422 THE INSTABILITY OF THE HOMOGENEOUS. 



cools, there arises that contrast, now so decided, between the 

 polar and equatorial regions. 



Along with these most marked physical differentiations 

 of the Earth, which are manifestly consequent on the insta- 

 bility of the homogeneous, there have been going on numer- 

 ous chemical differentiations, admitting of similar interpre- 

 tation. Without raising the question whether, as some 

 think, the so-called simple substances are themselves com- 

 pounded of unknown elements (elements which we cannot 

 separate by artificial heat, but which existed separately 

 when the heat of the Earth was greater than any which we 

 can produce), — without raising this question, it will suffice 

 the present purpose to show how, in place of that compara- 

 tive homogeneity of the Earth's crust, chemically consid- 

 ered, which must have existed when its temperature was 

 high, there has arisen, during its cooling, an increasing 

 chemical heterogeneity: each element or compound, being- 

 unable to maintain its homogeneity in presence of various 

 surrounding affinities, having fallen into heterogeneous 

 combinations. Let us contemplate this change somewhat in 

 detail. There is every reason to believe that at an 



extreme heat, the bodies we call elements cannot combine. 

 Even under such heat as can be generated artificially, some 

 very strong affinities yield; and the great majority of chemi- 

 cal compounds are decomposed at much lower temperatures. 

 Whence it seems not improbable that, when the Earth was 

 in its first state of incandescence, there were no chemical 

 combinations at all. But without drawing this inference, 

 let us set out with the unquestionable fact that the com- 

 pounds which can exist at the highest temperatures, and 

 which must therefore have been the first formed as the 

 Earth cooled, are those of the simplest constitutions. The 

 protoxides — including under that head the alkalies, earths, 

 <tc. — are, as a class, the most fixed compounds known: the 

 majority of them resisting decomposition by any heat we can 

 generate. These, consisting severally of one atom of each 



