1865.] CHEMISTRY OF NATURAL WATERS. 15 



§ 24. A consideration of the conditions of the ocean in earlier 

 geological periods will show that it must have contained a much 

 larger quantity of lime-salts than at present. The alkaline car- 

 bonates, whose origin has been described in § 13, and which from 

 the earliest times have been flowing into the sea, have gradually 

 modified the composition of its waters, separating the lime as car- 

 bonate, and thus replacing the chlorid of calcium by chlorid of 

 sodium, as I have long since pointed out.* This reaction has 

 doubtless 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. (§ 1 2). In this decomposition by carbonate of 

 soda, as already described in § 18, it results from the incompati- 

 bility of chlorid of calcium with hydrous carbonate of magnesia, 

 that the lime is first precipitated, with a little adhering carbonate 

 of magnesia ; and it is only when the chlorid of calcium is all 

 decomposed that the magnesian chlorid is transformed into car- 

 bonate of magnesia. This latter 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 carbonate. 

 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 predomi- 

 nance of lime-salts, would still contain large amounts of chlorid 

 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 equiv- 

 alent to the sulphuric acid; but the intervention of carbonate 

 of soda would be required. 



§ 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, which I have elsewhere 

 described, may intervene. f The addition of a solution of bicarbon- 



* Canadian Journal for 1858, p. 202 ; Silliman's Journal [2] xxv, 102, 

 and Comptes Rendus, June 9, 1862, p. 1191. 



f Silliman's Journal [2] xxviii, 174. 



