120 POPULAR LECTURES AND ADDRESSES. 



than enough to melt that amount of any kind of 

 surface rock under an) 7 moderate pressure. But 

 merely from consideration of thermal capacities, 

 and possible temperatures of the earth at great 

 depths, we arc not at present able to make any 

 much less vague estimate than that, of the possible 

 total amount of heat. 



35. Inasmuch as energy is being continually lost 

 from the earth by conduction through the upper 

 strata, the whole quantity of plutonic energy must 

 have been greater in past times than at present, 

 and the question forces itself upon us, how was it 

 first acquired ? As the earth, being finite, cannot 

 ever have had an infinite store of energy within it, 

 there must have been a time when it was not a 

 warm body, parting with energy, as it is now. If 

 the matter of the earth existed before that time, it 

 must have been under conditions which led to its 

 being warm, and to its commencing to part with 

 energy. It may have gained its heat by communi- 

 cation from other matter, or by work performed 

 upon it by matter not now forming part of itself. 

 But the only probable hypothesis is, that it has 



