heterogeneity. Without dwelling
upon the general fact that the forces which have been increasing the
variety and complexity of geological formations, have, at the same time,
been bringing into contact elements not previously exposed to each other
under conditions favourable to union, and so have been adding to the
number of chemical compounds, let us pass to the more important
complications that have resulted from the cooling of the Earth.

There is every reason to believe that at an extreme heat the elements
cannot combine. Even under such heat as can be artificially produced,
some very strong affinities yield, as for instance, that of oxygen for
hydrogen; and the great majority of chemical compounds are decomposed at
much lower temperatures. But without insisting upon the highly probable
inference, that when the Earth was in its first state of incandescence
there were no chemical combinations at all, it will suffice our purpose
to point to the unquestionable fact that the compounds that can exist at
the highest temperatures, and which must, therefore, have been the first
that were formed as the Earth cooled, are those of the simplest
constitutions. The protoxides--including under that head the alkalies,
earths, etc.--are, as a class, the most stable compounds we know: most
of them resisting decomposition by any heat we can generate. These,
consisting severally of one atom of each component element, are
combinations of the simplest order--are but one degree less homogeneous
than the elements themselves. More heterogeneous than these, less
stable, and therefore later in the Earth's history, are the deutoxides,
tritoxides, peroxides, etc.; in which two, three, four, or more atoms of
oxygen are united with one atom of metal or other element. Higher than
these in heterogeneity are the hydrates; in which an oxide of hydrogen,
united with an oxide of some other element, forms a substance whose
atoms severally contain at least four ultimate atoms of three different
kinds. Yet more heterogeneous and less stable still are the salts; which
present us with compound atoms 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 ente