. Discovery. Science. 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-
. Discovery. Science. 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
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