642 



GEOLOGY. 



The disintegration of the crystalhne rocks and the solution of Ume- 

 stone have consumed much carbon dioxide, and this is to be added to the 

 loss through organic action. On the other hand, there are inorganic 

 processes that supply carbon dioxide, and hence when the larger prob- 

 lem of the atmosphere is raised, the factors become so compHcated 

 that their consideration is best deferred to the historical chapters. 

 This passing reference may stand us in good part lest we forget, for the 

 moment, the inorganic factors in the atmospheric problem. 



The more inert factor. — Nitrogen in the free state is relatively inert 

 chemically, and it does not appear that it can be used directly by the 

 higher plants and animals in appreciable amounts. Certain bacteria, 

 and perhaps certain algae ^ and other low forms of plants, have the 

 power of using free nitrogen, and this is a principal way in which it is 

 put within the reach of higher plants. Nitrogen is also combined in 

 small quantity in the atmosphere by electric action, and thus made 

 available for plants. On account of the inertness of nitrogen and of 

 the relatively limited amount required for organic purposes, the nitro- 

 gen of the atmosphere has been less consumed than the carbon dioxide. 

 Besides this, the nitrogen compounds are very decomposable, and are 

 very generally and completely returned to their original state. Deposits 

 of nitrates or other nitrogenous compounds are relatively rare. 



It is obvious that if there is any considerable source of supply con- 

 current with this slight loss, the amount of nitrogen in the atmosphere 

 must have been increasing. We have seen that volcanoes give forth 

 considerable quantities of nitrogen, and that this may be a real addition 

 to the atmosphere, and not merely a return of the atmospheric nitrogen 

 that had been carried down previously by underground-water. It 

 has also been noted that crystalline rocks contain occluded nitrogen, 

 which is doubtless freed by their disintegration. It is, therefore, not 

 improbable that the nitrogen of the atmosphere has been increasing, 

 both actually and relatively. 



Probable fluctuations of atmospheric composition. — With this general 

 sketch of the interplay of the atmospheric elements under organic 

 influence, we are prepared for the further conception that if one or 

 another of these actions was relatively more vigorous than usual for 

 a period, it would bring about a variation in the proportions of the 

 atmospheric constituents. If, for example, vegetation flourished luxuri- 



' * Frank. Lehrbuch der Botanik, I, p. 576, 1892. 



