253 



by which the limit is reached. The argument is simple 

 enough. Assuming the earth to be nothing but a cool- 

 ing 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 mini- 

 mum 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/ 7 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 affirma- 

 tive answers are purely hypothetical, and that other 

 suppositions have an equal right to consideration. 



For example, it 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 distinct from 

 that which they possess as cooling bodies ? And has 

 it not also been proved by recent researches that the 

 quality of the atmosphere may immensely affect its 

 permeability to heat; and, consequently, profoundly 

 modify the rate of cooling the globe as a whole ? 



I do not think it can be denied that such conditions 

 may exist, and may so greatly affect the supply, and the 

 loss, of terrestrial heat as to destroy the value of any 

 calculations which leave them out of sight. 



My functions as your advocate are at an end. I 

 speak with more than the sincerity of a mere advocate 



