1176 
normal conditions were exactly the same, as those used in the case 
of nitrogen. For these we may therefore refer to the paper on the 
diameter of nitrogen quoted above. 
As regards the cryostat, only a small part of the temperature 
range of the measurements might have been covered with a liquefied 
gas, namely with neon (boiling point — 245°.92C., triple point — 
248°.67)'). Uniform and constant temperatures over the whole range 
can only be obtained by means of superheated vapour, as used in 
the hydrogen vapour cryostat described on a former occasion *). But 
this apparatus, being opaque, could not be used without alteration, 
since the position of the meniscus in the dilatometer has to be read. 
A description of the modified arrangement by which we succeeded 
in obtaining a transparent bath of superheated hydrogen vapour 
which answered all requirements, will be given in the next com- 
munication *). 
The hydrogen was freed from all impurities by freezing in liquid 
hydrogen *) and is therefore to be looked upon as having been 
absolutely pure. 
§ 3. The experiments were also conducted in the same manner 
as in the case of argon and nitrogen. 
The quantity of liquid or vapour in the dilatometer was found 
by reading the position of the meniscus on the scales of the upper 
tube or the appendix at the bottom of the dilatometer ; the hydrogen 
was then blown off into the carefully exhausted volumenometer and 
the quantity collected was finally measured under normal conditions. 
Although a series of vapour pressures of hydrogen between the 
critical point and the boiling point was available’) for the purpose 
of the necessary corrections, it seemed advisable to make measure- 
ments of the pressure regularly during the experiments. For this 
1) H. KAMERLINGH ONNES and C. A. CROMMELIN, these Proceedings 18, p. 515, 
Leiden Comm. N°. 147d. In this paper on p. 518, line 8 from the bottom, 
(Leiden Comm. p. 52, 1. 10) the pressure 76.00 should be replaced by 75.95 
(one atmosphere at Leiden). The temperature of the boiling point — 2459.92 
is correct. 
2) H. KAMERLINGH ONNES, these Proceedings 19, p. 1049, Leiden Comm. 
N°. 15la. 
3) H. KAMERLINGH ONNES and C. A. CROMMELIN, these Proceedings 23, 
p. 1185, Leiden Comm. N°. 154c. 
4) H. KAMERLINGH ONNES, these Proceedings 11, p. 883, Leiden Comm. 
NO, 1095. 
5) P. G. Carn and H. KAMERLINGH ONNES, these Proceedings 20, pp. 991, 
1155, Leiden Comm. No. 152a and P. G. Carn, Dissertation, Leiden 1917, p. 103. 
