e atoms of three different kinds.
Yet more heterogeneous and less stable still are the salts; which
present us with molecules each made up of five, six, seven, eight, ten,
twelve, or more atoms, of three, if not more, kinds. Then there are the
hydrated salts, of a yet greater heterogeneity, which undergo partial
decomposition at much lower temperatures. After them come the further
complicated supersalts and double salts, having a stability again
decreased; and so throughout. Without entering into qualifications for
which space fails, we believe no chemist will deny it to be a general
law of these inorganic combinations that, _other things equal_, the
stability decreases as the complexity increases. When we pass to the
compounds of organic chemistry, we find this general law still further
exemplified: we find much greater complexity and much less stability. A
molecule of albumen, for instance, consists of 482 ultimate atoms of
five different kinds. Fibrine, still more intricate in constitution,
contains in each molecule, 298 atoms of carbon, 49 of nitrogen, 2 of
sulphur, 228 of hydrogen, and 92 of oxygen--in all, 669 atoms; or, more
strictly speaking, equivalents. And these two substances are so unstable
as to decompose at quite ordinary temperatures; as that to which the
outside of a joint of roast meat is exposed. Thus it is manifest that
the present chemical heterogeneity of the Earth's surface has arisen by
degrees, as the decrease of heat has permitted; and that it has shown
itself in three forms--first, in the multiplication of chemical
compounds; second, in the greater number of different elements contained
in the more modern of these compounds; and third, in the higher and more
varied multiples in which these more numerous elements combine.

To say that this advance in chemical heterogeneity is due to the one
cause, diminution of the Earth's temperature, would be to say too much;
for it is clear that aqueous and atmospheric agencies have been
concerned; and further, that the affinities of the elements themselves
are implied. The cause has all along been a composite one: the cooling
of the Earth having been simply the most general of the concurrent
causes, or assemblage of conditions. And here, indeed, it may be
remarked that in the several classes of facts already dealt with
(excepting, perhaps, the first), and still more in those with which we
shall presently deal, the causes are more or less compound; as indeed