OXYGEN AND NITROGEN 253 



" The liquid thus produced is led to a point near the top of the 

 rectifying column, filled with glass marbles, over which it trickles 

 to the bottom. On its way down it meets with an ascending 

 current of gas from liquid below. This gas is rich in oxygen, and 

 this oxygen having a higher temperature of liquefaction than 

 the liquid air it meets in the rectifier, is condensed by an equi- 

 valent proportion of nitrogen being distilled off from the descend- 

 ing liquid. The latter therefore enters the tank at the bottom 

 enriched in oxygen, whilst the gases passing off above are nearly 

 pure nitrogen. To remove the remaining traces of oxygen, the 

 rectifying column is extended at the top above the point at 

 which the liquid air is introduced. The gases ascending through 

 this extension meet a downward flow of pure liquid nitrogen 

 which robs them completely of oxygen, so that pure gaseous 

 nitrogen alone escapes from the top of the column." In order to 

 obtain the liquid nitrogen needed in the upper part of the 

 rectifying column the gas is led into the second cylinder of the 

 compressor and thence, through intei mediate coolers, to the 

 other cylinders, from the last of which it is delivered at a pressure 

 of 120 atmospheres. 



The compressed gas is next passed through a coil contained in 

 a tank of liquid oxygen derived from the separator, and after 

 expansion through a throttle- valve the supply of liquid nitrogen 

 at low pressure needed in the rectification is obtained. 



The oxygen separated from the air is evaporated in a special 

 receiver, and is used in cooling the compressed air as already 

 explained. It may then be compressed into cylinders or used in 

 any other way. 



As the moisture and carbonic acid contained in the air drawn 

 from the atmosphere cannot be completely removed at the 

 beginning of the cycle of operations the separator gradually gets 

 choked up with ice and solid carbon dioxide. The working 

 period lasts from six to ten days, and the second half of the 

 plant is then brought into operation a few hours before the first 

 half is stopped, so that when this occurs the second half is already 

 yielding nitrogen. 



Chemical processes for the isolation of nitrogen, or at least 

 those which can be used on a large scale are always based on the 

 withdrawal of the oxygen from atmospheric air. 



Copper may be used for this purpose, as at a red heat it rapidly 

 unites with oxygen forming solid copper oxides. The metal is 



