April 23, 190S] 



NA TURE 



581 



LETTERS TO THE EDITOR. 



fl lie Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part 0/ Nature. 

 Ao notice is taken of anonymous communications.] 



The Condensation of Helium. 



In addition to my short notu printed in la^t week's 

 Nature (p. 559), let me begin by remarking that as 

 recently as last year, in an address to the Dutch Congress 

 of Natural Science and Medicine, I expressed the opinion 

 that it would be scarcely possible to liquefy helium. 

 Olszewski, from his expansion experiments, had deduced 

 that the critical temperature of helium was lower than 

 2° K. _ Dewar had no more succeeded in liquefying it by 

 expansion, and some experiences of my own on helium 

 gas sinking in liquid hydrogen seemed to indicate that 

 helium was nearly a perfect gas. At the same meeting I 

 indicated the determination of the isothermals of helium, 

 an investigation with which I was occupied, and which I 

 had prepared by a series of researches, as the direct way 

 to the calculation of the critical temperature. 



The first results I obtained with the isothermals changed 

 totally my views on the liquefaction of helium. From 

 the isothermals down to —217°, it followed that the critical 

 point of helium is at nearly 5° K., more in harmony with 

 the estimate of the boiling point at 5° or 6° K. by Dewar, 

 according to the helium absorbed in charcoal, and the 

 determinations at —252° C. and —259° C. confirmed the 

 result. It thence followed that it would be possible, by 

 rapid expansion of helium compressed at 100 atmospheres 

 at the melting point of hydrogen, to pass below the critical 

 temperature, and to cause a mist to appear in the gas. 

 .'\lso liquefaction by the Joule-Kelvin effect seemed possible, 

 h was to put the first conclusion to the test that I made 

 my recent experiments. 



The new features of my application of the expansion 

 method to helium were: — (i) the great quantity of gas; 

 (2) the application of a stop-cock on the tube to let off 

 the gas from the tube into a gas-holder, a gas-bag, or a 

 vacuum ; (3) an extremely thin-walled beaker, placed in 

 the thick-walled tube to protect the cooled gas against 

 heat conduction. These devices had been used by 

 Olszewski in his experiments on the expansion of hydrogen. 

 At the expansion a dense cloud appeared, from which 

 solid masses separated out, 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 seconds). There was no trace of melting. 



So far as I could judge, then, from my experiments, I 

 considered it probable that this .solid substance was helium. 

 The helium had been burned with copper oxide and passed 

 over charcoal at the temperature of boiling hydrogen, and 

 I trusted to have a gas with only very small admixtures. 

 If helium passed immediately to the solid state, then the 

 oosition of the vapour-line to the adiabatics would be more 

 favourable for condensation than if it passed into the 

 liquid state, and the voluminous aspect of the solid mass 

 was in harmony with this. By the above, and also by 

 other observations, w'hich afterwards gave rise to doubt 

 or proved incorrect, I was for some time under the 

 impression that I had seen solid helium rapidly giving off 

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

 15 atmospheres was shown). The continuation of my ex- 

 periments has shown that they must be explained in quite 

 a different way. By a not sufficiently explained cause, the 

 iras proved to be not so pure as was supposed, considering 

 the method of purification. In analysing what was 

 absorbed by charcoal at the temperature of boiling 

 hydrogen until the charcoal removed no more hydrogen, 

 ■;o that the gas could only contain traces of hvdrogcn, it 

 could be proved that in one case the gas had contained 

 onlv 0-45 and in another only 0'37 volume per cent, of 

 hydrogen at most. (.About a small possible quantity of 

 neon I could not vet be certain.) But this small admixture 

 must have had a very great influence ; for at a first 

 repetition of the experiment with the helium subjerted to 

 this new treatment no cloud at all was observed. In this 



XO. 200S, VOL. 77] 



exporimLin the velocity of expansion had been too small. 

 At a second repetition with the same gas, but with greater 

 velocity of expansion, a thin cloud appeared and vanished 

 extremely rapidly (i second}. The mist now had a different 

 aspect. 



The e.'iplanation of tlie previous observation is to be 

 found in solution phenomena of solid hydrogen in gaseous 

 helium. The phenomena which made the impression of 

 being the giving off of vapour had 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 con- 

 sequence. Helium at the temperatures that come into 

 account here can, accordirfg to the theory of mixtures, talie 

 up at every temperature a percentage of hydrogen, deter- 

 mined by that temperature in such a way that it is not 

 deposited at any pressure. With acceptable suppositions 

 one can deduce that at temperatures above the melting 

 point of hydrogen this percentage can be considerable, and 

 that at this melting point itself it can be more than i per 

 cent. From mixtures with smaller percentage, the 

 hydrogen is only deposited at lower temperatures, 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, fur the 

 quantity was so small that it could evaporate in the space 

 which it found at its disposal. It is remarkable that sn 

 small a quantity of admixture as the gas contained ha> 

 been able to give the total phenomenon of a substance 

 condensing to a solid and re-evaporating, though the rapid 

 evaporation is in harmony with the smallness of this 

 quantity of substance, considering that even denser masses 

 were seen to be blown away sometimes. There cannot 

 have been much more than i milligram or 15 cubic milli- 

 metres of solid hydrogen in round numbers in the tube — 

 probably there was less in it — and yet the tube of nearlv 

 7 cubic centimetres was over its w-hole length for almost 

 a quarter filled with dense, flaky substance. 



So far as the experiments on the expansion of helium 

 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 possi- 

 bility of realising with mixtures of hydrogen and helium 

 the rising or falling of the solid substance according to 

 the pressure exerted on the gas, the barotropic phenomenon 

 for a solid and a gas. But the question of condensing 

 helium is to be considered yet as an open one. 



Let me add a few words as to the mist observed in the 

 repetition of the expansion experiment with the " coal- 

 pure " gas. It is certain that this gas only contains very 

 small quantities of hydrogen. The spectroscopic test also 

 gives traces only. It is possible that the amount of the 

 traces will prove sufficient to attribute the mist to the 

 traces of hydrogen left in the gas. 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 what 

 I calculated from the isotherms, and helium would obey 

 tolerably well the laws of van der Waals. The tube broke, 

 and so I could 'not attain more certainty about the nature 

 of the cloud. 



The preceding experiments show very strikingly how 

 careful one has to be in arriving at conclusions from the 

 appearance or non-appearance of a cloud by expansion. .\ 

 decision about the critical point of helium is therefore only 

 to be obtained bv a prolonged systematic investigation, 

 which will take much time. 



.April 14. H. Kamerlincu Oxnes. 



Satellites of Yellow and Green Lines of Mercury. 



Being engaged with the investigation of the Zeen^an 

 effect bv using a 35-pIate Echelon spectroscope constructed 

 by Hilger. I made an experimental test of the resolving 

 power of the instrument on the yellow and green lines of 

 mercury. With a lamp of the .\ron type (30 volts. 

 6 amperes), and by eye observation with a micrometer. I 

 found the following satellites, some of which seem to bo 

 new. 5\ is given in Angstrom units. The measurements 

 by Janicki with an Echelon spectroscope, and by Baeyer 



