LAWS OF PRECIPITATION. 115 



be so, ideall y there must be a neutral point in which the volume of the 

 material is the same whether as a solution or as a solid. In this case 

 pressure would have no effect upon precipitation. However, the precipita- 

 tion of any part of the material from a solution modifies the character of 

 the remainder of the solution, and it is not to be supposed that a case is 

 likely to occur in which crystallization of material takes place without there 

 being any pressure effect. 



Where circulating waters are descending the pressure is increasing, 

 and where ascending the pressure is decreasing. Therefore, in the case of 

 ordinary ground-water solutions the direction of water circulation which is 

 favorable to precipitation is ascension. 



PRECIPITATION BY CHANGE OF TEMPERATURE. 



Change of temperature may result in supersaturation, and therefore in 

 precipitation. In general, in ground solutions increase in temperature 

 increases solubility. (See pp. 79-81.) Therefore decrease in temperature 

 is favorable to supersaturation and precipitation. While this statement is 

 true for most substances at temperatures below 100° C, and is correct for 

 many substances at temperatures considerably higher than this, at very 

 high temperatures the conditions are reversed for some substances. (See 

 p. 79.) In the common case, that of precipitation with decrease of 

 temperature, the freezing- point of the solution is lower than that of the 

 solvent." Apparently the amount of lowering is proportional to the 

 molecular weights, and is stated by Raoult as follows: "One molecule of 

 any compound when dissolved in 100 molecules of a liquid lowers the 

 solidification point of the liquid by an amount which is nearly constant, 

 viz, 0.62°;" or, the molecular depression, when the solvent is to the 

 solute as 1:100, is 0.62 . 6 In dilute solutions of salts in water, the 

 molecular depression may be larger than this, in which case the substance 

 is regarded by many as dissociated." It was by the application of the 

 principle of molecular depression that Kahlenberg and Lincoln were able 

 to reach the conclusion already given (p. 87), that silica goes into 

 solution as colloidal silicic acid.' When the silicates are dissolved and 



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

 York, 1891, p. 199 et seq. 



»Ost\vald, Solutions, tit., p. 208. 

 cOstwald, Solutions, tit., p. 214. 



