MUTUAL INFLUENCE OF COMPOUNDS. 77 



quartz grains are sometimes etched by meteoric waters, and the decompo- 

 sition and partial solution of the refractory silicates is universal. Under 

 conditions of deep-water circulation solution of quartz and the refractory 

 silicates vaa,j be accomplished with relative rapidity. This is illustrated 

 by the Calumet and Hecla conglomerate, many of the pebbles of which 

 have been partly or even completely dissolved and the space once occupied 

 by them taken by copper." 



Since underground solutions always contain a number of compounds, 

 and often many, the influence of one compound upon the solubility of 

 another is of consequence in various ways. For instance, when several 

 compounds are present, a unit quantity of water will not dissolve as much 

 of a given salt as it would if it were alone. But if a number of units of 

 water are each saturated with a single salt, and the solutions are mingled 

 without chemical reaction, the mixture is capable of taking additional quan- 

 tities of the salts into solution. In other words, a unit of solution simul- 

 taneously saturated with ' each of several compounds contains a greater 

 total of solids than a unit of solution saturated with fewer of these com- 

 pounds, but less of any individual salt than it would were it saturated with 

 that salt alone. 6 



In the ground solutions the different compounds frequently react upon 

 one another, and therefore important modifications in the above statement 

 are necessary, as is explained under "Precipitation," pp. 113-123. 



RELATIONS OP SOLUTION AND PRESSURE. 



In general, the volume of the solvent plus that of the dissolved 

 compound is greater than that of the solution. For a given quantity of 

 the solid the contraction is greater the more of the solvent is used. c In 

 some cases, however, the volume of the dissolved compound and solvent is 

 less than that of the solution, or expansion results from dissolving the solid. 

 Ammonium chloride in water is an illustration of this case. From the fore- 

 going relations we obtain a rule as to the relations of pressure to solubility. 1 * 

 In the common case in which the volume of the solution is less than that of 



« Pumpelly, R., The paragenesis and derivation of copper and its associates on Lake Superior: Am. 

 Jour. Sci., 3d ser., vol. 2, 1871, p. 34. 



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

 1891, pp. 83, 84. 



eOstwald, W., op. cit., p. 82. 



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

 1895, p. 567. 



