70 A TREATISE ON METAMORPHISM. 



The pressure. — "The quantity of a gas dissolved by a specified quantity of 

 a liquid is proportional to the pressure of the gas." a This statement is true 

 of each gas without reference to whether a gas be alone or mixed with other 

 gases. Thus the solubility of each of a number of mixed gases is controlled 

 by the pressure exerted by that gas, not by the total pressure exerted by 

 the mixture. It is therefore clear that under natural conditions the press- 

 ure of that part of any gas which is in the atmosphere and the pressure of 

 that part which is held in solution in the water immediately adjacent are 

 the same when the two are in equilibrium, and the water is therefore just 

 saturated. 



So far as ground waters are concerned, there are two cases; first, the 

 waters of the belt of weathering, or those to the level of ground water; 

 and second, those below the level of ground water, or the belt of satura- 

 tion. In the belt of weathering the pressure is atmospheric. Changes of 

 pressure are barometric. In so far as the atmospheric pressure varies — and 

 this is by fractions up to one-fifteenth — the solubility of the natural gases 

 in the water of the belt of weathering also varies directly as the pressure of 

 each of the gases varies, without reference to the pressure and solubility of 

 the other gases. 



In the belt of saturation, just at the level of ground water, the amount 

 of gases held in solution is proportional to atmospheric pressure; but 

 at greater depths higher degrees of concentration of gases are possible, 

 although it might at first be thought that the atmospheric pressure or vapor 

 pressure at the free surface of the water would determine the concentration 

 of the solution. The pressure which really is determinative as to the 

 amount of gas which ma}^ be held in solution is that of a column of water 

 extending to the free surface, plus the atmospheric pressure. Since, 

 however, water is so much heavier than the atmosphere, at considerable 

 depths below the level of ground water the atmospheric pressure may be 

 neglected; and the pressure, and therefore the solubility of underground 

 gases in water, is almost directly proportional to the depth below the level 

 of ground water. For instance, at a depth of only 100 meters below the 

 level of ground water the pressure of the atmosphere is only one-tenth 

 that of the water pressure; at a depth of 1,000 meters it is only one 



a Ostwald, W. , Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co. , New York, 

 1891, p. 9. 



