II.] IMPURITIES IN NITRATE OF SODA 51 



Sodium perchlorate is sometimes present in small 

 quantities as an impurity ; this has a very injurious 

 effect upon vegetation, but the instances of damage due 

 to this cause are uncommon. Adulteration of nitrate of 

 soda has become rare nowadays ; in the past (and to a 

 certain extent still) it was mixed with common salt, a 

 substance which it resembles in colour and crystalline 

 appearance. In the manufacture of gunpowder it is 

 customary to make potassium nitrate by mixing sodium 

 nitrate with potassium chloride and crystallising out the 

 less soluble nitre. The mother liquors contain sodium 

 chloride (common salt) with small quantities of the 

 more valuable potassium nitrate, and when evaporated 

 down yield what is sometimes known as " gunpowder 

 salt." On occasion this material has been sold at a 

 fraudulent price as " nitrate of salt," and credited with 

 being a combination of nitrate of soda and salt, more 

 valuable than either for such crops as mangolds. 



As there is no occasion for any admixture with 

 nitrate of soda, the farmer should always insist on 

 buying the unmixed substance of standard purity. 



As a manure, nitrate of soda is of course treated as 

 a source of nitrogen. It is not sufficiently realised how 

 valuable the soda base may be. This is not because 

 soda is in any way necessary to the nutrition of the plant, 

 but because of the action of any soluble salt upon the 

 insoluble potash compounds in the soil. The potash of 

 the soil is due to the partial weathering of double 

 silicates like felspar into clay, which is not to be 

 regarded as pure kaolinite, AlgOg 2Si02 sHgO, but as 

 containing a certain proportion of zeolitic bodies inter- 

 mediate between felspar and kaolinite — hydrated double 

 silicates containing potash, soda, magnesia, and lime 

 combined with alumina and silica. Any soluble salt, 

 and particularly a soluble soda salt, will react with 



