i 3 o SCIENCE PROGRESS 



the amount dissolved in sea water, the quantity of uncombined 

 oxygen as compared with the combined oxygen of water, for 

 example, is almost infinitesimal. It is computed that the 

 waters of the ocean, evenly distributed over the globe, would 

 form a spherical shell about 2,500 metres thick. 1 This would 

 represent an average pressure of 250 " atmospheres," and as 

 eight-ninths, by weight, of this water is oxygen, it follows that 

 this would be easily 1,000 or 1,200 times more than the un- 

 combined oxygen contained in the atmosphere, the ocean, etc. 



We have next the enormous quantity of oxygen locked up in 

 the earth's crust. The average thickness of the sedimentary 

 rocks exposed to view has been variously computed at from 

 10,000 to 50,000 metres. 2 Let us for the argument take the 

 lower of these estimates ; the relative density of the rocks is 

 approximately 2*5 and they are about one-half composed of 

 oxygen. 3 The total amount then is at least five times the 

 amount in the sea, it may readily be ten or twenty times. 

 It follows, therefore, that the amount of atmospheric oxygen 

 is certainly not a ^oW, possibly not a ^sbirs part, of the total 

 amount in the outer envelope of the earth. As to the amount in 

 the interior of the earth we of course know nothing whatever. 



It is very well known that large quantities of oxygen are 

 withdrawn from the air by the oxidation of the materials of 

 the rocks, the respiration of animals, etc., and that there is 

 practically but a single source of return supply, that of plant 

 activity. We may then regard the oxygen in the existing 

 atmosphere in one of three ways : 



1. As the residue of an enormously greater quantity 



in former times. 



2. As liberated under the influence of internal heat, vol- 



canic eruptions and the like from that occluded in the 

 rocks. 



3. As due entirely to plant action. 



1. So far as the first supposition is concerned, we are met 

 by the fact that throughout the entire lithosphere or rocky 

 crust are masses of oxidisable but unoxidised materials, large 

 areas of which must have been exposed at one time or another 

 to atmospheric action. This fact, familiar since the days of 



1 Ricbthofen, Das Mecr (Berlin, 1904). 

 * Geikie, Geo/. 3rd ed. (London, 1904). 

 3 Clarke, Data of Geochemistry, p. 18 (Washington, 1908). 



