CHAPTER XII. 



THE CIRCULATIOX OF NITROGEN. 



Chemical analysis shews that the animal and plant body is 

 mainly built up from the four elements, nitrogen, carbon, hydrogen 

 and oxygen. Added to these there are the metals, sodium, 

 potassium and iron, and the non-metals, chlorine, sulphur and 

 phosphorus. Calcium or silicon are also invariably present as the 

 bases of calcareous or siliceous skeletons. All these, with some 

 others, are indispensable constituents of the organic body, and in 

 an exhaustive study of the cycle of matter from the living to the 

 non-living phases, and vice versa, we should have to trace the course 

 of each. But w^e are accustomed to regard nitrogen as the charac- 

 teristic constituent of living substance and it will be sufficient to 

 consider this element alone. 



At any moment there exists, on the earth, a certain mass of 

 nitrogen, combined with other elements, forming the living substance 

 of animals and plants. Another fraction of the gas is present in 

 the atmosphere, in the soil, and in water, in combination with 

 other elements in the form of non-living organic and inorganic 

 substances, which are either being broken down into simpler 

 forms, or are being built up into living tissues. Yet a third mass 

 is present in the elementary condition in the atmosphere. Roughly 

 speaking there are about 434.5 billions of tons of nitrogen in the 

 latter form. How much is present in the shape of lifeless organic 

 and inorganic compounds we do not know, and still less have we any 

 idea of the mass of nitrogen which exists in the living tissues of 

 animals and plants : certainly it must be only a small fraction of 



J. F. 18 



