448 Mr. J. Stevenson on the Chemical and 



ample materials for reducing a very large quantity of oxide 

 of iron to the metallic condition. No doubt we should hardly 

 expect, even if there was a great deficiency of oxygen on the 

 earth relatively to the other elements, that all the carbon and 

 hydrogen would be originally unoxidized, but still after all 

 it is only a comparatively small proportion of the total 

 quantity of these elements that our theory requires to have 

 been present originally in the unoxidized condition, something 

 like J or ^ or even considerably less. We need not there- 

 fore go further into detail on this point, but it may be well 

 to state again how we conceive that the hydrogen or other 

 oxidisable gas, present by our hypothesis in the primitive 

 atmosphere, gradually disappeared. We assume that vege- 

 tation or organic life of some description could grow and 

 flourish in such an atmosphere, and that it may possibly even 

 have derived nourishment from the hydrogen or hydrocarbon 

 gases or perhaps also carbonic oxide (CO) which may have 

 been present. The coal or carbonaceous matter formed from 

 the remains of these vegetables or organisms would eventually 

 become heated along with materials containing oxide of iron, 

 with the resultant formation of metallic iron and carbonic acid. 

 If the hydrocarbon gases and carbonic oxide of the primitive 

 atmosphere were absorbed directly by vegetation (or the organic 

 life of the time) they would gradually come to be used up and 

 disappear, and after that period carbonic acid alone would 

 supply the carbon required for vegetation or organic growth. 

 The use of carbonic acid would involve the liberation of oxygen, 

 but as long as free hydrogen existed in the atmosphere the 

 oxygen liberated would be taken up by the hydrogen and water 

 formed. If carbonic acid was necessary from the very first 

 for the support of life, there w^ould probably be oxygen 

 evolved from the very earliest period, and this oxygen would 

 for a long time be used up in oxidizing the hydrogen, 

 hydrocarbon gases, and carbonic oxide or whatever oxidisable 

 gases may have been present. 



Whatever may be thought of the above theory and the 

 arguments adduced in support of it, we may be tolerably sure 

 that if the primitive atmosphere did not contain hydrogen, 

 hydrocarbon gases, or carbonic oxide it must have contained 

 a very large quantity of carbonic acid gas. It is quite 

 obvious that if the rocks composing the crust of the earth 

 were heated until they became liquefied, the limestone and 

 other carbonates which are present in enormous quantities 

 would be decomposed, with the result that silicates would be 

 formed and carbonic acid set free. Similarly we may infer 

 that if the earth was all highly heated and liquid in very 



