DISCOVERY 



283 



induced to combine, giving a compound which in 

 contact with more air reacts with water to give 

 nitric acid. The processes may be represented approxi- 

 mately by the following formulae ; 



Nj + Oa = 2NO (nitric oxide) 



2NO + 02 = 2NO2 (nitrogen dioxide) 



4X02;+ Pa + 2H2O = 4HNO3 (nitric acid) 



The addition of lime and ammonia then give respec- 

 tively calcium nitrate, which can be applied as a 

 fertiliser, and ammonium nitrate, which forms the basis 

 of explosives. 



The electric arcs used in chemical processes are of 

 two distinct types, the high-tension arc and the low- 

 tension. The former is identical with a miniature 

 lightning discharge continuously maintained, and is 

 produced by a small quantity or volume of electricity 

 at a very high potential or pressure, while the latter 

 is obtained by the passage of a large current at a low 

 potential between two adjacent poles or electrodes 

 which are thus raised to their volatilisation temperature 

 and yield the vapour needed to conduct the current. 



One of the main types of apparatus employed in 

 Norway for effecting the union of nitrogen and 

 oxygen is illustrated in Fig. i. In the type shown, 

 known as the Birkeland-Eyde process, a current of 



air is drawn through a narrow drum-shaped enclosure 

 in the centre of which a high-tension arc is formed 

 between hollow copper tubes through which water cir- 

 culates. By means of a magnetic force, which is applied 

 on the outside of the arc, a discharge is caused to spread 

 out in the form of a large fan and fill the space traversed 

 by the air, which emerges with a large content of 

 oxides of nitrogen. In the installation seen in Fig. i, 

 forty-five of these furnaces are in operation, and each 

 furnace consumes electric energy to the amount of 

 4,000 horse-power. 



In another type, designed by Schonherr, the air is 

 passed, at a high velocity, up vertical towers, and the 

 high-tension arc is adjusted to extend in rod form 

 axially through the centre of each tube, for a distance 

 of 25 feet. Each of these units consumes 1,300 

 horse-power of electric energy. 



The oxides of nitrogen, in all processes, are con- 

 ducted to a separate building, where they are passed 

 up a series of towers 100 feet high, containing pieces 

 of quartz down which water percolates. Nitric acid 

 is thus formed, and later converted into nitrates as 

 required. 



The factories in Norway are situated at Rjukan, a 

 somewhat isolated district possessing abundant water 

 power which furnishes all the energy required. One 



:.— BIRKELAND EYDE FIRNACES AT S.\.\HEIM, SWEDEN. 

 (The Norwegian Hydro-Electric Nitrogen Company.) 

 (By kind pcrmisiion of Messrs. Longmans, Green & Co.) 



