232 BACTERIOLOaiCAL AND ENZYME CHEMISTRY 



of course, that plants possess this property through the 

 activity of the chlorophyll in their cells, but instances of the 

 utilisation of the carbon in carbon dioxide by lower organisms 

 have not been frequently observed. 



Assimilation of Nitrogen. — ^It will be seen from the above 

 equations, representing de-nitrification changes, that these 

 must eventuate in escape of nitrogen into the atmosphere. 

 If this continued, it is evident that in time the stock of nitrogen 

 available for life would become depleted, as a certain percentage 

 of the nitrogen of all organic matter would be permanently 

 lost in this way. Fortunately a means exists for bringing 

 back this escaped nitrogen once more into the cycle of hfe. 

 A certain small quantity is returned as nitric acid, through 

 the combination of nitrogen and oxygen brought about by 

 the electric discharge of the lightning ; and of recent years 

 considerable developments have taken place in the production 

 of nitric acid by the union of the nitrogen and oxygen of the 

 atmosphere, by means of powerful electric discharges artificially 

 produced. Nitrogen has also been recovered artificially from 

 the atmosphere by the production of calcium cyanamide 

 in the electric furnace, by heating mixtures of lime or chalk 

 with charcoal at a temperature of 2000° C. in a ciurrent of air. 



Calcium carbide is first formed, which combines with 

 nitrogen to form calcium cyanamide, thus : — 



CaC2 + 2N = CaCNj + C 



Calcium cyanamide can be used as a source of nitrogen in 

 agriculture, as it decomposes readily in presence of moisture, 

 yielding calcium carbonate and ammonia, thus : — 



CaCNs + 3H,0 = CaCOs + 2NH3 



All these artificial methods are dependent upon cheap 

 electricity for their economic development, and the works 



