Mr Dalton has lately thrown out a very ingenious idea respecting chemical affinity, which deserves to be mentioned, According to him, bodies unite ei ther atom to atom, or two or three atoms of one to one atom of another. Thus water is composed of an atom of oxygen and an atom of hydrogen united to gether ; ammonia of an atom of hydrogen and an atom of azote united together ; carbonic acid of two atoms of oxygen united to one of carbon. Sulphate of potash consists of a particle of sulphuric acid united to a particle of potash ; supersulphate of pot ash of a particle of potash united to two particles of sulphuric acid. If we admit that matter is com posed of atoms, as is at least probable, it is difficult to refuse admission to this hypothesis, though it is probable that more complicated cases may exist. For example, two atoms of one body may combine with three of another, and so on. It is even possible that the proportion in unite cannot always be represented by numbers. But this hypothesis of Dalton is much more probable, and corresponds much better with the phenomena, than the opposite one of Berthollet, that bodies combine in all proportions whatever. Dalton's hypothesis is very useful, be
cause it facilitates the knowledge of the composition of bodies. For example, if water be composed of an atom of hydrogen and an atom of oxygen, it fol lows, from the known analysis of water, that the weight of an atom of hydrogen is to that of an atom of oxygen as 1 to 6. Hence we know, that when oxygen enters into a combination, it will always en ter as 6, or as some multiple of 6. This is very ob servable in the metallic oxides. The quantity of oxygen in the second oxide is usually double that in the first oxide, and that in the third triple that in the first. The same thing is equally remarkable in the salts; the supersulphate of potash contains just double the quantity of sulphuric acid that exists in the sul phate, and all the supersalts contain twice as much acid as the neutral salts, with the same acid and base. This atomic theory of Mr Dalton, plausible as it is, will be overturned, if Mr Davy succeeds in proving that azote is a 'compound of oxygen and hydrogen, as his experiments already published give us some reason to suspect.