ix SCIENTIFIC MOTIVE 245 



showed that when air treated in this way was absorbed 

 by a solution of caustic potash, the resulting product 

 was saltpetre, or nitrate of potash. This was not an 

 accidental discovery, but a result which was antici- 

 pated from the knowledge that saltpetre is made up 

 of certain definite proportions of potassium, nitrogen 

 and oxygen. 



The practical application of Cavendish's experiment 

 did not come until the beginning of the twentieth 

 century, when the demand for saltpetre or similar sub- 

 stances containing nitrogen in a particular form as one 

 of its constituents had reached such a high value that the 

 artificial production of the salt offered prospects of 

 commercial profit. Nitrogen is part of the constitution 

 of every explosive used until the middle of the nine- 

 teenth century for military purposes, and " villainous 

 saltpetre," which is chemically known as nitrate of 

 potash, was the only source from which it was obtained. 

 Nitrate of soda is a similar substance, but it was known 

 only as a laboratory product until the discovery of great 

 deposits of the salt in an arid region of the Andes in 

 1821. This Chile saltpetre can be converted into nitrate 

 of potash by a chemical process, and thus provides the 

 true saltpetre required for the manufacture of sporting 

 and military powders. 



But the demand for nitrates proceeds from arts of 

 peace as well as from those of war. Nitrogen in some 

 form or other is an essential fertiliser in agriculture, and 

 nitrate of soda has proved to be a most valuable means 

 of increasing the productiveness of the soil. Sir William 

 Crookes pointed out in 1898 that while the average yield 

 of wheat is but 127 bushels per acre, the yield could 

 be increased to 20 bushels by the use of 1 J cwt. of nitrate 

 of soda upon each acre each year. 



