VII. SPECIAL MANURES 133 



ning it into tanks, where, on cooling, crystals of sodium nitrate are de- 

 posited. The mother liquor is then run off the crystalline mass and 

 treated with sodium sulphite and sodium bisulphite (made on the spot) in 

 large wooden tanks lined with pitch. A precipitation of iodine then 

 takes place by the decomposition of the sodium iodate always present 

 in the caliche. The reaction is 



2NaIO 3 + 3Na 2 S0 8 + 2NaHS0 3 = 5Na 2 SO 4 + I 2 + H 2 O. 

 The iodine is then purified by sublimation and forms an important 

 source of profit. 



The crystals of nitrate are slightly rinsed with water, to wash out 

 the mother liquor adhering to them, and are then dried in the sun. 

 The average composition of the product, as it leaves the works, is said 

 to be 



Sodium nitrate 96-75 per cent. 



Water ........ 2-10 



Sodium chloride . . . . . 0-75 



Sulphates 0-30 



Insoluble matter . . . . . . 0-10 



The proportion of iodine obtained is about 50 grammes per 100 kilo- 

 grammes of crude nitrate. 1 



The composition of the caliche varies greatly, and as a rule the 

 larger the proportion of sodium nitrate present, the less iodine does it 

 contain. It is usual to mix the various qualities so that the mixture 

 becomes fairly constant in composition 



Earth, stones, etc 50 per cent. 



Sodium nitrate . 35 



Magnesium, calcium and sodium chlorides . . 10 



Water, sulphates and other salts ... 5 



Associated with the nitrate in the caliche, a large number of 

 different salts have been detected, including sulphates, nitrates, chlorides, 

 iodates and borates of calcium, magnesium and sodium. . There are 

 also traces of chromium, existing probably as calcium chromate. 2 



Pure sodium nitrate is a white crystalline salt containing no 

 water of crystallisation, but generally holding a small quantity of 

 hygroscopic moisture. Indeed, in moist air it is deliquescent. In 

 addition to its use as a manure, it finds application in the arts as a 

 source of nitric acid and in the manufacture of gunpowder and of 

 potassium nitrate. 



The product supplied for agricultural purposes is supposed to con- 

 tain 95 per cent or over, of real sodium nitrate and thus to yield more 

 than 15-6 per cent of nitrogen. Being extremely soluble and diffusible, 

 it is at once available to plants and should only be applied when the 

 crop is sufficiently grown to be capable of assimilating it ; otherwise, 

 since it is not retained by any constituent of the soil, considerable loss 

 in the drainage may occur. Nitrate of soda is poisonous to animals 

 and should not be left about in places to which cattle have access. Its 

 saline taste induces cattle to eat it, in districts where they show great 

 eagerness for salt, and the results are often fatal. 



1 Keport on the Nitrate Trade of Chili, by Consul-General Walker, 1890. 



2 Buchanan, Jour. Soc. Chem. Ind., 1893, 128. 



