On the Extraction of Salts from Sea-Water. 105 



with a saturated brine, and in this way a very fine quality of table 

 salt may be advantageously manufactured, 



During these successive concentrations the volume of the water 

 has become greatly diminished, 10,000 gallons of sea-water re- 

 duced to 25°, (the point at which it begins to deposit salt,) 

 measure only 935 gallons ; at 30°, 200 gallons ; at 31°, 50 gallons ; 

 and at 34°, a?e reduced to a volume of only 30 gallons. 



Preparation of Sulphate of Soda. — For this process the cold 

 of autumn and winter is required. The mixtures of sea-salt and 

 sulphate of magnesia, (A and B.) together with the pure sulphate 

 of magnesia obtained from the mother liquors at 32°, are dissolved 

 in water heated to 95° F., with the addition of such a quantity of 

 common salt as shall make the proportions of the two salts equal 

 to 80 parts of chlorid of sodium to 60 of anhydrous sulphate of 

 magnesia. The warm saturated solution is exposed in shallow ba- 

 sins to a cold of 32 ° F., when it deposits 120 parts of hydrated 

 sulphate of soda, equal to 54 of anhydrous sulphate, or three-fourths 

 of the sulphuric acid of the mixture. In theory, about equal 

 weights of the two salts are necessary for their mutual decompo- 

 sition, but an excess of common salt diminishes the solubility of 

 the sulphate of soda, and thus augments the product. From the 

 residual liquid, which contains chlorid of magnesium mixed with 

 common salt and a portion of sulphate of magnesia, the latter 

 salts may be separated by evaporation. The sulphate of soda is 

 converted into carbonate of soda by the usual process of calcina- 

 tion with carbonate of lime and coal. 



The Potash Salts. — The chlorid of potassium obtained by the 

 process already indicated, is decomposed by sulphuric acid, and 

 the resulting sulphate at once converted into carbonate of potash 

 by a process similar to that employed for the manufacture of car- 

 bonate of soda. The carbonate of potash thus prepared is free 

 from sulphate and chlorid, as well as from silica and alumina, and 

 those metallic impurities which like iron and manganese, are al- 

 ways present in the salt obtained from wood-ashes, and render the 

 potashes of America and Russia unfit for the fabrication of fine 

 crystal glass. The double sulphate of potash and magnesia may 

 be at once decomposed like the sulphate of potash, by limestone 

 and coal, and both it and the chlorid may be directly employed 

 in the fabrication of potash-alum, a salt which contains nearly 

 ten per cent, potash, and of which five thousand tons are annual- 

 ly manufactured in France. The high price of the salts of potash 



