T^R. ^r. W. TRAVERS, MR. (i. RENTER, AND DR. A. JAQUEROD 
170 
Tn the second exjieriment, the neouAvas Introdnced into the 1jnll)7nof the apparatus 
employed in determining the v’apoiir pressure of licpiid hydrogen (fig. 1). At a 
temperature of 15°’05 (helium scale), tlie vapour pressure of the neon was 2‘4 millims. 
of mercury. 
Appenpix V.— On the Frohahle Values of the Critical and Boiling-Points of 
IJeliurn, and. on an Attempt to Liquefij that Gas. 
As tlie foregoing results show, lielium hehaves at low temperature as a more perfect 
gas tlian iiydrogen, and even if no further data were forthcoming, one would exjoect 
to find that its critical and l)oiling-points were lower than those of the latter gas. 
This, as we shall presently show, receives further confirmation from theoretical 
considerations and from experimental results. 
In 1892, Olszewski {loc. cit., }). 174) showed that when helium, under a pressure 
of 80 atmos])heres, was cooled to — 210° 0., and suddenly allowed to exjiand till the 
pressure fell t(^ that of the atmosphere, no mist was seen in the compression tube. 
More recently (Mloy. Soc. Proc,,’ 1901, vol. G8, p. 3G0) Dewar performed a similar 
experiment, cooling the compression tube in liydrogen reduced to its freezing-point. 
He did not, however, succeed in liquefying the gas, which he believed to have been 
cooled to 9° or 10° absolute, the temperature which he assigns to the upper limiting 
value of its critical point. 
I'hough by compressing any gas, and subsecpiently allowing it to expand adiabatically, 
tlie relationship l)etween the initial and final pressures p, p' and the initial and final 
temj)eratures T, T' can he found hy tlie equation 
wlien h is the ratio of the sjiecific heats for tin* gas, this eipiation may not be 
applicable to the case in w hich a gas is compressed into a ca}nllary tube and then 
allow*ed to exjmnd, the thermal capacity of the gas being then very small compared 
wo'th that of the walls of the containino; tube. 
O 
It occurred to us that if we could cool a few centimetres of a compression-tube 
containing helium down to a temperature below the critical point of the gas, then, on 
compressing the gas, its volume would decrease and the pressure would rise till the 
'\aapour pressure of helium corresponding to that particular temperature Avas reached. 
It shmdd then be possible, siqiposing the gas to be homogeneous, to decrease the 
volume of the helium without increasing the pressure, as a change in the volume of 
tlie gas Mvndd iuiply licpielaction in the cooled jiortion of the t\d)e. Before describing 
our experiments aao* Avill first deal Avith the probalile A^alue of the critical and boiling- 
points of helium. 
