280 THE NEBUXAH HYPOTHESIS. 



Clearly this transition will be gone tlirougli by a planet like 

 the Earth, in a much shorter period than by such vast 

 planets as Jupiter and Saturn; in which, also, the centri- 

 fugal force is great. Hence, it is inferable from the Nebular 

 Hj^pothesis that, other things equal, the small celestial bod- 

 ies will have reached late stages of concentration and high 

 specific gravities, at a time when the large celestial bodies 

 are in early stages of concentration and have low specific 

 gravities. 



In considering the specific gra\dties of the heavenly 

 bodies, we have been obliged to speak of tlie heat evolved 

 by them. But we have yet to point out the fact that in 

 their present conditions with respect to temperature, we 

 find additional materials for building up our argument ; and 

 these too of the most substantial character. 



Heat must inevitably be generated by the aggregation of 

 diffused matter into a concrete form ; and throughout our 

 reasonings we have assumed that such generation of heat 

 has been an accompaniment of nebular condensation. If, 

 then, the Nebular Hypothesis be true, we ought to find in 

 all the heavenly bodies, either present high temperatures or 

 marks of past high temperatures. 



As far as observation can reach, the facts prove to be 

 what theory requires. Various evidences conspire to show 

 that, below a certain depth, the Earth is still molten. And 

 that it was once wholly molten, is implied by the circum- 

 stance that the rate at which the temperature increases on 

 descending below its surface, is such as would be found in 

 a mass that had been cooling for an indefinite jDeriod. The 

 Moon, too, shows us, by its corrugations and its conspicu- 

 ous volcanoes, that in it there has been a process of refrig- 

 eration and contraction, like that which had gone on in the 

 Earth. And in Venus, the existence of mountains simi- 

 larly indicates the shrinking of a soHdifj'ing crust, or an 

 igneous reaction of the interior upon it, or both. 



