FIXATION OF ATMOSPHERIC NITROGEN 407 



nitric acid formed is concentrated, so that the first tower yields 

 an acid containing 50 per cent of nitric acid. The soda towers 

 yield nitrite of soda. 



To carry the gases forward each row of towers is provided 

 with centrifugal fans made of aluminium which is not attacked 

 by nitric acid. 



The nitric acid collected from the towers is stored in granite 

 tanks, neutralised by calcium carbonate, and after the addition 

 of a small quantity of lime the solution is evaporated 1 and the 

 residue, after solidification and granulation in a mill, is run into 

 iron drums, in which it passes into commerce under the name 

 of " Norwegian Saltpetre " or " Air Saltpetre." 



The Birkeland-Eyde construction of furnaces is not the only 

 system which has come into use. A few years later Dr. Schonherr, 

 with the electrical engineer Hessberger of the Badische Company, 

 perfected an electric furnace for the oxidation of atmospheric 

 nitrogen based on different principles. The electro-magnet is 

 done away with, and in place of the great disc of electric flame a 

 long slender arc is formed in the axis of a narrow iron tube 

 through which the current of air passes. The iron tube contains 

 at one end an insulated electrode and itself forms the second 

 electrode. The furnace is built up of iron plates 7 metres in 

 height, and the air current passes into the internal reaction 

 chamber through a number of tangential openings or slits 

 arranged in several horizontal rows in the sides. As a rapid 

 cooling of the gases after exposure to the flame is of importance 

 the upper third of the tube has a water jacket through which the 

 gases pass, and reversal of the combination is prevented to a 

 notable degree. 



The Schonherr furnaces at Christiansand, shown in the illus- 

 tration (Fig. 150), take 600 horse-power, but larger furnaces have 

 since been built. 



According to Dr. Eyde the yield is practically the same as 

 that obtained with the Birkeland-Eyde furnace. 



At Notodden the furnaces are from 1000 to 3000 kilowatt 

 capacity, and are of the Birkeland-Eyde type. At Rjukan there 

 are furnaces of the Birkeland-Eyde system of 3000 kw. capacity 

 as well as furnaces of the Schonherr type all of 1000 kws. 



1 All the necessary evaporations are effected by the waste heat of the electric 

 furnaces, without the consumption of coal. This is a very important considera- 

 tion from the industrial point of view. 



