OF GEOLOGICAL DYNAMICS. 101 



" destroy the value of any calculations which leave 

 " them out of sight." 



I reply that I admit the first, and emphatically 

 deny the second, proposition of the last sentence. 

 Heat of combination of elements, present together 

 in a mixed mass and devoid of chemical affinity at 

 a high temperature, but acquiring chemical affinity 

 and consequently combining as the temperature 

 sinks, constitutes merely an addition to the sum of 

 the thermal capacities of the several elements 

 separately reckoned, to give the effective thermal 

 capacity of the composite mass. And the value 

 of " calculations " which leave this possibility " out 

 of sight " is not " destroyed " though an altered 

 figure in the result might be necessitated by an 

 altered estimate of specific heat. But in my calcu- 

 lations I have left a wide enough margin to give 

 due weight on Professor Huxley's side to the 

 smallness of our knowledge regarding specific 

 heats, thermal conductivities, and temperatures of 

 fusion, of the earth's material. And as to the 

 cloudiness or clearness of the atmosphere, I say 

 that the secular cooling of the earth is not affected 



