THE ATOMIC THEORY

The opening years of the nineteenth century were made memorable by the
promulgation of the atomic theory by John Dalton. The enunciation of
this theory, which affords a simple and adequate explanation of the
fundamental laws of chemical combination, marks an epoch in the history
of chemistry.

It may be desirable to trace, as briefly as possible, the successive
steps which led up to the generalisation which more than any other
has served to stamp chemistry as an exact science. That matter was
_discrete_—that is, that it was not continuous, but was composed of
ultimate particles—was, as already stated, imagined by the ancients,
and was part of the philosophy of Leukippus, Demokritus, and
Leucretius. But this supposition, although favoured by Newton and other
thinkers, had little or no scientific basis prior to the middle of the
eighteenth century. From that time onward a variety of chemical facts
gradually accumulated, many of which at the time of their discovery
had no obvious connection with pre-existing facts. It was reserved for
Dalton to point out how an extension and more precise definition of
the old doctrine would suffice to connect and explain them.

The first germ of an atomic theory based on chemical fact may be traced
in the observation of =Toburn Bergmann= (b. 1735, d. 1784), Professor
of Chemistry at Upsala, that neutral solutions of certain metals in
contact with other metals gave a precipitate without the neutrality of
the solution being disturbed, and without gas being evolved. One metal
had simply replaced the other in solution. Bergmann thus incidentally
discovered the fact of the chemical equivalence of metals. He was
of opinion, however, that the phenomenon meant a transference of
phlogiston from one metal to another, and that the process might be
made a mode of determining the relative amount of phlogiston in various
metals. Lavoisier extended Bergmann’s observations, and sought to show,
in effect, that the process afforded a means of determining the amounts
of the several metals which combined with one and the same quantity
of oxygen. But neither Bergmann nor Lavoisier really grasped the idea
of equivalence as we understand it to-day. It began to be appreciated
as the result of the work of =Jeremiah Benjamin Richter= (b. 1762,
d. 1807) and of =G. E. Fischer= on the mutual action of salts in
solutions, and on the determinations of the amounts of acid and bases
which respectively combine with one another. Methods of measurement of
the proportions in which substances combine were grouped by Richter
under the term _Stochiometry_.

However desirable it may be in the interests of history to indicate
the sequence of the surmises and facts which preceded the formulation
of the atomic theory, it is very doubtful whether Dalton was, to any
material extent, influenced by them. A self-educated man of lowly
origin, sturdily independent and highly original, he was accustomed
to rely upon his own faculty of observation and experiment for his
facts, and upon his own intellectual powers and mental energy for their
interpretation.

=John Dalton=, the son of a Quaker hand-loom weaver, was born at
Eaglesfield, in Cumberland, in 1766. While still a boy he took to
school-teaching, and acquired, in his leisure and by his own exertions,
a competent knowledge of mathematics and physical science. In 1793 he
was called to give instruction in mathematics, natural philosophy, and
chemistry at the Manchester New College, the Nonconformist academy—now
moved from Warrington—in which Priestley had formerly lectured. Here
he remained six years, leaving the college to take up an independent
position as a private tutor, so as to enable him the more freely to
pursue his scientific inquiries. In 1800 he became Secretary of the
Philosophical Society of Manchester, and remained connected, as an
official, with that institution until his death in 1844. The greater
number of his scientific communications were published by that society.
In the outset of his scientific career he was attracted to meteorology;
and it was probably its problems which led him in the first place to
experiment, and to speculate on the physical constitution of gases.
In the course of these observations he was led to the discovery
of the law of thermal expansion of gases, with which his name is
now generally associated. His speculations concerning the physical
constitution of gaseous substances, arising from the contemplation of
gaseous phenomena, led him to the conception that a gas is composed
of particles that repel one another with a force decreasing as the
distance of their centres from each other; and it is probable that in
this manner he familiarised himself with the idea of the existence of
atoms. His first insight into the laws of the chemical combination of
these atoms seems to have originated from his discovery that, when
two substances unite in different proportions, these proportions may
be expressed in simple multiples of whole numbers. Thus he found,
on examining the composition of marsh gas and of ethylene, both
hydrocarbons, that for the same weight of hydrogen there was twice the
amount of carbon in ethylene that there was in marsh gas. He then
examined the oxides of nitrogen, and found a similar regularity to
hold good in these compounds. Some time prior to the autumn of 1803
Dalton was led to the supposition that these regularities could be
satisfactorily explained by the assumption that matter is composed
of atoms having sizes and weights differing with each substance, but
of identical weight and size for any particular substance, and that
chemical combination consists in the approximation of these atoms. This
simple hypothesis explained all the facts then known. It explained the
constancy in the chemical composition of substances, which may be said
to have been established by Proust, and which is now formulated as the
Law of Constant Proportion—that the same body is invariably composed
of the same elements, united in the same proportion. It explained also
the fact discovered by Dalton that, when an element unites with another
in different proportions, the higher proportions are multiples of the
lowest—now formulated as the Law of Multiple Proportion. It further
explained the fact, which may be said to have been foreshadowed by
Richter, that when two bodies, A and B, separately combine with a third
body, C, the proportions of A and B which unite with C are measures or
multiples of the proportions in which A and B combine together. This
is known as the Law of Reciprocal Proportion.

Dalton’s theory was first made generally known by Thomas Thomson, in
the third edition of his _System of Chemistry_, published in 1807, and
was employed by Thomson in his paper on “The Oxalates of Strontium,”
published the same year in the _Philosophical Transactions_. The first
printed account by Dalton himself is contained in Part I. of his _New
System of Chemical Philosophy_, published in 1808, the substance of
which had been previously given in a course of lectures at the Royal
Institution, London, and subsequently repeated in Edinburgh and Glasgow.

The statement of his theory is contained in chapter iii. of this work,
under the heading “Of Chemical Synthesis,” and is accompanied by a
plate and explanation, of which a facsimile is given on pp. 130–1.

The facts upon which Dalton based his theory are incontrovertible; but
Dalton’s explanation of them was not universally accepted at the time
he gave it. Davy, who, of course, was familiar with the conception
of atoms as part of the Newtonian philosophy, objected to the term
“atomic weight” introduced by Dalton, and suggested the expression
“combining proportion”; and Wollaston, for similar reasons, proposed
the term “equivalent,” as denoting the constant quantity with which
bodies went in and out of combination. There is no doubt that the use
of these terms retarded the general acceptance of Dalton’s doctrine,
and, moreover, brought into the science a confusion which was not
finally dispelled, as we shall see, until during the second half of the
century.

[Illustration: ELEMENTS

_Simple_

_Binary_

_Ternary_

_Quaternary_

_Quinquenary_ & _Sextenary_

_Septenary_]

The illustration on the preceding page contains the arbitrary marks
or signs chosen to represent the several chemical elements or
ultimate particles.

Fig.
1. Hydro. its rel. weight 1
2. Azote 5
3. Carbone or charcoal 5
4. Oxygen 7
5. Phosphorus 9
6. Sulphur 13
7. Magnesia 20
8. Lime 23
9. Soda 28
10. Potash 42
11. Strontites 46
12. Barytes 68
13. Iron 38
14. Zinc 56
15. Copper 56
16. Lead 95
17. Silver 100
18. Platina 100
19. Gold 140
20. Mercury 167
21. An atom of water or steam, composed of 1 of
oxygen and 1 of hydrogen, retained in physical
contact by a strong affinity, and supposed
to be surrounded by a common atmosphere
of heat; its relative weight = 8
22. An atom of ammonia, composed of 1 of azote
and 1 of hydrogen 6
23. An atom of nitrous gas, composed of 1 of azote
and 1 of oxygen 12
24. An atom of olefiant gas, composed of 1 of carbone
and 1 of hydrogen 6
25. An atom of carbonic oxide composed of 1 of
carbone and 1 of oxygen 12
26. An atom of nitrous oxide, 2 azote + 1 oxygen 17
27. An atom of nitric acid, 1 azote + 2 oxygen 19
28. An atom of carbonic acid, 1 carbone + 2
oxygen 19
29. An atom of carburetted hydrogen, 1 carbone
+ 2 hydrogen 7
30. An atom of oxynitric acid, 1 azote + 3 oxygen 26
31. An atom of sulphuric acid, 1 sulphur + 3 oxygen 34
32. An atom of sulphuretted hydrogen, 1 sulphur
+ 3 hydrogen 16
33. An atom of alcohol, 3 carbone + 1 hydrogen 16
34. An atom of nitrous acid, 1 nitric acid + 1
nitrous gas 31
35. An atom of acetous acid, 2 carbone + 2 water 26
36. An atom of nitrate of ammonia, 1 nitric acid
+ 1 ammonia + 1 water 33
37. An atom of sugar, 1 alcohol + 1 carbonic acid 35

Dalton’s estimations of the relative weights of the atoms, or, to use
Davy’s phrase, the values of their combining proportions, were, as
might be expected, very rough approximations to the truth. This arose
partly from inadequate experimental data, and partly from uncertainty
as to the relative number of the constituent atoms which made up a
compound. Neither Dalton nor his immediate successors had any rational
or consistent method of determining the latter point. The view taken of
the composition of the compound decided what particular multiples or
sub-multiples of the values of the atomic weights of its constituents
were to be adopted. As Dalton, in many cases, had no real criterion to
guide him, he made the simplest possible assumptions; but these might
or might not be valid; and subsequent experience showed that in some
cases they were erroneous.

It was, however, generally recognised that these atomic weights,
combining proportions, or equivalents, as they were for a time
indifferently termed, were chemical constants of the highest
importance, both to the scientific chemist, who, apart from their
theoretic interest, had need of them in the course of quantitative
analysis, and to the manufacturing chemist, who required them for
the intelligent exercise of his operations; and accordingly a number
of chemists, very shortly after the promulgation of Dalton’s theory,
attempted to determine their values with all possible precision. Chief
among these was the Swedish chemist Berzelius, to whom science was
indebted for a series of estimations of atomic weights, which were long
regarded as models of quantitative accuracy, and stamped their author
as the greatest master of determinative chemistry of his age.

=Jöns Jakob Berzelius=, the son of a schoolmaster, was born near
Linköping, in East Gothland, Sweden, in 1779. Entering Upsala with
a view to the profession of medicine, he was attracted, under the
influence of Afzelius—or, rather, in spite of it—to the study of
chemistry, and, later, of voltaic electricity, then in its infancy.
While holding a number of minor appointments as a teacher of medicine,
pharmacy, physics, and chemistry, he was elected, in 1808, a member
of the Swedish Academy of Sciences, of which he became President in
1810. In 1818 he was made permanent Secretary of the Academy, and, by
means of a yearly subsidy, was enabled to devote himself wholly to
experimental science. He was ennobled in 1818, and on the occasion of
his marriage, in 1835, was created a baron of the Scandinavian kingdom.
He died in 1848.

Berzelius occupies a pre-eminent position in the history of chemistry,
and during a considerable portion of his lifetime exercised an almost
unassailable authority as a chemical philosopher. He is distinguished
as an experimenter, as a discoverer, as a critic and interpreter,
and as a lawgiver. His contributions to chemical knowledge range
over every department of the science. He shares with Davy the honour
of having established the fundamental laws of electro-chemistry.
His experimental work on the atomic weights of the elements—the
great work of his life—was of supreme importance at this particular
period of the development of chemistry: it served not only to give
precision to, and enhance the significance and value of, Dalton’s
generalisation, but it furnished chemists, for the first time, with
a set of constants, ascertained with the highest exactitude of which
operative chemistry was then capable, thereby contributing to the
expansion of quantitative analysis, and to a more exact knowledge of
the composition of substances. Berzelius, indeed, was an analyst of the
first rank—conscientious, patient, and painstaking; an ingenious and
skilful manipulator; inventive and resourceful. What determinative
chemistry owes to his labours, and not less to his example, is obvious
from even the most superficial examination of its literature during the
first third of the last century.

As a discoverer, Berzelius first made known the existence of _cerium_
(1803), of _selenium_ (1818), and of _thorium_ (1828); and he prepared
and investigated a large number of their combinations. He isolated
_silicon_ (1823), _zirconium_ (1824), _tantalum_ (1824), and studied
the compounds of _vanadium_, discovered by his countryman Sefström.
He largely extended our knowledge of groups of substances in which
sulphur replaces oxygen; investigated compounds of fluorine (1824),
platinum (1828), and tellurium (1831–1833), and made many analyses of
minerals, meteorites, and mineral-waters. He discovered _racemic acid_
and investigated the ferrocyanides. It was his investigation of racemic
acid—which has the same percentage composition as tartaric acid—that
first enabled him to grasp the conception of _isomerism_, a term which
we owe to him, and of _metamerism_ and _polymerism_. He was the first
to study the phenomena of contact-actions, which he comprehended under
the term _catalysis_.

[Illustration: JÖNS JAKOB BERZELIUS.

From a painting by J. G. Sandberg.]

As an author his literary activity was astonishing. His new system of
mineralogy marks an epoch in the history of that branch of science. His
text-book on chemistry was long the leading manual, and went through
many editions, being constantly revised by him. His annual reports on
the progress of physics and chemistry extended to twenty-seven volumes
and constitute a monument to his industry, thoroughness, perspicacity,
and critical ability.

Although holding no university appointment, and with a laboratory of
the most modest dimensions and character, Berzelius, exercised great
influence as a teacher. Some of the most notable chemists of the last
century, such as Heinrich and Gustav Rose, Dulong, Mitscherlich,
Wöhler, Chr. Gmelin, and Mosander, were among his pupils; and many of
them have testified to his stimulating power as an investigator of
nature, and to his merits as a worthy, genial man.

The reasonableness of Dalton’s conjecture received further support
from the discovery by Gay Lussac in 1808, that gases always combine
in simple proportions by volume, and that the volume of the gaseous
product formed, when measured under comparable conditions of
temperature and pressure, stands in a simple relation to the volumes
of the constituents. The law of pressure discovered by Boyle, that
of thermal expansion by Dalton, and of volumes by Gay Lussac (which,
it ought to be stated, was previously and independently made by
Dalton), are explained on the assumption that equal numbers of the
particles—either as simple particles or as compound particles—are
present in the same volume of the gas. This method of explanation was
first clearly stated by the Italian physicist =Avogadro= in 1811, but
its significance, as will be seen subsequently, was not appreciated
until half a century later.

As the values for the atomic weights gradually became more exact,
speculations arose as to the significance of the numerical relations
which were observed to exist among them. In 1815 =William Prout= threw
out the supposition that the atomic weights of the gaseous elements
are multiples by whole numbers of that of hydrogen. Extended into a
generalisation, this might be held to indicate that all kinds of matter
are so many forms of a primordial substance. Subsequent inquiry showed
that Prout’s “Law,” as it is sometimes called, was not tenable in its
original form. Certain elements, it was conclusively proved, had atomic
weights which were not whole numbers. Dumas subsequently modified the
law, after a redetermination of a large number of atomic weights,
by assuming that the substance common to the so-called elements had
a lower atomic weight than unity. Although there are a considerable
number of elements whose atomic weights, based upon the most
accurate determinations, are remarkably close to whole numbers, the
investigations of Stas and others afford no valid reason for believing
that Prout’s hypothesis, and the underlying supposition to which it has
been held to point, are justified by experimental evidence.

The first year of the nineteenth century is further memorable on
account of the invention of the voltaic pile, and by reason of its
application by =William Nicholson= and =Sir Anthony Carlisle= to the
electrolytic decomposition of water. This mode of resolving water into
its constituents made a great sensation at the time, mainly because
of the extraordinary method by which it was effected. It afforded an
independent and unlooked-for proof of the compound nature of water
by a method altogether differing in principle from that by which its
composition had been previously ascertained. The formation of water by
the combustion of hydrogen brought no conviction of its real nature
to a confirmed phlogistian like Priestley; and it is even doubtful
whether Cavendish ever fully realised the true significance of his
great discovery. But the fact that the quantitative results of the
analysis thus effected were identical with those of its synthesis, as
made by Cavendish and Lavoisier, admitted of only one interpretation.
This cardinal discovery may be said to have completed the downfall of
phlogiston.

The value of the voltaic pile as an analytical agent was nowhere more
quickly appreciated than in England. In the hands of Humphry Davy its
application to the analysis of the alkalis and alkaline earths led to
discoveries of the greatest magnitude.

=Humphry Davy= was born in Penzance in 1778. In the course of his
studies for the profession of medicine he was attracted to chemistry;
and he became chemical assistant to Dr. Beddoes, a former teacher of
chemistry at Oxford, but then living at Clifton, near Bristol. While
in the capacity of assistant and operator in Beddoes’s Pneumatical
Institute, Davy discovered the intoxicating properties of _nitrous
oxide_ (so called laughing gas), which brought him into prominence
and led to his engagement by the managers of the newly-created Royal
Institution in London as lecturer in chemistry in succession to
Garnett. He early began to experiment on galvanism, and soon succeeded
in developing the fundamental laws of electro-chemistry; and in 1807 he
effected the _decomposition of potash and soda_ by the application of
voltaic electricity—thereby establishing, what indeed had been surmised
previously, that the alkalis are compound substances. He subsequently
proved that this was also the case with the alkaline earths. Davy thus
added some five or six metallic elements to those already known.

These discoveries, perhaps the most brilliant of their time, afforded
additional evidence of the invalidity of Lavoisier’s assumption that
oxygen, as the name implies, was the “principle of acidity.” The
surmise, in fact, was already disproved by the case of water—a neutral
substance and devoid of all the recognised attributes of an acid. It
was still further disproved by the cases of potash and soda—strongly
alkaline compounds.

Additional evidence was adduced by Davy in demonstrating, in 1810,
that the so-called _oxymuriatic acid_, the _dephlogisticated
marine acid_ discovered by Scheele, contained no oxygen, but was
a simple, indivisible substance. For the old designation, which
connoted a compound body, he substituted the name _chlorine_, in
allusion to the characteristic colour of the element. In the course
of his investigation on this substance he discovered the _penta-
and trichloride of phosphorus_, _chlorophosphamide_ and _chlorine
peroxide_. He was also the discoverer of _telluretted hydrogen_ and an
independent discoverer of _nitrosulphonic acid_.

[Illustration: SIR HUMPHRY DAVY.

From a painting by Lawrence in the possession of the Royal Society.]

He worked on _iodine_ and the _iodates_, on the _diamond_, on the
so-called _fuming liquor of Cadet_, on _nitrogen chloride_, and on the
_pigments of the ancients_. Lastly, he invented the _miner’s safety
lamp_, with which his name will always be associated, effecting thereby
what was practically a revolution in coal-mining. He became President
of the Royal Society in 1820, and died at Geneva on May 29th, 1829, in
the fifty-first year of his age. Davy was a singularly gifted man, of
great mental vigour and imaginative power; quick, lively and ingenious;
an eloquent teacher and a daring and brilliant experimenter.

Another noteworthy name in the chemical history of this period is
Wollaston. =William Hyde Wollaston=, born at East Dereham, in Norfolk,
in 1766, was educated at Cambridge with a view to the profession of
medicine, but, failing to secure a practice, he devoted himself to
the pursuit of science, and especially to optics and chemistry. He
devised a method of _working platinum_, and was the first to make known
the existence of _palladium_ and _rhodium_. He was one of the most
ingenious and acute analysts of his time, and possessed remarkable
inventive powers. He investigated the nature of _urinary calculi_
and _chalk stones_. His paper on the _oxalates of potash_ was of
great service at the time as a demonstration of the law of multiple
proportions. He first drew attention to the existence in the solar
spectrum of what were subsequently termed the _Fraunhofer lines_; and
he invented the _reflecting goniometer_ and the _camera lucida_, and
a _slide rule_ for chemical calculations. He resembled Cavendish in
temperament and mental habitudes, and, like him, was distinguished
for the range and exactitude of his scientific knowledge, his habitual
caution, and his cold and reserved disposition. He died in 1828.

[Illustration: WILLIAM HYDE WOLLASTON.

From a painting by J. Jackson, R.A., in the possession of the Royal
Society.]

Almost immediately after the publication of Volta’s discovery attempts
were made—notably by Berzelius in Sweden and by Davy in England—to
prove that electrical and chemical phenomena are correlated and
mutually dependent. This assumption was more fully worked out by
Berzelius in 1812, and it served as the basis of a chemical system
which exercised considerable influence on chemical doctrine during the
first half of the nineteenth century.

Berzelius assumed that electric polarity was an attribute of all
atoms—that these were bipolar, in fact, but that in them either
positive or negative electricity predominated. Hence the elements
were capable of being divided into two classes—that is, positive or
negative, depending upon the excess of either charge. Which of the
electricities predominated might be ascertained by determining the
particular pole at which the element was separated on electrolysis.
Combinations of dissimilar elements—or, in other words, chemical
compounds—were also endowed with polarity. The chemical affinities of
elements and compounds were related to the excess of either kind of
electricity resident in them; and chemical combination resulted from,
and was a consequence of, the more or less perfect neutralisation
of the two kinds. From a study of the electrical deportment of the
elements Berzelius sought to arrange them in series, starting with
oxygen as the most electro-negative member.

These conceptions were employed by him as the basis of a method of
classification. The attempt is historically interesting as being the
first systematic endeavour to gain an insight into the constitution
of chemical compounds—that is, to determine the manner in which the
constituent atoms are grouped or arranged with respect to one another,
or, in other words, to distinguish between the empirical and the
rational composition of substances, which is the ultimate aim of modern
chemistry.

A necessary consequence of these views was that every compound was to
be considered as made up of two parts in electrically different states.
Thus baryta, consisted of a combination of the electro-positive barium,
combined with the electro-negative oxygen; it combined with sulphuric
oxide because the preponderating positive electricity it contained met
with the negative electricity which prevailed in the sulphuric oxide.
Generalising, it may be said that the basic oxides are invariably
the positive constituents of salts, whereas the acid oxides are the
negative constituents, as proved by the mode in which the two kinds of
oxides separated at the poles on electrolysis. Barium sulphate, then,
was to be regarded as made up of two entities—BaO and SO3—and hence was
to be called sulphate of baryta. Berzelius extended this conception
in order to explain the formation of double salts—such, for example,
as potash alum, which he regarded as a binary compound of positive
potassium sulphate and negative aluminium sulphate, each of which,
in its turn, could be resolved into an acidic and a basic oxide of
opposite electricities.

The dualistic notions of Berzelius led him to the construction of
a system of chemical nomenclature and notation which, in its main
features, has persisted to this day, and is universally current, with
certain modifications, in modern chemical literature. We owe to him
the grouping of the elements into metals and metalloids, and also our
present system of symbolic notation, whereby even complicated chemical
reactions may be expressed in a concise and intelligible manner.
Chemical symbols were used by the alchemists; but Berzelius first
suggested that a chemical symbol should not only represent the element
to which it refers, but also its relative atomic weight. Chemical
equations became quantitative as well as qualitative expressions of
the facts they denote. Such equations implicitly assumed that, to
use Davy’s words, chemistry had passed under the dominion of the
mathematical sciences. Professed mathematicians were, however, slow to
recognise that the phenomena of chemical action were capable of formal
mathematical treatment. Davy relates that on speaking to Laplace of
the atomic theory in chemistry, and expressing his belief that the
science would ultimately be referred to mathematical laws similar to
those he had so profoundly and successfully established with respect
to the mechanical properties of matter, the idea was treated in a tone
bordering on contempt.

Berzelius’s electro-chemical system, and the dualistic ideas associated
with it, were of considerable service when applied to the inorganic
branch of the science; but attempts to fit them to the facts of organic
chemistry, which began to accumulate rapidly after the first quarter of
the century, failed. Its inadequacy as a comprehensive generalisation
became more and more manifest, and it eventually fell. In fact, it may
be said to have received its death-blow by Davy’s discovery of the
elementary nature of chlorine, and by the recognition of the fact that
the acids do not necessarily contain oxygen. Davy and, later, Dulong
made it obvious that, if any one element was to be regarded as the
acidifying principle, it was hydrogen, and not oxygen; and, in a sense,
this view ultimately prevailed in the recognition of the acids as salts
of hydrogen.

In France the study of electro-chemistry was undertaken by Gay Lussac
and Thénard, largely owing to the action of the Emperor Napoleon,
who furnished the funds for the construction of a powerful galvanic
battery. The results were published, in 1811, under the title,
_Recherches Physico-Chimiques, faites sur la Pile_, etc. Gay Lussac,
whose name has already been mentioned as one of the discoverers of
the Law of Combination of Gases, played a considerable part in the
history of chemistry at this period. He was one of the earliest to
appreciate the importance of Dalton’s generalisation, and to point
out the significance of his own discovery in strengthening it. He was
probably led, in the first instance, to the recognition of the law of
gaseous combination by Berthollet’s work on the volumetric composition
of ammonia gas, and by his own discovery—made in 1805, in conjunction
with Humboldt, in the course of their analysis of atmospheric air—that
one volume of oxygen combined with exactly two volumes of hydrogen to
form water. The regularities thus indicated he found to be general: all
gases which are capable of chemical union combine in simple proportions
by volume, and the volume of the product, if a gas, always stands in
some simple relation to the volumes of the constituents.

=Joseph Louis Gay Lussac= was born in 1778, at Saint Leonard, studied
chemistry in Paris, and was associated in chemical inquiry with
Berthollet. As Eleve-Ingenieur in the École Nationale des Ponts et
des Chaussées he began the experimental work in physics and chemistry
upon which his fame rests. In 1804 he undertook, with Biot, a series
of balloon ascents for the purpose of investigating the physics and
chemistry of the upper regions of the atmosphere. In 1806 he became
Professor of Chemistry at the École Polytechnique, and in 1832
Professor at the Jardin des Plantes. He was one of the chief assayers
of the French Mint, and, as member of many commissions, exerted
considerable influence in official circles. He died in 1850.

Gay Lussac and Thénard were the first to devise a method of obtaining
potassium and sodium by a purely chemical process, whereby these
metals could be procured in far larger quantities than was at that
time possible by electrolytic means. They were thus enabled to make
use of the strong deoxidising power of these metals to effect a number
of reductions, notably that of boric oxide to _boron_. Gay Lussac and
Thénard were also the first to make known the existence of _boron
fluoride_. We further owe to Gay Lussac the discovery of _cyanogen_,
the first of the so-called compound radicals. He first prepared
ethyl iodide, investigated sulphovinic acid and grape sugar, studied
etherification and fermentation, etc. We are also indebted to him
for a method of determining vapour densities which proved of great
service in ascertaining the molecular weights of substances. He worked
on iodine and its compounds, discovered, with Welter, _thiosulphuric_
acid, and investigated fulminic acid in collaboration with Liebig.

Among his services to analytical chemistry were his method for the
analysis of gunpowder, his volumetric estimation of silver (wet silver
assay), chlorometric analysis, alkalimetry, etc. He devised the system
still in use in France for the estimation of alcohol in spirits of wine.

=Louis Jacques Thénard= was born in 1777 at Nogent-Sur-Seine, and was
a pupil of Vauquelin and of Berthollet. In 1797 he became _repétiteur_
at the Polytechnic School of Paris, and eventually its professor. He
subsequently occupied the chair of chemistry at the Collège de France,
and of the Faculty of Science of the University of Paris. He was
ennobled by Charles X. in 1824, and died at Paris in the eightieth year
of his age.

In addition to his work with Gay Lussac already mentioned, we owe
to Thénard the discovery of _hydrogen peroxide_ and _hydrogen
persulphide_. Together with Dulong he studied the catalytic action of
platinum on mixtures of oxygen and hydrogen. He investigated the fatty
acids, and worked on fermentation and on ether-formation; and he was
the first to isolate citric and malic acids. He also occupied himself
with the chemistry of bile, perspiration, albumen, the acids of urine
and milk, and with the theory of mordants.

In 1834 Faraday made known the important fact that on passing the same
galvanic current through a number of electrolytes—water, hydrochloric
acid, solutions of metallic chloride—these were decomposed in such
manner that definite amounts of hydrogen or metal were separated at the
negative pole, and corresponding amounts of oxygen or chlorine were
evolved at the positive pole. These observations were comprehended
by Faraday under his “law of definite electrolytic action.” The
electro-chemical equivalents thus obtained were in some cases identical
with the atomic weights deduced by Berzelius; in others they were
not; but, nevertheless, when they differed, they stood in some simple
relation to the assumed atomic weight. The significance of Faraday’s
observation was not lost sight of, although his anticipation that the
determination of electro-chemical equivalents would be of use in fixing
atomic weights was not immediately appreciated. A clear distinction
between the _equivalent_, the _atom_, and the _molecule_ was not
then apprehended. As will be subsequently shown, it was only during
the latter half of the nineteenth century that the discrepancies and
inconsistencies thus revealed were definitely reconciled and cleared
up.