110 CHEMISTRY OF NATURAL WATERS. [IX. 



being much the less soluble salt, especially in a strongly saline 

 liquid, is deposited as gypsum ; and subsequently the magne- 

 sian carbonate is precipitated in a hydrous form. The effect 

 of this reaction is to eliminate from the sea-water both the 

 sulphuric acid and the magnesia, without the permanent addi- 

 tion to it of any foreign element. 



27. Gypsum may thus be separated from sea-water by two 

 distinct processes, the one a reaction between sulphate of 

 magnesia and chloride of calcium, and the other between the 

 same sulphate and carbonate of lime. The latter, involving a 

 separation of bicarbonate of magnesia, can, as we have seen, 

 only take place when the whole of the chloride of calcium has 

 been eliminated ; and if we suppose the ancient ocean, unlike 

 the present, to have contained more than an equivalent of lime 

 for each equivalent of sulphuric acid, it is evident that a lake 

 or basin of sea-water free from lime-salts could only have been 

 produced by the intervention of carbonate of soda. The action 

 of this must have eliminated the whole of the lime as carbonate, 

 or at least have so far reduced the amount of tins base that the 

 sulphates present would be sufficient to separate the remainder 

 by evaporation in the form of gypsum, and still leave in the 

 mother-liquor a quantity of sulphate of magnesia for reaction 

 with bicarbonate of lime. 



The source of the magnesian carbonate, whose union, under 

 certain conditions, with the carbonate of lime, gives rise to 

 dolomite (ante, page 90), may thus be due either to the re- 

 action just described between bicarbonate of lime and solutions 

 holding sulphate of magnesia, or to the direct action of car- 

 bonate of soda upon waters containing magnesian salts ; but, 

 in either case, the previous elimination of the incompatible 

 chloride of calcium must be considered an indispensable pre- 

 liminary to the production of the magnesian carbonate. 



28. To the three principal sources of mineral matters in 

 mineral waters already enumerated, namely, decaying 01 

 matters, decomposing silicates, and the soluble saline matters 

 in rocks, a few other minor ones must be added. One of these 

 is the oxidation of metallic sulphurets, chiefly iron pyrites, 



