NITRATE OF SODA. 141 



from a few inches to 12 feet. It is extracted by boring through 

 the upper layers and introducing a charge of gunpowder, 

 which, when fired, exposes a considerable quantity of the 

 material. It is then broken up by means of picks and carried 

 to the refinery." There it is purified by crystallization. This 

 is done by dissolving in water by the aid of heat, allowing the 

 solution to settle, and then running it into tanks, where, on 

 cooling, crystals of sodium nitrate are deposited. The mother 

 liquid 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 + 3Na 2 S0 8 + 2NaHSO 3 = 5Na 2 SO 4 + 1 2 + H 2 O. 

 The iodine is then purified by sublimation and forms an impor- 

 tant 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% 



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



Sodium chloride ... ... 0'75% 



Sulphates ... ... O30% 



Insoluble matter ... ... 0-10% 



The proportion of iodine obtained is about 50 grammes per 

 100 kilogrammes of crude nitrate t. 



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, &c. ... ... ... 50% 



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, 



* See article by Aikman in " Blackwood's Magazine," March, 1892, and Report on 

 the Nitrate Trade of Chili, Jour. Soc. Chem. Ind. 1890, 664. \ 



t v. Report on the Nitrate Trade of Chili by Consul-General Walker, 1890. 



