

IX.] CHEMISTRY OF NATURAL WATERS. 109 



into the sea, have gradually modified the composition of its 

 waters, separating the lime as carbonate, and thus replacing 

 the chloride of calcium by chloride of sodium, as I have long 

 since pointed out (ante t page 2). This reaction has doubt- 

 less been the source of all the carbonate of lime in the earth's 

 crust, if we except that derived from the decomposition of 

 calcareous silicates. ( 12.) In this decomposition by car- 

 bonate of soda, as already described in 18, it results from 

 the incompatibility of chloride of calcium with hydrous car- 

 bonate of magnesia, that the lime is first precipitated, with 

 a little adhering carbonate of magnesia ; and it is only when 

 the chloride of calcium is all decomposed that the magnesian 

 chloride is transformed into carbonate of magnesia. This lat- 

 ter reaction can consequently take place only in limited basins, 

 or in portions cut off from the oceanic circulation. 



25. It follows from what has been said that the lime-salt 

 may be eliminated from sea- water either as sulphate or as car- 

 bonate. In the latter case no concentration is required ; while 

 in the former the conditions are two, a sufficient proportion 

 of sulphates to convert the whole of the lime into gypsum, 

 and such a degree of concentration of the water as to render 

 this insoluble. These conditions meet in the evaporation of 

 modern sea-water ; but the evaporated sea-water of earlier 

 periods, with its great predominance of lime-salts, would still 

 contain large amounts of chloride of calcium, the insolubility 

 of gypsum in this case serving to eliminate all the sulphates 

 from the mother-liquor. Evaporation alone would not suffice 

 to remove the whole of the lime-salts from waters in which 

 the calcium present was more than equivalent to the sulphuric 

 acid ; but the intervention of carbonate of soda would be re- 

 quired. 



26. In concentrated and evaporating waters freed from 

 lime-salts by either of the reactions just mentioned, but still 

 holding sulphate of magnesia, another process may intervene 

 (ante, page 90). The addition of a solution of bicarbonate 

 of lime to such a solution gives rise, by double decomposition, 

 to sulphate of lime and bicarbonate of magnesia. The former, 



