12 SOILS AND MANUEES 



Nitrogen. Nitrogen, which constitutes the remaining 

 four-fifths of the bulk of the air, is, like oxygen, an element 

 or simple body, but it exhibits very different properties. 

 Whereas oxygen is one of the most active elements known, 

 nitrogen is one of the most inert. The former unites 

 readily with other elements ; the latter does not. Many 

 substances burn readily in oxygen ; none of them will burn 

 in nitrogen at all. Animals and plants absorb and com- 

 bine with the oxygen of the air, but, although they take 

 in four parts of nitrogen along with each part of oxygen, 

 and although they require large quantities of it, they are 

 nevertheless quite unable to utilise the free nitrogen of 

 the air in this way (p. 8). 



But, though compounds of nitrogen cannot be formed, 

 like those of oxygen, by burning substances in air or in 

 pure nitrogen gas, such compounds are known and can be 

 prepared in various ways. Electric discharges in the air 

 thunder cause minute quantities of nitrogen and oxygen 

 to unite together forming oxide of nitrogen which, with 

 water, produces the traces of nitric acid found in the air. 

 The formation of nitric acid by the passage of electric 

 sparks through air can be experimentally demonstrated by 

 means of a small battery and induction coil. With greater 

 electric power more considerable quantities can be formed, 

 and a process has recently been devised for the manu- 

 facture of nitric acid, on a commercial scale, in this way. 



Ammonia, the most important compound of nitrogen 

 and hydrogen, can also be produced by a current of 

 electricity passing through a mixture of the two gases. 

 Only minute traces of hydrogen are, however, found in 

 the air, 1 and the compound is probably not formed in this 

 way in Nature. 



A small quantity of ammonia is, however, generally 



1 Dewar, Nature, December 20th, 1900. 



