838 GEOLOGICAL REFORM X 



But if, on the other hand, the 100,000,000 or 

 200,000,000 years appear to be insufficient for 

 geological purposes, we must closely criticise the 

 method by which the limit is reached. The 

 argument is simple enough. Assuming the earth 

 to be nothing but a cooling mass, the quantity of 

 heat lost per year, supposing the rate of cooling to 

 have been uniform, multiplied by any given 

 number of years, will be given the minimum 

 temperature that number of years ago. 



But is the earth nothing but a cooling mass, 

 "like a hot-water jar such as is used in carriages," 

 or " a globe of sandstone," and has its cooling 

 been uniform ? An affirmative answer to both 

 these questions seems to be necessary to the 

 validity of the calculations on which Sir W. 

 Thomson lays so much stress. 



Nevertheless it surely may be urged that such 

 affirmative answers are purely hypothetical, and 

 that other suppositions have an equal right to 

 consideration. 



For example, is it not possible that, at the 

 prodigious temperature which would seem to 

 exist at 100 miles below the surface, all the 

 metallic bases may behave as mercury does at a 

 red heat, when it refuses to combine with oxygen ; 

 while, nearer the surface, and therefore at a lower 

 temperature, they may enter into combination (as 

 mercury does with oxygen a few degrees below its 

 boiling-point), and so give rise to a heat totally 



