble

in carbon bisulphide and in some other liquids. Iron is insoluble

in carbon bisulphide, and the sulphur therefore can be dissolved

away from the iron.

[8] Decomposition of this kind is termed 'dry distillation,' because,

as in distillation, the substance is heated and vapours are

given off which, on cooling, condense into liquids. In general,

decomposition, in absorbing heat, presents much in common to a

physical change of state--such as, for example, that of a liquid

into a gas. Deville likened complete decomposition to boiling, and

compared partial decomposition, when a portion of a substance is

not decomposed in the presence of its products of decomposition

(or dissociation), to evaporation.

[9] A reaction of rearrangement may in certain cases take place with

one substance only; that is to say, a substance may by itself

change into a new isomeric form. Thus, for example, if hard yellow

sulphur be heated to a temperature of 250° and then poured into

cold water it gives, on cooling, a soft, brown variety. Ordinary

phosphorus, which is transparent, poisonous, and phosphorescent

in the dark (in the air), gives, after being heated at 270° (in

an atmosphere incapable of supporting combustion, such as steam),

an opaque, red, and non-poisonous isomeric variety, which is not

phosphorescent. Cases of isomerism point out the possibility of an

internal rearrangement in a substance, and are the result of an

alteration in the grouping of the same elements, just as a certain

number of balls may be grouped in figures and forms of different

shapes.

The majority of the chemical changes which occur in nature and

are made use of technically are very complicated, as they consist

of an association of many separate and simultaneous combinations,

decompositions, and replacements. It is chiefly due to this natural

complexity of chemical phenomena that for so many centuries chemistry

did not exist as an exact science; that is so say, that although many

chemical changes were known and made use of,[10] yet their real nature

was unknown, nor could they be predicted or directed at will. Another

reason for the tardy progress of chemical knowledge is the participation

of gaseous substances, especially air, in many reactions. The true

comprehension of air as a ponderable substance, and of gases in general

as peculiar elastic and dispersive states of matter, was only arrived

at in the sixteenth and seventeenth centuries, and it was only after

this that the transformations of substances could form a science. Up

to that time, without understanding the invisible and yet ponderable

gaseous and vaporous states of substances, it was impossible to obtain

any fundamental chemical evidence, because gases escaped from notice

between the reacting and resultant substances. It is easy from the

impression conveyed to us by the phenomena we observe to form the

opinion that matter is created and destroyed: a whole mass of trees

burn, and there only remains a little charcoal and ash, whilst from

one small seed there grows little by little a majestic tree. In one

case matter seems to be destroyed, and in the other to be created. This

conclusion is arrived at because the formation or consumption of gases,

being under the circumstances invisible to the eye, is not observed.

When wood burns it undergoes a chemical change into gaseous products,

which escape as smoke. A very simple experiment will prove this. By

collecting the smoke it may be observed that it contains gases which

differ entirely from air, being incapable of supporting combustion or

respiration. These gases may be weighed, and it will then be seen that

their weight exceeds that of the wood taken. This increase in weight

arises from the fact that, in burning, the component parts of the wood

combine with a portion of the air; in like manner iron increases in

weight by rusting. In burning gunpowder its substance is not destroyed,

but only converted into gases and smoke. So also in the growth of a

tree; the seed does not increase in mass of itself and from itself, but

grows because it absorbs gases from the atmosphere and sucks water and

substances dissolved therein from the earth through its roots. The sap

and solid substances which give plants their form are produced from

these absorbed gases and liquids by complicated chemical processes. The

gases and liquids are converted into solid substances by the plants

themselves. Plants not only do not increase in size, but die, in a gas

which does not contain the constituents of air. When moist substances

dry they decrease in weight; when water evaporates we know that it

does not disappear, but will return from the atmosphere as rain, dew,

and snow. When water is absorbed by the earth, it does not disappear

there for ever, but accumulates somewhere underground, from whence it

afterwards flows forth as a spring. Thus matter does not disappear

and is not created, but only undergoes various physical and chemical

transformations--that is to say, changes its locality and form. Matter

remains on the earth in the same quantity as before; in a word it is, so

far as we are concerned, everlasting. It was difficult to submit this

simple and primary truth of chemistry to investigation, but when once

made clear it rapidly spread, and now seems as natural and simple as

many truths which have been acknowledged for ages. Mariotte and other

savants of the seventeenth century already suspected the existence of

the law of the indestructibility of matter, but they made no efforts

to express it or to apply it to the requirements of science. The

experiments by means of which this simple law was arrived at were

made during the latter half of the last century by the founder of

modern chemistry, LAVOISIER, the French Academician and tax farmer. The

numerous experiments of this savant were conducted with the aid of the

balance, which is the only means of directly and accurately determining

the quantity of matter.

[10] Thus the ancients knew how to convert the juice of grapes

containing the saccharine principle (glucose) into wine or

vinegar, how to extract metals from the ores which are found

in the earth's crust, and how to prepare glass from earthy

substances.

Lavoisier found, by weighing all the substances, and even the apparatus,

used in every experiment, and then weighing the substances obtained

after the chemical change, that the sum of the weights of the substances

formed was always equal to the sum of the weights of the substances

taken; or, in other words: MATTER IS NOT CREATED AND DOES NOT DISAPPEAR,

or that, _matter is everlasting_. This expression naturally includes

a hypothesis, but our only aim in using it is to concisely express

the following lengthy period--That in all experiments, and in all the

investigated phenomena of nature, it has never been observed that the

weight of the substances formed was less or greater (as far as accuracy

of weighing permits[11]) than the weight of the substances originally

taken, and as weight is proportional to mass[11 bis] or quantity of

matter, it follows that no one has ever succeeded in observing a

disappearance of matter or its appearance in fresh quantities. The law

of the indestructibility of matter endows all chemical investigations

with exactitude, as, on its basis, an equation may be formed for every

chemical reaction. If in any reaction the weights of the substances

taken be designated by the letters A, B, C, &c., and the weights of the

substances formed by the letters M, N, O, &c., then

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

Therefore, should the weight of one of the re-acting or resultant

substances be unknown, it may be determined by solving the equation.

The chemist, in applying the law of the indestructibility of matter,

and in making use of the chemical balance, must never lose sight of any

one of the re-acting or resultant substances. Should such an over-sight

be made, it will at once be remarked that the sum of the weights of the

substances taken is unequal to the sum of the weights of the substances

formed. All the progress made by chemistry during the end of the last,

and in the present, century is entirely and immovably founded on the

law of the indestructibility of matter. It is absolutely necessary in

beginning the study of chemistry to become familiar with the simple

truth which is expressed by this law, and for this purpose several

examples elucidating its application will now be cited.

[11] The experiments conducted by Staas (described in detail in Chap.

XXIV. on Silver) form some of the accurate researches, proving

that the weight of matter is not altered in chemical reactions,

because he accurately weighed (introducing all the necessary

corrections) the reacting and resultant substances. Landolt

(1893) carried on various reactions in inverted and sealed glass

U-tubes, and on weighing the tubes before reaction (when the

reacting solutions were separated in each of the branches of

the tubes), and after (when the solutions had been well mixed

together by shaking), found that either the weight remained

perfectly constant or that the variation was so small (for

instance, 0·2 milligram in a total weight of about a million

milligrams) as to be ascribed to the inevitable errors of

weighing.

[11 bis] The idea of the mass of matter was first shaped into an exact

form by Galileo (died 1642), and more especially by Newton (born

1643, died 1727), in the glorious epoch of the development of

the principles of inductive reasoning enunciated by Bacon and

Descartes in their philosophical treatises. Shortly after the

death of Newton, Lavoisier, whose fame in natural philosophy

should rank with that of Galileo and Newton, was born on August

26, 1743. The death of Lavoisier occurred during the Reign

of Terror of the French Revolution, when he, together with

twenty-six other chief farmers of the revenue, was guillotined

on May 8, 1794, at Paris; but his works and ideas have made him

immortal.

1. It is well known that iron rusts in damp air,[12] and that when

heated to redness in air it becomes coated with scoria (oxide), having,

like rust, the appearance of an earthy substance resembling some of the

iron ores from which metallic iron is extracted. If the iron is weighed

before and after the formation of the scoria or rust, it will be found

that the metal has increased in weight during the operation.[13] It

can easily be proved that this increase in weight is accomplished at

the expense of the atmosphere, and mainly, as Lavoisier proved, at the

expense of that portion which is called oxygen. In fact, in a vacuum,

or in gases which do not contain oxygen, for instance, in hydrogen or

nitrogen, the iron neither rusts nor becomes coated with scoria. Had the

iron not been weighed, the participation of the oxygen of the atmosphere

in its transformation into an earthy substance might have easily passed

unnoticed, as was formerly the case, when phenomena like the above were,

for this reason, misunderstood. It is evident from the law of the

indestructibility of matter that as the iron increases in weight in its

conversion into rust, the latter must be a more complex substance than

the iron itself, and its formation is due to a reaction of combination.

We might form an entirely wrong opinion about it, and might, for

instance, consider rust to be a simpler substance than iron, and explain

the formation of rust as the removal of something from the iron. Such,

indeed, was the general opinion prior to Lavoisier, when it was held

that iron contained a certain unknown substance called 'phlogiston,' and

that rust was iron deprived of this supposed substance.

[12] By covering iron with an enamel, or varnish, or with unrustable

metals (such as nickel), or a coating of paraffin, or other

similar substances, it is protected from the air and moisture,

and so kept from rusting.

[13] Such an experiment may easily be made by taking the finest

(unrusted) iron filings (ordinary filings must be first washed

in ether, dried, and passed through a very fine sieve). The

filings thus obtained are capable of burning directly in air

(by oxidising or forming rust), especially when they hang (are

attracted) on a magnet. A compact piece of iron does not burn in

air, but spongy iron glows and smoulders like tinder. In making

the experiment, a horse-shoe magnet is fixed, with the poles

downwards, on one arm of a rather sensitive balance, and the iron

filings are applied to the magnet (on a sheet of paper) so as to

form a beard about the poles. The balance pan should be exactly

under the filings on the magnet, in order that any which might

fall from it should not alter the weight. The filings, having

been weighed, are set light to by applying the flame of a candle;

they easily take fire, and go on burning by themselves, forming

rust. When the combustion is ended, it will be clear that the

iron has increased in weight; from 5-1/2 parts by weight of iron

filings taken, there are obtained, by complete combustion, 7-1/2

parts by weight of rust.

[Illustration: FIG. 1.--Apparatus for the decomposition of red mercury

oxide.]

2. Copper carbonate (in the form of a powder, or as the well-known green

mineral called 'malachite,' which is used for making ornaments, or as an

ore for the extraction of copper) changes into a black substance called

'copper oxide' when heated to redness.[14] This black substance is also

obtained by heating copper to redness in air--that is, it is the scoria

or oxidation product of copper. The weight of the black oxide of copper

left is less than that of the copper carbonate originally taken, and

therefore we consider the reaction which occurred to have been one of

decomposition, and that by it something was separated from the green

copper carbonate, and, in fact, by closing the orifice of the vessel in

which the copper carbonate is heated with a well-fitting cork, through

which a gas delivery tube[15] passes whose end is immersed under water,

it will be observed that on heating, a gas is formed which bubbles

through the water. This gas can be easily collected, as will presently

be described, and it will be found to essentially differ from air in

many respects; for instance, a burning taper is extinguished in it as if

it had been plunged into water. If weighing had not proved to us that

some substance had been separated, the formation of the gas might easily

have escaped our notice, for it is colourless and transparent like air,

and is therefore evolved without any striking feature. The carbonic

anhydride evolved may be weighed,[16] and it will be seen that the sum

of the weights of the black copper oxide and carbonic anhydride is equal

to the weight of the copper carbonate[17] originally taken, and thus by

carefully following out the various stages of all chemical reactions we

arrive at a confirmation of the law of the indestructibility of matter.

[14] For the purpose of experiment, it is most convenient to take

copper carbonate, which may be prepared by the experimenter

himself, by adding a solution of sodium carbonate to a solution

of copper sulphate. The precipitate (deposit) so formed is

collected on a filter, washed, and dried. The decomposition of

copper carbonate into copper oxide is effected by so moderate

a heat that it may be performed in a glass vessel heated by a

lamp. For this purpose a thin glass tube, closed at one end, and

called a 'test tube,' may be employed, or else a vessel called a

'retort.' The experiment is carried on, as described in example

three on p. 11, by collecting the carbonic anhydride over water,

as will be afterwards explained.

[15] Gas delivery tubes are usually made of glass tubing of various

diameters and thicknesses. If of small diameter and thickness, a

glass tube is easily bent by heating in a gas jet or the flame

of a spirit lamp, and it may also be easily divided at a given

point by making a deep scratch with a file and then breaking the

tube at this point with a sharp jerk. These properties, together

with their impermeability, transparency, hardness, and regularity

of bore, render glass tubes most useful in experiments with

gases. Naturally they might be replaced by straws, india-rubber,

metallic, or other tubes, but these are more difficult to fix on

to a vessel, and are not entirely impervious to gases. A glass

gas delivery tube may be hermetically fixed into a vessel by

fitting it into a perforated cork, which should be soft and free

from flaws, and fixing the cork into the orifice of the vessel.

To protect the cork from the action of gases it is sometimes

previously soaked in paraffin, or it may be replaced by an

india-rubber cork.

[16] Gases, like all other substances, may be weighed, but, owing to

their extreme lightness and the difficulty of dealing with them

in large masses, they can only be weighed by very sensitive

balances; that is, in such as, with a considerable load, indicate

a very small difference in weight--for example, a centigram or a

milligram with a load of 1,000 grams. In order to weigh a gas,

a glass globe furnished with a tight-fitting stop-cock is first

of all exhausted of air by an air-pump (a Sprengel pump is the

best). The stop-cock is then closed, and the exhausted globe

weighed. If the gas to be weighed is then let into the globe,

its weight can be determined from the increase in the weight of

the globe. It is necessary, however, that the temperature and

pressure of the air about the balance should remain constant for

both weighings, as the weight of the globe in air will (according

to the laws of hydrostatics) vary with its density. The volume of

the air displaced, and its weight, must therefore be determined

by observing the temperature, density, and moisture of the

atmosphere during the time of experiment. This will be partly

explained later, but may be studied more in detail by physics.

Owing to the complexity of all these operations, the mass of a

gas is usually determined from its volume and density, or from

the weight of a known volume.

[17] The copper carbonate should be dried before weighing, as

otherwise--besides copper oxide and carbonic anhydride--water

will be obtained in the decomposition. Water forms a part of the

composition of malachite, and has therefore to be taken into

consideration. The water produced in the decomposition may be all

collected by absorbing it in sulphuric acid or calcium chloride,

as will be described further on. In order to dry a salt it must

be heated at about 100° until its weight remains constant, or

be placed under an air pump over sulphuric acid, as will also

be presently described. As water is met with almost everywhere,

and as it is absorbed by many substances, the possibility of its

presence should never be lost sight of.

3. Red mercury oxide (which is formed as mercury rust by heating

mercury in air) is decomposed like copper carbonate (only by heating

more slowly and at a somewhat higher temperature), with the formation

of the peculiar gas, oxygen. For this purpose the mercury oxide is

placed in a glass tube or retort,[18] to which a gas delivery tube is

attached by means of a cork. This tube is bent downwards, as shown in

the drawing (Fig. 1). The open end of the gas delivery tube is immersed

in a vessel filled with water, called a pneumatic trough.[19] When the

gas begins to be evolved in the retort it is obliged, having no other

outlet, to escape through the gas delivery tube into the water in the

pneumatic trough, and therefore its evolution will be rendered visible

by the bubbles coming from this tube. In heating the retort containing

the mercury oxide, the air contained in the apparatus is first partly

expelled, owing to its expansion by heat, and then the peculiar gas

called 'oxygen' is evolved, and may be easily collected as it comes off.

For this purpose a vessel (an ordinary cylinder, as in the drawing) is

filled quite full with water and its mouth closed; it is then inverted

and placed in this position under the water in the trough; the mouth

is then opened. The cylinder will remain full of water--that is, the

water will remain at a higher level in it than in the surrounding

vessel, owing to the atmospheric pressure. The atmosphere presses on

the surface of the water in the trough, and prevents the water from

flowing out of the cylinder. The mouth of the cylinder is placed over

the end of the gas delivery tube,[20] and the bubbles issuing from it

will rise into the cylinder and displace the water contained in it.

Gases are generally collected in this manner. When a sufficient quantity

of gas has accumulated in the cylinder it can be clearly shown that it

is not air, but another gas which is distinguished by its capacity for

vigorously supporting combustion. In order to show this, the cylinder is

closed, under water, and removed from the bath; its mouth is then turned

upwards, and a smouldering taper plunged into it. As is well known, a

smouldering taper will be extinguished in air, but in the gas which

is given off from red mercury oxide it burns clearly and vigorously,

showing the property possessed by this gas for supporting combustion

more energetically than air, and thus enabling it to be distinguished

from the latter. It may be observed in this experiment that, besides the

formation of oxygen, metallic mercury is formed, which, volatilising at

the high temperature required for the reaction, condenses on the cooler

parts of the retort as a mirror or in globules. Thus two substances,

mercury and oxygen, are obtained by heating red mercury oxide. In this

reaction, from one substance, two new substances are produced--that

is, a decomposition has taken place. The means of collecting and

investigating gases were known before Lavoisier's time, but he first

showed the real part they played in the processes of many chemical

changes which before his era were either wrongly understood (as will be

afterwards explained) or were not explained at all, but only observed

in their superficial aspects. This experiment on red mercury oxide has

a special significance in the history of chemistry contemporary with

Lavoisier, because the oxygen gas which is here evolved is contained in

the atmosphere, and plays a most important part in nature, especially in

the respiration of animals, in combustion in air, and in the formation

of rusts or scoriæ (earths, as they were then called) from metals--that

is, of earthy substances, like the ores from which metals are extracted.

[18] As the decomposition of red oxide of mercury requires so high a

temperature, near redness, as to soften ordinary glass, it is

necessary for this experiment to take a retort (or test tube)

made of hard glass, which is able to stand high temperatures

without softening. For the same reason, the lamp used must give

a strong heat and a large flame, capable of embracing the whole

bottom of the retort, which should be as small as possible for

the convenience of the experiment.

[19] [Illustration: FIG. 2.--Apparatus for distilling under a

diminished pressure liquids which decompose at their boiling

points under the ordinary pressure. The apparatus in which the

liquid is distilled is connected with a large globe from which the

air is pumped out; the liquid is heated, and the receiver cooled.]

The pneumatic trough may naturally be made of any material

(china, earthenware, or metal, &c.), but usually a glass one,

as shown in the drawing, is used, as it allows the progress

of the experiment to be better observed. For this reason, as

well as the ease with which they are kept clean, and from the

fact also that glass is not acted on by many substances which

affect other materials (for instance, metals), glass vessels

of all kinds--such as retorts, test tubes, cylinders, beakers,

flasks, globes, &c.--are preferred to any other for chemical

experiments. Glass vessels may be heated without any danger if

the following precautions be observed: 1st, they should be made

of thin glass, as otherwise they are liable to crack from the bad

heat-conducting power of glass; 2nd, they should be surrounded

by a liquid or with sand (Fig. 2), or sand bath as it is called;

or else should stand in a current of hot gases without touching

the fuel from which they proceed, or in the flame of a smokeless

lamp. A common candle or lamp forms a deposit of soot on a

cold object placed in their flames. The soot interferes with

the transmission of heat, and so a glass vessel when covered

with soot often cracks. And for this reason spirit lamps, which

burn with a smokeless flame, or gas burners of a peculiar

construction, are used. In the Bunsen burner the gas is mixed

with air, and burns with a non-luminous and smokeless flame.

On the other hand, if an ordinary lamp (petroleum or benzine)

does not smoke it may be used for heating a glass vessel without

danger, provided the glass is placed well above the flame in the

current of hot gases. In all cases, the heating should be begun

very carefully by raising the temperature by degrees.

[20] In order to avoid the necessity of holding the cylinder, its open

end is widened (and also ground so that it may be closely covered

with a ground-glass plate when necessary), and placed on a stand

below the level of the water in the bath. This stand is called

'the bridge.' It has several circular openings cut through it,

and the gas delivery tube is placed under one of these, and the

cylinder for collecting the gas over it.

4. In order to illustrate by experiment one more example of chemical

change and the application of the law of the indestructibility of

matter, we will consider the reaction between common table salt and

lunar caustic, which is well known from its use in cauterising wounds.

By taking a clear solution of each and mixing them together, it will at

once be observed that a solid white substance is formed, which settles

to the bottom of the vessel, and is insoluble in water. This substance

may be separated from the solution by filtering; it is then found to be

an entirely different substance from either of those taken originally in

the solutions. This is at once evident from the fact that it does not

dissolve in water. On evaporating the liquid which passed through the

filter, it will be found to contain a new substance unlike either table

salt or lunar caustic, but, like them, soluble in water. Thus table salt

and lunar caustic, two substances soluble in water, produced, by their

mutual chemical action, two new substances, one insoluble in water, and

the other remaining in solution. Here, from two substances, two others

are obtained, consequently there occurred a reaction of substitution.

The water served only to convert the re-acting substances into a liquid

and mobile state. If the lunar caustic and salt be dried[21] and

weighed, and if about 58-1/2 grams[22] of salt and 170 grams of lunar

caustic be taken, then 143-1/2 grams of insoluble silver chloride and

85 grams of sodium nitrate will be obtained. The sum of the weights

of the re-acting and resultant substances are seen to be similar and

equal to 228-1/2 grams, which necessarily follows from the law of the

indestructibility of matter.

[21] Drying is necessary in order to remove any water which may be held

in the salts (_see_ Note 17, and Chapter I., Notes 13 and 14).

[22] The exact weights of the re-acting and resulting substances are

determined with the greatest difficulty, not only from the

possible inexactitude of the balance (every weighing is only

correct within the limits of the sensitiveness of the balance)

and weights used in weighing, not only from the difficulty in

making corrections for the weight of air displaced by the vessels

holding the substances weighed and by the weights themselves, but

also from the hygroscopic nature of many substances (and vessels)

causing absorption of moisture from the atmosphere, and from the

difficulty in not losing any of the substance to be weighed in

the several operations (filtering, evaporating, and drying, &c.)

which have to be performed before arriving at a final result. All

these circumstances have to be taken into consideration in exact

researches, and their elimination requires very many special

precautions which are impracticable in preliminary experiments.

Accepting the truth of the above law, the question naturally arises as

to whether there is any limit to the various chemical transformations,

or are they unrestricted in number--that is to say, is it possible

from a given substance to obtain an equivalent quantity of any other

substance? In other words, does there exist a perpetual and infinite

change of one kind of material into every other kind, or is the cycle

of these transformations limited? This is the second essential problem

of Chemistry, a question of quality of matter, and one, it is evident,

which is more complicated than the question of quantity. It cannot be

solved by a mere superficial glance at the subject. Indeed, on seeing

how all the varied forms and colours of plants are built up from air

and the elements of the soil, and how metallic iron can be transformed

into colours such as inks and Prussian blue, we might be led to think

that there is no end to the qualitative changes to which matter is

susceptible. But, on the other hand, the experiences of everyday life

compel us to acknowledge that food cannot be made out of a stone, or

gold out of copper. Thus a definite answer can only be looked for in

a close and diligent study of the subject, and the problem has been

resolved in different way at different times. In ancient times the

opinion most generally held was that everything visible was composed of

four elements--Air, Water, Earth, and Fire. The origin of this doctrine

can be traced far back into the confines of Asia, whence it was handed

down to the Greeks, and most fully expounded by Empedocles, who lived

before 460 B.C. This doctrine was not the result of exact research,

but apparently owes its origin to the clear division of bodies into

gases (like air), liquids (like water), and solids (like the earth).

The Arabs appear to have been the first who attempted to solve the

question by experimental methods, and they introduced, through Spain,

the taste for the study of similar problems into Europe, where from

that time there appear many adepts in chemistry, which was considered

as an unholy art, and called 'alchemy.' As the alchemists were ignorant

of any exact law which could guide them in their researches, they

obtained most anomalous results. Their chief service to chemistry

was that they made a number of experiments, and discovered many new

chemical transformations; but it is well known how they solved the

fundamental problem of chemistry. Their view may be taken as a positive

acknowledgment of the infinite transmutability of matter, for they aimed

at discovering the Philosopher's Stone, capable of converting everything

into gold and diamonds, and of making the old young again. This solution

of the question was afterwards completely overthrown, but it must not,

for this reason, be thought that the hopes held by the alchemists were

only the fruit of their imaginations. The first chemical experiments

might well lead them to their conclusions. They took, for instance,

the bright metallic mineral galena, and extracted metallic lead from

it. Thus they saw that from a metallic substance which is unfitted for

use they could obtain another metallic substance which is ductile and

valuable for many technical purposes. Furthermore, they took this lead

and obtained silver, a still more valuable metal, from it. Thus they

might easily conclude that it was possible to ennoble metals by means of

a whole series of transmutations--that is to say, to obtain from them

those which are more and more precious. Having got silver from lead,

they assumed that it would be possible to obtain gold from silver. The

mistake they made was that they never weighed or measured the substances

used or produced in their experiments. Had they done so, they would have

learnt that the weight of the lead was much less than that of the galena

from which it was obtained, and the weight of the silver infinitesimal

compared with that of the lead. Had they looked more closely into

the process of the extraction of the silver from lead (and silver at

the present time is chiefly obtained from the lead ores) they would

have seen that the lead does not change into silver, but that it only

contains a certain small quantity of it, and this amount having once

been separated from the lead it cannot by any further operation give

more. The silver which the alchemists extracted from the lead was in the

lead, and was not obtained by a chemical change of the lead itself. This

is now well known from experiment, but the first view of the nature of

the process was very likely to be an erroneous one.[23] The methods of

research adopted by the alchemists could give but little success, for

they did not set themselves clear and simple questions whose answers

would aid them to make further progress. Thus though they did not

arrive at any exact law, they left nevertheless numerous and useful

experimental data as an inheritance to chemistry; they investigated, in

particular, the transformations proper to metals, and for this reason

chemistry was for long afterwards entirely confined to the study of

metallic substances.

[23] Besides which, in the majority of cases, the first explanation of

most subjects which do not repeat themselves in everyday

experience under various aspects, but always in one form, or

only at intervals and infrequently, is usually wrong. Thus

the daily evidence of the rising of the sun and stars evokes

the erroneous idea that the heavens move and the earth stands

still. This apparent truth is far from being the real truth,

and, as a matter of fact, is contradictory to it. Similarly, an

ordinary mind and everyday experience concludes that iron is

incombustible, whereas it burns not only as filings, but even as

wire, as we shall afterwards see. With the progress of knowledge

very many primitive prejudices have been obliged to give way to

true ideas which have been verified by experiment. In ordinary

life we often reason at first sight with perfect truth, only

because we are taught a right judgment by our daily experience.

It is a necessary consequence of the nature of our minds to reach

the attainment of truth through elementary and often erroneous

reasoning and through experiment, and it would be very wrong

to expect a knowledge of truth from a simple mental effort.

Naturally, experiment itself cannot give truth, but it gives the

means of destroying erroneous representations whilst confirming

those which are true in all their consequences.

In their researches, the alchemists frequently made use of two chemical

processes which are now termed 'reduction' and 'oxidation.' The rusting

of metals, and in general their conversion from a metallic into an

earthy form, is called 'oxidation,' whilst the extraction of a metal

from an earthy substance is called 'reduction.' Many metals--for

instance, iron, lead, and tin--are oxidised by heating in air alone,

and may be again reduced by heating with carbon. Such oxidised metals

are found in the earth, and form the majority of metallic ores. The

metals, such as tin, iron, and copper, may be extracted from these ores

by heating them together with carbon. All these processes were well

studied by the alchemists. It was afterwards shown that all earths and

minerals are formed of similar metallic rusts or oxides, or of their

combinations. Thus the alchemists knew of two forms of chemical changes:

the oxidation of metals and the reduction of the oxides so formed

into metals. The explanation of the nature of these two classes of

chemical phenomena was the means for the discovery of the most important

chemical laws. The first hypothesis on their nature is due to Becker,

and more particularly to Stahl, a surgeon to the King of Prussia.

Stahl writes in his 'Fundamenta Chymiæ,' 1723, that all substances

consist of an imponderable fiery substance called 'phlogiston' (materia

aut principium ignis non ipse ignis), and of another element having

particular properties for each substance. The greater the capacity of

a body for oxidation, or the more combustible it is, the richer it is

in phlogiston. Carbon contains it in great abundance. In oxidation or

combustion phlogiston is emitted, and in reduction it is consumed or

enters into combination. Carbon reduces earthy substances because it is

rich in phlogiston, and gives up a portion of its phlogiston to the

substance reduced. Thus Stahl supposed metals to be compound substances

consisting of phlogiston and an earthy substance or oxide. This

hypothesis is distinguished for its very great simplicity, and for this

and other reasons it acquired many supporters.[24]

[24] It is true that Stahl was acquainted with a fact which directly

disproved his hypothesis. It was already known (from the

experiments of Geber, and more especially of Ray, in 1630) that

metals increase in weight by oxidation, whilst, according to

Stahl's hypothesis, they should decrease in weight, because

phlogiston is separated by oxidation. Stahl speaks on this

point as follows:--'I am well aware that metals, in their

transformation into earths, increase in weight. But not only does

this fact not disprove my theory, but, on the contrary, confirms

it, for phlogiston is lighter than air, and, in combining

with substances, strives to lift them, and so decreases their

weight; consequently, a substance which has lost phlogiston

must be heavier.' This argument, it will be seen, is founded

on a misconception of the properties of gases, regarding them

as having no weight and as not being attracted by the earth,

or else on a confused idea of phlogiston itself, since it was

first defined as imponderable. The conception of imponderable

phlogiston tallies well with the habit and methods of the last

century, when recourse was often had to imponderable fluids for

explaining a large number of phenomena. Heat, light, magnetism,

and electricity were explained as being peculiar imponderable

fluids. In this sense the doctrine of Stahl corresponds entirely

with the spirit of his age. If heat be now regarded as motion

or energy, then phlogiston also should be considered in this

light. In fact, in combustion, of coals for instance, heat and

energy are evolved, and not combined in the coal, although the

oxygen and coal do combine. Consequently, the doctrine of Stahl

contains the essence of a true representation of the evolution

of energy, but naturally this evolution is only a consequence of

the combination occurring between the coal and oxygen. As regards

the history of chemistry prior to Lavoisier, besides Stahl's

work (to which reference has been made above), Priestley's

_Experiments and Observations on Different Kinds of Air_, London,

1790, and also Scheele's _Opuscula Chimica et Physica_, Lips.,

1788-89, 2 vols., must be recommended as the two leading works

of the English and Scandinavian chemists showing the condition

of chemical learning before the propagation of Lavoisier's

views, and containing also many important observations which lie

at the basis of the chemistry of our times. A most interesting

memoir on the history of phlogiston is that of Rodwell, in the

_Philosophical Magazine_, 1868, in which it is shown that the

idea of phlogiston dates very far back, that Basil Valentine

(1394-1415), in the _Cursus Triumphalis Antimonii_, Paracelsus

(1493-1541), in his work, _De Rerum Natura_, Glauber (1604-1668),

and especially John Joachim Becher (1625-1682), in his _Physica

Subterranea_, all referred to phlogiston, but under different

names.

[Illustration: FIG. 3.--Lavoisier's apparatus for determining the

composition of air and the reason of metals increasing in weight when

they are calcined in air.]

Lavoisier proved by means of the balance that every case of rusting of

metals or oxidation, or of combustion, is accompanied by an increase

in weight at the expense of the atmosphere. He formed, therefore, the

natural opinion that the heavier substance is more complex than the

lighter one.[25] Lavoisier's celebrated experiment, made in 1774,

gave indubitable support to his opinion, which in many respects was

contradictory to Stahl's doctrine. Lavoisier poured four ounces of pure

mercury into a glass retort (fig. 3), whose neck was bent as shown in

the drawing and dipped into the vessel R S, also full of mercury. The

projecting end of the neck was covered with a glass bell-jar P. The

weight of all the mercury taken, and the volume of air remaining in

the apparatus, namely, that in the upper portion of the retort, and

under the bell-jar, were determined before beginning the experiment.

It was most important in this experiment to know the volume of air in

order to learn what part it played in the oxidation of the mercury,

because, according to Stahl, phlogiston is emitted into the air, whilst,

according to Lavoisier, the mercury in oxidising absorbs a portion of

the air; and consequently it was absolutely necessary to determine

whether the amount of air increased or decreased in the oxidation of

the metal. It was, therefore, most important to measure the volume of

the air in the apparatus both before and after the experiment. For this

purpose it was necessary to know the total capacity of the retort,

the volume of the mercury poured into it, the volume of the bell-jar

above the level of the mercury, and also the temperature and pressure

of the air at the time of its measurement. The volume of air contained

in the apparatus and isolated from the surrounding atmosphere could

be determined from these data. Having arranged his apparatus in this

manner, Lavoisier heated the retort holding the mercury for a period

of twelve days at a temperature near the boiling point of mercury. The

mercury became covered with a quantity of small red scales; that is,

it was oxidised or converted into an earth. This substance is the same

mercury oxide which has already been mentioned (example 3). After the

lapse of twelve days the apparatus was cooled, and it was then seen that

the volume of the air in the apparatus had diminished during the time

of the experiment. This result was in exact contradiction to Stahl's

hypothesis. Out of 50 cubic inches of air originally taken, there only

remained 42. Lavoisier's experiment led to other equally important

results. The weight of the air taken decreased by as much as the weight

of the mercury increased in oxidising; that is, the portion of the air

was not destroyed, but only combined with mercury. This portion of

the air may be again separated from the mercury oxide and has, as we

saw (example 3), properties different from those of air. It is called

'oxygen.' That portion of the air which remained in the apparatus and

did not combine with the mercury does not oxidise metals, and cannot

support either combustion or respiration, so that a lighted taper is

immediately extinguished if it be dipped into the gas which remains

in the bell-jar. 'It is extinguished in the residual gas as if it had

been plunged into water,' writes Lavoisier in his memoirs. This gas is

called 'nitrogen.' Thus air is not a simple substance, but consists of

two gases, oxygen and nitrogen, and therefore the opinion that air is

an elementary substance is erroneous. The oxygen of the air is absorbed

in combustion and the oxidation of metals, and the earths produced by

the oxidation of metals are substances composed of oxygen and a metal.

By mixing the oxygen with the nitrogen the same air as was originally

taken is re-formed. It has also been shown by direct experiment that

on reducing an oxide with carbon, the oxygen contained in the oxide is

transferred to the carbon, and gives the same gas that is obtained by

the combustion of carbon in air. Therefore this gas is a compound of

carbon and oxygen, just as the earthy oxides are composed of metals and

oxygen.

[25] An Englishman, named Mayow, who lived a whole century before

Lavoisier (in 1666), understood certain phenomena of oxidation

in their true aspect, but was not able to develop his views

with clearness, or support them by conclusive experiments; he

cannot therefore be considered, like Lavoisier, as the founder of

contemporary chemical learning. Science is a universal heritage,

and therefore it is only just to give the highest honour in

science, not to those who first enunciate a certain truth, but to

those who are first able to convince others of its authenticity

and establish it for the general welfare. But scientific

discoveries are rarely made all at once; as a rule, the first

teachers do not succeed in convincing others of the truth they

have discovered; with time, however, a true herald comes forward,

possessing every means for making the truth apparent to all, but

it must not be forgotten that such are entirely indebted to the

labours and mass of data accumulated by many others. Such was

Lavoisier, and such are all the great founders of science. They

are the enunciators of all past and present learning, and their

names will always be revered by posterity.

The many examples of the formation and decomposition of substances which

are met with convince us that the majority of substances with which

we have to deal are compounds made up of several other substances. By

heating chalk (or else copper carbonate, as in the second example)

we obtain lime and the same carbonic acid gas which is produced by

the combustion of carbon. On bringing lime into contact with this gas

and water, at the ordinary temperature, we again obtain the compound,

carbonate of lime, or chalk. Therefore chalk is a compound. So also are

those substances from which it may be built up. Carbonic anhydride is

formed by the combination of carbon and oxygen; and lime is produced

by the oxidation of a certain metal called 'calcium.' By resolving

substances in this manner into their component parts, we arrive at

last at such as are indivisible into two or more substances by any

means whatever, and which cannot be formed from other substances. All

we can do is to make such substances combine together to act on other

substances. Substances which cannot be formed from or decomposed into

others are termed _simple substances_ (elements). Thus all homogeneous

substances may be classified into simple and compound substances. This

view was introduced and established as a scientific fact during the

lifetime of Lavoisier. The number of these elements is very small in

comparison with the number of compound substances which are formed

by them. At the present time, only seventy elements are known with

certainty to exist. Some of them are very rarely met with in nature, or

are found in very small quantities, whilst the existence of others is

still doubtful. The number of elements with whose compounds we commonly

deal in everyday life is very small. Elements cannot be transmuted

into one another--at least up to the present not a single case of such

a transformation has been met with; it may therefore be said that, as

yet, it is impossible to transmute one metal into another. And as yet,

notwithstanding the number of attempts which have been made in this

direction, no fact has been discovered which could in any way support

the idea of the complexity of such well-known elements[26] as oxygen,

iron, sulphur, &c. Therefore, from its very conception, an element is

not susceptible to reactions of decomposition.[27]

[26] Many of the ancient philosophers assumed the existence of one

elementary form of matter. This idea still appears in our times,

in the constant efforts which are made to reduce the number of

the elements; to prove, for instance, that bromine contains

chlorine or that chlorine contains oxygen. Many methods, founded

both on experiment and theory, have been tried to prove the

compound nature of the elements. All labour in this direction has

as yet been in vain, and the assurance that elementary matter

is not so homogeneous (single) as the mind would desire in its

first transport of rapid generalisation is strengthened from year

to year. All our knowledge shows that iron and other elements

remain, even at such a high temperature as there exists in the

sun, as different substances, and are not converted into one

common material. Admitting, even mentally, the possibility of one

elementary form of matter, a method must be imagined by which

it could give rise to the various elements, as also the _modus

operandi_ of their formation from one material. If it be said

that this diversitude only takes place at low temperatures, as is

observed with isomerides, then there would be reason to expect,

if not the transition of the various elements into one particular

and more stable form, at least the mutual transformation of some

into others. But nothing of the kind has as yet been observed,

and the alchemist's hope to manufacture (as Berthollet puts it)

elements has no theoretical or practical foundation.

[27] The weakest point in the idea of elements is the negative

character of the determinative signs given them by Lavoisier, and

from that time ruling in chemistry. They do _not_ decompose, they

do _not_ change into one another. But it must be remarked that

elements form the limiting horizon of our knowledge of matter,

and it is always difficult to determine a positive side on the

borderland of what is known. Besides, there is no doubt (from the

results of spectrum analysis) that the elements are distributed

as far as the most distant stars, and that they support the

highest attainable temperatures without decomposing.

The quantity, therefore, of each element remains constant in all

chemical changes: a fact which may be deduced as a consequence of

the law of the indestructibility of matter, and of the conception

of elements themselves. Thus the equation expressing the law of the

indestructibility of matter acquires a new and still more important

signification. If we know the quantities of the elements which occur in

the re-acting substances, and if from these substances there proceed, by

means of chemical changes, a series of new compound substances, then the

latter will together contain the same quantity of each of the elements

as there originally existed in the re-acting substances. The essence

of chemical change is embraced in the study of how, and with what

substances, each element is combined before and after change.

In order to be able to express various chemical changes by equations,

it has been agreed to represent each element by the first or some two

letters of its (Latin) name. Thus, for example, oxygen is represented by

the letter O; nitrogen by N; mercury (hydrargyrum) by Hg; iron (ferrum)

by Fe; and so on for all the elements, as is seen in the tables on

page 24. A compound substance is represented by placing the symbols

representing the elements of which it is made up side by side. For

example, red mercury oxide is represented by HgO, which shows that it

is composed of oxygen and mercury. Besides this, the symbol of every

element corresponds with a certain relative quantity of it by weight,

called its 'combining' weight, or the weight of an atom; so that the

chemical formula of a compound substance not only designates the nature

of the elements of which it is composed, but also their quantitative

proportion. Every chemical process may be expressed by an equation

composed of the formulæ corresponding with those substances which take

part in it and are produced by it. The amount by weight of the elements

in every chemical equation must be equal on both sides of the equation,

since no element is either formed or destroyed in a chemical change.

On pages 24, 25, and 26 a list of the elements, with their symbols and

combining or atomic weights, is given, and we shall see afterwards on

what basis the atomic weights of elements are determined. At present we

will only point out that a compound containing the elements A and B is

designated by the formula A_n_ B_m_, where _m_ and _n_ are the

coefficients or multiples in which the combining weights of the elements

enter into the composition of the substance. If we represent the

combining weight of the substance A by _a_ and that of the substance B by

_b_, then the composition of the substance A_n_ B_m_ will be expressed

thus: it contains _na_ parts by weight of the substance A and _mb_ parts

by weight of the substance B, and consequently 100 parts of our compound

contain _na_ 100/_na_ + _mb_ percentage parts by weight of the substance

A and _mb_ 100/_na_ + _mb_ of the substance B. It is evident that as a

formula shows the relative amounts of all the elements contained in a

compound, the actual weights of the elements contained in a given weight

of a compound may be calculated from its formula. For example, the

formula NaCl of table salt shows (as Na = 23 and Cl = 35·5) that 58·5

lbs. of salt contain 23 lbs. of sodium and 35·5 lbs. of chlorine, and

that 100 parts of it contain 39·3 per cent. of sodium and 60·7 per cent.

of chlorine.

What has been said above clearly limits the province of chemical

changes, because from substances of a given kind there can be obtained

only such as contain the same elements. Even with this limitation,

however, the number of possible combinations is infinitely great. Only

a comparatively small number of compounds have yet been described or

subjected to research, and any one working in this direction may easily

discover new compounds which had not before been obtained. It often

happens, however, that such newly-discovered compounds were foreseen by

chemistry, whose object is the apprehension of that uniformity which

rules over the multitude of compound substances, and whose aim is the

comprehension of those laws which govern their formation and properties.

The conception of elements having been established, the next objects

of chemistry were: the determination of the properties of compound

substances on the basis of the determination of the quantity and kind of

elements of which they are composed; the investigation of the elements

themselves; the determination of what compound substances can be formed

from each element and the properties which these compounds show; and

the apprehension of the nature of the connection between the elements

in different compounds. An element thus serves as the starting point,

and is taken as the primary conception on which all other substances are

built up.

When we state that a certain element enters into the composition of a

given compound (when we say, for instance, that mercury oxide contains

oxygen) we do not mean that it contains oxygen as a gaseous substance,

but only desire to express those transformations which mercury oxide

is capable of making; that is, we wish to say that it is possible to

obtain oxygen from mercury oxide, and that it can give up oxygen to

various other substances; in a word, we desire only to express those

transformations of which mercury oxide is capable. Or, more concisely,

it may be said that the _composition_ of a compound is the expression

of those transformations of which it is capable. It is useful in this

sense to make a clear distinction between the conception of an element

as a _separate_ homogeneous substance, and as a _material_ but invisible

_part_ of a compound. Mercury oxide does not contain two simple bodies,

a gas and a metal, but two elements, mercury and oxygen, which, when

free, are a gas and a metal. Neither mercury as a metal nor oxygen as

a gas is contained in mercury oxide; it only contains the substance of

these elements, just as steam only contains the substance of ice, but

not ice itself, or as corn contains the substance of the seed, but not

the seed itself. The existence of an element may be recognised without

knowing it in the uncombined state, but only from an investigation

of its combinations, and from the knowledge that it gives, under

all possible conditions, substances which are unlike other known

combinations of substances. Fluorine is an example of this kind. It was

for a long time unknown in a free state, and nevertheless was recognised

as an element because its combinations with other elements were known,

and their difference from all other similar compound substances was

determined. In order to grasp the difference between the conception of

the visible form of an element as we know it in the free state, and of

the intrinsic element (or 'radicle,' as Lavoisier called it) contained

in the visible form, it should be remarked that compound substances

also combine together forming yet more complex compounds, and that

they evolve heat in the process of combination. The original compound

may often be extracted from these new compounds by exactly the same

methods as elements are extracted from their corresponding combinations.

Besides, many elements exist under various visible forms whilst the

intrinsic element contained in these various forms is something which is

not subject to change. Thus carbon appears as charcoal, graphite, and

diamond, but yet the element carbon alone, contained in each, is one

and the same. Carbonic anhydride contains carbon, and not charcoal, or

graphite, or the diamond.

Elements alone, although not all of them, have the peculiar lustre,

opacity, malleability, and the great heat and electrical conductivity

which are proper to metals and their mutual combinations. But elements

are far from all being _metals_. Those which do not possess the physical

properties of metals are called _non-metals_ (or _metalloids_). It

is, however, impossible to draw a strict line of demarcation between

metals and non-metals, there being many intermediary substances. Thus

graphite, from which pencils are manufactured, is an element with the

lustre and other properties of a metal; but charcoal and the diamond,

which are composed of the same substance as graphite, do not show any

metallic properties. Both classes of elements are clearly distinguished

in definite examples, but in particular cases the distinction is not

clear and cannot serve as a basis for the exact division of the elements

into two groups.

The conception of elements forms the basis of chemical knowledge, and

in giving a list of them at the very beginning of our work, we wish to

tabulate our present knowledge on the subject. Altogether about seventy

elements are now authentically known, but many of them are so rarely met

with in nature, and have been obtained in such small quantities, that

we possess but a very insufficient knowledge of them. The substances

most widely distributed in nature contain a very small number of

elements. These elements have been more completely studied than the

others, because a greater number of investigators have been able to

carry on experiments and observations on them. The elements most widely

distributed in nature are:--

Hydrogen, H = 1. In water, and in animal and vegetable

organisms.

Carbon, C = 12. In organisms, coal, limestones.

Nitrogen, N = 14. In air and in organisms.

Oxygen, O = 16. In air, water, earth. It forms the greater

part of the mass of the earth.

Sodium, Na = 23. In common salt and in many minerals.

Magnesium, Mg = 24. In sea-water and in many minerals.

Aluminium, Al = 27. In minerals and clay.

Silicon, Si = 28. In sand, minerals, and clay.

Phosphorus, P = 31. In bones, ashes of plants, and soil.

Sulphur, S = 32. In pyrites, gypsum, and in sea-water.

Chlorine, Cl = 35·5. In common salt, and in the salts of sea-water.

Potassium, K = 39. In minerals, ashes of plants, and in nitre.

Calcium, Ca = 40. In limestones, gypsum, and in organisms.

Iron, Fe = 56. In the earth, iron ores, and in organisms.

Besides these, the following elements, although not very largely

distributed in nature, are all more or less well known from their

applications to the requirements of everyday life or the arts, either in

a free state or in their compounds:--

Lithium, Li = 7. In medicine (Li_{2}CO_{3}), and in photography

(LiBr).

Boron, B = 11. As borax, B_{4}Na_{2}O_{7}, and as boric

anhydride, B_{2}O_{3}.

Fluorine, F = 19. As fluor spar, CaF_{2}, and as hydrofluoric

acid, HF.

Chromium, Cr = 52. As chromic anhydride, CrO_{3}, and potassium

dichromate, K_{2}Cr_{2}O_{7}.

Manganese, Mn = 55. As manganese peroxide, MnO_{2}, and potassium

permanganate, MnKO_{4}.

Cobalt, Co = 59·5 In smalt and blue glass.

Nickel, Ni = 59·5 For electro-plating other metals.

Copper, Cu = 63. The well-known red metal.

Zinc, Zn = 65. Used for the plates of batteries, roofing, &c.

Arsenic, As = 75. White arsenic (poison), As_{2}O_{3}.

Bromine, Br = 80. A brown volatile liquid; sodium bromide, NaBr.

Strontium, Sr = 87. In coloured fires (SrN_{2}O_{6}).

Silver, Ag = 109. The well-known white metal.

Cadmium, Cd = 112. In alloys. Yellow paint (CdS).

Tin, Sn = 119. The well-known metal.

Antimony, Sb = 120. In alloys such as type metal.

Iodine, I = 127. In medicine and photography; free, and as KI.

Barium, Ba = 137. "Permanent white," and as an adulterant in

white lead, and in heavy spar, BaSO_{4}.

Platinum, Pt = 196.}

Gold, Au = 197.}

Mercury, Hg = 200.} Well-known metals.

Lead, Pb = 207.}

Bismuth, Bi = 209. In medicine and fusible alloys.

Uranium, U = 239. In green fluorescent glass.

The compounds of the following metals and semi-metals have fewer

applications, but are well known, and are somewhat frequently met with

in nature, although in small quantities:--

Beryllium, Be = 9. Palladium, Pd = 107.

Titanium, Ti = 48. Cerium, Ce = 140.

Vanadium, V = 51. Tungsten, W = 184.

Selenium, Se = 79. Osmium, Os = 192.

Zirconium, Zr = 91. Iridium, Ir = 193.

Molybdenum, Mo = 96. Thallium, Tl = 204.

The following rare metals are still more seldom met with in nature, but

have been studied somewhat fully:--

Scandium, Sc = 44. Germanium, Ge = 72.

Gallium, Ga = 70. Rubidium, Rb = 86.

Yttrium, Y = 89. Cæsium, Cs = 133.

Niobium, Nb = 94. Lanthanum, La = 138.

Ruthenium, Ru = 102. Didymium, Di = 142.

Rhodium, Rh = 103. Ytterbium, Yb = 173.

Indium, In = 114. Tantalum, Ta = 183.

Tellurium, Te = 125. Thorium, Th = 232.

Besides these 66 elements there have been discovered:--Erbium, Terbium,

Samarium, Thullium, Holmium, Mosandrium, Phillipium, and several

others. But their properties and combinations, owing to their extreme

rarity, are very little known, and even their existence as independent

substances[28] is doubtful.

[28] Possibly some of their compounds are compounds of other

already-known elements. Pure and incontestably independent

compounds of these substances are unknown, and some of them have

not even been separated, but are only supposed to exist from the

results of spectroscopic researches. There can be no mention

of such contestable and doubtful elements in a short general

handbook of chemistry.

It has been incontestably proved from observations on the spectra of

the heavenly bodies that many of the commoner elements (such as H, Na,

Mg, Fe) occur on the far distant stars. This fact confirms the belief

that those forms of matter which appear on the earth as elements are

widely distributed over the entire universe. But we do not yet know why,

in nature, the mass of some elements should be greater than that of

others.[28 bis]

[28 bis] Clark in America made an approximate calculation of the amount

of the different elements contained in the earth's crust (to a

depth of 15 kilometres), and found that the chief mass (over 50

per cent.) is composed of oxygen; then comes silicon, &c.; while

the amount of hydrogen is less than 1 per cent., carbon scarcely

0·25 per cent., nitrogen even less than 0·03 per cent. The

relative masses of such metals as Cu, Ni, Au is minute. Judging

from the density (see Chapter VIII.) of the earth, a large

proportion of its mass must be composed of iron.

The capacity of each element to combine with one or another element,

and to form compounds with them which are in a greater or less degree

prone to give new and yet more complex substances, forms the fundamental

character of each element. Thus sulphur easily combines with the

metals, oxygen, chlorine, or carbon, whilst gold and silver enter

into combinations with difficulty, and form unstable compounds, which

are easily decomposed by heat. The cause or force which induces the

elements to enter into chemical change must be considered, as also the

cause which holds different substances in combination--that is, which

endues the substances formed with their particular degree of stability.

This cause or force is called _affinity_ (_affinitas_, _affinité_,

_Verwandtschaft_), or chemical affinity.[29] Since this force must

be regarded as exclusively an attractive force, like gravity, many

writers (for instance, Bergmann at the end of the last, and Berthollet

at the beginning of this, century) supposed affinity to be essentially

similar to the universal force of gravity, from which it only differs

in that the latter acts at observable distances whilst affinity only

evinces itself at the smallest possible distances. But chemical

affinity cannot be entirely identified with the universal attraction

of gravity, which acts at appreciable distances and is dependent

only on mass and distance, and not on the quality of the material on

which it acts, whilst it is by the quality of matter that affinity is

most forcibly influenced. Neither can it be entirely identified with

cohesion, which gives to homogeneous solid substances their crystalline

form, elasticity, hardness, ductility, and other properties, and to

liquids their surface tension, drop formation, capillarity, and other

properties, because affinity acts between the component parts of a

substance and cohesion on a substance in its homogeneity, although both

act at imperceptible distances (by contact) and have much in common.

Chemical force, which makes one substance penetrate into another,

cannot be entirely identified with even those attracting forces which

make different substances adhere to each other, or hold together (as

when two plane-polished surfaces of solid substances are brought into

close contact), or which cause liquids to soak into solids, or adhere

to their surfaces, or gases and vapours to condense on the surfaces

of solids. These forces must not be confounded with chemical forces,

which cause one substance to penetrate into the substance of another

and to form a new substance, which is never the case with cohesion.

But it is evident that the forces which determine cohesion form a

connecting-link between mechanical and chemical forces, because they

only act by intimate contact. For a long time, and especially during

the first half of this century, chemical attraction and chemical forces

were identified with electrical forces. There is certainly an intimate

relation between them, for electricity is evolved in chemical reactions,

and has also a powerful influence on chemical processes--for instance,

compounds are decomposed by the action of an electrical current. And the

exactly similar relation which exists between chemical phenomena and the

phenomena of heat (heat being developed by chemical phenomena, and heat

being able to decompose compounds) only proves the unity of the forces

of nature, the capability of one force to produce and to be transformed

into others. For this reason the identification of chemical force

with electricity will not bear experimental proof.[30] As of all the

(molecular) phenomena of nature which act on substances at immeasurably

small distances, the phenomena of heat are at present the best

(comparatively) known, having been reduced to the simplest fundamental

principles of mechanics (of energy, equilibrium, and movement), which,

since Newton, have been subjected to strict mathematical analysis, it

is quite natural that an effort, which has been particularly pronounced

during recent years, should have been made to bring chemical phenomena

into strict correlation with the already investigated phenomena of

heat, without, however, aiming at any identification of chemical with

heat phenomena. The true nature of chemical force is still a secret

to us, just as is the nature of the universal force of gravity, and

yet without knowing what gravity really is, by applying mechanical

conceptions, astronomical phenomena have been subjected not only to

exact generalisation but to the detailed prediction of a number of

particular facts; and so, also, although the true nature of chemical

affinity may be unknown, there is reason to hope for considerable

progress in chemical science by applying the laws of mechanics to

chemical phenomena by means of the mechanical theory of heat. As yet

this portion of chemistry has been but little worked at, and therefore,

while forming a current problem of the science, it is treated more

fully in that particular field which is termed either 'theoretical'

or 'physical' chemistry, or, more correctly, _chemical mechanics_. As

this province of chemistry requires a knowledge not only of the various

homogeneous substances which have yet been obtained and of the chemical

transformations which they undergo, but also of the phenomena (of heat

and other kinds) by which these transformations are accompanied, it

is only possible to enter on the study of chemical mechanics after an

acquaintance with the fundamental chemical conceptions and substances

which form the subject of this book.[31]

[29] This word, first introduced, if I mistake not, into chemistry

by Glauber, is based on the idea of the ancient philosophers that

combination can only take place when the substances combining

have something in common--a medium. As is generally the case,

another idea evolved itself in antiquity, and has lived until

now, side by side with the first, to which it is exactly

contradictory; this considers union as dependent on contrast, on

polar difference, on an effort to fill up a want.

[30] Especially conclusive are those cases of so-called metalepsis

(Dumas, Laurent). Chlorine, in combining with hydrogen, forms

a very stable substance called 'hydrochloric acid,' which is

split up by the action of an electrical current into chlorine

and hydrogen, the chlorine appearing at the positive and

the hydrogen at the negative pole. Hence electro-chemists

considered hydrogen to be an electro-positive and chlorine

an electro-negative element, and that they are held together

in virtue of their opposite electrical charges. It appears,

however, from metalepsis, that chlorine can replace hydrogen

(and, inversely, hydrogen can replace chlorine) in its compounds

without in any way changing the grouping of the other elements,

or altering their chief chemical properties. For instance, acetic

acid in which hydrogen has been replaced by chlorine is still

capable of forming salts. It must be observed, whilst considering

this subject, that the explanation suggesting electricity as the

origin of chemical phenomena is unsound, since it attempts to

explain one class of phenomena whose nature is almost unknown by

another class which is no better known. It is most instructive

to remark that together with the electrical theory of chemical

attraction there arose and survives a view which explains the

galvanic current as being a transference of chemical action

through the circuit--_i.e._, regards the origin of electricity as

being a chemical one. It is evident that the connection is very

intimate, although both phenomena are independent and represent

different forms of molecular (atomic) motion, whose real nature

is not yet understood. Nevertheless, the connection between

the phenomena of both categories is not only in itself very

instructive, but it extends the applicability of the general idea

of the unity of the forces of nature, conviction of the truth of

which has held so important a place in the science of the last

ten years.

[31] I consider that in an elementary text-book of chemistry, like the

present, it is only possible and advisable to mention, in

reference to chemical mechanics, a few general ideas and some

particular examples referring more especially to gases, whose

mechanical theory must be regarded as the most complete. The

molecular mechanics of liquids and solids is as yet in embryo,

and contains much that is disputable; for this reason, chemical

mechanics has made less progress in relation to these substances.

It may not be superfluous here to remark, with respect to the

conception of chemical affinity, that up to the present time

gravity, electricity, and heat have all been applied to its

elucidation. Efforts have also been made to introduce the

luminiferous ether into theoretical chemistry, and should that

connection between the phenomena of light and electricity

which was established by Maxwell be worked out more in detail,

doubtless these efforts to elucidate all or a great deal by

the aid of luminiferous ether will again appear in theoretical

chemistry. An independent chemical mechanics of the material

particles of matter, and of their internal (atomic) changes,

would, in my opinion, arise as the result of these efforts.

Two hundred years ago Newton laid the foundation of a truly

scientific theoretical mechanics of external visible motion, and

on this foundation erected the edifice of celestial mechanics.

One hundred years ago Lavoisier arrived at the first fundamental

law of the internal mechanics of invisible particles of matter.

This subject is far from having been developed into a harmonious

whole, because it is much more difficult, and, although many

details have been completely investigated, it does not possess

any starting points. Newton only came after Copernicus and

Kepler, who had discovered empirically the exterior simplicity

of celestial phenomena. Lavoisier and Dalton may, in respect to

the chemical mechanics of the molecular world, be compared to

Copernicus and Kepler. But a Newton has not yet appeared in the

molecular world; when he does, I think that he will find the

fundamental laws of the mechanics of the invisible motions of

matter more easily and more quickly in the chemical structure

of matter than in physical phenomena (of electricity, heat,

and light); for these latter are accomplished by particles of

matter already arranged, whilst it is now clear that the problem

of chemical mechanics mainly lies in the apprehension of those

motions which are invisibly accomplished by the smallest atoms of

matter.

As the chemical changes to which substances are liable proceed from

internal forces proper to these substances, as chemical phenomena

certainly consist of motions of material parts (from the laws

of the indestructibility of matter and of elements), and as the

investigation of mechanical and physical phenomena proves the law of

the _indestructibility of forces_, or the conservation of energy--that

is, the possibility of the transformation of one kind of motion into

another (of visible or mechanical into invisible or physical)--we are

inevitably obliged to acknowledge the presence in substances (and

especially in the elements of which all others are composed) of a store

of _chemical energy_ or invisible motion inducing them to enter into

combinations. If heat be evolved in a reaction, it means that a portion

of chemical energy is transformed into heat;[32] if heat be absorbed

in a reaction,[33] that it is partly transformed (rendered latent) into

chemical energy. The store of force or energy going to the formation

of new compounds may, after several combinations, accomplished with an

absorption of heat, at last diminish to such a degree that indifferent

compounds will be obtained, although these sometimes, by combining with

energetic elements or compounds, give more complex compounds, which

may be capable of entering into chemical combination. Among elements,

gold, platinum, and nitrogen have but little energy, whilst potassium,

oxygen, and chlorine have a very marked degree of energy. When

dissimilar substances enter into combination they often form substances

of diminished energy. Thus sulphur and potassium when heated easily burn

in air, but when combined together their compound is neither inflammable

nor burns in air like its component parts. Part of the energy of the

potassium and of the sulphur was evolved in their combination in the

form of heat. Just as in the passage of substances from one physical

state into another a portion of their store of heat is absorbed or

evolved, so in combinations or decompositions and in every chemical

process, there occurs a change in the store of chemical energy, and at

the same time an evolution or absorption of heat.[34]

[32] The theory of heat gave the idea of a store of internal motion

or energy, and therefore with it, it became necessary to

acknowledge chemical energy, but there is no foundation whatever

for identifying heat energy with chemical energy. It may be

supposed, but not positively affirmed, that heat motion is

proper to molecules and chemical motion to atoms, but that as

molecules are made up of atoms, the motion of the one passes to

the other, and that for this reason heat strongly influences

reaction and appears or disappears (is absorbed) in reactions.

These relations, which are apparent and hardly subject to doubt

on general lines, still present much that is doubtful in detail,

because all forms of molecular and atomic motion are able to pass

into each other.

[33] The reactions which take place (at the ordinary or at a high

temperature) directly between substances may be clearly divided

into exothermal, which are accompanied by an evolution of heat,

and endothermal, which are accompanied by an absorption of heat.

It is evident that the latter require a source of heat. They are

determined either by the directly surrounding medium (as in the

formation of carbon bisulphide from charcoal and sulphur, or in

decompositions which take place at high temperatures), or else by

a secondary reaction proceeding simultaneously, or by some other

form of energy (light, electricity). So, for instance, hydrogen

sulphide is decomposed by iodine in the presence of water at the

expense of the heat which is evolved by the solution in water

of the hydrogen iodide produced. This is the reason why this

reaction, as exothermal, only takes place in the presence of

water; otherwise it would be accompanied by a cooling effect. As

in the combination of dissimilar substances, the bonds existing

between the molecules and atoms of the homogeneous substances

have to be broken asunder, whilst in reactions of rearrangement

the formation of any one substance proceeds simultaneously with

the formation of another, and, as in reactions, a series of

physical and mechanical changes take place, it is impossible to

separate the heat directly depending on a given reaction from

the total sum of the observed heat effect. For this reason,

thermochemical data are very complex, and cannot by themselves

give the key to many chemical problems, as it was at first

supposed they might. They ought to form a part of chemical

mechanics, but alone they do not constitute it.

[34] As chemical reactions are effected by heating, so the heat

absorbed by substances before decomposition or change of

state, and called 'specific heat,' goes in many cases to the

preparation, if it may be so expressed, of reaction, even when

the limit of the temperature of reaction is not attained. The

molecules of a substance A, which is not able to react on a

substance B below a temperature _t_, by being heated from a

somewhat lower temperature to _t_, undergoes that change which

had to be arrived at for the formation of A B.

For the comprehension of chemical phenomena as mechanical

processes--_i.e._, the study of the _modus operandi_ of chemical

phenomena--it is most important to consider: (1) the facts gathered

from stoïchiometry, or that part of chemistry which treats of the

quantitative relation, by weight or volume, of the reacting substances;

(2) the distinction between the different forms and classes of

chemical reactions; (3) the study of the changes in properties

produced by alteration in composition; (4) the study of the phenomena

which accompany chemical transformation; (5) a generalisation of the

conditions under which reactions occur. As regards stoïchiometry,

this branch of chemistry has been worked out most thoroughly, and

comprises laws (of Dalton, Avogadro-Gerhardt, and others) which bear

so deeply on all parts of chemistry that at the present time the chief

problem of chemical research consists in the application of general

stoïchiometrical laws to concrete examples, _i.e._, the quantitative

(volumetric or gravimetric) composition of substances. All other

branches of chemistry are clearly subordinate to this most important

portion of chemical knowledge. Even the very signification of reactions

of combination, decomposition, and rearrangement, acquired, as we shall

see, a particular and new character under the influence of the progress

of exact ideas concerning the quantitative relations of substances

entering into chemical changes. Furthermore, in this sense there arose

a new--and, till then, unknown--division of compound substances into

_definite_ and _indefinite_ compounds. Even at the beginning of this

century, Berthollet had not made this distinction. But Prout showed

that a number of compounds contain the substances of which they are they

break up, in exact definite proportions by weight, which are unalterable

under any conditions. Thus, for example, red mercury oxide always

contains sixteen parts by weight of oxygen for every 200 parts by weight

of mercury, which is expressed by the formula HgO. But in an alloy of

copper and silver one or the other metal may be added at will, and in an

aqueous solution of sugar, the relative proportion of the sugar and water

may be altered and nevertheless a homogeneous whole with the sum of the

independent properties will be obtained--_i.e._, in these cases there was

indefinite chemical combination. Although in nature and chemical practice

the formation of indefinite compounds (such as alloys and solutions)

plays as essential a part as the formation of definite chemical

compounds, yet, as the stoïchiometrical laws at present apply chiefly to

the latter, all facts concerning indefinite compounds suffer from

inexactitude, and it is only during recent years that the attention of

chemists has been directed to this province of chemistry.

In chemical mechanics it is, from a qualitative point of view, very

important to clearly distinguish at the very beginning between

_reversible_ and _non-reversible reactions_. Substances capable of

reacting on each other at a certain temperature produce substances which

at the same temperature either can or cannot give back the original

substances. For example, salt dissolves in water at the ordinary

temperature, and the solution so obtained is capable of breaking up

at the same temperature, leaving salt and separating the water by

evaporation. Carbon bisulphide is formed from sulphur and carbon at

about the same temperature at which it can be resolved into sulphur

and carbon. Iron, at a certain temperature, separates hydrogen from

water, forming iron oxide, which, in contact with hydrogen at the same

temperature, is able to produce iron and water. It is evident that if

two substances, A and B, give two others C and D, and the reaction be

reversible, then C and D will form A and B, and, consequently, by taking

a definite mass of A and B, or a corresponding mass of C and D, we shall

obtain, in each case, all four substances--that is to say, there will be

a state of _chemical equilibrium_ between the reacting substances. By

increasing the mass of one of the substances we obtain a new condition

of equilibrium, so that reversible reactions present a means of studying

the _influence of mass_ on the _modus operandi_ of chemical changes.

Many of those reactions which occur with very complicated compounds or

mixtures may serve as examples of non-reversible reactions. Thus many of

the compound substances of animal and vegetable organisms are broken up

by heat, but cannot be re-formed from their products of decomposition at

any temperature. Gunpowder, as a mixture of sulphur, nitre, and carbon,

on being exploded, forms gases from which the original substances cannot

be re-formed at any temperature. In order to obtain them, recourse

must be had to an indirect method _of combination at the moment of

separation_. If A does not under any circumstances combine directly

with B, it does not follow that it cannot give a compound A B. For A

can often combine with C and B with D, and if C has a great affinity

for D, then the reaction of A C or B D produces not only C D, but also

A B. As on the formation of C D, the substances A and B (previously in

A C and B D) are left in a peculiar state of separation, it is supposed

that their mutual combination occurs because they meet together in this

_nascent state_ at the moment of separation (_in statu nascendi_). Thus

chlorine does not directly combine with charcoal, graphite, or diamond;

there are, nevertheless, compounds of chlorine with carbon, and many

of them are distinguished by their stability. They are obtained in the

action of chlorine on hydrocarbons, as the separation products from the

direct action of chlorine on hydrogen. Chlorine takes up the hydrogen,

and the carbon liberated at the moment of its separation, enters into

combination with another portion of the chlorine, so that in the end the

chlorine is combined with both the hydrogen and the carbon.[35]

[35] It is possible to imagine that the cause of a great many of such

reactions is, that substances taken in a separate state, for

instance, charcoal, present a complex molecule composed of

separate atoms of carbon which are fastened together (united, as

is usually said) by a considerable affinity; for atoms of the

same kind, just like atoms of different kinds, possess a mutual

affinity. The affinity of chlorine for carbon, although unable

to break this bond asunder, may be sufficient to form a stable

compound with atoms of carbon, which are already separate. Such

a view of the subject presents a hypothesis which, although

dominant at the present time, is without sufficiently firm

foundation. It is evident, however, that not only does chemical

reaction itself consist of motions, but that in the compound

formed (in the molecules) the elements (atoms) forming it are in

harmonious stable motion (like the planets in the solar system),

and this motion will affect the stability and capacity for

reaction, and therefore the mechanical side of chemical action

must be exceedingly complex. Just as there are solid, physically

constant non-volatile substances like rock, gold, charcoal,

&c., so are there stable and chemically constant bodies; while

corresponding to physically volatile substances there are bodies

like camphor, which are chemically unstable and variable.

As regards those phenomena which accompany chemical action, the most

important circumstance in reference to chemical mechanics is that not

only do chemical processes produce a mechanical displacement (a motion

of particles), heat, light, electrical potential and current; but that

all these agents are themselves capable of changing and governing

chemical transformations. This reciprocity or reversibility naturally

depends on the fact that all the phenomena of nature are only different

kinds and forms of visible and invisible (molecular) motions. First

sound, and then light, was shown to consist of vibratory motions, as

the laws of physics have proved and developed beyond a doubt. The

connection between heat and mechanical motion and work has ceased to

be a supposition, but has become an accepted fact, and the mechanical

equivalent of heat (425 kilogrammetres of mechanical work correspond

with one kilogram unit of heat or Calorie) gives a mechanical measure

for thermal phenomena. Although the mechanical theory of electrical

phenomena cannot be considered so fully developed as the theory of heat,

both statical and dynamical electricity are produced by mechanical

means (in common electrical machines or in Gramme or other dynamos),

and conversely, a current (in electric motors) can produce mechanical

motion. Thus by connecting a current with the poles of a Gramme dynamo

it may be made to revolve, and, conversely, by rotating it an electrical

current is produced, which demonstrates the reversibility of electricity

into mechanical motion. Accordingly chemical mechanics must look for

the fundamental lines of its advancement in the correlation of chemical

with physical and mechanical phenomena. But this subject, owing to its

complexity and comparative novelty, has not yet been expressed by a

harmonious theory, or even by a satisfactory hypothesis, and therefore

we shall avoid lingering over it.

A chemical change in a certain direction is accomplished not only by

reason of the difference of masses, the composition of the substances

concerned, the distribution of their parts, and their affinity or

chemical energy, but also by reason of the _conditions_ under which the

substances occur. In order that a certain chemical reaction may take

place between substances which are capable of reacting on each other, it

is often necessary to have recourse to conditions which are sometimes

very different from those in which the substances usually occur in

nature. For example, not only is the presence of air (oxygen) necessary

for the combustion of charcoal, but the latter must also be heated to

redness. The red-hot portion of the charcoal burns--_i.e._ combines with

the oxygen of the atmosphere--and in so doing evolves heat, which raises

the temperature of the adjacent parts of charcoal, so that they burn.

Just as the combustion of charcoal is dependent on its being heated

to redness, so also every chemical reaction only takes place under

certain physical, mechanical, or other conditions. The following are the

chief conditions which exert an influence on the progress of chemical

reactions.

(_a_) _Temperature._--Chemical reactions of combination only take

place within certain definite limits of temperature, and cannot be

accomplished outside these limits. We may cite as examples not only

that the combustion of charcoal begins at a red heat, but also that

chlorine and salt only combine with water at a temperature below 0°.

These compounds cannot be formed at a higher temperature, for they are

then wholly or partially broken up into their component parts. A certain

rise in temperature is necessary to start combustion. In certain cases

the effect of this rise may be explained as causing one of the reacting

bodies to change from a solid into a liquid or gaseous form. The

transference into a fluid form facilitates the progress of the reaction,

because it aids the intimate contact of the particles reacting on

each other. Another reason, and to this must be ascribed the chief

influence of heat in exciting chemical action, is that the physical

cohesion, or the internal chemical union, of homogeneous particles is

thereby weakened, and in this way the separation of the particles of

the original substances, and their transference into new compounds, is

rendered easier. When a reaction absorbs heat--as in decomposition--the

reason why heat is necessary is self-evident.

At the present day it may be asserted upon the basis of existing data,

respecting the action of high temperature, that all compound bodies are

decomposed at a more or less high temperature. We have already seen

examples of this in describing the decomposition of mercury oxide into

mercury and oxygen, and the decomposition of wood under the influence

of heat. Many substances are decomposed at a very moderate temperature;

for instance, the fulminating salt which is employed in cartridges is

decomposed at a little above 120°. The majority of those compounds

which make up the mass of animal and vegetable tissues are decomposed

at 200°. On the other hand, there is reason to think that at a very

low temperature no reaction whatever can take place. Thus plants cease

to carry on their chemical processes during the winter. Raoul Pictet

(1892), employing the very low temperatures (as low as -200°C.) obtained

by the evaporation of liquefied gases (_see_ Chap. II.), has recently

again proved that at temperatures below -120°, even such reactions as

those between sulphuric acid and caustic soda or metallic sodium do not

take place, and even the coloration of litmus by acids only commences

at temperatures above -80°. If a given reaction does not take place at

a certain low temperature, it will at first only proceed slowly with

a rise of temperature (even if aided by an electric discharge), and

will only proceed rapidly, with the evolution of heat, when a certain

definite temperature has been reached. Every chemical reaction requires

certain limits of temperature for its accomplishment, and, doubtless,

many of the chemical changes observed by us cannot take place in the

sun, where the temperature is very high, or on the moon, where it is

very low.

The influence of heat on reversible reactions is particularly

instructive. If, for instance, a compound which is capable of being

reproduced from its products of decomposition be heated up to the

temperature at which decomposition begins, the decomposition of a mass

of the substance contained in a definite volume is not immediately

completed. Only a certain fraction of the substance is decomposed, the

other portion remaining unchanged, and if the temperature be raised, the

quantity of the substance decomposed increases; furthermore, for a given

volume, the ratio between the part decomposed and the part unaltered

corresponds with each definite rise in temperature until it reaches that

at which the compound is entirely decomposed. This partial decomposition

under the influence of heat is called _dissociation_. It is possible

to distinguish between the temperatures at which dissociation begins

and ends. Should dissociation proceed at a certain temperature, yet

should the product or products of decomposition not remain in contact

with the still undecomposed portion of the compound, then decomposition

will go on to the end. Thus limestone is decomposed in a limekiln

into lime and carbonic anhydride, because the latter is carried off

by the draught of the furnace. But if a certain mass of limestone be

enclosed in a definite volume--for instance, in a gun barrel--which is

then sealed up, and heated to redness, then, as the carbonic anhydride

cannot escape, a certain proportion only of the limestone will be

decomposed for every increment of heat (rise in temperature) higher

than that at which dissociation begins. Decomposition will cease

when the carbonic anhydride evolved presents a maximum _dissociation

pressure_ corresponding with each rise in temperature. If the pressure

be increased by increasing the quantity of gas, then combination begins

afresh; if the pressure be diminished decomposition will recommence.

Decomposition in this case is exactly similar to evaporation; if the

steam given off by evaporation cannot escape, its pressure will reach a

maximum corresponding with the given temperature, and then evaporation

will cease. Should steam be added it will be condensed in the liquid;

if its quantity be diminished--_i.e._ if the pressure be lessened,

the temperature being constant--then evaporation will go on. We shall

afterwards discuss more fully these phenomena of dissociation, which

were first discovered by Henri St. Claire Deville. We will only remark

that the products of decomposition re-combine with greater facility the

nearer their temperature is to that at which dissociation begins, or, in

other words, that the initial temperature of dissociation is near to the

initial temperature of combination.

(_b_) _The influence of an electric current_, and of electricity in

general, on the progress of chemical transformations is very similar

to the influence of heat. The majority of compounds which conduct

electricity are decomposed by the action of a galvanic current, and as

there is great similarity in the conditions under which decomposition

and combination proceed, combination often proceeds under the influence

of electricity. Electricity, like heat, must be regarded as a peculiar

form of molecular motion, and all that refers to the influence of heat

also refers to the phenomena produced by the action of an electrical

current, with this difference, only that a substance can be separated

into its component parts with much greater ease by electricity, since

the process goes on at the ordinary temperature. The most stable

compounds may be decomposed by this means, and a most important fact

is then observed--namely, that the component parts appear at the

different poles of electrodes by which the current passes through the

substance. Those substances which appear at the positive pole (anode)

are called 'electro-negative,' and those which appear at the negative

pole (cathode, that in connection with the zinc of an ordinary galvanic

battery) are called 'electro-positive.' The majority of non-metallic

elements, such as chlorine, oxygen, &c., and also acids and substances

analogous to them, belong to the first group, whilst the metals,

hydrogen, and analogous products of decomposition appear at the negative

pole. Chemistry is indebted to the decomposition of compounds by the

electric current for many most important discoveries. Many elements

have been discovered by this method, the most important being potassium

and sodium. Lavoisier and the chemists of his time were not able to

decompose the oxygen compounds of these metals, but Davy showed that

they might be decomposed by an electric current, the metals sodium and

potassium appearing at the negative pole. Now that the dynamo gives the

possibility of producing an electric current by the combustion of fuel,

this method of Sir H. Davy is advantageously employed for obtaining

metals, &c. on a large scale, for instance, sodium from fused caustic

soda or chlorine from solutions of salt.

(_c_) Certain unstable compounds are also decomposed by _the action of

light_. Photography is based on this property in certain substances

(for instance, in the salts of silver). The mechanical energy of those

vibrations which determine the phenomena of light is very small, and

therefore only certain, and these generally unstable, compounds can be

decomposed by light--at least under ordinary circumstances. But there is

one class of chemical phenomena dependent on the action of light which

forms as yet an unsolved problem in chemistry--these are the processes

accomplished in plants under the influence of light. Here there take

place most unexpected decompositions and combinations, which are often

unattainable by artificial means. For instance, carbonic anhydride,

which is so stable under the influence of heat and electricity, is

decomposed and evolves oxygen in plants under the influence of light. In

other cases, light decomposes unstable compounds, such as are usually

easily decomposed by heat and other agents. Chlorine combines with

hydrogen under the influence of light, which shows that combination, as

well as decomposition, can be determined by its action, as was likewise

the case with heat and electricity.

(_d_) _Mechanical causes_ exert, like the foregoing agents, an action

both on the process of chemical combination and of decomposition. Many

substances are decomposed by friction or by a blow--as, for example,

the compound called iodide of nitrogen (which is composed of iodine,

nitrogen, and hydrogen), and silver fulminate. Mechanical friction

causes sulphur to burn at the expense of the oxygen contained in

potassium chlorate. Pressure affects both the physical and chemical

state of the reacting substances, and, together with the temperature,

determines the state of a substance. This is particularly evident when

the substance occurs in an elastic-gaseous form since the volume, and

hence also the number of points of encounter between the reacting

substances is greatly altered by a change of pressure. Thus, under equal

conditions of temperature, hydrogen when compressed acts more powerfully

upon iodine and on the solutions of many salts.

(_e_) Besides the various conditions which have been enumerated above,

the progress of chemical reactions is accelerated or retarded by the

_condition of contact_ in which the reacting bodies occur. Other

conditions remaining constant, the rate of progress of a chemical

reaction is accelerated by increasing the number of points of contact.

It will be enough to point out the fact that sulphuric acid does not

absorb ethylene under ordinary conditions of contact, but only after

continued shaking, by which means the number of points of contact

is greatly increased. To ensure complete action between solids, it

is necessary to reduce them to very fine powder and to mix them as

thoroughly as possible. M. Spring, the Belgian chemist, has shown

that finely powdered solids which do not react on each other at the

ordinary temperature may do so under an increased pressure. Thus, under

a pressure of 6,000 atmospheres, sulphur combines with many metals at

the ordinary temperature, and mixtures of the powders of many metals

form alloys. It is evident that an increase in the number of points

or surfaces must be regarded as the chief cause producing reaction,

which is doubtless accomplished in solids, as in liquids and gases, in

virtue of an internal motion of the particles, which motion, although

in different degrees and forms, must exist in all the states of matter.

It is very important to direct attention to the fact that the internal

motion or condition of the parts of the particles of matter must be

different on the surface of a substance from what it is inside; because

in the interior of a substance similar particles are acting on all sides

of every particle, whilst at the surface they act on one side only.

Therefore, the condition of a substance at its surfaces of contact with

other substances must be more or less modified by them--it may be in a

manner similar to that caused by an elevation of temperature. These

considerations throw some light on the action in the large class of

_contact reactions_; that is, such as appear to proceed from the mere

presence (contact) of certain special substances. Porous or powdery

substances are very prone to act in this way, especially spongy platinum

and charcoal. For example, sulphurous anhydride does not combine

directly with oxygen, but this reaction takes place in the presence of

spongy platinum.[36]

[36] Contact phenomena are separately considered in detail in the work

of Professor Konovaloff (1884). In my opinion, it must be

held that the state of the internal motions of the atoms in

molecules is modified at the points of contact of substances,

and this state determines chemical reactions, and therefore,

that reactions of combination, decomposition, and rearrangement

are accomplished by contact. Professor Konovaloff showed that a

number of substances, under certain conditions of their surfaces,

act by contact; for instance, finely divided silica (from the

hydrate) acts just like platinum, decomposing certain compound

ethers. As reactions are only accomplished under close contact,

it is probable that those modifications in the distribution of

the atoms in molecules which come about by contact phenomena

prepare the way for them. By this the _rôle_ of contact phenomena

is considerably extended. Such phenomena should explain the fact

why a mixture of hydrogen and oxygen yields water (explodes) at

different temperatures, according to the kind of heated substance

which transmits this temperature. In chemical mechanics,

phenomena of this kind have great importance, but as yet they

have been but little studied. It must not be forgotten that

contact is a necessary condition for every chemical reaction.

The above general and introductory chemical conceptions cannot be

thoroughly grasped in their true sense without a knowledge of the

particular facts of chemistry to which we shall now turn our attention.

It was, however, absolutely necessary to become acquainted on the

very threshold with such fundamental principles as the laws of the

indestructibility of matter and of the conservation of energy, since it

is only by their acceptance, and under their direction and influence,

that the examination of particular facts can give practical and fruitful

results.

CHAPTER I

ON WATER AND ITS COMPOUNDS

Water is found almost everywhere in nature, and in all three physical

states. As vapour, water occurs in the atmosphere, and in this form

it is distributed over the entire surface of the earth. The vapour of

water in condensing, by cooling, forms snow, rain, hail, dew, and fog.

One cubic metre (or 1,000,000 cubic centimetres, or 1,000 litres, or

35·316 cubic feet) of air can contain at 0° only 4·8 grams of water, at

20° about 17·0 grams, at 40° about 50·7 grams; but ordinary air only

contains about 60 per cent. of this maximum. Air containing less than

40 per cent. is felt to be dry, whilst air which contains more than

80 per cent. of the same maximum is considered as distinctly damp.[1]

Water in the liquid state, in falling as rain and snow, soaks into

the soil and collects together into springs, lakes, rivers, seas, and

oceans. It is absorbed from the soil by the roots of plants, which, when

fresh, contain from 40 to 80 per cent. of water by weight. Animals

contain about the same amount of water. In a solid state, water appears

as snow, ice, or in an intermediate form between these two, which is

seen on mountains covered with perpetual snow. The water of rivers,[2]

springs, oceans and seas, lakes, and wells contains various substances

in solution mostly salt,--that is, substances resembling common table

salt in their physical properties and chief chemical transformations.

Further, the quantity and nature of these salts differ in different

waters.[3] Everybody knows that there are salt, fresh, iron, and

other waters. The presence of about 3-1/2 per cent. of salts renders

sea-water[4] bitter to the taste and increases its specific gravity.

Fresh water also contains salts, but only in a comparatively small

quantity. Their presence may be easily proved by simply evaporating

water in a vessel. On evaporation the water passes away as vapour,

whilst the salts are left behind. This is why a crust (incrustation),

consisting of salts, previously in solution, is deposited on the insides

of kettles or boilers, and other vessels in which water is boiled.

Running water (rivers, &c.) is charged with salts, owing to its being

formed from the collection of rain water percolating through the soil.

While percolating, the water dissolves certain parts of the soil. Thus

water which filters or passes through saline or calcareous soils becomes

charged with salts or contains calcium carbonate (chalk). Rain water and

snow are much purer than river or spring water. Nevertheless, in passing

through the atmosphere, rain and snow succeed in catching the dust held

in it, and dissolve air, which is found in every water. The dissolved

gases of the atmosphere are partly disengaged, as bubbles from water on

heating, and water after long boiling is quite freed from them.

[1] In practice, the chemist has to continually deal with gases, and

gases are often collected over water; in which case a certain

amount of water passes into vapour, and this vapour mixes with

the gases. It is therefore most important that he should be able

to calculate the amount of water or of _moisture in air and other

gases_. Let us imagine a cylinder standing in a mercury bath, and

filled with a dry gas whose volume equals _v_, temperature _t_°,

and pressure or tension _h_ mm. (_h_ millimetres of the column

of mercury at 0°). We will introduce water into the cylinder in

such a quantity that a small part remains in the liquid state, and

consequently that the gas will be saturated with aqueous vapour;

the volume of the gas will then increase (if a larger quantity

of water be taken some of the gas will he dissolved in it, and

the volume may therefore he diminished). We will further suppose

that, after the addition of the water, the temperature remains

constant; then since the volume increases, the mercury in the

cylinder falls, and therefore the pressure as well as the volume

is increased. In order to investigate the phenomenon we will

artificially increase the pressure, and reduce the volume to the

original volume _v_. Then the pressure or tension will be greater

than _h_, namely _h_ + _f_, which means that by the introduction

of aqueous vapour the pressure of the gas is increased. The

researches of Dalton, Gay-Lussac, and Regnault showed that this

increase is equal to the maximum pressure which is proper to

the aqueous vapour at the temperature at which the observation

is made. The maximum pressure for all temperatures may be found

in the tables made from observations on the pressure of aqueous

vapour. The quantity _f_ will be equal to this maximum pressure

of aqueous vapour. This may be expressed thus: the maximum

tension of aqueous vapour (and of all other vapours) saturating

a space in a vacuum or in any gas is the same. This rule is

known as _Dalton's law_. Thus we have a volume of dry gas _v_,

under a pressure _h_, and a volume of moist gas, saturated with

vapour, under a pressure _h_ + _f_. The volume _v_ of the dry gas

under a pressure _h_ + _f_ occupies, from Boyle's law, a volume

_vh_/_h_ + _f_; consequently the volume occupied by the aqueous

vapour under the pressure _h_ + _f_ equals _v_-_vh_/(_h_ + _f_),

or _vf_/(_h_ + _f_). Thus the volumes of the dry gas and of the

moisture which occurs in it, at a pressure _h_ + _f_, are in the

ratio _f_ : _h_. And, therefore, if the aqueous vapour saturates

a space at a pressure _n_, the volumes of the dry air and of the

moisture which is contained in it are in the ratio (_n_-_f_) :

_f_, where _f_ is the pressure of the vapour according to the

tables of vapour tension. Thus, if a volume N of a gas saturated

with moisture be measured at a pressure H, then the volume of

the gas, when dry, will be equal to N[(H-f)/H]. In fact, the

entire volume N must be to the volume of dry gas _x_ as H is to

H-_f_; therefore, N : _x_ = H : H-_f_, from which _x_ = N[(H-f)/H].

Under any other pressure--for instance, 760 mm.--The volume of

dry gas will be _x_H/760, or (H-_f_)/760, and we thus obtain

the following practical rule: If a volume of a gas saturated

with aqueous vapour be measured at a pressure H mm., then the

volume of dry gas contained in it will be obtained by finding the

volume corresponding to the pressure H, less the pressure due

to the aqueous vapour at the temperature observed. For example,

37·5 cubic centimetres of air saturated with aqueous vapour were

measured at a temperature of 15·3°, and under a pressure of 747·3

mm. of mercury (at 0°). What will be the volume of dry gas at 0°

and 760 mm.?

The pressure of aqueous vapour corresponding to 15·3° is equal to

12·9 mm., and therefore the volume of dry gas at 15·3° and 747·3

mm. is equal to 37·5 × (747·3-12·9)/747·3; at 760 mm. it will be

equal to 37·5 × (734·4/760); and at 0° the volume of dry gas will

be 37·5 × (734·4/760) × 273/(273 + 15·3) = 34·31 c.c.

From this rule may also be calculated what fraction of a volume

of gas is occupied by moisture under the ordinary pressure at

different temperatures; for instance, at 30° C. _f_ = 31·5,

consequently 100 volumes of a moist gas or air, at 760 mm.,

contain a volume of aqueous vapour 100 × (31·5/760), or 4·110; it

is also found that at 0° there is contained 0·61 p.c. by volume,

at 10° 1·21 p.c., at 20° 2·29 p.c., and at 50° up to 12·11 p.c.

From this it may be judged how great an error might be made in

the measurement of gases by volume if the moisture were not taken

into consideration. From this it is also evident how great are

the variations in volume of the atmosphere when it loses or gains

aqueous vapour, which again explains a number of atmospheric

phenomena (winds, variation of pressure, rainfalls, storms, &c.)

If a gas is not saturated, then it is indispensable that the

degree of moisture should be known in order to determine the

volume of dry gas from the volume of moist gas. The preceding

ratio gives the maximum quantity of water which can be held in

a gas, and the degree of moisture shows what fraction of this

maximum quantity occurs in a given case, when the vapour does

not saturate the space occupied by the gas. Consequently, if the

degree of moisture equals 50 p.c.--that is, half the maximum--then

the volume of dry gas at 760 mm. is equal to the volume of dry

gas at 760 mm. multiplied by (_h_-0·5_f_)/760, or, in general, by

(_h_-_rf_)/760 where _r_ is the degree of moisture. Thus, if it

is required to measure the volume of a moist gas, it must either

be thoroughly dried or quite saturated with moisture, or else

the degree of moisture determined. The first and last methods

are inconvenient, and therefore recourse is usually had to the

second. For this purpose water is introduced into the cylinder

holding the gas to be measured; it is left for a certain time so

that the gas may become saturated, the precaution being taken

that a portion of the water remains in a liquid state; then the

volume of the moist gas is determined, from which that of the

dry gas may be calculated. In order to find the _weight of the

aqueous vapour_ in a gas it is necessary to know the weight of a

cubic measure at 0° and 760 mm. Knowing that one cubic centimetre

of air in these circumstances weighs 0·001293 gram, and that

the density of aqueous vapour is 0·62, we find that one cubic

centimetre of aqueous vapour at 0° and 760 mm. weighs 0·0008 gram,

and at a temperature _t_° and pressure _h_ the weight of one cubic

centimetre will be 0·0008 × _h_/760 × 273/(273 + _t_). We already

know that _v_ volumes of a gas at a temperature _t_° pressure _h_

contain _v_ × _f_/_h_ volumes of aqueous vapour which saturate it,

therefore the weight of the aqueous vapour held in _v_ volumes of

a gas will be

_v_ x 0·0008 × _f_/760 × 273/(273 + _t_).

Accordingly, the weight of water which is contained in one volume

of a gas depends only on the temperature and not on the pressure.

This also signifies that evaporation proceeds to the same extent

in air as in a vacuum, or, in general terms (this is _Dalton's

law_), vapours and gases diffuse into each other as if into a

vacuum. In a given space, at a given temperature, a constant

quantity of vapour enters, whatever be the pressure of the gas

filling that space.

From this it is clear that if the weight of the vapour contained

in a given volume of a gas be known, it is easy to determine the

degree of moisture _r_ = _p_/(_v_ × 0·0008) × 760/_t_ × (273 +

_t_)/273. On the is founded the very exact determination of the

degree of moisture of air by the weight of water contained in a

given volume. It is easy to calculate from the preceding formula

the number of grams of water contained at any pressure in one

cubic metre or million cubic centimetres of air saturated with

vapour at various temperatures; for instance, at 30° _f_ = 31·5,

hence _p_ = 29·84 grams.

The laws of Mariotte, Dalton, and Gay-Lussac, which are here

applied to gases and vapours, are not entirely exact, but are

approximately true. If they were quite exact, a mixture of several

liquids, having a certain vapour pressure, would give vapours of

a very high pressure, which is not the case. In fact the pressure

of aqueous vapour is slightly less in a gas than in a vacuum,

and the weight of aqueous vapour held in a gas is slightly less

than it should be according to Dalton's law, as was shown by the

experiments of Regnault and others. This means that the tension

of the vapour is less in air than in a vacuum. The difference

does not, however, exceed 5 per cent. of the total pressure of

the vapours. This _decrement in vapour tension_ which occurs in

the intermixture of vapours and gases, although small, indicates

that there is then already, so to speak, a beginning of chemical

change. The essence of the matter is that in this case there

occurs, as on contact (see preceding footnote), an alteration in

the motions of the atoms in the molecules, and therefore also a

change in the motion of the molecules themselves.

In the uniform intermixture of air and other gases with aqueous

vapour, and in the capacity of water to pass into vapour and

form a uniform mixture with air, we may perceive an instance of

a physical phenomenon which is analogous to chemical phenomena,

forming indeed a transition from one class of phenomena to

the other. Between water and dry air there exists a kind of

affinity which obliges the water to saturate the air. But such a

homogeneous mixture is formed (almost) independently of the nature

of the gas in which evaporation takes place; even in a vacuum

the phenomenon occurs in exactly the same way as in a gas, and

therefore it is not the property of the gas, nor its relation to

water, but the property of the water itself, which compels it to

evaporate, and therefore in this case chemical affinity is not yet

operative--at least its action is not clearly pronounced. That it

does, however, play a certain part is seen from the deviation from

Dalton's law.

[2] In falling through the atmosphere, water dissolves the gases of

the atmosphere, nitric acid, ammonia, organic compounds, salts

of sodium, magnesium, and calcium, and mechanically washes

out a mixture of dust and microbes which are suspended in the

atmosphere. The amount of these and certain other constituents

is very variable. Even in the beginning and end of the same

rainfall a variation which is often very considerable may be

remarked. Thus, for example, Bunsen found that rain collected

at the beginning of a shower contained 3·7 grams of ammonia per

cubic metre, whilst that collected at the end of the same shower

contained only O·64 gram. The water of the entire shower contained

an average of 1·47 gram of ammonia per cubic metre. In the course

of a year rain supplies an acre of ground with as much as 5-1/2

kilos of nitrogen in a combined form. Marchand found in one cubic

metre of snow water 15·63, and in one cubic metre of rain water

10·07, grams of sodium sulphate. Angus Smith showed that after a

thirty hours' fall at Manchester the rain still contained 34·3

grams of salts per cubic metre. A considerable amount of organic

matter, namely 25 grams per cubic metre, has been found in rain

water. The total amount of solid matter in rain water reaches 50

grams per cubic metre. Rain water generally contains very little

carbonic acid, whilst river water contains a considerable quantity

of it. In considering the nourishment of plants it is necessary

to keep in view the substances which are carried into the soil by

rain.

_River water_, which is accumulated from springs and sources

fed by atmospheric water, contains from 50 to 1,600 parts by

weight of salts in 1,000,000 parts. The amount of solid matter,

per 1,000,000 parts by weight, contained in the chief rivers is

as follows:--the Don 124, the Loire 135, the St. Lawrence 170,

the Rhone 182, the Dnieper 187, the Danube from 117 to 234, the

Rhine from 158 to 317, the Seine from 190 to 432, the Thames at

London from 400 to 450, in its upper parts 387, and in its lower

parts up to 1,617, the Nile 1,580, the Jordan 1,052. The Neva is

characterised by the remarkably small amount of solid matter it

contains. From the investigations of Prof. G. K. Trapp, a cubic

metre of Neva water contains 32 grams of incombustible and 23

grams of organic matter, or altogether about 55 grams. This is one

of the purest waters which is known in rivers. The large amount

of impurities in river water, and especially of organic impurity

produced by pollution with putrid matter, makes the water of many

rivers unfit for use.

The chief part of the soluble substances in river water consists

of the calcium salts. 100 parts of the solid residues contain

the following amounts of calcium carbonate--from the water of

the Loire 53, from the Thames about 50, the Elbe 55, the Vistula

65, the Danube 65, the Rhine from 55 to 75, the Seine 75, the

Rhone from 82 to 94. The Neva contains 40 parts of calcium

carbonate per 100 parts of saline matter. The considerable amount

of calcium carbonate which river water contains is very easily

explained from the fact that water which contains carbonic acid

in solution easily dissolves calcium carbonate, which occurs all

over the earth. Besides calcium carbonate and sulphate, river

water contains magnesium, silica, chlorine, sodium, potassium,

aluminium, nitric acid, iron and manganese. The presence of salts

of phosphoric acid has not yet been determined with exactitude

for all rivers, but the presence of nitrates has been proved with

certainty in almost all kinds of well-investigated river water.

The quantity of calcium phosphate does not exceed 0·4 gram in the

water of the Dnieper, and the Don does not contain more than 5

grams. The water of the Seine contains about 15 grams of nitrates,

and that of the Rhone about 8 grams. The amount of ammonia is

much less; thus in the water of the Rhine about 0·5 gram in June,

and 0·2 gram in October; the water of the Seine contains the same

amount. This is less than in rain water. Notwithstanding this

insignificant quantity, the water of the Rhine alone, which is not

so very large a river, carries 16,245 kilograms of ammonia into

the ocean every day. The difference between the amount of ammonia

in rain and river water depends on the fact that the soil through

which the rain water passes is able to retain the ammonia. (Soil

can also absorb many other substances, such as phosphoric acid,

potassium salts, &c.)

The waters of springs, rivers, wells, and in general of those

localities from which it is taken for drinking purposes, may

be injurious to health if it contains much organic pollution,

the more so as in such water the lower organisms (bacteria) may

rapidly develop, and these organisms often serve as the carriers

or causes of infectious diseases. For instance, certain pathogenic

(disease-producing) bacteria are known to produce typhoid, the

Siberian plague, and cholera. Thanks to the work of Pasteur,

Metchnikoff, Koch, and many others, this province of research has

made considerable progress. It is possible to investigate the

number and properties of the germs in water. In bacteriological

researches a gelatinous medium in which the germs can develop and

multiply is prepared with gelatin and water, which has previously

been heated several times, at intervals, to 100° (it is thus

rendered sterile--that is to say, all the germs in it are killed).

The water to be investigated is added to this prepared medium in

a definite and small quantity (sometimes diluted with sterilised

water to facilitate the calculation of the number of germs), it

is protected from dust (which contains germs), and is left at

rest until whole families of lower organisms are developed from

each germ. These families (colonies) are visible to the naked eye

(as spots), they may be counted, and by examining them under the

microscope and observing the number of organisms they produce,

their significance may be determined. The majority of bacteria

are harmless, but there are decidedly pathogenic bacteria, whose

presence is one of the causes of malady and of the spread of

certain diseases. The number of bacteria in one cubic centimetre

of water sometimes attains the immense figures of hundreds of

thousands and millions. Certain well, spring, and river waters

contain very few bacteria, and are free from disease-producing

bacteria under ordinary circumstances. By boiling water, the

bacteria in it are killed, but the organic matter necessary for

their nourishment remains in the water. The best kinds of water

for drinking purposes do not contain more than 300 bacteria in a

cubic centimetre.

The amount of gases dissolved in river water is much more constant

than that of its solid constituents. One litre, or 1,000 c.c., of

water contains 40 to 55 c.c. of gas measured at normal temperature

and pressure. In winter the amount of gas is greater than in

summer or autumn. Assuming that a litre contains 50 c.c. of

gases, it may be admitted that these consist, on an average, of

20 vols. of nitrogen, 20 vols of carbonic anhydride (proceeding

in all likelihood from the soil and not from the atmosphere), and

of 10 vols. of oxygen. If the total amount of gases be less, the

constituent gases are still in about the same proportion; in many

cases, however, carbonic anhydride predominates. The water of many

deep and rapid rivers contains less carbonic anhydride, which

shows their rapid formation from atmospheric water, and that they

have not succeeded, during a long and slow course, in absorbing a

greater quantity of carbonic anhydride. Thus, for instance, the

water of the Rhine, near Strasburg, according to Deville, contains

8 c.c. of carbonic anhydride, 16 c.c. of nitrogen, and 7 c.c. of

oxygen per litre. From the researches of Prof. M. R. Kapoustin and

his pupils, it appears that in determining the quality of a water

for drinking purposes, it is most important to investigate the

composition of the dissolved gases, more especially oxygen.

[3] _Spring water_ is formed from rain water percolating through the

soil. Naturally a part of the rain water is evaporated directly

from the surface of the earth and from the vegetation on it. It

has been shown that out of 100 parts of water falling on the earth

only 36 parts flow to the ocean; the remaining 64 are evaporated,

or percolate far underground. After flowing underground along

some impervious strata, water comes out at the surface in many

places as springs, whose temperature is determined by the depth

from which the water has flowed. Springs penetrating to a great

depth may become considerably heated, and this is why hot mineral

springs, with a temperature of up to 30° and higher, are often met

with. When a spring water contains substances which endow it with

a peculiar taste, and especially if these substances are such as

are only found in minute quantities in river and other flowing

waters, then the spring water is termed a _mineral water_. Many

such waters are employed for medicinal purposes. Mineral waters

are classed according to their composition into--(_a_) saline

waters, which often contain a large amount of common salt; (_b_)

alkaline waters, which contain sodium carbonate; (_c_) bitter

waters, which contain magnesia; (_d_) chalybeate waters, which

hold iron carbonate in solution; (_e_) aërated waters, which are

rich in carbonic anhydride; (_f_) sulphuretted waters, which

contain hydrogen sulphide. Sulphuretted waters may be recognised

by their smell of rotten eggs, and by their giving a black

precipitate with lead salts, and also by their tarnishing silver

objects. Aërated waters, which contain an excess of carbonic

anhydride, effervesce in the air, have a sharp taste, and redden

litmus paper. Saline waters leave a large residue of soluble

solid matter on evaporation, and have a salt taste. Chalybeate

waters have an inky taste, and are coloured black by an infusion

of galls; on being exposed to the air they usually give a brown

precipitate. Generally, the character of mineral waters is mixed.

In the table below the analyses are given of certain mineral

springs which are valued for their medicinal properties. The

quantity of the substances is expressed in millionths by weight.

Column Headings:

A: Calcium salts

B: Sodium chloride

C: Sodium sulphate

D: Sodium carbonate

E: Potassium iodide and bromide

+-------+-------+--------+-------+-------+-----+------+

| | | | | | | |

| | [A] | [B] | [C] | [D] | [E] | [F] |

| | | | | | | |

+-------+-------+--------+-------+-------+-----+------+

| | | | | | | |

| I. | 1,928 | -- | 152 | -- | -- | 24 |

| II. | 816 | 386 | 1,239 | 26 | -- | 43 |

| III. | 1,085 | 1,430 | 1,105 | -- | 4 | 90 |

| IV. | 343 | 3,783 | 16 | 3,431 | -- | 14 |

| V. | 3,406 | 15,049 | -- | -- | 2 | -- |

| VI. | 352 | 3,145 | -- | 95 | 35 | 50 |

| VII. | 308 | 1,036 | 2,583 | 1,261 | 4 | 178 |

| VIII. | 1,726 | 9,480 | -- | -- | 40 | 120 |

| IX. | 551 | 2,040 | 1,150 | 999 | -- | 1 |

| X. | 285 | 558 | 279 | 3,813 | -- | -- |

| | | | | | | |

| XI. | 340 | 910 | Iron and aluminium {1,020 |

| | | | sulphates: {1,660 |

+-------+-------+--------+----------------------------+

Column Headings:

G: Iron carbonate

H: Magnesium salts

I: Silica

J: Carbonic anhydride

K: Sulphuretted hydrogen

L: Total solid contents

+-------+------+-------+-----+-------+-----+-----------+

| | | | | | | |

| | [G] | [H] | [I] | [J] | [K] | [L] |

+-------+------+-------+-----+-------+-----+-----------+

| | | | | | | |

| I. | -- | 448 | 152 | 1,300 | 80 | 2,609 |

| II. | 9 | 257 | 46 | 1,485 | -- | 2,812 |

| III. | -- | 187 | 65 | 1,326 | 11 | 3,950 |

| IV. | -- | 251 | 112 | 2,883 | -- | 7,950 |

| V. | 17 | 1,587 | 229 | -- | 76 | 20,290 |

| VI. | 1 | 260 | 11 | 20 | -- | 3,970 |

| VII. | 4 | 178 | 75 | -- | -- | 5,451 |

| VIII. | 26 | 208 | 40 | -- | -- | 11,790 |

| IX. | 30 | 209 | 50 | 2,740 | -- | 4,070 |

| X. | 7 | 45 | 45 | 2,268 | -- | 5,031 |

| | | | | {Sulphuric |

| XI. | | 940 | 190 | 2,550 {and hydrochloric |

| | | | | 330 {acids |

+-------+------+-------+-----+-------------------------+

I. Sergieffsky, a sulphur water, Gov. of Samara (temp. 8° C.),

analysis by Clause. II. Geléznovodskya water source No. 10,

near Patigorsk, Caucasus (temp. 22·5°), analysis by Fritzsche.

III. Aleksandroffsky, alkaline-sulphur source, Patigorsk (temp.

46·5°), average of analyses by Herman, Zinin and Fritzsche.

IV. Bougountouksky, alkaline source, No. 17, Essentoukah,

Caucasus (temp. 21·6°), analysis by Fritzsche. V. Saline water,

Staro-Russi, Gov. of Novgorod, analysis by Nelubin. VI. Water from

artesian well at the factory of state papers, St. Petersburg,

analysis by Struve. VII. Sprüdel, Carlsbad (temp. 83·7°), analysis

by Berzelius. VIII. Kreuznach spring (Elisenquelle), Prussia

(temp. 8·8°), analysis by Bauer. IX. Eau de Seltz, Nassau,

analysis by Henry. X. Vichy water, France, analysis by Berthier

and Puvy. XI. Paramo de Ruiz, New Granada, analysis by Levy; it is

distinguished by the amount of free acids.

[4] _Sea water_ contains more non-volatile saline constituents than

the usual kinds of fresh water. This is explained by the fact that

the waters flowing into the sea supply it with salts, and whilst a

large quantity of vapour is given off from the surface of the sea,

the salts remain behind. Even the specific gravity of sea water

differs considerably from that of pure water. It is generally

about 1·02, but in this and also in respect of the amount of salts

contained, samples of sea water from different localities and from

different depths offer rather remarkable variations. It will be

sufficient to point out that one cubic metre of water from the

undermentioned localities contains the following quantity in grams

of solid constituents:--Gulf of Venice, 19,122; Leghorn Harbour

24,312; Mediterranean, near Cetta, 37,665; the Atlantic Ocean

from 32,585 to 35,695,; the Pacific Ocean from 35,233 to 34,708.

In closed seas which do not communicate, or are in very distant

communication, with the ocean, the difference is often still

greater. Thus the Caspian Sea contains 6,300 grams; the Black

Sea and Baltic 17,700. Common salt forms the chief constituent

of the saline matter of sea or ocean water; thus in one cubic

metre of sea water there are 25,000-31,000 grams of common salt,

2,600-6,000 grams of magnesium chloride, 1,200-7,000 grams of

magnesium sulphate, 1,500-6,000 grams of calcium sulphate, and

10-700 grams of potassium chloride. The small amount of organic

matter and of the salts of phosphoric acid in sea water is very

remarkable. Sea water (the composition of which is partially

discussed in Chapter X.) contains, in addition to salts of common

occurrence, a certain and sometimes minute amount of the most

varied elements, even gold and silver, and as the mass of water of

the oceans is so enormous these 'traces' of rare substances amount

to large quantities, so that it may be hoped that in time methods

will be found to extract even gold from sea water, which by means

of the rivers forms a vast reservoir for the numerous products

of the changes taking place on the earth's surface. The works of

English, American, German, Russian, Swedish, and other navigators

and observers prove that a study of the composition of sea water

not only explains much in the history of the earth's life, but

also gives the possibility (especially since the researches of

C. O. Makaroff of the St. Petersburg Academy) of fixing one's

position in the ocean in the absence of other means, for instance,

in a fog, or in the dark.

In general terms water is called pure when it is clear and free from

insoluble particles held in suspension and visible to the naked eye,

from which it may be freed by filtration through charcoal, sand, or

porous (natural or artificial) stones, and when it possesses a clean

fresh taste. It depends on the absence of any taste, decomposing organic

matter, on the quantity of air[5] and atmospheric gases in solution, and

on the presence of mineral substances to the amount of about 300 grams

per ton (or 1000 kilograms per cubic metre, or, what is the same, 300

milligrams to a kilogram or a litre of water), and of not more than 100

grams of organic matter.[6] Such water is suitable for drinking and

every practical application, but evidently it is not pure in a chemical

sense. A _chemically pure water_ is necessary not only for scientific

purposes, as an independent substance having constant and definite

properties, but also for many practical purposes--for instance, in

photography and in the preparation of medicines--because many properties

of substances in solution are changed by the impurities of natural

waters. Water is usually purified by distillation, because the solid

substances in solution are not transformed into vapours in this process.

Such _distilled_ water is prepared by chemists and in laboratories by

boiling water in closed metallic boilers or stills, and causing the

steam produced to pass into a condenser--that is, through tubes (which

should be made of tin, or, at all events, tinned, as water and its

impurities do not act on tin) surrounded by cold water, and in which

the steam, being cooled, condenses into water which is collected[7] in

a receiver. By standing exposed to the atmosphere, however, the water

in time absorbs air, and dust carried in the air. Nevertheless, in

distillation, water retains, besides air, a certain quantity of volatile

impurities (especially organic) and the walls of the distillation

apparatus are partly corroded by the water, and a portion, although

small, of their substance renders the water not entirely pure, and a

residue is left on evaporation.[8]

[5] The taste of water is greatly dependent on the quantity of

dissolved gases it contains. These gases are given off on

boiling, and it is well known that, even when cooled, boiled

water has, until it has absorbed gaseous substances from the

atmosphere, quite a different taste from fresh water containing

a considerable amount of gas. The dissolved gases, especially

oxygen and carbonic anhydride, have an important influence on

the health. The following instance is very instructive in this

respect. The Grenelle artesian well at Paris, when first opened,

supplied a water which had an injurious effect on men and animals.

It appeared that this water did not contain oxygen, and was in

general very poor in gases. As soon as it was made to fall in

a cascade, by which it absorbed air, it proved quite fit for

consumption. In long sea voyages fresh water is sometimes not

taken at all, or only taken in a small quantity, because it spoils

by keeping, and becomes putrid from the organic matter it contains

undergoing decomposition. Fresh water may he obtained directly

from sea-water by distillation. The distilled water no longer

contains sea salts, and is therefore fit for consumption, but

it is very tasteless and has the properties of boiled water. In

order to render it palatable certain salts, which are usually held

in fresh water, are added to it, and it is made to flow in thin

streams exposed to the air in order that it may become saturated

with the component parts of the atmosphere--that is, absorb gases.

[6] _Hard water_ is such as contains much mineral matter, and

especially a large proportion of calcium salts. Such water,

owing to the amount of lime it contains, does not form a lather

with soap, prevents vegetables boiled in it from softening

properly, and forms a large amount of incrustation on vessels

in which it is boiled. When of a high degree of hardness, it

is injurious for drinking purposes, which is evident from the

fact that in several large cities the death-rate has been found

to decrease after introducing a soft water in the place of a

hard water. _Putrid water_ contains a considerable quantity of

decomposing organic matter, chiefly vegetable, but in populated

districts, especially in towns, chiefly animal remains. Such

water acquires an unpleasant smell and taste, by which stagnant

bog water and the water of certain wells in inhabited districts

are particularly characterised. Water of this kind is especially

injurious at a period of epidemic. It may be partially purified

by being passed through charcoal, which retains the putrid and

certain organic substances, and also certain mineral substances.

Turbid water may be purified to a certain extent by the addition

of alum, which aids, after standing some time, the formation of

a sediment. Condy's fluid (potassium permanganate) is another

means of purifying putrid water. A solution of this substance,

even if very dilute, is of a red colour; on adding it to a putrid

water, the permanganate oxidises and destroys the organic matter.

When added to water in such a quantity as to impart to it an

almost imperceptible rose colour it destroys much of the organic

substances it contains. It is especially salutary to add a small

quantity of Condy's fluid to impure water in times of epidemic.

The presence in water of one gram per litre, or 1,000 grams per

cubic metre, of any substance whatsoever, renders it unfit and

even injurious for consumption by animals, and this whether

organic or mineral matter predominates. The presence of 1 p.c. of

chlorides makes water quite salt, and produces thirst instead of

assuaging it. The presence of magnesium salts is most unpleasant;

they have a disagreeable bitter taste, and, in fact, impart to sea

water its peculiar taste. A large amount of nitrates is only found

in impure water, and is usually injurious, as they may indicate

the presence of decomposing organic matter.

[7] [Illustration: FIG. 4.--Distillation by means of a metallic still.

The liquid in C is heated by the fire F. The vapours rise through

the head A and pass by the tube T to the worm S placed in a vessel

R, through which a current of cold water flows by means of the

tubes D and P.]

Distilled water may be prepared, or distillation in general

carried on, either in a metal still with worm condenser (fig. 4)

or on a small scale in the laboratory in a glass retort (fig.

5) heated by a lamp. Fig. 5 illustrates the main parts of the

usual glass laboratory apparatus used for distillation. The steam

issuing from the retort (on the right-hand side) passes through a

glass tube surrounded by a larger tube, through which a stream of

cold water passes, by which the steam is condensed, and runs into

a receiver (on the left-hand side).

[Illustration: FIG. 5.--Distillation from a glass retort. The neck

of the retort fits into the inner tube of the Liebig's condenser.

The space between the inner and outer tube of the condenser is

filled with cold water, which enters by the tube _g_ and flows out

at _f_.]

[8] One of Lavoisier's first memoirs (1770) referred to this question.

He investigated the formation of the earthy residue in the

distillation of water in order to prove whether it was possible,

as was affirmed, to convert water into earth, and he found that

the residue was produced by the action of water on the sides of

the vessel containing it, and not from the water itself. He proved

this to be the case by direct weighing.

For certain physical and chemical researches, however, it is necessary

to have perfectly pure water. To obtain it, a solution of potassium

permanganate is added to distilled water until the whole is a light

rose colour. By this means the organic matter in the water is destroyed

(converted into gases or non-volatile substances). An excess of

potassium permanganate does no harm, because in the next distillation it

is left behind in the distillation apparatus. The second distillation

should take place in a platinum retort with a platinum receiver.

Platinum is a metal which is not acted on either by air or water, and

therefore nothing passes from it into the water. The water obtained

in the receiver still contains air. It must then be boiled for a long

time, and afterwards cooled in a vacuum under the receiver of an air

pump. Pure water does not leave any residue on evaporation; does not

in the least change, however long it be kept; does not decompose like

water only once distilled or impure; and it does not give bubbles of

gas on heating, nor does it change the colour of a solution of potassium

permanganate.

Water, purified as above described, has constant _physical_ and

_chemical properties_. For instance, it is of such water only that

one cubic centimetre weighs one gram at 4° C.--_i.e._ it is only such

pure water whose specific gravity equals 1 at 4° C.[9] Water in a

solid state forms crystals of the hexagonal system[10] which are seen

in snow, which generally consists of star-like clusters of several

crystals, and also in the half-melted scattered ice floating on rivers

in spring time. At this time of the year the ice splits up into spars

or prisms, bounded by angles proper to substances crystallising in the

hexagonal system.

[9] Taking the generally-accepted specific gravity of water at its

greatest density--_i.e._ at 4° as one--it has been shown by

experiment that the specific gravity of water at different

temperatures is as follows:

At 0° 0·99987 | At 30° 0·99574

" +10° 0·99974 | " 40° 0·99233

" 15° 0·99915 | " 50° 0·98817

" 20° 0·99827 | " 100° 0·95859

A comparison of all the data at present known shows that the

variation of the specific gravity S_{t} with the temperature

_t_ (determined by the mercurial thermometer) maybe expressed

(Mendeléeff 1891) by the formula

S_{t} = 1 - (_t_-4)^{2}/(94·1 + _t_) (703·51-_t_) 1·9

+-----------+-------------+---------------------------+-----------+

| t° C. | | Variation of sp. gr. with | |

| according |Sp. gr. S_{t}| a rise of | Volume |

| to the |(at 4° = +--------------+------------+taking vol.|

+-----------+-------------+--------------+------------+-----------+

| -10 | 998,281 | +264 | +54 | 1,001,722 |

| 0 | 999,873 | +65 | +50 | 1,000,127 |

| 10 | 999,738 | -85 | +47 | 1,000,262 |

| 20 | 998,272 | -203 | +45 | 1,001,731 |

| 30 | 995,743 | -299 | +43 | 1,004,276 |

| 50 | 988,174 | -450 | +40 | 1,011,967 |

| 70 | 977,948 | -569 | +39 | 1,022,549 |

| 90 | 965,537 | -670 | +41 | 1,035,692 |

| 100 | 958,595 | -718 | +42 | 1,043,194 |

| 120 | 943,814 | -810 | +43 | 1,060,093 |

| 160 | 907,263 | -995 | +55 | 1,102,216 |

| 200 | 863,473 | -1,200 | +73 | 1,158,114 |

+-----------+-------------+--------------+------------+-----------+

If the temperature be determined by the hydrogen thermometer,

whose indications between 0° and 100° are slightly lower than the

mercurial (for example, about 0·1° C. at 20°), then a slightly

smaller sp. gr. will be obtained for a given _t_. Thus Chappuis

(1892) obtained 0·998233 for 20°. Water at 4° is taken as the

basis for reducing measures of length to measures of weight and

volume. The _metric, decimal, system_ of measures of weights and

volumes is generally employed in science. The starting point of

this system is the metre (39·37 inches) divided into decimetres

(= 0·1 metre), centimetres (= 0·01 metre), millimetres (= 0·001

metre), and micrometres (= one millionth of a metre). A cubic

decimetre is called a _litre_, and is used for the measurement

of volumes. The weight of a litre of water at 4° in a vacuum is

called a kilogram. One thousandth part of a kilogram of water

weighs one _gram_. It is divided into decigrams, centigrams,

and milligrams (= 0·001 gram). An English pound equals 453·59

grams. The great advantage of this system is that it is a decimal

one, and that it is universally adopted in science and in most

international relations. _All the measures cited in this work are

metrical._ The units most often used in science are:--Of length,

the centimetre; of weight, the gram; of time, the second; of

temperature, the degree Celsius or Centigrade. According to the

most trustworthy determinations (Kupfer in Russia 1841, and Chaney

in England 1892), the weight of a c. dcm. of water at 4° in vacuo

is about 999·9 grms. For ordinary purposes the weight of a c.

dcg. may be taken as equal to a kg. Hence the litre (determined

by the weight of water it holds) is slightly greater than a cubic

decimetre.

[10] As solid substances appear in independent, regular, crystalline

forms which are dependent, judging from their cleavage or

lamination (in virtue of which mica breaks, up into laminae,

and Iceland spar, &c., into pieces bounded by faces inclined to

each other at angles which are definite for each substance), on

an inequality of attraction (cohesion, hardness) in different

directions which intersect at definite angles the determination

of crystalline form therefore affords one of the most important

characteristics for identifying definite chemical compounds. The

elements of crystallography which comprise a special science

should therefore he familiar to all who desire to work in

scientific chemistry. In this work we shall only have occasion

to speak of a few crystalline forms, some of which are shown in

figs. 6 to 12.

[Illustration: FIG. 6.--Example of the form belonging to the

regular system. Combination of an octahedron and a cube. The

former predominates. Alum, fluor spar, suboxide of copper, and

others.]

[Illustration: FIG. 7.--Rhombic Dodecahedron of the regular

system. Garnet.]

[Illustration: FIG. 8.--Hexagonal prism terminated by hexagonal

pyramids. Quartz, &c.]

[Illustration: FIG. 9.--Rhombohedron. Calc spar, &c.]

[Illustration: FIG. 10.--Rhombic system. Desmine.]

[Illustration: FIG. 11.--Triclinic pyramid.]

[Illustration: FIG. 12.--Triclinic system. Albite, &c.]

The temperatures at which water passes from one state to another are

taken as fixed points on the thermometer scale; namely, the zero

corresponds with the temperature of melting ice, and the temperature

of the steam disengaged from water boiling at the normal barometric

pressure (that is 760 millimetres measured at 0°, at the latitude of

45°, at the sea level) is taken as 100° of the Celsius scale. Thus, the

fact that water liquefies at 0° and boils at 100° is taken as one of

its properties as a definite chemical compound. The weight of a litre

of water at 4° is 1,000 grams, at 0° it is 999·8 grams. The weight of a

litre of ice at 0° is less--namely, 917 grams; the weight of the same

cubic measure of water vapour at 760 mm. pressure and 100° is only 0·60

gram; the density of the vapour compared with air = 0·62, and compared

with hydrogen = 9.

These data briefly characterise the physical properties of water as a

separate substance. To this may be added that water is a mobile liquid,

colourless, transparent, without taste or smell, &c. Its latent heat

of vaporisation is 534 units, of liquefaction 79 units of heat.[11]

The large amount of heat stored up in water vapour and also in liquid

water (for its specific heat is greater than that of other liquids)

renders it available in both forms for heating purposes. The chemical

reactions which water undergoes, and by means of which it is formed,

are so numerous, and so closely allied to the reactions of many other

substances, that it is impossible to describe the majority of them at

this early stage of chemical exposition. We shall become acquainted

with many of them afterwards, but at present we shall only cite certain

compounds formed by water. In order to see clearly the nature of the

various kinds of compounds formed by water we will begin with the most

feeble, which are determined by purely mechanical superficial properties

of the reacting substances.[12]

[11] Of all known liquids, water exhibits the greatest _cohesion_ of

particles. Indeed, it ascends to a greater height in capillary

tubes than other liquids; for instance, two and a half times as

high as alcohol, nearly three times as high as ether, and to a

much greater height than oil of vitriol, &c. In a tube one mm. in

diameter, water at 0° ascends 15·3 mm., measuring from the height

of the liquid to two-thirds of the height of the meniscus, and at

100° it rises 12·5 mm. The cohesion varies very uniformly with

the temperature; thus at 50° the height of the capillary column

equals 13·9 mm.--that is, the mean between the columns at 0°

and 100°. This uniformity is not destroyed even at temperatures

near the freezing point, and hence it may be assumed that at

high temperatures cohesion will vary as uniformly as at ordinary

temperatures; that is, the difference between the columns at 0°

and 100° being 2·8 mm., the height of the column at 500° should

be 15·2-(5 × 2·8) = 1·2 mm.; or, in other words, at these high

temperatures the cohesion between the particles of water would he

almost _nil_. Only certain solutions (sal ammoniac and lithium

chloride), and these only with a great excess of water, rise

higher than pure water in capillary tubes. The great cohesion of

water doubtless determines many of both its physical and chemical

properties.

The quantity of heat required to raise the temperature of one

part by weight of water from 0° to 1°, _i.e._ by 1° C., is called

the _unit of heat_ or calorie; the _specific heat of liquid

water_ at 0° is taken as equal to unity. The variation of this

specific heat with a rise in temperature is inconsiderable in

comparison with the variation exhibited by the specific heats of

other liquids. According to Ettinger, the specific heat of water

at 20° = 1·016, at 50° = 1·039, and at 100° = 1·073. The specific

heat of water is greater than that of any other known liquid;

for example, the specific heat of alcohol at 0° is 0·55--_i.e._

the quantity of heat which raises 55 parts of water 1° raises

100 parts of alcohol 1°. The specific heat of oil of turpentine

at 0° is 0·41, of ether 0·53, of acetic acid 0·5274, of mercury

0·033. Hence water is the best condenser or absorber of heat.

This property of water has an important significance in practice

and in nature. Water prevents rapid cooling or heating, and thus

tempers cold and heat. The specific heats of ice and aqueous

vapour are much less than that of water; namely, that of ice is

0·504, and of steam 0·48.

With an increase in pressure equal to one atmosphere, the

compressibility of water (_see_ Note 9) is 0·000047, of mercury

0·00000352, of ether 0·00012 at 0°, of alcohol at 13° 0·000095.

The addition of various substances to water generally decreases

both its compressibility and cohesion. The compressibility of

other liquids increases with a rise of temperature, but for water

it decreases up to 53° and then increases like other liquids.

The _expansion of water_ by heat (Note 9) also exhibits many

peculiarities which are not found in other liquids. The expansion

of water at low temperatures is very small compared with other

liquids; at 4° it is almost zero, and at 100° it is equal to

0·0008; below 4° it is negative--_i.e._ water on cooling then

expands, and does not decrease in volume. In passing into a solid

state, the specific gravity of water decreases; at 0° one c.c.

of water weighs 0·999887 gram, and one c.c. of ice at the same

temperature weighs only 0·9175 gram. The ice formed, however,

contracts on cooling like the majority of other substances. Thus

100 volumes of ice are produced from 92 volumes of water--that

is, water expands considerably on freezing, which fact determines

a number of natural phenomena. The freezing point of water falls

with an increase in pressure (0·007° per atmosphere), because in

freezing water expands (Thomson), whilst with substances which

contract in solidifying the melting point rises with an increase

in pressure; thus, paraffin under one atmosphere melts at 46°,

and under 100 atmospheres at 49°.

When liquid water passes into vapour, the cohesion of its

particles must be destroyed, as the particles are removed to

such a distance from each other that their mutual attraction

no longer exhibits any influence. As the cohesion of aqueous

particles varies at different temperatures, the quantity of heat

which is expended in overcoming this cohesion--or the _latent

heat of evaporation_--will for this reason alone be different at

different temperatures. The quantity of heat which is consumed in

the transformation of one part by weight of water, at different

temperatures, into vapour was determined by Regnault with great

accuracy. His researches showed that one part by weight of water

at 0°, in passing into vapour having a temperature _t_°, consumes

606·5 + 0·305_t_ units of heat, at 50° 621·7, at 100° 637·0, at

150° 652·2, and at 200° 667·5. But this quantity includes also

the quantity of heat required for heating the water from 0° to

_t_°--_i.e._ besides the latent heat of evaporation, also that

heat which is used in heating the water in a liquid state to a

temperature _t_°. On deducting this amount of heat, we obtain the

latent heat of evaporation of water as 606·5 at 0°, 571 at 50°,

534 at 100°, 494 at 150°, and only 453 at 200°, which shows that

the conversion of water at different temperatures into vapour

at a constant temperature requires very different quantities of

heat. This is chiefly dependent on the difference of the cohesion

of water at different temperatures; the cohesion is greater at

low than at high temperatures, and therefore at low temperatures

a greater quantity of heat is required to overcome the cohesion.

On comparing these quantities of heat, it will be observed that

they decrease rather uniformly, namely their difference between

0° and 100° is 72, and between 100° and 200° is 81 units of

heat. From this we may conclude that this variation will be

approximately the same for high temperatures also, and therefore

that no heat would be required for the conversion of water into

vapour at a temperature of about 400°. At this temperature, water

passes into vapour whatever be the pressure (see Chap. II. The

absolute boiling point of water, according to Dewar, is 370°,

the critical pressure 196 atmospheres). It must here be remarked

that water, in presenting a greater cohesion, requires a larger

quantity of heat for its conversion into vapour than other

liquids. Thus alcohol consumes 208, ether 90, turpentine 70,

units of heat in their conversion into vapour.

The whole amount of heat which is consumed in the conversion of

water into vapour is not used in overcoming the cohesion--that

is, in internal accomplished in the liquid. A part of this heat

is employed in moving the aqueous particles; in fact, aqueous

vapour at 100° occupies a volume 1,659 times greater than that

of water (at the ordinary pressure), consequently a portion of

the heat or work is employed in lifting the aqueous particles, in

overcoming pressure, or in external work, which may be usefully

employed, and which is so employed in steam engines. In order

to determine this work, let us consider the variation of the

maximum _pressure_ or _vapour tension of steam_ at different

temperatures. The observations of Regnault in this respect,

as on those preceding, deserve special attention from their

comprehensiveness and accuracy. The pressure or tension of

aqueous vapour at various temperatures is given in the adjoining

table, and is expressed in millimetres of the barometric column

reduced to 0°.

+------------+---------+-------------+----------+

+------------+---------+-------------+----------+

| -20° | 0·9 | 70° | 233·3 |

| -10° | 2·1 | 90° | 525·4 |

| 0° | 4·6 | 100° | 760·0 |

| +10° | 9·1 | 105° | 906·4 |

| 15° | 12·7 | 110° | 1075·4 |

| 20° | 17·4 | 115° | 1269·4 |

| 25° | 23·5 | 120° | 1491·3 |

| 30° | 31·5 | 150° | 3581·0 |

| 50° | 92·0 | 200° | 11689·0 |

+------------+---------+-------------+----------+

The table shows the boiling points of water at different

pressures. Thus on the summit of Mont Blanc, where the average

pressure is about 424 mm., water boils at 84·4°. In a rarefied

atmosphere water boils even at the ordinary temperature, but in

evaporating it absorbs heat from the neighbouring parts, and

therefore it becomes cold and may even freeze if the pressure

does not exceed 4·6 mm., and especially if the vapour be rapidly

absorbed as it is formed. Oil of vitriol, which absorbs the

aqueous vapour, is used for this purpose. Thus ice may be

obtained artificially at the ordinary temperature with the aid

of an air-pump. This table of the tension of aqueous vapour also

shows the temperature of water contained in a closed boiler if

the pressure of the steam formed be known. Thus at a pressure

of five atmospheres (a pressure of five times the ordinary

atmospheric pressure--_i.e._ 5 × 760 = 3,800 mm.) the temperature

of the water would be 152°. The table also shows the pressure

produced on a given surface by steam on issuing from a boiler.

Thus steam having a temperature of 152° exerts a pressure of

517 kilos on a piston whose surface equals 100 sq. cm., for the

pressure of one atmosphere on one sq. cm. equals 1,033 kilos,

and steam at 152° has a pressure of five atmospheres. As a

column of mercury 1 mm. high exerts a pressure of 1·35959 grams

on a surface of 1 sq. cm., therefore the pressure of aqueous

vapour at 0° corresponds with a pressure of 6·25 grams per

square centimetre. The pressures for all temperatures may be

calculated in a similar way, and it will be found that at 100°

it is equal to 1,033·28 grams. This means that if a cylinder

be taken whose sectional area equals 1 sq. cm., and if water

be poured into it and it be closed by a piston weighing 1,033

grams, then on heating it in a vacuum to 100° no steam will be

formed, because the steam cannot overcome the pressure of the

piston; and if at 100° 534 units of heat be transmitted to each

unit of weight of water, then the whole of the water will be

converted into vapour having the same temperature; and so also

for every other temperature. The question now arises, to what

height does the piston rise under these circumstances? that is,

in other words, What is the volume occupied by the steam under

a known pressure? For this we must know the weight of a cubic

centimetre of steam at various temperatures. It has been shown by

experiment that the density of steam, which does not saturate a

space, varies very inconsiderably at all possible pressures, and

is nine times the density of hydrogen under similar conditions.

Steam which saturates a space varies in density at different

temperatures, but this difference is very small, and its

average density with reference to air is 0·64. We will employ

this number in our calculation, and will calculate what volume

the steam occupies at 100°. One cubic centimetre of air at 0°

and 760 mm. weighs 0·001293 gram, at 100° and under the same

pressure it will weigh 0·001293/1·368 or about 0·000946 gram,

and consequently one cubic centimetre of steam whose density

is 0·64 will weigh 0·000605 gram at 100°, and therefore one

gram of aqueous vapour will occupy a volume of about 1·653 c.c.

Consequently, the piston in the cylinder of 1 sq. cm. sectional

area, and in which the water occupied a height of 1 cm., will

be raised 1,653 cm. on the conversion of this water into steam.

This piston, as has been mentioned, weighs 1,033 grams, therefore

the _external work of the steam_--that is, that work which the

water does in its conversion into steam at 100°--is equal to

lifting a piston weighing 1,033 grams to a height of 1,653 cm.,

or 17·07 kilogram-metres of work--_i.e._ is capable of lifting

17 kilograms 1 metre, or 1 kilogram 17 metres. One gram of water

requires for its conversion into steam 534 gram units of heat or

0·534 kilogram unit of heat--_i.e._ the quantity of heat absorbed

in the evaporation of one gram of water is equal to the quantity

of heat which is capable of heating 1 kilogram of water 0·534°.

Each unit of heat, as has been shown by accurate experiment,

is capable of doing 424 kilogram-metres of work. Hence, in

evaporating, one gram of water expends 424 × 0·534 = (almost) 227

kilogram-metres of work. The external work was found to be only

17 kilogram-metres, therefore 210 kilogram-metres are expended in

overcoming the internal cohesion of the aqueous particles, and

consequently about 92 p.c. of the total heat or work is consumed

in overcoming the internal cohesion. The following figures are

thus calculated approximately:--

+------------+----------------+-----------------+--------------+

+------------+----------------+-----------------+--------------+

| 0° | 255 | 13 | 242 |

| 50° | 242 | 15 | 227 |

| 100° | 226 | 17 | 209 |

| 150° | 209 | 19 | 190 |

| 200° | 192 | 20 | 172 |

+------------+----------------+-----------------+--------------+

The work necessary for overcoming the internal cohesion of

water in its passage into vapour decreases with the rise in

temperature--that is, corresponds with the decrease of cohesion;

and, in fact, the variations which take place in this case are

very similar to those which are observed in the heights to which

water rises in capillary tubes at different temperatures. It is

evident, therefore, that the amount of external--or, as it is

termed, useful--work which water can supply by its evaporation

is very small compared with the amount which it expends in its

conversion into vapour.

In considering certain physico-mechanical properties of water, I

had in view not only their importance for theory and practice,

but also their purely chemical significance; for it is evident

from the above considerations that even in a physical change

of state the greatest part of the work done is employed in

overcoming cohesion, and that an enormous amount of internal

energy must be expended in overcoming chemical cohesion or

affinity.

[12] When it is necessary to heat a considerable mass of liquid in

different vessels, it would be very uneconomical to make use of

metallic vessels and to construct a separate furnace for each;

such cases are continually met with in practice. Steam from a

boiler is introduced into the liquid, or, in general, into the

vessel which it is required to heat. The steam, in condensing and

passing into a liquid state, parts with its latent heat, and as

this is very considerable a small quantity of steam will produce

a considerable heating effect. If it be required, for instance,

to heat 1,000 kilos of water from 20° to 50°, which requires

approximately 30,000 units of heat, steam at 100° is passed into

the water from a boiler. Each kilogram of water at 50° contains

about 50 units of heat, and each kilogram of steam at 100°

contains 637 units of heat; therefore, each kilogram of steam in

cooling to 50° gives up 587 units of heat, and consequently 52

kilos of steam are capable of heating 1,000 kilos of water from

20° to 50°. Water is very often applied for heating in chemical

practice. For this purpose metallic vessels or pans, called

'water-baths,' are made use of. They are closed by a cover formed

of concentric rings lying on each other. The vessels--such as

beakers, evaporating basins, retorts, &c.--containing liquids,

are placed on these rings, and the water in the bath is heated.

The steam given off heats the bottom of the vessels to be heated,

and thus effects the evaporation or distillation.

Water is mechanically attracted by many substances; it adheres to their

surfaces just as dust adheres to objects, or one piece of polished glass

adheres to another. Such attraction is termed 'moistening,' 'soaking,'

or 'absorption of water.' Thus water moistens clean glass and adheres

to its surface, is absorbed by the soil, sand, and clay, and does

not flow away from them, but lodges itself between their particles.

Similarly, water soaks into a sponge, cloth, hair, or paper, &c., but

fat and greasy substances in general are not moistened. Attraction

of this kind does not alter the physical or chemical properties of

water. For instance, under these circumstances water, as is known from

everyday experience, may be expelled from objects by drying. Water

which is in any way held mechanically may be dislodged by mechanical

means, by friction, pressure, centrifugal force, &c. Thus water is

squeezed from wet cloth by pressure or centrifugal machines. But objects

which in practice are called dry (because they do not feel wet) often

still contain moisture, as may be proved by heating the object in a

glass tube closed at one end. By placing a piece of paper, dry earth,

or any similar object (especially porous substances) in such a glass

tube, and heating that part of the tube where the object is situated,

it will be remarked that water condenses on the cooler portions of

the tube. The presence of such absorbed, or 'hygroscopic,' water is

generally best detected in non-volatile substances by drying them at

100°, or under the receiver of an air-pump and over substances which

attract water chemically. By weighing a substance before and after

drying, it is easy to determine the amount of hygroscopic water from

the loss in weight.[13] Only in this case the amount of water must be

judged with care, because the loss in weight may sometimes proceed

from the decomposition of the substance itself, with disengagement of

gases or vapour. In making exact weighings the hygroscopic capacity

of substances--that is, their capacity to absorb moisture--must be

continually kept in view, as otherwise the weight will be untrue from

the presence of moisture. The quantity of moisture absorbed depends on

the degree of moisture of the atmosphere (that is, on the tension of the

aqueous vapour in it) in which a substance is situated. In an entirely

dry atmosphere, or in a vacuum, the hygroscopic water is expelled, being

converted into vapour; therefore, substances containing hygroscopic

water may be completely dried by placing them in a dry atmosphere or

in a vacuum. The process is aided by heat, as it increases the tension

of the aqueous vapour. Phosphoric anhydride (a white powder), liquid

sulphuric acid, solid and porous calcium chloride, or the white powder

of ignited copper sulphate, are most generally employed in drying

gases. They absorb the moisture contained in air and all gases to a

considerable, but not unlimited, extent. Phosphoric anhydride and

calcium chloride deliquesce, become damp, sulphuric acid changes from an

oily thick liquid into a more mobile liquid, and ignited copper sulphate

becomes blue; after which changes these substances partly lose their

capacity of holding water, and can, if it be in excess, even give up

their water to the atmosphere. We may remark that the order in which

these substances are placed above corresponds with the order in which

they stand in respect to their capacity for absorbing moisture. Air

dried by calcium chloride still contains a certain amount of moisture,

which it can give up to sulphuric acid. The most complete desiccation

takes place with phosphoric anhydride. Water is also removed from

many substances by placing them in a dish over a vessel containing a

substance absorbing water under a glass bell jar.[14] The bell jar,

like the receiver of an air pump, should be hermetically closed. In this

case desiccation takes place; because sulphuric acid, for instance,

first dries the air in the bell jar by absorbing its moisture, the

substance to be dried then parts with its moisture to the dry air,

from which it is again absorbed by the sulphuric acid, &c. Desiccation

proceeds still better under the receiver of an air pump, for then the

aqueous vapour is formed more quickly than in a bell jar full of air.

[13] [Illustration: FIG. 13.--Drying oven, composed of brazed copper.

It is heated by a lamp. The object to be dried is placed on the

gauze inside the oven. The thermometer indicates the temperature.]

In order to dry any substance at about 100°--that is, at the

boiling point of water (hygroscopic water passes off at this

temperature)--an apparatus called a 'drying-oven' is employed.

It consists of a double copper box; water is poured into the

space between the internal and external boxes, and the oven is

then heated over a stove or by any other means, or else steam

from a boiler is passed between the walls of the two boxes.

When the water boils, the temperature inside the inner box will

be approximately 100° C. The substance to be dried is placed

inside the oven, and the door is closed. Several holes are cut

in the door to allow the free passage of air, which carries

off the aqueous vapour by the chimney on the top of the oven.

Often, however, desiccation is carried on in copper ovens

heated directly over a lamp (fig. 13). In this case any desired

temperature may be obtained, which is determined by a thermometer

fixed in a special orifice. There are substances which only

part with their water at a much higher temperature than 100°,

and then such air baths are very useful. In order to determine

directly the amount of water in a substance which does not part

with anything except water at a red heat, the substance is placed

in a bulb tube. By first weighing the tube empty and then with

the substance to be dried in it, the weight of the substance

taken may be found. The tube is then connected on one side with

a gas-holder full of air, which, on opening a stop-cock, passes

first through a flask containing sulphuric acid, and then into a

vessel containing lumps of pumice stone moistened with sulphuric

acid. In passing through these vessels the air is thoroughly

dried, having given up all its moisture to the sulphuric acid.

Thus dry air will pass into the bulb tube, and as hygroscopic

water is entirely given up from a substance in dry air even at

the ordinary temperature, and still more rapidly on heating,

the moisture given up by the substance in the tube will be

carried off by the air passing through it. This damp air then

passes through a U-shaped tube full of pieces of pumice stone

moistened with sulphuric acid, which absorbs all the moisture

given off from the substance in the bulb tube. Thus all the water

expelled from the substance will collect in the [U] tube, and so,

if this be weighed before and after, the difference will show the

quantity of water expelled from the substance. If only water (and

not any gases) come over, the increase of the weight of the [U]

tube will be equal to the decrease in the weight of the bulb tube.

[14] Instead of under a glass bell jar, drying over sulphuric acid is

often carried on in a desiccator consisting of a shallow

wide-mouthed glass vessel, closed by a well-fitting ground-glass

cover. Sulphuric acid is poured over the bottom of the

desiccator, and the substance to be dried is placed on a glass

stand above the acid. A lateral glass tube with a stop-cock is

often fused into the desiccator in order to connect it with an

air pump, and so allow drying under a diminished pressure, when

the moisture evaporates more rapidly. The fact that in the usual

form of desiccator the desiccating substance (sulphuric acid) is

placed beneath the substance to be dried has the disadvantage

that the moist air being lighter than dry air distributes itself

in the upper portion of the desiccator and not below. Hempel,

in his desiccator (1891), avoids this by placing the absorbent

above the substance to be dried. The process of desiccation

can be further accelerated by cooling the upper portion of the

desiccator, and so inducing ascending and descending currents of

air within the apparatus.

From what has been said above, it is evident that the transference of

moisture to gases and the absorption of hygroscopic moisture present

great resemblance to, but still are not, chemical combinations with

water. Water, when combined as hygroscopic water, does not lose its

properties and does not form new substances.[15]

[15] Chappuis, however, determined that in wetting 1 gram of charcoal

with water 7 units of heat are evolved, and on pouring carbon

bisulphide over 1 gram of charcoal as much as 24 units of heat

are evolved. Alumina (1 gram), when moistened with water, evolves

2-1/2 calories. This indicates that in respect to evolution of

heat moistening already presents a transition towards exothermal

combinations (those evolving heat in their formation).

The attraction of water for substances which dissolve in it is of a

different character. In the solution of substances in water there

proceeds a peculiar kind of indefinite combination; a new homogeneous

substance is formed from the two substances taken. But here also the

bond connecting the substances is very unstable. Water containing

different substances in solution boils at a temperature near to its

usual boiling point. From the solution of substances which are lighter

than water itself, there are obtained solutions of a less density than

water--as, for example, in the solution of alcohol in water; whilst a

heavier substance in dissolving in water gives it a higher specific

gravity. Thus salt water is heavier than fresh.[16]

[16] Strong acetic acid (C_{2}H_{4}O_{2}), whose specific gravity at

15° is 1·055, does not become lighter on the addition of water

(a lighter substance, sp. gr. = 0·999), but heavier, so that a

solution of 80 parts of acetic acid and 20 parts of water has a

specific gravity of 1·074, and even a solution of equal parts of

acetic acid and water (50 p.c.) has a sp. gr. of 1·065, which is

still greater than that of acetic acid itself. This shows the

high degree of contraction which takes place on solution. In

fact, solutions--and, in general, liquids--on mixing with water,

decrease in volume.

We will consider _aqueous solutions_ somewhat fully, because, among

other reasons, solutions are constantly being formed on the earth and

in the waters of the earth, in plants and in animals, in chemical

processes and in the arts, and these solutions play an important part

in the chemical transformations which are everywhere taking place,

not only because water is everywhere met with, but chiefly because a

substance in solution presents the most favourable conditions for the

process of chemical changes, which require a mobility of parts and a

possible distension of parts. In dissolving, a solid substance acquires

a mobility of parts, and a gas loses its elasticity, and therefore

reactions often take place in solutions which do not proceed in the

undissolved substances. Further, a substance, distributed in water,

evidently breaks up--that is, becomes more like a gas and acquires a

greater mobility of parts. All these considerations require that in

describing the properties of substances, particular attention should be

paid to their relation to water as a solvent.

[Illustration: FIG. 14.--Method of transferring a gas into a cylinder

filled with mercury and whose open end is immersed under the mercury

in a bath having two glass sides. The apparatus containing the gas is

represented on the right. Its upper extremity is furnished with a tube

extending under the cylinder. The lower part of the vessel communicates

with a vertical tube. If mercury be poured into this tube, the pressure

of the gas in the apparatus is increased, and it passes through the

gas-conducting tube into the cylinder, where it displaces the mercury,

and can be measured or subjected to the action of absorbing agents, such

as water.]

It is well known that water dissolves many substances. Salt, sugar,

alcohol, and a number of other substances, dissolve in water and form

homogeneous liquids with it. To demonstrate the solubility of gases

in water, a gas should be taken which has a high co-efficient of

solubility--for instance, ammonia. This is introduced into a bell jar

(or cylinder, as in fig. 14), which is previously filled with mercury

and stands in a mercury bath. If water be then introduced into the

cylinder, the mercury will rise, owing to the water dissolving the

ammonia gas. If the column of mercury be less than the barometric

column, and if there be sufficient water to dissolve the gas, all the

ammonia will be absorbed by the water. The water is introduced into

the cylinder by a glass pipette, with a bent end. The bent end is put

into water, and the air is sucked out from the upper end. When full of

water, its upper end is closed with the finger, and the bent end placed

in the mercury bath under the orifice of the cylinder. On blowing into

the pipette the water will rise to the surface of the mercury in the

cylinder owing to its lightness. The solubility of a gas like ammonia

may be demonstrated by taking a flask full of the gas, and closed

by a cork with a tube passing through it. On placing the tube under

water, the water will rise into the flask (this may be accelerated by

previously warming the flask), and begin to play like a fountain inside

it. Both the rising of the mercury and the fountain clearly show the

considerable affinity of water for ammonia gas, and the force acting

in this dissolution is rendered evident. A certain period of time is

required both for the homogeneous intermixture of gases (diffusion) and

the process of solution, which depends, not only on the surface of the

participating substances, but also on their nature. This is seen from

experiment. Solutions of different substances heavier than water, such

as salt or sugar, are poured into tall jars. Pure water is then very

carefully poured into these jars (through a funnel) on to the top of the

solutions, so as not to disturb the lower stratum, and the jars are then

left undisturbed. The line of demarcation between the solution and the

pure water will be visible, owing to their different co-efficients of

refraction. Notwithstanding that the solutions taken are heavier than

water, after some time complete intermixture will ensue. Gay Lussac

convinced himself of this fact by this particular experiment, which

he conducted in the cellars under the Paris Astronomical Observatory.

These cellars are well known as the locality where numerous interesting

researches have been conducted, because, owing to their depth under

ground, they have a uniform temperature during the whole year; the

temperature does not change during the day, and this was indispensable

for the experiments on the diffusion of solutions, in order that no

doubt as to the results should arise from a daily change of temperature

(the experiment lasted several months), which would set up currents in

the liquids and intermix their strata. Notwithstanding the uniformity

of the temperature, the substance in solution in time ascended into the

water and distributed itself uniformly through it, proving that there

exists between water and a substance dissolved in it a particular kind

of attraction or striving for mutual interpenetration in opposition to

the force of gravity. Further, this effort, or rate of diffusion, is

different for salt or sugar or for various other substances.[16 bis] It

follows therefore that a peculiar force acts in solution, as in actual

chemical combinations, and solution is determined by a particular kind

of motion (by the chemical energy of a substance) which is proper to the

substance dissolved and to the solvent.

[16 bis] Graham, in the jelly formed by gelatine, and De Vries in

gelatinous silica (Chapter XVIII.) most frequently employed

coloured (tinted) substances, for instance, K_{2}Cr_{2}O_{7},

which showed the rate of diffusion with very great clearness.

Prof. Oumoff employed the method described in Chapter X., Note

17, for this purpose.

Graham made a series of experiments similar to those above described,

and showed that the _rate of diffusion_[17] in water is very

variable--that is, a uniform distribution of a substance in the water

dissolving it is attained in different periods of time with different

solutions. Graham compared diffusive capacity with volatility. There are

substances which diffuse easily, and there are others which diffuse with

difficulty, just as there are more or less volatile substances. Seven

hundred cubic centimetres of water were poured into a jar, and by means

of a syphon (or a pipette) 100 cub. centimetres of a solution containing

10 grams of a substance were cautiously poured in so as to occupy the

lower portion of the jar. After a lapse of several days, successive

layers of 50 cubic centimetres were taken from the top downwards, and

the quantity of substance dissolved in the different layers determined.

Thus, common table salt, after fourteen days, gave the following amounts

(in milligrams) in the respective layers, beginning from the top: 104,

120, 126, 198, 267, 340, 429, 535, 654, 766, 881, 991, 1,090, 1,187,

and 2,266 in the remainder; whilst albumin in the same time gave, in

the first seven layers, a very small amount, and beginning from the

eighth layer, 10, 15, 47, 113, 343, 855, 1,892, and in the remainder

6,725 milligrams. Thus, the diffusive power of a solution depends on

time and on the nature of the substance dissolved, which fact may

serve, not only for explaining the process of solution, but also for

distinguishing one substance from another. Graham showed that substances

which rapidly diffuse through liquids are able to rapidly pass through

membranes and crystallise, whilst substances which diffuse slowly and do

not crystallise are _colloids_, that is, resemble glue, and penetrate

through a membrane slowly, and form jellies; that is, occur in insoluble

forms,[18] as will be explained in speaking of silica.

[17] The researches of Graham, Fick, Nernst, and others showed that

the quantity of a dissolved substance which is transmitted

(rises) from one stratum of liquid to another in a vertical

cylindrical vessel is not only proportional to the time and to

the sectional area of the cylinder, but also to the amount and

nature of the substance dissolved in a stratum of liquid, so that

each substance has its corresponding co-efficient of diffusion.

The cause of the diffusion of solutions must be considered

as essentially the same as the cause of the diffusion of

gases--that is, as dependent on motions which are proper to their

molecules; but here most probably those purely chemical, although

feebly-developed, forces, which incline the substances dissolved

to the formation of definite compounds, also play their part.

[18] [Illustration: FIG. 15.--Dialyser. Apparatus for the separation

of substances which pass through a membrane from those which do

not. Description in text.]

The rate of diffusion--like the rate of transmission--through

membranes, or _dialysis_ (which plays an important part in the

vital processes of organisms and also in technical processes),

presents, according to Graham's researches, a sharply defined

change in passing from such crystallisable substances as the

majority of salts and acids to substances which are capable

of giving jellies (gum, gelatin, &c.) The former diffuse

into solutions and pass through membranes much more rapidly

than the latter, and Graham therefore distinguishes between

_crystalloids_, which diffuse rapidly, and _colloids_, which

diffuse slowly. On breaking solid colloids into pieces, a total

absence of cleavage is remarked. The fracture of such substances

is like that of glue or glass. It is termed a 'conchoidal'

fracture. Almost all the substances of which animal and vegetable

bodies consist are colloids, and this is, at all events, partly

the reason why animals and plants have such varied forms,

which have no resemblance to the crystalline forms of the

majority of mineral substances. The colloid solid substances in

organisms--that is, in animals and plants--almost always contain

water, and take most peculiar forms, of networks, of granules,

of hairs, of mucous, shapeless masses, &c., which are quite

different from the forms taken by crystalline substances. When

colloids separate out from solutions, or from a molten state,

they present a form which is similar to that of the liquid from

which they were formed. Glass may he taken as the best example of

this. Colloids are distinguishable from crystalloids, not only

by the absence of crystalline form, but by many other properties

which admit of clearly distinguishing both these classes of

solids, as Graham showed. Nearly all colloids are capable of

passing, under certain circumstances, from a soluble into an

insoluble state. The best example is shown by white of eggs

(albumin) in the raw and soluble form, and in the hard-boiled

and insoluble form. The majority of colloids, on passing into

an insoluble form in the presence of water, give substances

having a gelatinous appearance, which is familiar to every one

in starch, solidified glue, jelly, &c. Thus gelatin, or common

carpenter's glue, when soaked in water, swells up into an

insoluble jelly. If this jelly be heated, it melts, and is then

soluble in water, but on cooling it again forms a jelly which

is insoluble in water. One of the properties which distinguish

colloids from crystalloids is that the former pass very slowly

through a membrane, whilst the latter penetrate very rapidly.

This may be shown by taking a cylinder, open at both ends, and

by covering its lower end with a bladder or with vegetable

parchment (unsized paper immersed for two or three minutes in a

mixture of sulphuric acid and half its volume of water, and then

washed), or any other membranous substance (all such substances

are themselves colloids in an insoluble form). The membrane must

be firmly tied to the cylinder, so as not to leave any opening.

Such an apparatus is called a _dialyser_ (fig. 15), and the

process of separation of crystalloids from colloids by means of

such a membrane is termed _dialysis_. An aqueous solution of a

crystalloid or colloid, or a mixture of both, is poured into the

dialyser, which is then placed in a vessel containing water, so

that the bottom of the membrane is covered with water. Then,

after a certain period of time, the crystalloid passes through

the membrane, whilst the colloid, if it does pass through at all,

does so at an incomparably slower rate. The crystalloid naturally

passes through into the water until the solution attains the same

strength on both sides of the membrane. By replacing the outside

water with fresh water, a fresh quantity of the crystalloid may

be separated from the dialyser. While a crystalloid is passing

through the membrane, a colloid remains almost entirely in the

dialyser, and therefore a mixed solution of these two kinds of

substances may be separated from each other by a dialyser. The

study of the properties of colloids, and of the phenomena of

their passage through membranes, should elucidate much respecting

the phenomena which are accomplished in organisms.

Hence, if it be desired to increase the rate of solution, recourse must

be had to stirring, shaking, or some such mechanical motion. But if once

a uniform solution is formed, it will remain uniform, no matter how

heavy the dissolved substance is, or how long the solution be left at

rest, which fact again shows the presence of a force holding together

the particles of the body dissolved and of the solvent.[19]