( 745 ) 



at —253° C. and —259^ C, fi-om whicli [ luul calouliitcd nearly 

 5° K. for (lie critical temperature of helium ^). It thence followed 

 that it would be possible by rapid ex|)ansion of helium compressed 

 at 100 atm. at the meltingpoint of hydrogen to pass below the 

 critical temperature and to cause a mist to appear in the gas "). It 

 was to put this conclusion to the test, that 1 compressed nearly 7 

 liters of helium, purified by burning with copperoxyde and leading 

 over charcoal at the temperature of li(piid hydrogen (so that 1 could 

 trust to have a gas with only very small admixtures) in a thick 

 walled tube placed in a non silvered vacuum glass with liquid 

 hydrogen, and provided with a stopcock through which the helium 

 could be let otf from tiie tube into a gasholdei-, a gasbag or a 

 vacuum. The liquid hydrogen round the tube was exhausted at such 

 a pressure that hydrogen crystals Just appeared at the surface of the 

 liquid. The vacuumglass with hydrogen was surrounded by a second 

 non silvered vacuumglass with liquid air. In the thickwalled tube, 

 leaving only a small clearance, there was placed an extremely thin 

 walled beaker ') for protecting the gas which was cooled by expan- 

 sion against conduction of heat from the walls, the layer of gas, 

 between the beaker and the walls of the tube, though it was very 

 thin, being a bad conductor. 



At the expansion of the helium a dense gray cloud appeared from 

 which separated out solid masses floating in the gaseous helium, 

 resembling partly cotton wool, partly also denser masses, as if floating 

 in a syrupy liquid, adhering to the walls and sliding downward 

 while at the same time vanishing rapidly (20"). There was no trace 

 of melting. 



As far as I could Judge then from the experiments 1 considered it 

 prot)able that this solid substance was for the greater part helium. 



If helium passed immediately to the so'id state then the position 

 of the vapour line in respect to the adiabatics would be more favourable 

 for condensation than was to be expected according to the formula 

 of VAN DER Waals. The voluminous aspect of the solid mass was 

 in harmony with this. By the above and also bv other observations 



1) Olszewski from expansion experiments has deduced that tlie critical tempe- 

 rature of helium lies below 2^ K. Dewar estimates tlie boiling point according to 

 the absorption in charcoal at higher tlian 5'' K. (This would agree witli a critical 

 temperature of 8 K. Note added in the translation). 



2) Liquefaction by making use of the Joule Kelvin process would also be 

 possible. (Note added in the translation). 



3) This device has been used by Olszewski in his experiments on the expan- 

 sion of hydrogen (Note added in the translation). 



