1050 
up in a satisfactory manner *). In its present construction the 
hydrogen-vapour cryostat is not yet suitable for experiments in which 
the phenomena in the experimental space have to be followed by 
the eye, as this space is completely surrounded by copper walls. But 
we hope to remove this objection by a modification of the apparatus. 
Since the hydrogen-vapour cryostat has proved to fulfil its 
object, a  helium-vapour cryostat will be built on the same 
principles, in order also to bridge the other gap which still remains 
in the series of low temperatures for which appliances are available 
which guarantee the constancy and uniformity of the temperature 
necessary for experimental work, viz. the very important ®) interval 
from 14° K. to 4°,25 K. (freezing point of hydrogen to boiling point 
-of helium). 
‘ 
§ 2. Description of the apparatus. The cryostat (see fig. 1) *) 
consists of the evaporator V and the eryostat-glass B, which latter 
contains the experimental chamber Z. The air-tight german-silver 
caps Vn and By, by which the two parts are closed immovably, 
are connected together by means of strong tinned iron strips g,, 9,, 9; 
(see fig. 2) and clamping rings g, and 7,.*) 
A continuous current of superheated hydrogen-vapour is needed 
to keep the walls of the experimental chamber as well as the gas 
and measuring apparatus inside at a constant and uniform tempera- 
ture. This current is supplied by the evaporator. 
The unsilvered lower part of the vacuumglass of this evaporator 
V, contains liquid hydrogen. The hydrogen is transferred to the 
hydrogen-vapour cryostat in view of the difficulty of providing large quantities of 
the gas. 
(In Proc. XVIII, 1, p. 508 1. 1 from below, insert after “most experi- 
ments”: “at temperatures between 25° K and 27° K”). Perhaps we shall find 
that it will be possible with the hydrogen-vapour cryostat to descend almost to 
the boiling point of hydrogen and thus to embrace the region where otherwise 
— as mentioned |. c. — it might be possible to use a baih of liquid hydrogen 
boiling under enhanced pressure. 
1) The proper working of the vapour cryostat is much impeded when experi- 
ments are carried out with it, in which heat-actions take place inside the experi- 
mental chamber. 
*) We need only point out the desirability of this interval being filled up for accurate _ 
determinations of the critical data of helium and for deciding whether lead becomes 
supra-conducting continuously or, in the same way as tin and mercury, suddenly. 
8) The section of this figure is taken along the line shown in fig. 2. 
*) This firm connection is necessary because of two glass syphon-tubes con- 
necting the two parts. With a view’to the expansion by change of temperature 
of the tubes and the strips, the latter were made of iron. 
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