96 MANUFACTURE OF FERTILIZING MATERIALS 



quantities of electrical energy. There are at 

 present two chief methods of manufacture: (1) 

 the Frank and Caro process, (2) the Birkeland 

 and Eyde process. The first consists in com- 

 bining atmospheric nitrogen dry and deprived 

 of its oxygen, with calcium carbide, obtained by 

 fusion in the electrical furnace of equal amounts 

 of coal and lime. The product so obtained is 

 termed nitrogen or cyanamide of calcium. The 

 second process consists in oxidizing atmospheric 

 nitrogen by electrical means, and converting it 

 into nitric acid, which is put into commerce 

 as nitrate of lime with 13 per cent of nitrogen, 

 which has the greater analogy with nitrate of 

 soda and which, like the latter, is assimilable 

 by plants. The two products come on the market 

 as more or less dark, dirty-gray powders. 



As just mentioned, this process for the man- 

 ufacture of nitrate of lime consists in oxidizing 

 atmospheric nitrogen by electrical means. In 

 1903 Prof. Birkeland of Christiania observed 

 that the electrical discharges from the alternate 

 current, at an average tension, dispersed in the 

 magnetic field, brought about the combus- 

 tion of the nitrogen in the air. This process 

 had the advantage over similar ones of requir- 

 ing a much lower electric tension, say 5,000 

 volts in place of 15,000, and to furnish much 

 higher yields of nitric acid. The air is burnt 

 in an electrical oven having the form of a drum. 

 This furnace was modified and improved by 



