8 HUMAN BIOLOGY 



We are so used to the existence of free oxygen in the 

 atmosphere, so abjectly dependent on it in fact, that it is 

 hard for us to realize how remarkable it is that a gas so 

 chemically active should be present in large proportions. 

 Free chlorine would be only a httle more surprising. There are 

 many purely inorganic reactions that take oxygen out of the 

 atmosphere, and that on a very large scale, and no known 

 inorganic processes which put any in. Should all earthly 

 hfe perish, while geological processes went on, we might 

 expect a steady depletion of the atmospheric oxygen. 

 Practically all volcanic rocks contain considerable quantities 

 of the ferrous compounds of iron, which are incompletely 

 oxidized, and on weathering pass into the ferric state, 

 with absorption of oxygen. Volcanic gases, moreover, have 

 never been found to contain free oxygen, but often contain 

 elements avid for combination with oxygen, so that they are 

 sometimes actually combustible. The evidence indicates, 

 indeed, that the materials of the Earth's crust as a whole, 

 down to a depth of say twenty or thirty miles, are definitely 

 unsaturated with respect to oxygen. If the whole crust 

 should be fused and thoroughly mixed, it appears very prob- 

 able that all the oxygen of the atmosphere would be 

 absorbed by the ferrous iron of the molten lava, and might 

 not nearly suffice to oxidize it. 



There is little reason to suppose that the Earth is of an 

 exceptionally unoxidized composition (especially as compared 

 with bodies of similar density, which alone can be planets 

 with land and water areas), and it does not therefore seem 

 probable that free oxygen should be an initial constituent of 

 a planetary atmosphere. It might indeed be liberated by 

 inorganic means, for example, as has been suggested, by the 

 dissociation of water-vapor at a high temperature, and the 

 escape of the fast-moving hydrogen atoms from the atmos- 

 phere, leaving oxygen in excess. But as the molten mass 

 cooled, it is hard to see how this oxygen could escape chemical 

 combinations with the vast mass of incompletely oxidized 

 lavas, rendered more active chemically by the presence of 

 water at high pressure and temperature. 



Vegetation, on the other hand, is continually breaking 

 up carbon dioxide, and pouring oxygen into the atmosphere, 



