504 



NA TURE 



[July 25, 1901 



atmosphere that the pressure falls in expansion, air at 

 normal temperature is cooled about a quarter of a degree 

 Centigrade. But expanding from three-quarters of that 

 temperature, or - 56' C, it is cooled nearly twice as 

 much, or half a degree, for each atmosphere that the 

 pressure falls. And Thomson considered that, if allowed to 

 expand from an initial temperature of 100 C, it would 

 undergo no cooling at all. The differences of cooling for 

 different substances point in the same direction. Joule 

 and Thomson found that gases which are at a lower 

 point in the scale of corresponding states show more cool- 

 ing on free expansion than others. Thus oxygen, which 

 is not so far above its critical point as nitrogen, shows 

 more cooling ; and carbonic acid, which is actually below 

 its critical point, shows much more. It was a reasonable 

 conclusion then that hydrogen also, if it were made a 

 much less perfect gas by being cooled down to a tem- 

 perature not much above its critical point, would undergo 

 considerable cooling on free expansion. Early in 1896 

 Onnes calculated that if hydrogen were cooled to - 210 C. 

 before free expansion it would be in the same position in 

 the scale of corresponding states as oxygen expanding 

 from - 20" C. Now oxygen expanding from -20 is in a 

 very favourable condition for cooling on free expansion, 

 for it can be liquefied by that method from an initial 

 temperature of -fjO" C, and hydrogen can readily be 

 cooled below - 200' C. by air boiling at low pressure. 



In 1S9S Dewar had an apparatus constructed to work 

 on the principle above described, and succeeded in 

 collecting hydrogen as a stable liquid, thus obtain- 

 ing temperature which he subsequently estimated by 

 platinum-resistance thermometer at - 2384^ C.,or 34'6' A., 

 by constant- volume hydrogen thermometer at — 253'C , or 

 20' A. Later, he boiled liquid hydrogen at low pressure 

 and found it to be, like nitrogen and carbonic acid, one 

 of the substances which readily freeze themselves by 

 evaporation. In the solid hydrogen thus obtained he 

 reached the lowest temperature known, which he 

 estimated at from 13' to 15' .A. — temperatures confirmed 

 by his subsequent observations by helium thermometer. 



Liquid hydrogen has already been turned to useful account 

 in scientific work by Ramsay and Travers in their researches 

 on the rare inactive gases of the atmosphere. For the 

 purpose of obtaining pure neon by a process of fractional 

 distillation, it was necessary to have so low a temperature 

 that liquid hydrogen had to be employed as a cooling 

 agent. To make this, Travers designed an apparatus oa 

 the plan described above, to work in combination with 

 a Hampson air-liquefier which they had at their disposal. 

 The plan involved the preliminary cooling of hydrogen 

 by liquid air at low pressure and its further cooling by 

 free expansion with intensification by counter-current 

 interchange. The Dewar form of the apparatus appears, 

 from such descriptions of it as have been published, to be 

 on the same general plan. The Travers apparatus is 

 fully described, with a drawing to scale, in a paper by its 

 designer in \.\\t Philosopliidxl Magazine for April 1901. 

 For the present purpose a clearer idea of its working 

 will be obtained from a simplified diagram of it, such as 

 is here given. To avoid complexity, the insulation, the 

 joints and many other details have been omitted. 



The operation is as follows : hydrogen is compressed 

 in a pump, the plungers of which are lubricated with 

 water, to a pressure of about 200 atmospheres. The 

 lubrication water and any hydrogen dissolved in it or 

 blown off with it pass together from the water-separator 

 by the tube t for further separation at low pressure in 

 a chamber guarded by a water-seal, whence the gas 

 returns by U and O to the gas-holder. The high-pressure 

 gas from the compressor and a drying purifier passes by 

 the tube a through a coil in the vessel 1;, containing 

 solid carbonic acid in methylated spirit, by which the 

 hydrogen is cooled to - 79" C. Thence it passes through 

 the coil C in another vessel containing liquid air, and the 



NO. 1656, VOL. 64] 



temperature is thereby reduced to about — 185° C. In 

 the next vessel lower it is reduced, in the coil D, to a 

 temperature below - 200° C. by liquid air boiling at 

 reduced pressure. The liquid air for this purpose is 

 admitted, as required, by the valve E from the vessel 

 above ; and the low pressure is maintained in the vessel 

 II by an exhaust-pump connected with jt by the tube F 

 and the passage i^ s The compressed hydrogen at the 

 temperature of low-pressure liquid air then passes through 

 the coil R in the vessel p, forming the counter-current inter- 

 changer, and so reaches the expansion-valve M. It is in 

 this lowest vessel that the operation takes place which has 

 made the liquefaction of hydrogen possible. The vessel 

 and coil K have been previously reduced to the tempera- 

 ture of low-pressure air in the following way. The vessel 

 is connected with the exhaust-pump through the annular 

 passage l and the tube F by opening the tap (; and 

 closing H. The tap ij at the bottom of the vessel having 



waler-s?parator of compre: 



, e.\pansion-valv 

 haust-pump ; T, fron 



been opened, a supply of liquid air is drawn up into the 

 vessel by the suction of the exhaust-pump ; and the tap 

 being then closed, the exhaust pressure causes the liquid 

 air to boil at a reduced temperature, cooling the vessel 

 H, the coil K, and the compressed hydrogen within it to 

 about -205' C. The vessel is now cut off from the 

 exhaust-pump by reversing the taps v. and H, which 

 remain in the position shown, and the remaining liquid 

 air is drawn off again through Q, which is then closed. 

 The valve M is now opened by turning the spindle con- 

 trolled by the hand-wheel n, and the hydrogen at about 

 - 205= C, issuing into the chamber P, is cooled by free 

 expansion through, say, 10", to -215' C, and then 

 returns by the passage L and the pipe o to the gasholder. 

 But before doing so it begins the process of intensification 

 by passing over the coil K and giving up to this coil and 

 the high-pressure hydrogen within it the ten degrees of 



