410 PRINCIPLES OF CHEMISTRY 



being about 2^ p.c. The remaining portion of the table salt sepa- 

 rates out, intermixed with the bitter salts of magnesium which form 

 the chief admixture with table salt in sea water. These salts of 

 magnesium, owing to their degree of solubility and the small amount in 

 which they are present (less than 1 p.c.), only separate out, in the first 

 crystallisations, in traces with the table salt ; whilst, in the succeeding 

 crystallisations, they are deposited together with the table salt. Gypsum, 

 or calcium sulphate, CaSO 4 2H 2 O, owing to its sparing solubility, 

 separates together with or even before the table salt. When about 

 half of the table salt has separated, then, on further evaporation, the 

 sea water gives a mixture of table salt and magnesium sulphate, and, 

 on still further evaporation, the chlorides of potassium and magnesium 

 begin to separate in a state of combination, forming the double salt 

 KMgCl 3 ,6H 2 O, which occurs in nature as carnallite. This is a 

 crystallo-hydrated compound of KC1 and MgCl 2 . 4 After the separa- 

 tion of this salt from sea water, there remains a mother liquor, con- 

 taining a large amount of magnesium chloride in admixture with 

 various other salts. 5 The extraction of sea salt is usually carried on 

 for the purpose of procuring table salt, and therefore directly it begins 

 to separate mixed with a considerable proportion 6 of magnesium salts 

 (when it acquires a bitter taste) the remaining liquor is run back into 

 the sea. 



The same process which is employed for artificially obtaining salt 

 in a crystalline form from sea water has been repeatedly accomplished 

 during the geological evolution of the earth on a gigantic scale ; up- 

 heavals of the earth have cut off portions of the sea from the remainder 

 (as the Dead Sea was formerly a part of the Mediterranean, and the Sea 



4 The double salt, KCl,MgCl 2 , is only formed from solutions containing an excess 

 of magnesium chloride, because water decomposes this double salt, extracting the more 

 soluble magnesium chloride from it. 



5 Owing to the fundamental property of salts of interchanging their metals, it 

 cannot be said that sea water contains this or that salt, but only that it contains certain 

 amounts of certain metals M (univalent like Na and K, and bivalent like Mg and Ca), and 

 haloids X (univalent like Cl, Br, and bivalent like SO 4 , CO 3 ), which are disposed in 

 every possible kind of grouping ; for instance, K as KC1, KBr, K 2 SO 4 , Mg as MgCL, 

 MgBr 2 , MgSO 4 , and so on for all the other metals. In evaporation different salts separate 

 out consecutively only because they reach saturation. A proof of this may be seen in 

 the fact that a solution of a mixture of sodium chloride and magnesium sulphate (both 

 of which salts are obtained from sea water, as was mentioned above), when evaporated, 

 separates out crystals of these salts, but when refrigerated (if the solution be sufficiently 

 saturated) the salt Na 2 SO 4 ,10H 2 O is first deposited because it is the first to arrive at 

 saturation at low temperatures. Consequently, this solution contains MgCl 2 and Na-^SO.,, 

 besides MgSO 4 and NaCl. So it is with sea water. 



e The salt extracted from water is piled up in heaps and left under the action of 

 rain water, which purifies the salt, owing to its becoming saturated with table-salt and 

 then no longer dissolving it, but washing out the impurities. 



