UTILIZATION OF ATMOSPHERIC NITROGEN 79 



The system, therefore, of progressive choking is carried out 

 through the entire ascending column of liquefying nitrogen. 



By other methods air is passed over heated copper, 

 oxygen being taken up by the copper and nitrogen passed 

 on, the copper oxide formed being subsequently reduced by 

 generator gas. Another method is to use producer gas, 

 which consists of nitrogen, oxygen, carbon mon-oxide and 

 carbon dioxide. The producer gas, mixed with a little air, 

 is passed over a heated mixture of copper and copper oxide, 

 when oxygen combines with the copper, forming copper 

 oxide, and the carbon mon-oxide acts on the copper oxide, 

 reducing it to metallic copper. The carbon dioxide so 

 obtained can be absorbed either by lime or by a solution of 

 sodium carbonate. When the latter process is used, pure 

 carbon dioxide can be obtained as a by-product, after steam- 

 ing the gas out of the sodium bi-carbonate formed. Another 

 method of removing carbon dioxide is to compress the gas 

 in the presence of water, when carbon dioxide dissolves. The 

 water saturated with carbon dioxide is removed and the 

 gas extracted by exhaustion, when the water can be used over 

 again. 



The subsequent utilization of nitrogen obtained in any 

 of these ways will depend upon the particular process used. 

 It may be converted into ammonia by the Haber process 

 (p. 89), or it may be absorbed by calcium carbide and con- 

 verted into calcium cyanamide (see p. 105). For the 

 purpose of forming nitric acid by the arc process unaltered 

 air can be used (see p. 97) . 



The fixation of nitrogen is also carried out in the field 

 by the action of bacteria, either with or without the growth 

 of plants to act as hosts. Experimental results have shown 

 that among the bacteria which are most active, azotobacter 

 is one of the most important. Such results can be observed by 

 inoculating about o'l gramme of soil into 1000 c.c. of tap 

 water containing 2 % of mannitol, o'02 % of potassium phos- 

 phate, in the presence of calcium carbonate ; the mixture is 

 kept for some weeks at 27-3o C. in a thin well-aerated layer 

 in a conical flask. As an average of several results, about 



