INORGANIC NITROGEN FERTILIZERS 97 



Competition with synthetic nitrates will prove the surest 

 incentive to the utilization of weaker deposits. The amount 

 of nitrate actually extracted to date, in comparison with 

 what is still in Chili, is a very small fraction. 



Nitrate of Lime. Nitric acid can be produced directly 

 from the air by the action of an electric arc or any other means 

 of obtaining a high temperature. The first really successful 

 method of working this process was the Birkeland and Eyde 

 Arc Flame. In this type of furnace an electric arc is produced 

 by an alternating two-phase current between water-cooled 

 electrodes, which are placed between the poles of a powerful 

 electro-magnet in such a way that the terminals of the 

 electrodes are in the centre of the magnetic field. The 

 electric flame, so produced, assumes the form of a disc, and 

 the arc is drawn into a half disc at every half period of the 

 alternating current. When the arc starts, the air is heated, 

 the hot air acts as a conductor and is attracted to one 

 of the poles of the magnet. Since the magnet poles are 

 shaped around the path of the incoming air, the hot air 

 assumes a disc shape. As the length of the electric arc 

 increases, the resistance becomes greater, until it becomes so 

 great that a new electric arc starts from the points of the 

 electrode ; the current then breaks, only to re-form in the 

 opposite direction, and be attracted to the opposite mag- 

 netic pole. For the purpose of using this disc flame, it is 

 enclosed in a special iron furnace lined with firebrick, which 

 lasts very well, because the temperature of the walls does 

 not rise above 800 C. (1500 F.) in normal working, in spite 

 of the fact that the flame is over 3000 C. (5432 F.). Air 

 is driven, by means of a blower, into the furnace, and 

 thence successively through the tubes of a multi-tubular 

 boiler, a cooler, and the oxidation tower, as described under 

 the head of the Kilburn Scott process. The temperature 

 of the arc flame may exceed 3000 C. (say 5400 F.), but 

 the temperature of the escaping gases is only between 

 800 and 1000 C. (say I5oo-i8oo F.). 



The Schonherr arc flame differs essentially from that of 

 the Birkeland, as in place of magnets and magnetic fields the 

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