ON THP] MEASUKE.MENT OF TEMPERATURE. 
177 
Of the five gases, helium, neon, argon, krypton, and xenon, the vapour pressures 
and critical constants of the last three only have been determined. They are as 
follows (Ramsay and Travers. ‘Phil. Trans.,’ A, 1901, vol. 197, p. 47). 
Atomic iveight. 
Boiling-point. 
Critical point. 
Ratio. 
Helium .... 
4 
Neon. 
20 
— 
_ 
— 
Argon . 
40 
87 abs. 
1.55 abs. 
0-561 
Krypton .... 
82'.3 
121 „ 
210 „ 
0-576 
Xenon . 
128 
164 „ 
288 „ 
0-568 
It will be observed that the ratios of the absolute boiling-points to the absolute 
critical ijoints has a nearly constant value, the mean of the actual ratios being 0'57. 
This relationship is common among similar substances, for which the boiling and 
critical temperatures may be considered as corresponding states. 
The boiling and critical points of neon are not known. We are certain, however, 
that the critical point lies below 60'^ abs., and, as the vapour pressure is 12’8 millims. 
at 20°’4 abs. tlie boiling-point must lie above 25° abs. If the ratio for the critical 
and boiling-points is the same as for the other gases, viz., 0‘57, we may calculate 
the temperature of the boiling and critical points which vail correspond. 
Boiling-point . . . 28°‘5 . . 31° . . 34°. 
Critical point . . . 50° . . 55° . . 00°. 
In the 25aper in the ‘ Philoso^fiiical Transactions’ already referred to, it was j^ointed 
out that argon, krypton, and xenon resemble one another, though kryjDton bears 
a stronger resemblance to neon and helium, lorming, in fact, a link between the two 
sub-series of this grou]) of elements. If now we plot (fig. 5) on a diagram the critical 
and boiling-points of the elements as ordinates and their atomic weights as abscissae, 
the i^oints representing argon, kryjjton, and xemjii lie on a straight line. Taking 
30 abs. as the boiling-^^oint and 53 abs. as the critical point of neon, and plotting 
these points on the diagram agaiiist the atomic weight of neon, we find that the 
lines joining these points to the corresponding points for krypton, when prolonged, 
pass through the origin of the axes. If the 23oints corresponding to helium lie also on 
these lines, the critical and boiling-jjoints of this gas will be 10° and 6° abs. Even if 
we assign to the critical point of neon its n2J23er limiting value 60° abs., and su23pose 
the points on tlie diagram representing kry23ton, neon, and helium to lie on a curve, 
the critical point of helium cannot be far above 12° abs. 
In our experiments the helium was compressed in the conqiression tube of an 
apparatus similar to that employed by Cailletet or Amagat, only the jiressure was 
transmitted to the surface of the mercury by means ot compressed air, instead of 
