Before going on to show how organic progress also depends upon the universal law that every force produces more than one change, we have to notice the manifestation of this law in yet another species of inorganic progress—­namely, chemical.  The same general causes that have wrought out the heterogeneity of the Earth, physically considered, have simultaneously wrought out its chemical 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 entering into qualifications for which we lack space, 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.