104 On the Extraction of Salts from Sea-Water. 



of magnesia, and the quantity of this latter salt goes on increasing 

 as the evaporation advances toward 35 Q . This mixture of salts 

 (A) is carefully collected, and reserved for the manufacture of the 

 sulphate of soda. 



When the bittern at 35 Q is still further evaporated by the heat 

 of the sun, it deposits a mixture which is called set cVete, and con- 

 tains a large amount of potash. By a second crystallization of this 

 product, a double sulphate of potash and magnesia is obtained, 

 which holds 24 per cent, of potash ; but this mode of treating the 

 mother liquors of 35° is less advantageous than the following 

 which is now adopted. The liquors are placed in large basins 

 and preserved until the first frosts, when at the temperature of 35° 

 or 40° Farenbeit, they deposit the greater part of their sulphate 

 of magnesia in large crystals. This sulphate, which is pure Epsom 

 salt, is either sold to the apothecaries, or used to prepare sulphate 

 of soda by the process about to be described. When the sulphate 

 of magnesia has been thus separated, the liquid is run off into 

 large reservoirs, and preserved until the next summer, when it is 

 again evaporated in shallow basins by the sun's rays. It now de- 

 posits a large amount of a fine granular salt, which is a double chlo- 

 rid of potassium and magnesium. This double salt can only be 

 crystallized from solutions containing a large quantity of chlorid' 

 of magnesium,and when re-dissolved in pure water gives pure chlorid 

 of potassium by ev poration. The double chlorid is raked up 

 from the tables and placed in piles on the earth, where the mois- 

 ture causes the salt to decompose ; the magnesian salt deliques- 

 cing drains off, and the chlorid of potassium remains behind. 



The mother liquors having acquired a density of 38°, have de- 

 posited all their potash, and are now evaporated by-artificial heat 

 to 44° ; during this evaporation they still deposite a portion of 

 common salt mixed with sulphate of magnesia (B), and on 

 cooling, the liquid becomes a solid mass of hydrated chlorid of 

 magnesium, which may be employed to furnish caustic and carbo- 

 nated maguesia by decomposition. When calcined in a current 

 of steam, it is completely decomposed into hydrochloric acid and 

 an impure magnesia, still containing some sulphates and chlorids, 

 which may be removed by water. 



By mingling in proper proportions the solution of chlorid of 

 magnesium at 44° with brine at 24°, nearly the whole of the sea- 

 salt is precipitated in the form of minute crystals of great pure- 

 ness aud beauty; the mother liquors are then removed by washing 



