44 



THE POPULAR SCIENCE MONTHLY 



Pig. 8. Notodden. Portion of the Saltpeter Works near Notodden. 



it to say that it is now accomplished industrially and it was my privi- 

 lege to see the working of the whole process at Odda. The air is first 

 liquified as in ordinary liquid-air machines, and then the constituent 

 gases separated by rectification, using much the same process as that 

 by which alcohol is separated from water. The difficulty of the process 

 depends upon the low temperatures necessary. Oxygen boils at — 182° 

 and nitrogen at — 194° Centrigrade. The air must of course be com- 

 pletely freed from moisture and also from carbon dioxid, for at the 

 temperatures used both are solids and would clog the pipes. All the 

 difficulties have been successfully overcome and from the stills the 

 nitrogen is boiled off in an almost pure condition. Delicate tests in the 

 laboratory show that on the average not over 0.2 per cent, of oxygen is 

 present. The commercial weakness of the process is the fact that there 

 is no use for the fairly pure oxygen which is left, which in many places 

 would be very valuable and probably pay the whole cost of operating. 

 The calcium carbid is ground and exposed for two days in an atmos- 

 phere of the pure nitrogen. While the absorption of nitrogen is an 

 exothermic reaction, it must be started and supported in the initial 

 stages by a supply of heat from some external source, and for this elec- 

 tric heating with carbon anodes is used. The resultant mass is a fairly 

 pure cyanamid, with uniform nitrogen content of 20 per cent. From 

 the cyanamid ammonia is easily obtained by the action of water, and 

 this being absorbed by sulfuric acid gives the ammonium sulfate so ex- 

 tensively used as a fertilizer. At present the sulfuric acid for this ab- 

 sorption must be imported, but an electric zinc smelter is in process of 



