30 FERTILISERS CONTAINING NITROGEN [chap. 



land receives about 4 to 5 lb. per acre per annum of 

 combined nitrogen from the atmosphere, an amount 

 which only forms a small fraction of the requirements 

 of the crop : — 



Table VII.— Nitrogen as Ammonia and Nitric Acid 

 IN Rain. 



The tenacity with which in the face of such evidence 

 the opinion has been held that the leaf of the plant can 

 obtain nitrogen as well as carbon from the atmosphere, 

 is due to the difficulty of explaining how the world's 

 original stock of combined nitrogen can have arisen. 

 Assuming the world to have cooled down from the 

 state of incandescent gas, it must have started with all 

 its nitrogen in the free gaseous state ; yet as we see it 

 to-day, the whole stock of combined nitrogen is of 

 organic origin. 



The circulatory process through which combined 

 nitrogen passes is very plain. Animals only use the 

 highly organised compounds like the proteins ; these 

 they break down during their vital processes to simpler 

 compounds like urea and the amides, which in turn are 



