118 CHEMISTRY OF NATURAL WATERS. [IX. 



of sodium, varying from about one third to more than two thirds 

 of the solid contents of the above waters, it is apparent that in 

 most cases the process of evaporation had gone so far as to 

 separate a part of the common salt ; and thus successive strata 

 of this ancient saliferous formation must be impregnated with 

 solid or dissolved salts of unlike composition. The mingling 

 of these in varying proportions affords the only apparent ex- 

 planation of the differences which appear in the relative amounts 

 of the several chlorides in waters from the same region, and 

 even from adjacent sources. 



38. The great solubility of chloride of calcium renders it 

 difficult to suppose its separation from the mother-liquors so as 

 to be deposited in a solid state in the strata.* The same re- 

 mark applies to chloride of magnesium. It is however to be 

 remarked that the double chloride of potassium and magne- 

 sium (carnallite) is decomposed by deliquescence into solid 

 chloride of potassium and a solution of chloride of magne- 

 sium ; and thus strata like those which at Stassfurth contain 

 large quantities of carnallite ( 22), might give rise to solu- 

 tions of magnesian chloride. This, however, would require the 

 presence of a large amount of chloride of potassium in the 

 early seas. It appears from the analyses above referred to that 

 the chloride of magnesium sometimes surpasses in amount the 

 chloride of calcium ; and sometimes, on the contrary, is equal 

 to only one half or one fourth of the latter salt. While it is 

 not impossible that the predominance of the magnesian chloride, 

 in some waters may be traced to the decomposition of carnal- 

 lite, it is undoubtedly in most cases connected with the action 

 of solutions of carbonate of soda ; the effect of which, as already 

 pointed out, is to first separate the soluble lime-salt as carbon- 

 ate, leaving to a subsequent stage the magnesian chloride. 

 (18.) As this reaction replaces the calcium-salt by chloride 

 of sodium, it might be expected that there would be an increase 

 in the amount of the latter salt in the water wherever the 

 magnesian chloride predominates, did we not remember that 



* [A hydrated double chloride of calcium and magnesium (tachydrite) has 

 since been found at Stassfurth.] 



