Physics. — “Methods and apparatus used in the cryogenic laboratory. 
XVIII. Improved form of a hydrogen vapour cryostat for 
temperatures between — 217° C. and — 258° C.”. By H. 
KAMERLINGH ONNES and C. A. CROMMELIN. Communication 
N°. 154c from the Physical Laboratory at Leiden. (Commu- 
nicated by Prof. H. KAMERLINGH Onnus). 
(Communicated at the meeting of January 29, 1921). 
$ 1. Introduction. In a previous communication one of us has 
described a hydrogen vapour cryostat for temperatures between the 
melting point of oxygen and the boiling point of hydrogen *). During 
the preliminary determinations of the critical points of neon’) and 
of hydrogen’), made in this cryostai, it was found that in principle 
the apparatus properly performed its function, but at the same time 
we discovered certain faults and deficiencies. When therefore the 
modification, which had been foreseen all the time and by which 
it would become possible to take readings on a tube inside the 
cryostat, became necessary, we resolved with the assistance of Mr. 
Frrm, chief mechanic of the cryogenic laboratory, to revise the con- 
struction in all details and rather than execute the modifications as 
originally planned to build a new apparatus of somewhat larger 
dimensions. This new cryostat which in the various measurements 
has given almost complete satisfaction and may therefore be con- 
sidered to have approached its final form, will be described in 
this paper. 
The apparatus is shown in fig. 1, partly in section, partly in 
direct view, as appears easily from the different details. Moreover 
three sections are given at different heights, a view of the turnable 
drum and diagramatically the electric connections, as will be further 
explained later on. 
1) H. KAMERLINGH ONNEs, these Proceedings..19, p. 1049, Leiden Comm. 
N°. 151a. ; 
2) H. KAMERLINGH ONNES, C. A. CROMMELIN and P. G. Carn, these Proceedings 
19, p. 1058, Leiden Comm. N°. 1515. 
3) H. KAMERLINGH ONNES, C. A. CROMMELIN and P. G. Carn, these Proceedings 
20, p. 178, Leiden Comm. N°. 15lc. 
