(178 ) 
Until a cryostat is obtained suitable for such temperatures as here 
come into account, 2.75 atm. can be regarded only as a provisional 
value, and it is perhaps better to round it off to 3 atmospheres. 
In the meantime the value obtained in connection with more 
recent determinations which bring the boiling point still closer to 
4° K. confirms the estimate of the critical temperature as being at 
5° K. But as has already been mentioned, determinations concerning 
temperatures and densities will be reserved for a later communication. 
The determination of the critical pressure took place practically 
in the same manner as that of neon. It was done in a small 
pyknometer, a vessel of 0.5 c.c. capacity with a narrow graduated 
stem, that was placed along with the reservoir of the helium ther¬ 
mometer (see Comm. N°. 108) in that part of the liquefier destined 
for the reception of the liquid helium; it was attached to a steel 
capillary and came outside the apparatus just as the capillary of 
the helium thermometer. This capillary was then connected with a 
manometer tube, an admission valve, and a reservoir, just as with 
neon. (See $ 1). 
According as the level of the-liquid helium in the liquefier was 
more or less lowered *), this pyknometer was partially or wholly 
immersed in the liquid helium, or came wholly above it; helium 
from the reservoir connected' with the pyknometer could be condensed 
in it, so that the meniscus could be seen in it or in its graduated 
stem. For density determinations the quantity of helium admitted 
to the pyknometer was so regulated that the position of the meniscus 
upon the graduated stem could be accurately determined, while for 
determinations of the critical pressure the vaporisation was followed 
while the meniscus was lowering in the pyknometer. The critical 
pressure was read at the moment when the meniscus when standing 
half-way up the pyknometer reservoir disappeared, when the helium 
surrounding it was sufficiently vaporised. The meniscus could be 
brought back again for a short time by expansion. 
The estimate that temperatures below 2°.5 K. have already been 
reached depends also upon the critical pressure given. An attempt 
will be made to reach still lower temperatures by causing the helium 
to evaporate from still lower pressures; for this purpose the apparatus 
in which the helium is to be cooled by being allowed to evaporate 
under pressures still lower than 2 m.m., will be surrounded by a 
layer of liquid helium to protect it from conduction of heat, and to 
prevent the formation of unmanageable volumes of vapour. 
!) This was brought about by allowing insufficiently cooled helium to blow 
through the regenerator spiral into the helium liquefier. 
