56 THE FITNESS OF THE ENVIRONMENT 



drogen and carbon must exist at or near 

 the surface when a crust forms upon a cool- 

 ing star. 



The nature of the chemical combinations 

 into which these elements at first enter is 

 perhaps open to some question. But as the 

 temperature falls in the cooling of a sun or 

 planet the affinities of carbon and hydrogen 

 for oxygen increase, so that carbonic acid and 

 water must normally result. For oxygen 

 is almost certainly present in the sun; it is 

 found in meteorites, and the vast store of it 

 in the earth's atmosphere and crust (roughly 

 one half of their total mass) justifies the be- 

 lief that it is everywhere one of the commonest 

 of elements. Hence an atmosphere contain- 

 ing water and carbonic acid appears to be a 

 normal envelope of a new crust upon a cooling 

 body. Even were not these substances at 

 first present in such an atmosphere, volcanoes 

 must soon belch them forth in enormous 

 quantities to relieve the pressure which inevi- 

 table chemical processes set up. 



It is clear that no one can give an exhaus- 

 tive description of the formation of the earth's 

 atmosphere and the changes which underlie 

 vulcanism, so long as the theoretical considera- 

 tions involved remain often more obscure than 

 the facts. However, be the process what it 



