126 POPULAR LECTURES AND ADDRESSES. 



materials of the earth would be over estimated at 

 3,000 units centigrade that is, 3,000 times the 

 quantity of heat required to raise the temperature 

 of a ton of water by i cent., or, according to 

 Joule's equivalent, 1,270,000 metre-tons of energy. 

 39. The number 4,250,000 previously found 

 C 37) f r the amount of potential energy of 

 gravitation exhausted in the coalition of the 

 earth's mass, is 3j times this estimate of the 

 potential energy of the chemical affinity of its 

 elements. The whole amount of energy due to 

 the two causes together is about 5|- million metre- 

 tons, or 13,000 thermal units centigrade, per ton of 

 the earth's mass. This, being about 700 times as 

 much heat as would raise the temperature of an 

 equal mass of surface rock from o to 100 cent., 

 is three and a-half times the amount stated in 

 34, as an over-estimate of the whole amount of 

 heat at present in the earth. But considering, as 

 in $ 37, how much heat must have been dissipated 

 during the conglomeration of the materials which 

 now constitute the earth, we are rather compelled 

 to contract than permitted to enlarge our ideas of 



