Where:
T, p are the initial temperature and pressure of the gas.
T1, p1 are the final temperature and pressure of the gas.
k is the ratio (cp/cv) which, for a monatomic gas, is 1.66.
In the first series of experiments the gas, under a pressure of 128
atmospheres, was cooled down to -210° C.
p T p1 T1
At. Deg. At. Deg. Deg.
125 -210 C. 50 -229.3 C. 43.7 A.
... ... 20 -242.7 C. 30.3 A.
... ... 10 -250.1 C. 22.9 A.
... ... 5 -255.6 C. 17.4 A.
... ... 1 -263.9 C. 9.1 A.
The results of these calculations tend to show that the boiling point
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. Ramsay's determination, has a density of 2.133,
more than double that of hydrogen, should liquefy at a much higher
temperature. Both argon and helium have much lower boiling points than
might be expected, judging from their densities. This anomalous
condition may be 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 the gas thermometer. The helium thermometer could be
used to advantage in the determination of the critical temperature and
boiling point of hydrogen. To determine whether the hydrogen
thermometer is of any value at temperatures below -198° C. I carried
out a series of experiments, in which I measured the temperature of
liquid oxygen boiling under reduced pressure. I made use of the
identical thermometer tube employed by T. Estreicher (Phil. Mag. [5]
40, 54, 1898) as a hydrogen thermometer for the same purpose, and
applied the same corrections as were made in his experiments.
Temperature.
Pressure Helium Thermometer. Hydrogen Thermometer.
Mm. Deg. Deg.
741 -182.6 C. -182.6 C.
240 -191.8 C. -191.85 C.
90.4 -198.7 C. -198.75 C.
12 -209.3 C. -209.2 C.
9 -210.57 C. -210.6 C.
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
trustworthy instrument even when employed to measure the very lowest
temperatures.
I have already pointed out (Wied. Ann., Bd. xxxi, 869, 1887) that the
gas thermometer 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. (Wied. Ann., 56, 133; Phil. Mag. [5] 40,
202, 1898) can be determined by means of a hydrogen thermometer. The
helium thermometer could be used at much lower temperatures, and would
probably give a more exact value for the boiling point of hydrogen
than it is possible to obtain by means of a platinum thermometer.
* * * * *