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.