32 UNIVERSAL EVOLUTION 



earth extend from thirty to forty-five miles only from 

 the surface. At a greater depth than this the matter 

 is fairly homogeneous. By an ingenious method, start- 

 ing with the fact that from radio-active bodies the gas 

 helium is given out, it is found that a specimen of 

 thoranite, one of the metals of the earth, must be more 

 than 240,000,000 years old. Of course, the earth would 

 be of equal age. Another curious fact is, were there 

 no carbonic acid, and invisible aqueous vapor in the 

 atmosphere, there would be little continuous heat on 

 the earth. The internal heat of the earth has little 

 effect at the surface, about one-twentieth of a degree, 

 but the heat increases about one degree every sixty 

 feet from the surface toward the center. 



