72 A TREATISE ON METAMORPHISM. 



The temperature. — Increase of temperature generally results in decrease of 

 solubility of a gas." Increase in temperature with depth, or because of 

 volcanism, lessens the solubility of gases in ground water, and to this 

 extent works against the effect of increased pressure. 



solids in solution. — There is still another factor which enters to a slight 

 extent into the solubility of gases. Water holding solids in. solution, in 

 most cases, absorbs less of a gas at a given pressure than does pure water. 6 

 However, the solutions near the surface are ordinarily so dilute that this 

 law is probably not important, but at depth it may be of some consequence 

 in working against the effect of increased pressure. 



SOLUTION OF SOLIDS IX GROUND WATER. 



Where a solid is placed in a liquid some or all of it dissolves, and thus 

 forms a homogeneous mixture composed of the two, or a solution. 



It has been found that if a liquid be placed in a vessel having two com- 

 partments separated by a membrane through which the solvent but not the 

 dissolved substance may pass, when a soluble compound — for instance, 

 sugar — is dissolved in the liquid in one of the compartments, pressure 

 against the membrane is produced. This pressure has been called osmotic 

 pressure, to distinguish it from ordinary gas pressure, known as vapor 

 pressure. According to van't Hoff, the osmotic pressure "is independent 

 of the nature of the solvent, and in general obeys the laws of gases." That 

 is to say, "the osmotic pressure is proportional to the concentration; the 

 osmotic pressure is proportional to the absolute temperature; the same 

 osmotic pressure can be obtained by equimolecular quantities of the most 

 various substances in the same solvent; the osmotic pressure is exactly the 

 same as the gas pressure which would be observed if the solvent were 

 removed and the dissolved substance were left filling the same space in the 

 gaseous state at the same temperature." These somewhat sweeping state- 

 ments need various modifications. For instance, where the solutions are 

 very concentrated the molecules in solution are believed to be so close to 



"Ostwald, W., Outlines of general chemistry, translated by James Walker, Macmillan & Co., 

 London, 2d ed., 1895, p. 121. 



&Ostwald, op. cit. , p. 121. 



c Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895, 

 pp. 134-137. Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., 

 New York, 1891, pp. 112-117. 



