62 THE GASES IN ROCKS. 



tallized, its gas-solvent powers would be increasing, allowing some of the 

 gas to pass into solution. At the same time free gas might be occluded by 

 the growing crystals. The experiments upon the reabsorption of gas by ex- 

 hausted rock powder indicate that a portion of the gas unites chemically as 

 the heat diminishes. Because of these processes, liquid lavas may be sup- 

 plied with free gas, even when the solidified rocks retain but little free gas. 



As the imprisoned carbon dioxide frequently remains in the liquid 

 form up to the critical point (30.9 C.), it must be subjected to a pressure 

 of at least 73 atmospheres, which is the critical pressure of this gas. Since 

 a pressure of 73 atmospheres corresponds to a column of water 2 ; 470 feet 

 in height, quartz crystals formed from aqueous solution, under hydrostatic 

 pressure simply, can not contain liquid carbon dioxide up to 30.9 unless 

 developed at depths exceeding 2,470 feet. It is to be recognized, however, 

 that such crystals might be formed at lesser depths if mechanical pressure 

 operated with hydrostatic pressure or replaced it. 



If the quartz crystallized from a lava, say at 1100C., the effect of 

 cooling down to ordinary temperatures upon both the size of the cavity 

 and the pressure of the inclosed carbon dioxide must be taken into account. 

 If we take the case of a cavity found to be entirely filled with carbon dioxide 

 at the critical point (30.9 C. and 73 atmospheres), it is possible, by the 

 use of Van der Waal's equation, to calculate the pressure to which the gas 

 would be subjected if the quartz were heated to 1100. This pressure is 

 found to be 756 atmospheres, 1 provided the size of the cavity remains 

 constant. But as most minerals contract on cooling, the volume of the 

 cavity diminishes at the same rate as though it were filled with the material 

 of the inclosing walls. 2 The coefficient of expansion of quartz is given as 

 0.00003618. Assuming for the sake of simplicity that the rate of expansion 

 does not vary greatly with changing temperatures, 3 quartz, cooling from 

 1100 to 31, would contract to an extent of about 3.87 per cent of its 

 original volume. Since the contraction of the quartz diminishes the size 

 of the cavity and increases the pressure by 3.87 per cent, the original 

 pressure need be only 727 atmospheres, which corresponds to the pressure 

 beneath 9,100 feet of average rock. To fill cavities forming in crystals at 

 1100 with carbon dioxide which is so condensed that it will pass into the 

 liquid state just at the critical temperature when the rock cools down, 

 a pressure corresponding to a depth of at least 9,100 feet, or its mechanical 

 equivalent, would seem to be required. If, when warmed under the micro- 

 scope, the liquid carbon dioxide is found to pass into the gaseous state at 

 temperatures below 30.9, and the cavity contains only carbon dioxide, 

 or carbon dioxide and water, these must have been entrapped under a 

 pressure less than 727 atmospheres, or else the crystal was formed at a 

 temperature above 1100. 



n 



1 By starting with the equation p = --- -at the critical point where the values 



of the constants are taken as # = 0.003684; a = 0.00874; 6 = 0.0029; t; = 36 = 0.0087, 

 and substituting for the critical temperature, T= 1100 + 273, the theoretical value of 756 

 atmospheres for the pressure at 1100 is obtained. 



2 Daniell, Principles of Physics, p. 379. 



3 It would, however, slowly increase with the increase of temperature. 



