PHOSPHORUS 57 



on the arsenic present in the sulphuric acid in 

 which the ammonia is absorbed. Commercial 

 ammonium sulphate usually contains about 24.5 

 per cent of ammonia. It should be free from am- 

 monium sulphocyanide, which is a plant poison. 

 This impurity is rarely present, save when the am- 

 monium sulphate has been made by the direct sat- 

 uration of gas liquor with sulphuric acid. 



Sodium Nitrate. Sodium nitrate is obtained 

 from the deposits of crude nitrate in Chili 

 and Peru. The deposits, which lie about six to 

 ten feet below the surface, are known as caliche, 

 and form a layer four to six feet deep. It is asso- 

 ciated with clayey substances; the composition is, 

 however, very various, and the content of sodium 

 nitrate may reach 50 per cent. 



The caliche is broken in a stone breaker, and 

 systematically lixiviated in tanks heated by closed 

 steam. Water being a rare commodity in nitrate 

 districts, it has to be used for repeated extractions. 

 When the liquid reaches a specific gravity of 1.55, 

 it is run into crystallizing vats, in which it remains 

 from four to six days; the mother liquor is then 

 run off, and used for the recovery of iodine. 

 The iodine characteristically present in crude ni- 

 trate probably exists as iodate. As much as 1 per 

 cent of perchlorate is also found in some samples 

 of caliche, and lowers the value of the latter con- 

 siderably, since even small quantities of perchlorate 

 are injurious to plants. The nitrate deposited 

 after having been sun-dried has the following com- 



