atomic theory; and he had the merit of convincing Davy that his former opinions on the subject were wrong. What arguments he employed I do not know; but they must have been convincing ones, for Davy ever after became a strenuous supporter of the atomic theory. The only alteration which he made was to substitute _proportion_ for Dalton's word, _atom_. Dr. Wollaston substituted for it the term _equivalent_. The object of these substitutions was to avoid all theoretical annunciations. But, in fact, these terms, _proportion_, _equivalent_, are neither of them so convenient as the term _atom_: and, unless we adopt the hypothesis with which Dalton set out, namely, that the ultimate particles of bodies are _atoms_ incapable of further division, and that chemical combination consists in the union of these atoms with each other, we lose all the new light which the atomic theory throws upon chemistry, and bring our notions back to the obscurity of the days of Bergman and of Berthollet. In the year 1808 Mr. Dalton published the first volume of his New System of Chemical Philosophy. This volume consists chiefly of two chapters: the first, on _heat_, occupies 140 pages. In it he treats of all the effects of heat, and shows the same sagacity and originality which characterize all his writings. Even when his opinions on a subject are not correct, his reasoning is so ingenious and original, and the new facts which he contrives to bring forward so important, that we are always pleased and always instructed. The second chapter, on the _constitution of bodies_, occupies 70 pages. The chief object of it is to combat the peculiar notions respecting elastic fluids, which had been advanced by Berthollet, and supported by Dr. Murray, of Edinburgh. In the third chapter, on _chemical synthesis_, which occupies only a few pages, he gives us the outlines of the atomic theory, such as he had conceived it. In a plate at the end of the volume he exhibits the symbols and atomic weights of thirty-seven bodies, twenty of which were then considered as simple, and the other seventeen as compound. The following table shows the atomic weight of the simple bodies, as he at that time had determined them from the best analytical experiments that had been made: Weight of atom. Hydrogen 1 Azote 5 Carbon 5 Oxygen 7 Phosphorus 9 Sulphur 13 Magnesia 20 Lime 23 Soda 28 Potash 42 Strontian 46 Barytes 68 Iron 38 Zinc 56 Copper 56 Lead 95 Silver 100 Platinum 100 Gold 140 Mercury 167