LAWS OF PRECIPITATION. 119 



To illustrate: "If one adds some KNO s to AgBr0 3 , a number of molecules 

 of Ag-NOg and also of KBr0 3 will be formed. This will result in a diminution 

 of the number of the molecules of AgBr0 3 , which must be replaced from 

 the solid salt,-" or the solubility will be increased." 



Cameron gives two illustrations of this converse which are of great 

 importance in ground solutions : 



Gypsum, which is essentially the salt calcium sulphate containing some water, 

 is sparingly soluble in water. But the addition of an electrolyte with no common 

 ion, such as sodium chloride, will considerably increase the solubility of the gypsum. 

 Some experiments made in this laboratory have shown that in moderately strong 

 brines containing only sodium chloride gypsum can be regarded as a soluble salt. 

 The reason for this is readily seen when the substances which are formed are con- 

 sidered, both the calcium chloride and the sodium sulphate being very soluble salts. 

 The transportation of large quantities of lime by the drainage and ground waters in 

 arid regions where these salts are found is readily explicable from this point of view. 



Calcium carbonate, so abundant and so important in nature, is dissolved in a 

 precisely similar waj T ; but the ionization of carbonates being relatively small, the 

 effect is not so striking and relatively much less lime is transported in the solution. 

 Treadwell and Reuter* have recently published investigations on this point and find 

 the solubility of calcium carbonate in sodium chloride solutions does not become 

 markedly large until considerable concentrations of the latter salt are reached. The 

 effect of carbon dioxide in forming the more soluble bicarbonate of lime undoubtedly 

 is an important element in this connection, but as the ionization is but little affected 

 by its presence its influence must lie small in the presence of such a salt as sodium 

 chloride/ 



PRECIPITATION BY REACTIONS BETWEEX A(^UE0l'S SOLUTIONS AND (iASES. 



Another case of precipitation occurring in nature follows as a result 

 of mixing' solutions, one of which is a gas which acts upon the compounds 

 in the aqueous solution, producing ions of a different kind from those before 

 present, and in some cases forming compounds, the solubilitv of which is 

 so small that precipitation results. Perhaps the most important case of 

 this kind is the mixing of oxvgen with a solution containing' salts of iron 

 protoxide. As a result of this the iron is changed from ferrous to ferric 

 form, and the latter is precipitated as a sesquioxide or l^drosesquioxide of 

 iron. In the latter case hydration occurs simultaneously with oxidation. 



« Nernst, cit., p. 450. 



''Treadwell, F. P., and Renter, M., Ueber die Loslichkeit der Bikarbonate des Calciums und 

 Magnesiums: Zeitschr. fur anorgan. Cheruie, vol. 17, 1S9S, p. 170. 



c Cameron, F. K., Application of the theory of solutions to the study of soils: Kept. No. 64, Field 

 Operations of Division of Soils, 1899, TJ. S. Dept. of Agric, 1900, pp. 150-151. 



