378 



NA TURE 



[August 20, 1896 



could not detect the slightest indication that liquefaction had 

 taken place. The first time that I compressed the gas I had, 

 indeed, noticed that a small quantity of a white substance 

 separated out and remained at the bottom of the tube when the 

 ]iressure was released. Possibly this may have been due to the 

 presence of a small trace of impurity in tlic helium, but it could 

 not have constituted more than i per ci.nl. of the total volume 

 of the gas. 



In the second series of experiments I employed liquid air, 

 boiling under a pressure of lo mm. of mercury. The helium 

 was first confined under a pressure of 140 atmospheres, and then 

 allowed to expand till the pressure fell to twenty atmo.spheres, or in 

 some cases to one atmosphere The results of these experiments 

 were also negative, the gas remained perfectly clear durinjr the 

 expansiim, and not the slightest trace of liquid could be 

 detected. The boiling point of liquid air was taken, from my 

 previous determination, to be - 220° C. (Cowttes rendtis^ 1885, 

 p. 238). This number cannot, however, be taken as a constant, 

 as the liquid air, boiling under reduced pressure, becomes 

 gr.idually poorer in nitrogen. Further, the quantity of nitrogen 

 lost by the liquid air on partial evaporation, varies not only with 

 the rate of lioilins,', Ijut even according to the manner in which 

 it has been liquefied. 



If air, under high pressure, be cooled first to the temperature 

 of boiling ethylene, and then to - l-,o° C, it liquefies, and, on 

 reducing the pressure slowly, liquid air is obtained boiling under 

 atmospheric pressure During the process a considerable 

 quantity of the liquid air evaporates, and the proportion of 

 nitrogen to oxygen in the remaining liquid is less than in air 

 liquefied under high pressure. If the liquid air, obtained by this 

 process, be made to boil under a pressure of 10 mm. of mercury, 

 the proportion of nitrogen in the mixture continues to decrease, 

 but, on account of the large quantity of oxygen present, the 

 liquid does not solidify, although its temperature is some 6° 

 below the freezing point of nitrogen. When, as in some of my 

 former experiments, the air was litjuefied under normal pressure, 

 by means of liquid oxygen boiling under a pressure of 10 mm. of 

 mercury, the ratio of nitrogen to the oxygen in the liquid air 

 was the same as in the gaseous air from which it had been pro- 

 duced. The liquid air, obtained by direct condensation at 

 normal pressure, appeared to lose oxygen and nitrogen with about 

 eqvial rapidity, and at the end of the experiment a considerable 

 quantity of liquid nitrogen remained behind in the apparatus 

 On reducing the jiressure to 10 mm. of mercury the nitrogen 

 solidified. Prof. Dewar has stated (Nature, February 6, 1S96, 

 p. 329) that liquid air solidifies as such, the solid product con- 

 taining a slightly smaller percentage of nitrogen than is present 

 in the atmosphere My experiment? have proved this statement 

 to be incorrect ; liquid oxygen does not solidify even when 

 boiling under a pressure of 2 mm. of mercury. 



After carrying the.se experiments to a successful conclusion. I 

 found that it was yet necessary to jirove that, on reducing the 

 vapour pres.sure of boiling oxygen to a minimum, no correspond- 

 ing fall of temperature lakes place. The vessel, f, was partially 

 filled with liquid oxygen, and, by means of a small syphon, a 

 small quantity of the liquid was allowed lo flow into the tube, a. 

 The inner vessel, ir, was then connected with the air-pump and 

 manometer, and the pressure was reduced to 2 mm. of mercury. 

 The oxygen remained liquid and quite clear. In a second 

 experiment the temperature of the liquid oxygen, boiling under 

 2 mm. of mercury pressuie, was measured by means of a 

 thermometer. The temperalure indicated lay above - 220° C, a 

 temperature easily arrived at by means of liquid air. I therefore 

 concluded that liquid air was a much more efficient cooling agent 

 than liquid oxygen, and that it would be quite unnecessary to 

 make further experiments on the liquefaction of helium. 



In every single in.stance I have obtained negative results, and, 

 as far as my experiments go, helium remains a permanent gas, 

 and apparently much more difficult to liquefy than even 

 hydrogen. The small quantity of the gas at my disposal, and, 

 indeed, the extreme rarity of the minerals from which it is 

 obtained, compelled me to carry out my investigation on a very 

 small scale. Using a larger apparatus, and working at a much 

 higher pre.ssure, I could have submitted the gas to greater 

 expan,sion. Further, I should have been able to measure the 

 temperature of the gas at the moment of expansion by means of 

 a platinum thermometer, as I did when working with hydrogen ; 

 but to male such experiments I .should have required 10, if not 

 100 litres of the gas. As I was unable to determine the tem- 



peratures lo which I cooled the gas, by any experimental means, 

 I have been obliged lo calculate them from Laplace's anii 

 Poisson's formula (or the change of temperature in a gas during 

 adiabatic expansion. 



T/T, = (////■ - ■/* 

 Where : — 



T,/ are the initial temperature and pressure of the gas. 

 T,,/, are the final temperature and prcs.'.ure of the gas. 

 k is the ratio (c//i?') which, for a monatomic gas, is i '66. 



In the first series of experiments the gas, under a pressure of 

 128 atmospheres, was cooled down to - 210' C. 



The results of these calculations tend to show that the boiling- 

 |ioint of helium lies below - 264° C, at least 20° lower than the 

 value I have found for the boiling-point of hydrogen. If the 

 boiling-point of a gas be taken as a simple function of its density, 

 helium, which, according to Prof. Kamsay's determination, has a 

 density 2-133, more Ihan double that of hydrogen, should liquefy 

 at a much higher temperature. Both argon and helium have much 

 lower boiling-points lhan might be expected, judging from their 

 densities. This anomalous condition may lie accounted for by 

 the fact that in each case the molecular .structure is monatomic, 

 as shown by the values obtained for the ratios of their .specific 

 heats. 



The permanent character of helium might be taken advantage 

 of in its application to ihe gas thermomeler. The helium 

 Iheimometer could be used to advantage in the determination of 

 Ihe critical temperature and boiling-point of hydrogen. To 

 deteimine whether the hydrogen Iheimometer is of any value at 

 temperatures below —198° C, I carried out a series of experi- 

 ments, in which I measured the temperalure of liquid oxygen 

 boiling under reduced pressure. I made use of the identical 

 Iheimometer lube emplojed by T. Estreicher {/V/;7. Mag. [5] 

 40, 54, 1S9S) as a hydrogen thermometer for the same pur- 

 pose, and applied ihe same correclions as were made in his 

 exjieriments. 



NO. 1399, VOL, 54] 



The results of these experiments prove that the coefficient of 

 expansion of hydrogen does not change between these limits of 

 temperature, and that the hydrogen thermometer is a perfectly 

 tru.stworthy instrument even when employed to measure the 

 very lowest temperatures. 



I have already pointed out (IVictl. Aim., Bd. xxxi. ?6g, 1887) 

 that the gas thermomeler can be used to measure temperatures 

 which lie even below the critical point of the gas with which the 

 instrument is filled. For instance, the critical temperature of 

 hydrogen, which I have found to be - 234-5" C. {IVicd. Ann., 

 56, 133 : Phil. Mag. [5] 40, 202, 189S) can be determined by 

 means of a hydrogen ihermometer. The helium thermometer 

 could be used at much lower temperatures, and would probably 

 give a more exact value for the boiling-poini of hydrogen than 

 it is possible to obtain by means of a platinum thermometer. 



