406 CHEMICAL DISCOVERY AND INVENTION 



the terminals of the electrodes. While the length of thfc arc 

 increases its electric resistance also increases, so that the tension 

 is heightened until it becomes sufficient to create a new arc at 

 the points of the electrodes. The resistance of this short arc is 

 very small, and the tension of the electrodes sinks suddenly, 

 with the consequence that the outer long arc is extinguished. 

 It is assumed that while this is taking place the strength of the 

 current is regulated by an inductive resistance in series with the 

 flame. In an alternating current all the arcs with a positive 

 direction of current run one way, while all with a negative 

 direction run the opposite way, presupposing the magnetising 

 being effected by direct currents. In this way a complete 

 luminous circular disc is presented to the eye. When the flame is 

 burning it emits a loud noise, from which alone an impression 

 may be obtained of the number of arcs per second formed in the 

 flames which, however, may be more minutely investigated by 

 means of an oscillograph." 



The interior of the furnaces is lined with fire-clay brick, 

 through the walls of which the air is admitted to the flame. 

 The air is brought into each furnace by aid of centrifugal fans 

 which drive it through tubes from the basement. The nitrous 

 gases formed escape through a channel made along the casing of 

 the furnace. This air at a temperature of about 1000 C. is then 

 conducted to the steam boilers where its heat is utilised, and 

 afterwards through a number of aluminium pipes cooled ex- 

 ternally by water. The gases then pass into vertical iron cylinders 

 lined with acid-proof stone, where the oxidation of the nitric 

 oxide to nitric peroxide by the oxygen present is completed. 

 The mixed gases then are driven into the absorption towers. 



These towers are tall stone structures, of which the height, 

 approximately 65 feet, 1 may be judged by the figures of two 

 men visible in the adjoining illustration. They are filled with 

 broken quartz over which water is continually trickling. The 

 gases enter at the base of the first tower, and by means of a 

 large earthenware pipe pass to the top of the second, through 

 which they pass downwards to the bottom of the third tower. 

 They then pass through two wooden towers in which they are 

 washed by a solution of carbonate of soda. The liquid collected 

 in the first series is pumped backward, the product of the third 

 into the second and from the second into the first, and thus the 



1 Inside measure. 6 x 20 metres. 



