( 676 ) 
that the pitch remains constant in each section A to 5, B to C, or 
C to D. At A, B, C, and D platinum wires a, b, c, d, e, f,g, and 
h about 15 em. long and 0.5 mm. thick are soldered in pairs. At 
the other ends they are connected to copper wires. In order to pre- 
vent faults in insulation the spirally wound wires lay in shellae they 
were also covered with a layer of tissue paper for purposes of pro- 
tection. The portions A to B and C to D were enveloped in succes- 
sive layers of fishglue and writing paper to about a thickness of 0.25 em., 
in order that the distribution of temperature should be as even as 
possible along the rod. This protection was found to be proof against the 
action of either liquid nitrous oxide or oxygen. To allow of contraction 
on cooling the paper layers were only pasted together at both ends. 
2d. The cylindrical vacuum jacket. The bath for the liquid gases 
has the form of a tubular vacuum glass. Usually vacuum glasses 
are made so that there is but one edge connecting the cooled and 
uncooled walls. When it is necessary to remove liquid at the bottom of a 
vacuum glass the lower surfaces are connected by a spiral tube. However 
we required something quite different i.e. a double-walled tube open at 
both ends and capable of holding a rubber stopper in one. Ifsuch a vacuum 
tube were made by blowing simply together inner and outer walls 
it would certainly crack when cooled, owing to the different expan- 
sion of the outer and inner walls. Also it did not appear to be pos- 
sible to make the outer wall sufficiently elastic by blowing several 
spherical portions in it (see fig. 1). 
EE Awe eee ee Eee 
TI 
Fig. 1. 
Hence the outer wall was divided by a thin brass case V,, PI. I, 
which allows a compression or expansion of 2 mm. This copper 
box was inserted by platinising and coppering the two glass surfaces 
and then soldering them to the copper box. The vacuum tube thus 
produced was silvered and evacuated in the usual manner. In the 
first arrangement the top was left clear in order to allow of the observation 
of the surface of the liquid. In later arrangements we preferred a float, 
Such tubes with compound elastic walls appeared to be suitable for our 
purpose and will probably also be found to be useful for the solution of 
various other problems. An example of how easily tensions arise which 
cause such glass apparatus to crack, was found when the rubber 
stopper at the bottom was pushed in too far. On admitting the liquid 
oxygen the rubber became hard before it had reached the temperature of 
the liquid, which temperature the glass immediately above had reached 
