EL] GEOL GICAL REFORM. ^51 



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 rendered it almost certain that he had not 

 illuminated 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." 1 



I do not wish to " Hansardize " Sir William Thomson 

 by laying much stress on the fact that, only fifteen years 

 ago, he entertained a totally different view of the origin 

 of the sun's heat, and believed that the energy radiated 

 from year to year was supplied from year to year — 

 a doctrine which would have suited Hutton perfectly. 

 But the fact that so eminent a physical philosopher has, 

 thus recently, held views opposite to those which he now 

 entertains, and that he confesses his own estimates to 

 be "very vague/' justly entitles us to disregard those 

 estimates, if any distinct facts on our side go against 

 them. However, I am not aware that such facts exist. 

 As I have already said, for anything I know, one, two, 

 or three hundred millions of years may serve the needs 

 of geologists perfectly well. 



III. The third line of argument is based upon the 

 temperature of the interior of the earth. Sir W. 

 Thomson refers to certain investigations which prove 

 that the present thermal condition of the interior of 

 the earth implies either a heating of the earth within the 

 last 20,000 years of as much as 100° R, or a greater 

 heating all over the surface at some time further back 

 than 20,000 years, and then proceeds thus ; — 



1 Loc. cit., p. on. 



