SALTS DEPOSITED ON EVAPOEATION. 183 



become rich in calcium chloride. In fact, owing to its deliquescent nature, 

 natural evaporation seldom proceeds for enough to cause the precipitation 

 of the chloride. The early deposition of calcium carbonate when nat- 

 ural waters are evaporated is rendered the more certain for the reason that 

 it is by far the most abundant salt found in surface waters. 



The fact that various salts are deposited in a regular succession when 

 minei'al waters are evaporated, is of great service in securing certain ones 

 in a pure state by the method of fractional crystallization. In evaporating 

 the brines of Syracuse the precipitation of ferric oxide and calcium sulj)l)ate 

 is first secured by moderate concentration; the water is then conducted to 

 lower vats and evaporation is continued until the sodium chloride has mostly 

 crystallized. The mother liquor, rich in magnesium and calcium, is the^i 

 allowed to go to waste. In the Soda lakes near Ragtown, Nevada, sodium 

 and calcium carbonates crystallize out as the mineral gaylussite, bN" the 

 natural concentration of the waters; when evaporation is continued, the 

 deposition of sodium sulphate and carbonate takes place previous to the 

 crystallization of common salt. 



On concentrating sea water it has been found that calcium carbonate 

 is usually the first constituent to be precipitated. This salt is not always 

 found when the waters of the ocean are analyzed, but may usually be de- 

 tected when the sample examined is taken near shore. The quantity deliv- 

 ered to the ocean by the drainage of the land, seems to be almost exactly 

 counterbalanced by its secretion in the tissues of animals and plants. 



The separation of sodium sulphate, potassium chloride, and common 

 salt from the mother liquor derived from the concentration of sea water, 

 b}' alternate evaporation and cooling, is the principle of Balard's well-known 

 process so largely used in the south of Europe. In Merel's modification of 

 this process a low temperature is obtained artificiall}^. When sea water is 

 concentrated until its specific gravity is 1.24 (28°B.) it deposits about four- 

 fifths of the common salt it originally contained; after adding 10 per cent, 

 of fresh water to the mother liquor remaining, it is passed through a refrig- 

 erating machine and its temperature lowered to — 18° C. The low tempera- 

 ture causes double decomposition to take place between the magnesium 

 sulphate and the sodium chloride; sodium sulphate being deposited and the 



