SOURCE OF OXYGEN FOR THE ATMOSPHERE. 949 



within the lithosphere is combined, and there is no evidence that this is not 

 true of the centrosphere also. ■ 



It might be supposed, as has been suggested by various men in 

 reference to carbon dioxide, that oxygen has been attracted to the earth by 

 the force of gravity from the interplanetary spaces. To what extent this 

 has occurred, if at all, must long, and perhaps forever, be a matter of 

 conjecture. 



The chief certain source of oxygen for the atmosphere is the reducing 

 action of organic material upon carbon dioxide. As plants grow they 

 decompose carbon dioxide of the atmosphere, thereby liberating oxygen, 

 and build the carbon into their bodies. Animals take their carbon com- 

 pounds directly or indirectly from plants. So far as the plants and animals 

 decompose, the carbon is again oxidized to carbon dioxide by the ox}"gen 

 of the atmosphere, thus consuming an amount of oxygen equivalent to that 

 originally liberated. Therefore, so far as life products decompose there is 

 no permanent gain of oxygen to the atmosphere by the cycle. But a portion 

 of the organic compounds do not completely decompose, and in so far as 

 this is true and they remain in the rocks in carbonaceous shale, graphitic 

 slates, schists, or gneisses, in peat, in coal, and other rocks, the oxygen 

 liberated by the reduction of the carbon dioxide is a permanent gain to the 

 atmosphere. It will be seen under carbon, pp. 962-974, that the quantity of 

 carbon which is thus locked up in the earth is enormous. Since no quanti- 

 tative estimate has ever been made of the amount of such carbon, there is 

 no way in which one can ascertain to what extent the air has gained in 

 oxygen in consequence of the reduction of carbon dioxide. But it is 

 suspected that a considerable percentage of the oxygen now in the atmos- 

 phere could be thus accounted for. Indeed the reduction of carbon dioxide 

 by plants and the liberation of oxygen to the atmosphere in consequence of 

 the formation of the rock coals, peats, etc., has so impressed Koene and 

 Phipson as to make them think that originally the relations of carbon dioxide 

 and oxygen in the atmosphere were probably reversed, but that as a result 

 of the continuous reducing action of vegetation and the decay of only a part 

 of it the atmosphere has become one in which carbon dioxide is very subor- 

 dinate and oxygen one of the chief constituents. ° 



"Chemical News, vol. 67, 1893, p. 135; vol. 68, 1893, pp. 45, 75, and 259; vol. 70, 1894, p. 223. 



