Foundations of the molecular theory / comprising papers and extracts by John Dalton, Joseph-Louis Gay-Lussac, and Amedeo Avogadro (1808-1811).

Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Foundations of the molecular theory / comprising papers and extracts by John Dalton, Joseph-Louis Gay-Lussac, and Amedeo Avogadro (1808-1811). Source: Wellcome Collection.

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    volumes of the gaseous compounds, which depend partly on the division of molecules entirely unsuspected by this physicist. Thus Dalton supposes* that water is formed by the union of hydrogen and oxygen, molecule to molecule. From this, and from the ratio by weight of the two com- ponents, it would follow that the mass of the molecule of oxygen would be to that of hydrogen as 7-^ to i nearly, or, according to Dalton’s evaluation, as 6 to i. This ratio on our hypothesis is, as we saw, twice as great, namely, as 15 to i. As for the molecule of water, its mass ought to be roughly expressed by 15 -f 2 = 17 (taking for unity that of hydrogen), if there were no division of the molecule into two ; but on account of this division it is reduced to half, 8^, or more exactly 8.537, as may also be found directly by dividing the density of aqueous vapour 0.625 (Gay-Lussac) by the density of hydrogen 0.0732. This mass only differs from 7, that assigned to it by Dalton, by the difference in the values for the com- position of water; so that in this respect Dalton’s result is approximately correct from the combination of two compensating errors,—the error in the mass of the molecule of oxygen, and his neglect of the division of the molecule. Dalton supposes that in nitrous gas the combination of nitrogen and oxygen is molecule to molecule : we have seen on our hypothesis that this is actually the case. Thus Dalton would have found the same molecular mass for nitrogen as we have, always supposing that of hydrogen to be unity, if he had not set out from a different value for that of oxygen, and if he had taken precisely the same value for the quantities of the elements * In what follows I shall make use of the exposition of Dalton’s ielcas given in Thomson’s System of Chemistry. [See AlemDr Club Reprints, No. 2, ji. 42.]