46 OUTLINES OF PHYSICAL CHEMISTEY
conducting material. 1 The conditions of the operation may be so combined that the increase in the cooling, due to the expansion of the gas, brings about a partial conden- sation. A liquid mass at a very low temperature collects in the receiver and can be preserved for a time (even under
Air, oxygen, nitrogen (and argon) have been liquefied in this way in considerable quantities. cubic centimetres of liquid hydrogen at a temperature which is only about thirty degrees from the absolute zero. Into this liquid he immersed a glass tube closed at the lower end, and almost immediately the cooled part of the tube be- came filled with solidified air. 4 A tube of helium was intro- duced into the liquid hydrogen and the helium liquefied.
The following table gives numerical data for some gases, relating to the critical state and the boiling point :
1 This receiver generally consists of a double-walled metallic vessel, with the space between the walls evacuated.
3 The following briefly indicates how Linde's apparatus for the liquefaction of air is used (German Patent, No. 88824, June 1'895, Class 12). The process is continuous. Air under the pressure #i and at temperature t x is compressed (p 2t t 2 ) and at the same time cooled by means of an ordinary refrigerator (jo 2 , t 3 ). This air is then forced into a very powerful refrigerator, whose temperature becomes lower and lower as the process goes on. This refrigerator consists of two worm-tubes cr and s, the one wound coaxially outside and close to the other. The compressed air passes through the inner worm s $ and its temperature is reduced to t A \ it then passes through a regulating valve to an expansion-receiver. The pressure has now been reduced to p x and the temperature is further reduced to t b . The air thus cooled now passes through the outer worm <r, and lowers the tem- perature of the whole refrigerator, and consequently also of the compressed air which enters the inner worm s. The temperature of this air entering the inner worm is thus progressively diminished, and there comes a time when the expansion (in the receiver) must be accompanied by a partial liquefaction.
8 Jow. Chem. Soc. 73, 528 (1898).
4 Dewar proposes to utilise this for the production of a very perfect vacuum.
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