PLANETARY EVOLUTION 



erful argument in favour of the nebular hypo- 

 thesis. 



But* the evidence does not end with these 

 mechanical illustrations. In the present physical 

 condition of the various planets, so far as it can 

 be determined, we shall find further corrob- 

 orative testimony. It is a corollary from the 

 nebular hypothesis that all the planets, having 

 successively originated from the same vaporous 

 mass, must be composed in the main of similar 

 chemical elements ; and this inference has thus 

 far been uniformly corroborated by spectro- 

 scopic observation wherever there has been an 

 opportunity to employ it. Hence it follows 

 that the process through which the earth has 

 passed in contracting to its present dimensions 

 has been, or will be, repeated to a certain ex- 

 tent upon all the other planets. Upon any 

 planet there must eventually occur a solidifica- 

 tion of the crust, an extensive evaporation and 

 precipitation of water, an upheaval of moun- 

 tains, an excavation of river-beds, and a deposit 

 of alluvium, resulting in sedimentary strata. 

 But obviously the time at which these pheno- 

 mena occur must depend, not merely upon the 

 antiquity of the planet, but also upon the rate 

 with which it parts with the heat generated dur- 

 ing its contraction. Since the outer planets are 

 so much older than the inner ones, it might 

 at first be supposed that they must have pro- 

 279 



