64 THE GASES IN ROCKS. 



of the former and 1.3 per cent of the latter. A critical temperature of 28 

 would indicate 98.1 per cent of liquid carbon dioxide and 1.9 per cent of 

 liquid nitrogen, while 27 would mean 97.5 per cent of the dioxide and 2.5 

 per cent nitrogen. The figures for carbon dioxide and hydrogen are of the 

 same general order. In the estimates of the depths at which cavity-bearing 

 crystals were formed, made by different methods, 1 it has been usual to 

 assume that only the pressure arising from the weight of the overlying rock 

 was involved. 



Sorby examined those cavities which contained only water, or a saline 

 solution, and a vacuole left by the contraction of the liquid, as a result of 

 the lowering of the temperature. By noting the relative size of the bubble 

 and the volume of the liquid, he estimated the temperature to which the 

 mineral would have to be heated for the liquid to completely fill the cavity, 

 and from this, together with the elastic force of the water-vapor, he com- 

 puted the necessary existing pressure in feet of rock. The highest tempera- 

 ture found by this method was only 356 C., at which point Sorby believed 

 that the trachyte of Ponza solidified, while the lowest temperature was 

 89 C., obtained from a study of the main mass of granite at Aberdeen. 

 But Sorby considered it more probable that the granite crystallized at 

 about the same temperature as the trachyte and, assuming that the solidifi- 

 cation took place at 360, he computed that the granite of Aberdeen was 

 formed under a pressure of 78,000 feet of rock. 2 These estimates are based 

 upon the unwarranted supposition that when the crystals were formed 

 the volume of liquid water was such as to just fill the cavities, and that in 

 each case a meniscus at once appeared with a loss of heat. He overlooked 

 the fact that the meniscus could not appear until the water reached the 

 liquid condition, no matter at what temperature the growing crystal sur- 

 rounded the vesicle of highly compressed water-gas. 



The highest temperature at which a vacuole of this sort can appear 

 must, therefore, be the critical temperature for water, or 365 C. In order 

 to study this problem, we may, perhaps, best take the special case in which 

 the inclosed water passed through its critical state (at 365 and 200.5 

 atmospheres pressure) during the cooling of the crystal. The vesicle formed 

 in this case may be termed the critical vacuole. It may be assumed that 

 the growing crystal inclosed the water at some temperature in the neigh- 

 borhood of 1100, which is an average temperature for the solidification of 

 lavas. Starting thus with a cavity formed at 1100, in order to allow the 

 water on cooling to pass through the critical state, an original pressure 

 of 1,070 atmospheres is necessary according to Van der Waal's equation, 3 

 provided the size of the cavity remained constant. But if 2.66 per cent is 

 allowed for the shrinkage of the cavity while cooling down 4 from 1100 



'See Geikie's Textbook of Geology, vol. 1, pp. 144-145. 



2 Sorby, Quart. Jour. Geol. Soc. London, vol. 14, p. 494. 



KT n 



3 p = 'r j-. In this case the values of the constants for the critical point 



were taken to be # = .003607; a = .01173; = .00151; v = 36 = . 00453. By substituting 

 for the critical temperature, T = 1100 + 273, the equation gives the theoretical value of 

 1,070 atmospheres. 



4 Ante, p. G2. 



