414 .Dr. M. W. Travers on the 



Olszewski and Dewar. There is no cooling of the gas in the 

 coil of the Dewar apparatus owing to the performance of 

 work ; this is done entirely by the pump, and the gas merely 

 flows along the tube in a steady stream and transmits the 

 pressure to the jet. The cooling must be attributed entirely 

 to the performance of internal work consequent on change of 

 volume only. In Olszewski's experiments the pressure in 

 the cylinder is not maintained, and work is done by the gas 

 which it contains. 



The temperature at which the Joule-Thomson effect for 

 hydrogen changes sign has yet to be determined ; it probably 

 lies very low. 



I shall now proceed to describe my own experiments. In 

 a preliminary experiment the gas, under a pressure of two 

 hundred atmospheres, was cooled to —80° C. by passing 

 through a coil immersed in a mixture of carbonic acid and 

 alcohol, and was then allowed to expand at the jet of a 

 Hampson air-liquefier, the coil of which had previously been 

 cooled to the temperature of liquid air. Under these con- 

 ditions it appeared that progressive cooling did not take 

 place, and it may be concluded that at —80° C. hydrogen is 

 still a perfect gas. 



Four attempts were made to liquefy hydrogen before an 

 apparatus was constructed which gave satisfactory results. 

 These experiments, which occupied about three months, I 

 shall not describe ; it suffices to state that they served to 

 show that hydrogen remains a perfect gas down to very low 

 temperatures. 



The details of the structure of the apparatus finally employed 

 in liquefying hydrogen are shown in Plate Y. ; text-fig. 1 

 indicates the general arrangement of compressor, &c. 



The hydrogen from the compressor under a pressure of 

 200 atmospheres enters the liquefier through the tube, and 

 passes through a coil A, which is cooled to —80° C. in a 

 mixture of solid carbonic acid and alcohol. It then enters 

 the coil contained in the chamber B, which is continually 

 replenished with liquid air during an experiment. The lower 

 portion of this coil passes into the chamber O, which is closed 

 and communicates through the pipe// with an exhaust-pump; 

 liquid air flows continuously from B into through a pin- 

 valve, controlled by a lever b, and boiling under a pressure 

 of 100 mm. of mercury lowers the temperature to —200° O. 

 The gas now passes into the regenerator-coil D which is 

 enclosed in the vacuum-vessel H, and, expanding at the valve 

 E, passes upwards through the interstices of the coil and the 

 annular space F, surrounding B and C, to the outlet G, 



