198 



HEAT 



He greatly aided the study of the liquids ana tneir use as cooling agents 

 by the introduction of a special containing vessel, to catch the condensed 

 gas issuing from the expansion nozzle and to keep it for a long time 

 liquid at the atmospheric pressure. The vessel is a double-walled glass 

 globe with a vacuum between the walls, as in 

 Fig. 116, the surfaces in the evacuated space 

 being silvered or, later, covered with a film of 

 mercury, secured merely by condensation of 

 mercury vapour rising from a few drops of 

 mercury left in the space. The evaporation from 

 liquid air or oxygen in such a globe is only a 

 small fraction of what it is in a single-walled 

 globe. 



About 1894-95 a new step was taken in the 

 introduction of the "regenerative" method. There 

 has been much discussion as to priority of dis- 

 covery, but it is enough here to say that it was 

 used very nearly at the same time by Dewar, 

 Hampson, Kammerlingh-Onnes, Linde, and Tripler. 



FIG 11 0. Dewar's ^ e P r i n ciple of the method may be understood 



Vacuum Vessel (Sec- from Fig. 117, which is merely a diagrammatic 



tion). sketch. The gas is forced by a pump P at very high 



pressure through a cooler C, which removes the 



heat developed by the compression, and the gas flows on at the ordinary 

 temperature to the regenerator R, where it passes down a spiral pipe to 

 a nozzle N. From this it issues with a sudden fall of pressure from 

 that of the pump to that of the atmosphere. There is considerable 

 cooling of the issuing gas, which in turn cools the spiral pipe by which 

 succeeding portions are 

 coming to the nozzle. 

 Thus the next issuing 

 gas falls still lower in 

 temperature, cools the 

 spiral still more, and 

 so on, till ultimately 

 the gas issues as liquid 

 and falls into the col- 

 lecting vessel V. No 

 cooler below the ordi- 

 nary temperature is 

 required, the cooler 

 merely reducing the 

 gas to the temperature 

 of the room. 



It is to be noted 

 that the expansion at 

 the nozzle is not adiabatic, i.e. not one in which the gas has to use up 

 its own energy in pushing out its surroundings. The pump behind is 

 continually doing work and supplying very nearly all the energy which 

 the gas requires to push its way onwards, when it conies out of the 

 nozzle. The conditions are much more nearly those of the porous 



Fia. 117. Diagram of Regenerator Process for 

 Liquefying Gases. 



