1920] 



on Low Temperature Studies 



253 



into the vacuum vessel above the cell. An annular space filled with 

 charcoal formed part of the construction of the quartz vessel, to 

 ensure the maintenance of highest vacuum isolation at low tempera- 

 ture. Since the metal connecting- tube of the cell passes through 

 the liquid in the outer vessel, it is necessary that the liquid air used 

 there should be of the same temperature and composition as that in 

 the inner vessel. The safest procedure is to have liquid oxygen 

 in both. 



The simpler form shown in B was commonly employed, and was 



Fig. 5. — Forms of Cell. 



the most simple to use for transmission measurements. Both inner 

 and outer vessels were of the silvered cylindrical type. The isolation 

 of the inner by the cooling of the outer was not so effective as in A, 

 but was reinforced by the use of a set of thin polished metal discs 

 with an axial support of an ebonite or thin german silver tube 

 (closed at the ends). This device very effectively reduced the radia- 

 tion and convection losses consequent upon the open unprotected 

 neck. In addition a light sheet metal hood and cover plate were 

 fixed to the neck to protect it from disturbance when opened. 

 Attached by thread to the disc stopper was some uranium nitrate, 



