Proust is the discoverer of what is now styled “the law of constant proportion,” which states that the same body is invariably composed of the same elements, united in the same proportion. He was a skilful analyst, and made numerous analyses of minerals; and he was one of the earliest to undertake a systematic study of metallic salts of organic acids. 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_.