[I.] ORIGIN OF Nil ROGEN IN VEGETATION 31 



taken by plants and built up once more into the protein 

 complexes. The nitrogen, however, only circulates 

 from one form of combination to another, with 

 occasional losses when a compound is broken down 

 as far as elementary nitrogen ; there is never any 

 bringing of fresh elementary nitrogen into the account. 

 The stocks of combined nitrogen that have been handed 

 down from past ages all speak of the same organic 

 circulation, never of fixation. Coal is but the debris of 

 an extinct vegetation ; nitrate of soda represents the 

 glorified result of the same decay processes which give 

 rise to nitrate of potash in India and nitrate of lime in 

 the old nitre beds. Virgin soils with their vast stores 

 of nitrogenous humus are often looked upon as having 

 gained nitrogen by the accumulation of long epochs of 

 vegetable growth ; but if plants cannot fix nitrogen there 

 can have been no gain, however long the growth, but 

 only a circulation of the pre-existing combined stock- 

 At first sight there seem to exist no processes which can 

 either bring about the original combination or renew 

 the stock from time to time. Inorganic agencies are 

 certainly trifling, because nitrogen is a difficult element 

 to bring into combination, so great an initial expenditure 

 of energy is required to separate the atoms in the 

 gaseous molecule. Electric sparks will effect a com- 

 bination of nitrogen and oxygen, and lightning flashes 

 through the air have been invoked to account for the 

 trace of nitric acid to be found in the atmosphere and 

 in rain water. Again, it has been supposed that during 

 the evaporation of water there is always a slight com- 

 bination of nitrogen with the elements of water to form 

 ammonium nitrite, but more recent and refined experi- 

 ments are against the existence of any such reaction. 



There has, however, of late years been discovered 

 one vital process capable of fixing nitrogen, which has 



