ox THE MEASUREMENT OF TEMPERATURE. 
167 
Vapour Pressures of Liquid Hydrogen. 
June 12th, 1902. 
Pressure on gas 
in thermometer. 
Temperature of 
dead-space. 
Temperature 
of stem. 
Vapour pressures 
of 
liquid hydrogen. 
Temperature 
Found. 
Calculated. 
1 
niillim.s. 
^ C. 
°He. 
iTiillims. 
°He. 
°IIe. 
*69-79 
15-4 
28 
749-0 
20-36 
20-36 
155•45 
16-1 
26 
169-2 
16-17 
16-19 
t51 -70 
16-3 
25 
96-2 
15-08 
15-07 
t50•95 
16-4 
24 
83-0 
14-86 
14-82 
150-80 
16-6 
26 
80-2 
14-81 
14-76 
150-55 
16-6 
30 
76-3 
14-74 
14-70 
53 - 70 
16-4 
33 
129-8 
15-65 
15-63 
For the measurement of the vapour pressures of liquid oxygen ^vith this 
thermometer, see Part 11., 11 (IV.). 
8. Treatment of the Resvlts.—Their Prohahle Accuracij. 
As we shall presently find, the melting point of hydrogen is 14‘1 on the helium 
scale, and consequently the whole of the observations recorded in the foregoing tables 
refer to liquid hydrogen only. As in the case of liquid oxygen (Part II., p. 151), the 
experimental results were smoothed by the method of Pamsay and Youxg, and the 
vapour-pressure curves for liquid hydrogen on the scales of the hydrogen and helium 
thermometers were plotted on a diagram as in Plate 1. The last columns of the 
preceding tables contain the temperatures read off the curves at points corresponding 
to the observed pressures. 
It will be observed that at pressures near 760 millims. the polTits representing 
actual observation He either on, or very close to, the ciu ve. Of these points there 
are four on the curve ie})resenting the vapour pressures on the helium scale, obtained 
by means of three separate thermometers ; there are two similar points on the curve 
representing the vapour pressures on the hydrogen scale, the results of observations 
with two different thermometers. We can therefore consider that the boiling-point 
of liquid hydrogen, which is 20°'22 on the hydrogen scale and 20°‘41 on the helium 
scale, has been determined with a high degree of accuracy. 
* The mean of two nearly coincident observations. 
t In these observations the vapour pressures of hydrogen were measured on two manometers, connected 
with two bulbs containing pure liquid hydrogen, and immersed in the vacuum vessel surrounding the 
thermometer bulb to the depth of the top and the bottom of the latter respectively. The two sets of 
readings, of which the mean is given above, did not differ by more than 1 millim., indicating that the 
temperature of the thermometer bulb was uniform. 
