( 665 ) 
is made air-tight by means of the rubber sleeve D a which is smeared 
with rubber solution and bound with copper wire. The lower end 
of the glass tube C is cemented to a second bronze ring, which is 
soldered to the brass tube B of the cover. To the centre of this 
brass tube is attached a ring B i carrying the bolts of the supporting 
rods B t which hold the vacuum glass in position. 
The Dewar tube it elf consists of a narrow lower portion A, com¬ 
pletely silvered and a wider upper portion that is silvered up to 
A s (the upper portion is left transparent so that we might be sure 
that we were not allowing too much liquid hydrogen to enter the 
glass). It fits into the brass tube B x and is protected by a 'wooden 
ring. The supporting rods B t keep the vacuum tube in position and 
at such a height that it is just clear of the wooden safety ring. 
Fig. 7 shows how, by means of the screw B t0 , the vacuum glass 
protected by a layer of paper is clamped to the thin brass ring B tf to 
which are attached the ends of the supporting rods B 4 . The lower 
portion of the vacuum tube has an external diameter of 8 mm. and 
an internal diameter of 5 mm. The glass walls are 0,5 mm. thick, 
which leaves only 0,5 mm. as the distance between the two silvered 
walls. 
The apparatus is centred by placing it on an auxiliary support by 
means of the ring B % . Before the vacuum tube it yet in position, the 
narrow portion f t of the adjusting tube is adjusted by a central ring 
in an adjustable centring-plate. The loose ring is then removed from 
. the plate and a second is fitted such that it just fits the narrow 
portion of the lower end of the vacuum tube. The nuts B sl serve 
to bring the vacuum glass to its proper position, and, as before, it 
is made air-tight by a rubber sleeve B s , which is smeared with 
rubber solution and bound with copper wire. By adopting this method 
of attaching the vacuum tube one need not fear alteration of the 
cover when the apparatus is evacuated, and only small further 
adjustments are necessary for recentring the apparatus after evacuation. 
In the tube B is soldered the steel capillary 0 3 (figs. 1 and 3) of a 
helium thermometer *} with german silver reservoir 6 X (figs. 3 and 7) and 
glass stem 0 4 , which is permanently attached to this portion of the 
cover. The quantity of helium is so chosen that at the boiling point 
of oxygen the mercury stands at a mark in the lower portion of the 
stem, and at the melting point of hydrogen at one in the upper 
portion. If, as is the case with hydrogen boiling under ordinary 
atmospheric pressure, the temperature is sufficiently well known 
~ _i ) Compare the apparatus for the liquefaction of helium. H. Kamerungh Onnes 
These Proe May/June 1908, Comm. Leid. N". 108. 
