592 Sir James Dewar [Jan. 17. 



UTILISATION OF AIR XITROGEK. 



For a loiJg time oxygen alone was in inclnstrial demand, and there 

 vfas no nse for the huge quantities of nitrogen obtained simultaneously. 



Processes for the fixation of nitrogen and the production of 

 fertilisers and nitric acid were soon developed. The German dve 

 makers exploited electrical methods of fixation, and spent some three 

 millions sterling in acquiring water power in Sweden for the produc- 

 tion of the necessary cheap energy. 



Then came Haber's discovery that when a mixture of 3 volumes 

 hydrogen and 1 of nitrogen under 200 to 300 atmospheres pressure, 

 at a temperature rather below red heat, is passed over a suita])le 

 catalytic agent (osmium was used first, but was ultimately replaced 

 by finely divided iron), the issuing stream of gas contains a proportion 

 of ammonia, dependent on the temperature and pressure, which can 

 be absorbed out of the hydrogen and nitrogen mixture in circulation, 

 by water or by cooling. 



The oxidation of this ammonia yields nitric acid, a substance that 

 is indispensable for the manufacture of modern high explosives. The 

 importance attached by Germany to this process is indicated by the 

 fact that the Haber process was made a State monopoly. It is 

 impossible to say how many tons of liquid air were made in Germany 

 during the war, but we know that a single liquid air unit of plant 

 was producing nearly as much as the total daily production in this 

 country. 



SOME OF THE CHARACTERISTIC PROPERTIES OF 

 LIQUID OXYGEN. 



These were demonstrated as follows : — 



1. Absorption Spectrum : As would l)e expected from the greenish 

 blue tint of the liquid, definite absorption bands are present in the 

 spectrum. A clear cylindrical vacuum vessel containing liquid oxygen 

 was placed in front of a slit, through which an arc lamp cast a 

 parallel beam of light. When this was projected through a direct 

 vision prism on to the screen three well-defined dark bands were 

 seen in the orange and green in the neighbourhood of the CD. and 

 F. lines.''' 



2. The magnetic properties were shown by the action of a 

 magnetic field on the liquid as it fell from a special silvered vacuum 

 vessel t between the poles of an electro-magnet. By rotating a 

 sector wheel in the illuminating beam the drops remainCvi separate 

 while no current passed round the coils of the magnet, but the 

 excitation of the field at once caused the drops to coalesce into a 



* Proc. Roy. Soc, xlvi. 222. 

 t Proc. Roy. Inst., xx. 586. 



