The molecule of any elementary body is, on the ground of the hypothesis,
assumed to be a compound of two atoms, and the molecule of carbon
consequently C_2=24; that of nitrogen N_2=28. Combination of the two,
according to the same hypothesis, takes place by substitution; the
atoms are supposed to be set free and to exchange places, forming a
new compound different from the original only in this: that each new
particle contains an atom of each of the two different substances, while
each original particle consists of two identical atoms. The product is,
therefore, assumed to be, and can, under the circumstances, be no other
than particles of the composition CN and weight 26. These particles are
molecules, according to the definition laid down, just as C_2 and N_2;
but there is this essential difference, that the specific gravity of
cyanogen gas, 26, coincides with the molecular weight, while the assumed
molecular weight, N_2=28, is twice as great as the specific gravity of
the gas, N=14.
In using the term molecular weight, it is to be remembered that it does
not express the weight of single molecules, but only their relative
weight, millions of millions molecules being contained in the unit of
volume. But on the hypothesis that there is the same number of molecules
in the same volume of any gas, the specific gravities of gases can be,
and are, identified with their molecular weights, and, on the ground of
the hypothesis again, the unit of the numbers which enter into every
chemical reaction and constitute the molecular weight, is stipulated to
be that contained in two volumes.
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