( 177 ) 



in liquid air. Here whatever traces of air might have been absorbed 

 during the circulation, remain behind. 



d. through a refrigerating tube B s lying in the liquid air, which 

 keeps the cover of the hydrogen space and of the helium space 

 cooled down. 



e. through a refrigerating tube B„ in which it is cooled by the 

 evaporated liquid hydrogen. 



ƒ. through the refrigerating tube B x lying in the liquid hydrogen 

 evaporating under a pressure of 6 cm., here the compressed helium 

 is cooled down to 15° K; 



g. and from here in the regenerator coil A, which has been 

 fourfold wound as in Hampson's apparatus for air, and in the 

 hydrogen liquefactor of Comm. 94/. 



Then it expands through the cock M x , if it should allow too much 

 gas to pass, this can escape through a safety tube. When the tem- 

 perature has descended so low that the liquid helium flows out, 

 the latter collects in the lower part of the vacuum glass Ea, which 

 is transparent up to the level of the cock, and is silvered above it. 



The outflowing gaseous helium can be made to circulate again 

 by the compressor of the circulation, or be pressed in the supply 

 cylinders 2?,. 



At some distance under the expansion cock M l} the german silver 

 reservoir Th x of a helium thermometer has been adjusted, it is 

 soldered to a steel capillary 7%,, which is connected with the mano- 

 meter reservoir Th i with stem Th 3 . If the mercury has been adjusted 

 in such a way that at 15° K its level is at the lower end of the 

 just mentioned stem, the stem has sufficient length to prevent the 

 mercury from overflowing into the capillary with further fall of 

 the temperature. 



The circulation is provided with numerous arrangements for dif- 

 ferent operations (for the compressor comp. Comm. n°. 54). Worth 

 mentioning is an auxiliary tube Z filled with exhausted charcoal, 

 which is cooled by liquid air when used After the whole apparatus 

 has been filled with pure gas, the gas is circulated through this side- 

 conduit (along 11 and 8) while the tube Cb of charcoal belonging to 

 the liquefactor, is shut off (by M and 9), to free it from the last traces 

 of air which might have remained in the compressor and the conduits. 



It now remains to describe in what way it has been arranged that 

 the liquid helium can be observed. Round the transparent bottom 

 part of the vacuum glass a protection of liquid hydrogen has been 

 applied. The second vacuum glass Eh , which serves this purpose, 

 forms a closed space together with the former E a , and the construction 



12 



Proceedings Royal Acad. Amsterdam. 'Vol. XI. 



