128 THE NEW WORLD OF SCIENCE 



until the temperature becomes very high. At 1600 Centigrade 

 0.4 per cent, (by volume) of a mixture of equal parts of ni- 

 trogen and oxygen is converted into nitric oxide; at 1900 i.o 

 per cent. ; and at 2400, 2.2 per cent. It is clear, then, that we can 

 get a considerable production of nitric oxide only by operating 

 at a high temperature. But not only is it necessary to do this, 

 but the gases must be cooled so quickly that in the process of 

 cooling the reaction does not reverse itself, with decomposition 

 of the nitric oxide into oxygen and nitrogen. The only really 

 practical way in which these conditions can be realized is by 

 passing through air a powerful electric discharge. An electric 

 arc produces locally an extremely high temperature, and the 

 gas can be drawn rapidly away from the arc and quickly 

 cooled. 



The nitric oxide in the gases coming from the arc must now 

 be converted into nitric acid (HNO 3 ). This is done by caus- 

 ing the two chemical changes expressed by the following equa- 

 tions to take place successively : 



2NO + O 2 = 2 NO 2 (nitrogen peroxide). 

 3 N0 2 + H 2 O = 2 HN0 3 + NO. 



This first chemical reaction takes place of itself when the gas 

 cools to below 150 ; but time must be allowed for its comple- 

 tion, which is accomplished by passing the nitrous gases 

 through a large empty chamber. The second reaction is then 

 brought about by passing the cool gases through a series of 

 high granite towers, often sixty feet high and sixteen to 

 twenty feet in diameter, filled with quartz pebbles over which 

 water is trickling. As this second reaction reconverts one 

 third of the nitrogen into nitric oxide, and the first reaction 

 must again take place before the nitrous vapors can be ab- 

 sorbed by the water, the process is a slow one, and an elaborate 

 absorption system is required. From the towers flows a dilute 

 (30 per cent.) nitric acid, which can be concentrated by well- 

 known processes to the strength needed for the manufacture of 

 explosives. 



