SECTION VI.-THE UTILIZATION OF 

 ATMOSPHERIC NITROGEN 



FOR the purpose of obtaining nitrogen from the air in a form 

 suitable for use by plants, almost all processes demand that 

 some purification of the nitrogen in the air is necessary. 

 Under modern conditions it is a common practice to obtain 

 the necessary nitrogen by liquefying the air by one of the 

 modern methods of converting gas into'liquid by pressure and 

 automatic cooling. These methods depend upon the follow- 

 ing principles : (i) that if compressed air is allowed to expand 

 without doing external work, there is a fall of temperature, 

 due to the fact that internal work must be done in such 

 expansion. With low temperatures and high pressures all 

 gases can be liquefied. (2) That the boiling point of oxygen 

 is 77 '5 absolute, and that of nitrogen 91 -5 absolute. (3) That 

 by fractionation such liquids can be separated. 



Air, compressed to about four atmospheres, is usually 

 drawn through towers, and cooled by passing through a 

 water tube to remove the heat caused by compressing the gas. 

 It is then still further cooled by the waste gas proceeding 

 from the rest of the process, by which means the moisture is 

 condensed with the formation of ice, which is periodically 

 removed. The air next passes through a cooler, which may 

 be worked by ammonia, by which means the temperature 

 is further reduced, and the gas purified from water and car- 

 bonic acid. Almost pure and dry air passes away from the 

 fore cooler to the L,inde nitrogen gas separator, where it 

 is expanded through a valve and cooled down by an oppositely 

 flowing current of nitrogen. The air then passes at a very 

 low temperature through a coil, where it liquefies. The coils 

 through which the gases flow surround a rectifier which 

 works on the same principle as a distillation fractionating 



