( 746 ) 



which afterwards gave rise to doubt or proved incorrect, I had for some 

 time the conviction tliat I had seen solid helinm rapidly giving 

 off vapours of the pressure shown by the gas (once more than 15 

 atm. was observed). 



The continuation of my experiments has shown that they must be 

 explained in quite a different way. By a not sufficiently explained 

 cause the gas proved to be not so pure as was to be expected con- 

 sidering the method of purification. In analysing what was absorbed 

 by charcoal at the temperature of boiling hydrogen till the charcoal 

 did no more absorb hydrogen, (so that the gas could only contain 

 traces of hydrogen) it could be proved that in one case the gas has 

 contained only 0.45 and in another only 0.37 volume percents of 

 hydrogen at most ^). But this small admixture must have had a very 

 great influence. 



For at a repetition of the experiment with the helium subjected 

 to the new^ treatment no cloud at all was observed. The experiment 

 is not decisive as the velocity of expansion had been too small, but 

 it is difficult before further investigation to find in the difference of 

 velocity of expansion the cause that the helium in the tube remained 

 now perfectly clear. 



The explication of the previous observations is to be found in 

 solution phenomena of solid hydrogen in gaseous helium. The pheno- 

 mena which made the impression of being the giving off of vapour 

 liad been the solution of deposited solid hydrogen in the gaseous 

 helium, the latter rapidly returning from the lower temperature to 

 that of melting hydrogen, and the pressure increasing in consequence. 

 Helium at the temperatures, that come into account here can accord- 

 ing to the theory of mixtures take up at every temperature a per- 

 centage of hydrogen determined by that temperature in such a way 

 that it is not deposited at any pressure. On plausible suppositions 

 one can deduce that at temperatures above the melting point of 

 iiydrogen this percentage can be considerable and that at this melting 

 point itself it can be more than one percent. From mixtures with 

 sn)aller percentage the 'hydrogen is only deposited at lower tempe- 

 ratures e.g. by expansion. By the smallness of the quantity of hydrogen 

 present it is also explained that after prolonged blowing off of the 

 helium no solid hydrogen was left. For the quantity left was so 

 small that it could evaporate in the space which it found at its 



disposal. 



It remains remarkable that as small a quantity of admixture as 

 the gas contained has been able to give the total phenomenon of a 



ly About a small possible quantity of neon I could not yet be certain. 



