SODIUM CHLOKIDE BERTHOLLET'S LAWS 421 



mixed with the bitter salts of magnesium which, owing to their solu- 

 bility and the small amount in which they are present (less than 

 1 p.c.), only separate out, in the first crystallisations, in traces. 

 Gypsum, or calcium sulphate, CaS0 4 2H 2 O, because of its sparing 

 solubility, separates together with or even before the table salt. When 

 about half of the latter has separated, then a mixture of table salt 

 and magnesium sulphate separates out, and on still further evapora- 

 tion 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.* After the separation of this 

 salt from sea water, there remains a mother liquor containing 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 

 of Aral of the Caspian), and their water has evaporated and formed 

 (if the mass of the inflowing fresh water were less than that of the 

 mass evaporated) deposits of rock salt. It is always accompanied by 

 gypsum, because the latter is separated from sea water with or before 

 the sodium chloride. For this reason rock salt may always be looked for 



4 The double salt KCl,MgCl 2 is a crystallohydrate of KC1 and MgCl 2 , and is only 

 formed from solutions containing an excess of magnesium chloride, because water de- 

 composes 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 S0 4 , C0 3 ), which are disposed in 

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

 MgBr 2 , MgSO4, 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, 

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

 saturated) the salt Na 2 S0 4 ,10H.>O is first deposited because it is the first to arrive at 

 saturation at low temperatures. Consequently this solution contains MgCl 3 and Na-zSO^ 

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



6 The salt extracted from water is piled up in heaps and left exposed to the action of 

 rain water, which purifies it, owing to the water becoming saturated with sodium chloride 

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



