1 64 CHEMICAL FERTILIZERS 



above certain temperatures. When equivalent amounts 

 of hydrated sodium sulphate, Na 2 SO 4 , ioH 2 O, and hydrated 

 magnesium sulphate, MgSO 4 , 7H 2 O, are treated with a small 

 quantity of water at 15 C. (59 F.) a solution is obtained 

 which is saturated with both these salts, and is in equi- 

 librium with the solid present. At 22 C. (72 F.), there 

 occurs a transition point for the mixture of these salts and 

 the double compound Na 2 SO 4 . MgSO 4 . 4H 2 O, and a certain 

 amount of that double salt is formed. When the temper- 

 ature rises above 22 C. the single salts continue to 

 dissolve, but the less soluble double compound separates 

 out. The whole of the solid is not immediately converted 

 into the double salt, because magnesium sulphate is more 

 soluble than sodium sulphate, and the solid present is a mixture 

 of sodium sulphate and the double compound. After 

 rising to 25 C. (77 F.) the solid double compound is 

 left with the saturated solution, which contains both 

 sodium and magnesium sulphates. The temperature from 

 22 to 25 C. is called the transition interval. Carnallite, 

 KCl.MgCl 2 .6H 2 O, is completely decomposed by water 

 at 167 C. (335 F.) into the chlorides of magnesium 

 and potassium. It is also partly decomposed at all temper- 

 atures below this with the separation of potassium chloride, 

 so that a pure saturated solution of carnallite in water cannot 

 be prepared at ordinary temperatures. The alums, on the 

 other hand, are much more stable in the presence of their 

 solutions, and ordinary potash alum is stable up to a temper- 

 ature of 92 J C. (200 F.). 



Super-saturated Salts. A clear saturated solution of 

 a salt, if allowed to cool quietly in a closed vessel, may fail 

 to deposit crystals when the solution contains more salt 

 than necessary to make a saturated solution at that temper- 

 ature ; under such conditions the solution is said to be super- 

 saturated. If, however, a small crystal of the same salt is 

 added, rapid crystallization occurs, generally accompanied by 

 a rapid rise in temperature, and the solution becomes exactly 

 saturated at the temperature prevailing. In the chemical 

 manufacture of pure salts by means of crystallization, super- 



