INTBODUCTOBY 13 



present in the air. It is derived from the incomplete com- 

 bustion of coal, decomposition of organic matter, etc. Some 

 of it is oxidised, forming nitric acid ; some, perhaps a con- 

 siderable quantity, is directly absorbed by the soil. The 

 remainder of the ammonia, together with all the nitric 

 acid and any other compounds of nitrogen present in the 

 air, are removed by solution in rain water, by which they 

 are carried down, and so ultimately reach the soil. The 

 amount of nitrogen gained by the soil in this way, how- 

 ever, is probably not very great (p. 16). 



Nitrogen does not unite directly with carbon under ordi- 

 nary conditions, but, in the presence of a third substance, 

 they may be brought into combination. It has long beer 

 known that when carbon is heated with potassium car- 

 bonate, and nitrogen gas is passed into the mixture, potas- 

 sium cyanide is formed. This substance contains a 

 compound of carbon and nitrogen. More recently it has 

 been discovered that similar compounds are produced when 

 lime and carbon are heated together in an electric furnace 

 in the presence of nitrogen, and that these compounds may 

 be used as a source of nitrogen for nourishment of plants. 



It is evident that electricity greatly facilitates the forma- 

 tion of compounds of nitrogen, and it may be asked whether 

 the element could not be rendered directly available to 

 plants by this means. Some observers have held that this 

 question is to be answered in the affirmative. It is known 

 that electric currents have a stimulating effect on the 

 growth of plants, but it is not clear that this effect is due 

 to the direct assimilation of nitrogen. 



Under natural conditions nitrogen enters into combina- 

 tion with other elements through the agency of certain 

 vegetable organisms. It is true, as has been previously 

 mentioned, that the higher plants, such as agricultural 

 crops, cannot absorb and utilise the free nitrogen of the 

 air. But it appears that certain simple plants called algae 



