78 



A TREATISE ON METAMOKPHISM. 



solvent and solid, pressure increases solubility; for in that case solution 

 tends to bring the molecules nearer together and works in conjunction with 

 the pressure. A mixture of water and ice furnishes an excellent illustration 

 of this principle. At any moment the volume of the water is less than 

 that of the equivalent water and ice. Hence pressure promotes solution and 

 prevents freezing, or in other words, crystallization. In the reverse case, 

 that in which the volume of the solution is greater than that of solvent and 

 solid, pressure decreases the solubility, the reason being the reverse of that 

 of the previous case. 



The above law may be illustrated by fig. 1. A given amount of salt, 

 say 10 cc. in volume, may be supposed to be placed in 90 cc. of water, and 

 the salt be of such a nature as to saturate the water at that 

 temperature and pressure. Before solution begins the 

 space occupied is 100 cc. After solution this space may 

 be greater or less than 100 cc, say 105 cc. or 95 cc; that 

 is, the water surface instead of being at aa will be at cc or 

 bb. If it be at bb, where the volume is less, and the pres- 

 sure be increased, an additional amount of salt ma} be 

 added and taken into solution. If it be at cc, and the 

 pressure be increased, a part of the salt already in solution 

 will be precipitated from the solution. 



It is well known that the solubility of calcium car- 

 bonate and of some other carbonates is increased by pres- 

 sure a It is a fair inference from Barus's work that the 

 solubility of the silicates is also increased by pressure. 

 Barus b found that when soft glass is dissolved in water at temperatures 

 above 210° C, the volume is 20 to 30 per cent less than the two sepa- 

 rately. This glass is one which contains alkalies, alkaline earths, and 

 lead, and therefore is somewhat similar in composition to many natural 

 silicates. The carbonates and the silicates are the dominant compounds 

 in" undei'ground solutions. The solubility of many other salts, besides 

 the carbonates and silicates, occurring underground is increased by pres- 

 sure. Therefore, in the majority of the complex underground solutions 



a Lindgren, W., Gold-quartz veins of Nevada City and Grass Valley, California: Seventeenth 

 Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, pp. 176-178. 



6 Barus, C, Hot water and soft glass in their thermodynamic relations: Am. Jour. Sci., 4th 

 ser., vol. 9, 1900, p. 173. 



Fig. 1.— Change of vol- 

 ume resulting from so- 

 lution, and relations of 

 solution and pressure. 



