Liquefaction of Hydrogen, 413 



performing internal work. In these experiments the gas 

 was compressed by a pump and escaped at constant tempera- 

 ture and pressure through a plug of silk fibre. Work was 

 done on the gas by the pump, and the heat generated 

 (pv + Q) was absorbed by passing it through a coil immersed 

 in water. On its way to the plug the gas flowed in a steady 

 stream, doing no work. In the plug, work was done against 

 friction and the heat generated was absorbed by the gas, so 

 that any temperature-change which occurred could be con- 

 sidered as consequent on the performance of internal work only. 

 If the gas were allowed to escape at a jet, instead of passing 

 through a plug, it is possible that, even in the case of a perfect 

 gas, a fall of temperature would occur close to the jet owing 

 to the conversion o£ molecular energy into kinetic energy : 

 the effect would, however, be entirely local, and would disappear 

 as the velocity of the stream was reduced in the formation of 

 eddies. The results of Joule and Lord Kelvin, and of others 

 •of a more recent date, show that in the case of the commoner 

 gases a fall of temperature takes place on free expansion. 

 With hydrogen, however, and probably with helium also, the 

 temperature rises ; these gases being, to use Regnault's 

 expression, "plus que parfait." 



It was first suggested by Hampson in England and Linde 

 in Germany, that the principle of free expansion might be 

 applied to the liquefaction of air. In the Hampson-Linde 

 process the compressed air flows through a coiled copper tube, 

 and expanding at a jet becomes cooled. The expanded gas 

 passes back over the outside of the coil, losing any velocity 

 it may have attained in forming eddies, so that any external 

 work done results in the formation of heat which is absorbed 

 directly by the gas. The effective cooling is the result of the 

 work done against internal stresses only; and since the tem- 

 perature of the expanded gas is lower than that of the coil, 

 the latter together with the compressed gas it contains becomes 

 cooled. 



Within the last two years Dewar has shown that at a 

 temperature close to —200° C. hydrogen also behaves as an 

 imperfect gas and becomes cooled when suffered to expand 

 freely. His experiments, which are described in the ' Chemical 

 News ' for March 1900, led him to apply this discovery to 

 the liquefaction of hydrogen in quantity, and this he has 

 . successfully accomplished. The details of the method em- 

 ployed have not however been published. 



Since there has been some confusion in dealing with the 

 subject, it may be well to point out once more here that there 

 as an essential difference between the processes employed by 



