1865.] CHEMISTRY OF NATURAL WATERS. 169 



§ 41 . We have already shown in § 38 how the action of carbonate 

 of soda upon sea-water or bittern will destroy the normal propor- 

 tion between the two chlorids of magnesium and calcium by con- 

 verting the latter into an insoluble carbonate, and leaving at last 

 only salts of sodium and magnesium in solution. A process the 

 reverse of this has evidently intervened for the production of waters 

 like that from Cape Breton, and some others noticed by Lersch, in 

 which chlorid of calcium abounds, with little or no sulphate or 

 chlorid of magnesium. This process is probably one connected 

 with the formation of a silicate of magnesia. Bischof has already 

 insisted upon the sparing solubility of this silicate ; and he observed 

 that silicates of alumina, both artificial and natural, when digested 

 with a solution of magnesian chlorid, exchange a portion of their 

 base for magnesia, thus giving rise to solutions of alumina - 

 which, being decomposed by carbonates, may have been the source 

 of many of the aluminous deposits referred to in § 9. He also 

 observed a similar decomposition between a solution of an artificial 

 silicate of lime and soluble magnesian salts. (Bischof, Chem.. 

 Geology, i, 13, also chap, xxiv.) In repeating and extending 

 his experiments, I have confirmed his observation that a solution 

 of silicate of lime precipitates silicate of magnesia from the 

 sulphate and the chlorid of magnesium ; and have moreover found 

 that by digestion at ordinary temperatures with an excess of 

 freshly precipitated silicate of lime, chlorid of magnesium is com- 

 pletely decomposed ; an insoluble silicate of magnesia being formed, 

 while nothing but chlorid of calcium remains in solution. It is 

 clear that the greater insolubility of the magnesian silicate, as com- 

 pared with silicate of lime, determines a result the very reverse of 

 that produced by carbonates with solutions of the two earthy bases- 

 In the one case the lime is separated as carbonate, the magnesia 

 remaining in solution ; while in the other by the action of silicate 

 of soda (or of lime), the magnesia is removed and the lime remains. 

 Hence carbonate of lime and silicate of magnesia are everywhere 

 found in nature ; while carbonate of magnesia and silicate of lime 

 are produced only under local and exceptional conditions. The 

 detailed results of some experiments on this subject are reserved 

 for another place. It is evident that the production from the 

 waters of the early seas of beds of sepiolite, talc, serpentine, and 

 other rocks in which a magnesian silicate abounds, must, in closed 

 basins, have given rise to waters in which chlorid of calcium would 

 predominate. 



