EELATIONS OF PRESSURE AND SOLUTION. 71 



one-hundredth, and at still gi.ater depths the fraction of pressure due to 

 the atmosphere is insignificant. 



But in order that saturation for any gas corresponding to the pressure at 

 any given depth shall occur, it is necessary that a sufficient amount of gas 

 shall there exist. Gases may be produced below the level of ground water 

 by the chemical reactions, as by the liberation of carbon dioxide in the 

 process of silication. Later it will be seen (see Chapter VIII, pp. 677-679) 

 that this is one of the fundamental processes of the lower physical-chemical 

 zone. It follows from the above that at depth the amount of carbon 

 dioxide or other gas in solution per unit of water may be many score times 

 greater than near the surface. The pressure of carbon dioxide at the 

 surface is only about 0.0006 of an atmosphere. The water pressure at a 

 depth of 1,000 meters is almost 100 atmospheres; therefore the amount of 

 free carbon dioxide which may be held in solution, if pressure were the 

 only factor concerned, might be 166666 times as great as that held in 

 solution in the belt of weathering. 



But it must be remembered that, as shown below, the increase of 

 temperature due to increase of depth somewhat reduces this multiple. 



It should be remembered also that carbon dioxide combines with 

 water, producing carbonic acid, and the amount of this compound which 

 may be held in solution at the surface of ground water is not dependent 

 upon the pressure of the atmospheric carbon dioxide. But it is evident 

 that deep ground waters, where the pressure is great, may hold a vastly 

 greater quantity of carbon dioxide than can be held in solution near the 

 level of ground water. 



As already pointed out, the law which obtains in reference to geological 

 work is that the activity of the carbon dioxide increases in direct ratio with 

 its quantity. 



The theoretical conclusion that the action of carbon dioxide would be 

 increased by pressure, and consequent greater quantity, has been experi- 

 mentally verified bv Mueller" and Struve, who found that strong pressure 

 increased the action of carbon dioxide in the decomposition of the silicates 

 more than did increase of time. 



a Mueller, Richard, Untersuchungen iiber die Einwirkung des kohlensaurehaltigen Wassers auf 

 einige Mineralien und Gesteine: Tschermaks mineral. Mittheil., vol. 7, 1877, p. 47. 



