( 747 ) 



substance coiideiising to a solid and i-cevaporatiiig, though the rapid 

 evaporation, in which even denser masses were seen to be blown 

 awav sometimes, is in harmony with the smallness of this quantity 

 of substance. There cannot have been much more than 1 mgr. or 15 

 cubic millimetres of hydrogen in round numbers in the tube — 

 probably there was less in it — and yet the tube of nearly 7 cubic 

 centimetres was over its whole length for almost a quarter fdled 

 with a dense flaky substance. 



As far as the experiments on the expansion of helium at the melting 

 point of hydrogen are now advanced they show the curious forms 

 that the solution phenomena of a solid in a gas take in the case of 

 helium and hydrogen. They further point to the possibility of reali- 

 sing with mixtures of hydrogen and helium the rising or falling of 

 the solid substance according to the pressure exerted on the gas, the 

 barotropic jihenomenon for a solid and a gas. But the question of 

 condensing helium is to be considered yet as an open one, which will 

 ask an extensive investigation. 



POSTSCRIPTUM. 



1 have had the occasion to repeat the experiment with the gas 

 that remained perfectly clear in the last expansion experiment ; and 

 which also according to the spectroscopic test contained only traces 

 of hydrogen. I now used a greater velocity of expansion. A thin cloud 

 appeared and vanished extremely rapidly (in 1" nearly). The mist 

 now had another aspect. 



It is possible that the traces of hydrogen left in the gas will 

 prove sufficient to cause this mist. But it is also possible that the 

 mist has been a liquid cloud and the changed aspect seemed to 

 point to this. If this might prove to be the case then the critical 

 point would be nearly as I calculated it from the isothermals and 

 helium would follow tolerably well the laws of van der Waals. 

 The tube broke and I could not attain more cei'tainty about the 

 nature of the cloud. 



The preceding experiments show very strikingly how careful one 

 has to be in making conclusions from the appearing or not appearing 

 of a cloud by expansion. A decision about the critical temperature 

 of helium is therefore only to be obtained by a prolonged systematical 

 investigation which will take much time. 



(April, 24, 1908). 



