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.
[Illustration: JOHN DALTON.
From a painting by B. R. Faulkner in the possession of the Royal
Society]
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.