Geological History of the Atmosphere. 403 



the history of free oxygen, but it is closely related to it, and 

 we will therefore discuss it separately for a little. 



A simple calculation will show that our total free oxygen 

 is only a small fraction of the total oxygen of the earth. In the 

 sea alone, which weighs roughly 1,400,000,000,000,000,000 

 (fourteen hundred thousand million million) tons, and con- 

 tains about 85 per cent, by weight of oxygen, there is nearly 

 1000 times as much oxygen as there is in the atmosphere. This 

 oxygen is very nearly all in the combined condition, the free 

 oxygen dissolved in sea-water being very small in amount, even 

 when compared with the amount of atmospheric oxygen itself. 

 Further, the crust, or solid envelope of the earth as known to 

 geologists, is estimated to contain about 50 per cent, of its 

 weight of oxygen, and all in the combined condition. Now 

 if we suppose the crust having this average composition to 

 be 10 miles thick, and its specific gravity to be about 2| times 

 that of water, it will weigh about 10 times as much as the 

 sea, and the oxygen contained in it will be about 6 times as 

 much as that in the sea, or 6000 times as much as that in the 

 atmosphere. It is quite clear therefore that the total amount of 

 free oxygen in the world is very small when compared with that 

 of combined oxygen. Therefore, if there is an excess or 

 surplus of oxygen on the earth after satisfying the require- 

 ments of the elements for which it has an affinity, that surplus 

 must be relatively a very small one. 



But further still, if the whole earth, which weighs 1,200,000 

 times as much as the atmosphere, or 5,000,000 times as much 

 as the free o>:vgen of the atmosphere, should contain 50 per 

 cent, of oxygen throughout, then the total combined oxygen 

 of the earth will be 2,500,000 times as much as the free. 

 However, even if we assume that the earth is thoroughly 

 oxidized throughout, this estimate will have to be modified 

 considerably in order to bring it into better conformity with 

 the well-known fact that the specific gravity of the earth as 

 a whole is much higher than that of the crust or envelope 

 with which we are acquainted. The principal substances 

 which exist in the interior of the earth must therefore be 

 much heavier than those found near the surface ; but still if 

 they are oxides, or oxygen compounds, we might reasonably 

 expect them — still judging from the specific gravity of the 

 earth — to contain roughly about 20 per cent, of oxygen. 

 This would make the total combined oxygen of the earth 

 1,000,000 times as much as the free. That is to say, if the 

 earth as a whole is composed of well-oxidized materials, it 

 would have to contain something like this amount of oxygen; 

 and whatever the exact figure may be, it is obvious that there 



