ON GEOLOGICAL TIME. 53 



of heat that the sun could have, dealing with it as 

 you would with a stone, or a piece of matter, only 

 taking into account the sun's dimensions, which 

 showed it to be possible, that the sun may have 

 already illuminated the earth for as many as one 

 hundred million years, but at the same time also 

 rendered it almost certain that he had not illumin- 

 ated the earth for five hundred millions of years. 

 The estimates here are necessarily very vague, but 

 yet vague as they are, I do not know that it is 

 possible, upon any reasonable estimate, founded on 

 known properties of matter, to say that we can 

 believe the sun has really illuminated the earth for 

 five hundred million years. 



25. But Play fair looks to the earth, and says 

 that while the heavenly bodies give every evidence 

 of having gone on for ever as now, the earth, in the 

 phenomena presented all through its crust, to 

 unprejudiced observers, gives similar evidence, and 

 seems to indicate no evidence of a beginning, and 

 no progress or advance towards an end. Now, let 

 us consider the question of underground heat. 

 The earth, if \ve bore into it anywhere, is warm, 



