76 A. IV. Steivart on 



The human body contains between fifteen and twenty per cent, 

 of nitrogen, and can neither be built up nor maintained in working 

 order by a ])ure carbohydrate diet. Proteins are essential to life 

 and niti'ogen must be supplied to the animal frame if life is to be 

 kept in existence. 



But our bodies can make no direct use of the nitrogen which 

 forms fouT'-fifths of the air around us ; we breathe it but expel 

 it again unchanged. Nor can we utilize the simple nitrogen 

 compounds. Nitrogen must be supplied in the form of the 

 proteins which make up a lai'ge part of both animal and vegetable 

 tissue. Our vital mechanism seems incapable of profiting by 

 nitrogen unless i)resented in the form of protoplasm, which is a 

 mass of extremely complicated chemical derivatives of nitrogen. 



Human diet is partly vegetable and partly animal, but if the 

 animal part of it is traced back it will be found to depend 

 ultimately upon the vegetable kingdom. Thus the nitrogenous 

 compounds have been traced back to the plants ; but where do 

 the plants themselves obtain the niti'ogen for their tissues'] In 

 the majority of cases plants are unable to tap the store of nitrogen 

 in the atmosphei'e. Some, like i)eas, beans, &c., are actually able 

 to utilise nitrogen drawn direct from the air, but as a whole the 

 nitrogenous sui)plies are derived by plants from the earth by means 

 of their roots. Vegetables are in this way able to assimilate the 

 simple compounds of nitrogen, such as ammonia, nitrites and 

 nitrates, and to build the nitrogen thus obtained into the complex 

 derivatives which the animals can utilise as food. The main 

 reservoir of nitrogen is the atmosjjhere, from which certain bacteria 

 and some plants are able to draw nitrogen which the plants 

 convert into the organic compounds from which vegetable tissue 

 is built up. When the plants die their in'otoplasm is broken up, 

 either spontaneously or by the act of digestion of some animal 

 which has swallowed the plant. The nitiogen then yields the 

 derivative ammonia, containing one nitrogen and three hydrogen 

 atoms. 



If this ammonia is returned to the soil it is attacked by 



