220 LAY SERMONS, ESSAYS, AND REVIEWS. [xi. 



the reply is, once more, that, for anything that can be proved 

 to the contrary, one or two hundred million years might serve 

 the purpose, even of a thorough-going Huttonian uniformitarian, 

 very well. 



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, &quot; like a hot-water 

 jar such as is used in carriages,&quot; or &quot; a globe of sandstone,&quot; 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 

 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. 



