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this purpose lad afterwards been closed again by copper plates for 
safety, in order to avoid fluctuations of the temperature in consequence 
of the presence of these apertures in connection with the small 
heat-capacity of the gaseous hydrogen. On the latter account the 
vapour cryostat requires much greater precautions than an ordinary 
liquid cryostat and for this reason in our first trials all superfluous 
complications were avoided. The difficulty which was then feared 
(correctly, as we found later on) is met in the present model by 
providing the copper mantle with two diametrically opposite narrow 
glass windows and surrounding the mantle with a second one M 
of german silver (also shown separately in the figure). In this mantle 
two screw shaped slits have been cut. Thus a tield K is left clear 
for ilJumination from behind and observation from the front. 
The mantle M may be turned by means of a glass rod G and 
the cogwheel 7’ which works in a rack fitted on the outside of the 
mantle at the top: by this means AK may be moved vertically up 
or down. An experimental tube mounted axially in the space # 
may thus successively be observed from the bottom to the top. The 
resistance thermometers which do not need to be seen may be 
mounted excentrally. 
In this manner the aperture for visual observation which might 
produce fluctuations of the temperature by radiation is reduced toa 
minimum, and moreover the mantle can be closed by turning far 
enough as soon as an observation is finished. 
We have systematically investigated whether the opening did still 
cause any change of temperature and for this purpose have tried 
various sources of light, for instance a metal wire lamp with or 
without alum filter, diffuse daylight, ete. A lamp without a filter 
immediately produces a rise in the temperature of a few hundredths 
of a degree. When a heat filter (alum solution) was placed between 
the lamp and the cryostat, and the light was used with caution, no 
changes of the temperature of as much as .01° could be observed. 
In the silver coating of the vessels B and B, narrow slots have 
naturally also been left open, and similarly in the coating of vessel 
V, in order that the evaporation of the liquid hydrogen may be 
followed. V is for the greater part unsilvered. 
The construction of these vessels, which was particularly difficult 
in the case of the large outer ones, we thank to the exceptional 
ability of the chief glassblower Mr. O. KessrLrING. It seems as if 
with these largest vessels the limit of what can be done with cylin- 
drical vessels has almost been reached. They have to be treated 
with the utmost care and have to be very slowly cooled, at least 
