252 CHEMICAL DISCOVERY AND INVENTION 



on the chemical reaction, 1 nor leads to the burning of the elec- 

 trodes. The plant . . . was put down for a production of 375 

 cubic metres (about 13,000 cubic feet) 2 per hour, and is run by a 

 200 horse-power electric motor. 3 All parts are in duplicate in 

 order to prevent any long interruptions in the working, the 

 second half of the plant serving as a stand-by. The diagram 

 shows how the separation of oxygen and nitrogen from the air 

 takes place, and will better explain the process than would 

 sections through the various apparatus. The right half of the 

 diagram represents the part of the plant in actual working ; the 

 left half shows the part held as a stand-by. We shall deal only 

 with the former part. The air to be treated is drawn from the 

 atmosphere by the largest of the four cylinders of the compressor, 

 and through two towers, through which a soda liquor is made to 

 trickle, the object being to free the air as much as possible from 

 carbonic acid. The air is compressed to approximately 4 atmo- 

 spheres (57 Ib. per square inch) ; it is then cooled, first in a 

 water tower, down to the temperature of the cooling water, and 

 further by being passed in pipes through a reversing air cooler. 

 Around these pipes flow cold oxygen or nitrogen from other 

 portions of the apparatus. The water condensed is drained off 

 at the bottom. In the rising leg of the apparatus ice is apt to 

 form, as the temperature is below freezing-point. It is got rid of 

 by periodically reversing the direction of the flow by the valves 

 indicated. The air next passes to an ammonia cooler, where its 

 temperature is further reduced to about -20 deg. or -25 deg. 

 Cent. In this almost the whole of the remaining moisture is 

 abstracted. On leaving this, almost absolutely pure and dry, 

 the air passes next to the separator or still, which it enters 

 through a counter-current interchanger consisting of the usual 

 system of concentric pipes. Flowing itself through the inner of 

 these pipes it is cooled by an oppositely flowing current of 

 nitrogen or oxygen evaporating from the liquid state. The in- 

 coming air thus passes from the interchanger at a very low 

 temperature, and being led next into a coil immersed in a tank 

 of liquid oxygen liquefies there, since it is at a pressure of 4 

 atmospheres. From this coil it expands through a throttle valve 

 with result that a large portion of it is obtained in the liquid 

 state, and at about atmospheric pressure. 



1 This refers to the use of the nitrogen in the production of cyanaraide. 



2 Of nitrogen. 



3 The power is derived from falling water. See later " Cyanamide." 



