§ =75.= At the beginning of the nineteenth century, John Dalton (see plate 15) put forward his Atomic Theory in explanation of these facts. This theory assumes (1) that all matter is made up of small indivisible and indestructible particles, called “atoms”; (2) that all atoms are _not_ alike, there being as many different sorts of atoms as there are elements; (3) that the atoms constituting any one element are exactly alike and are of definite weight; and (4) that compounds are produced by the combination of different atoms. Now, it is at once evident that if matter be so constituted, the stoichiometric laws must necessarily follow. For the smallest particle of any definite compound (now called a “molecule”) must consist of a definite assemblage of different atoms, and these atoms are of definite weight: whence the law of constant proportion. One atom of one substance may combine with 1, 2, 3 . . . atoms of some other substance, but it cannot combine with some fractional part of an atom, since the atoms are indivisible: whence the law of multiple proportions. And these laws holding good, and the atoms being of definite weight, the law of combining weights necessarily follows. Dalton’s Atomic Theory gave a simple and intelligible explanation of these remarkable facts regarding the weights of substances entering into chemical combination, and, therefore, gained universal acceptance. But throughout the history of Chemistry can be discerned a spirit of revolt against it as an explanation of the absolute constitution of matter. The tendency of scientific philosophy has always been towards Monism as opposed to Dualism, and here were not merely two eternals, but several dozen; Dalton’s theory denied the unity of the Cosmos, it lacked the unifying principle of the alchemists. It is only in recent times that it has been recognised that a scientific hypothesis may be very useful without being altogether true. As to the usefulness of Dalton’s theory there can be no question; it has accomplished that which no other hypothesis could have done; it rendered the concepts of a chemical element, a chemical compound and a chemical reaction definite; and has, in a sense, led to the majority of the discoveries in the domain of Chemistry that have been made since its enunciation. But as an expression of absolute truth, Dalton’s theory, as is very generally recognised nowadays, fails to be satisfactory. In the past, however, it has been the philosophers of the materialistic school of thought, rather than the chemists _quâ_ chemists, who have insisted on the absolute truth of the Atomic Theory; Kekulé, who by developing Franklin’s theory of atomicity or valency[91] made still more definite the atomic view of matter, himself expressed grave doubts as to the absolute truth of Dalton’s theory; but he regarded it as _chemically_ true, and thus voices what appears to be the opinion of the majority of chemists nowadays, namely, there are such things as chemical