CHAPTER IX

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.

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_.

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.