1920] 



on Low Temperature Studies 



257 



necessary accuracy, which 1 minute readings would not secure, was 

 attained. The time taken to re-equilibrate after such a reading is 

 no longer than is necessary after an ordinary J to 1 minute's expo- 

 sure to unscreened radiation, as the volume of gas expelled is no 

 greater. 



It was not intended, with such a cell, to get absolute measure- 

 ments, because, as already seen, the charcoal is only partially isolated ; 

 hence only comparative measures were taken, everything being related 

 to the unscreened radiation from the black Leslie cube at ordinary 

 temperature, taken on each occasion for comparison. 



When exposed, therefore, to a uniform flux of radiation, the 

 absorbed gas will be liberated up to a steady limit defined by the 

 small maintained increment of temperature of the charcoal above 

 that of the bath. Directly the radiation is shut off, by lowering the 



01 s * 



Mai|omt b 

 12 



RADIATION OFF, SHUTTER REPLACED 

 (RE ABSORPTION) 



Fig. 



shutter over the cell, the charcoal again falls to the original tempe- 

 rature, and the expelled gas is re-absorbed. The general character 

 of the curve is shown in Fig. 8. 



An approximate idea of the possible sensibility may be gathered 

 from a knowledge of the latent heat of gases in cooled charcoal. 

 Thus a measurement made with 300 cc. of oxygen in 1 gramme of 

 charcoal gave for a lowering of temperature from 90 - 23°Abs. to 

 83 "2° Abs. a change of occlusion pressure from 1*0634 mm. to 

 0-2029 mm., i.e. a dp of 0*8605 mm. for a dT of 7*03% or 

 0*123 mm. Hg. per 1°. This on a sulphuric acid manometer would 

 be 0*905 mm. per degree. 



Assuming, then, that the latent heat of the oxygen in charcoal 

 would, at the saturation pressure of one atmosphere, be the same at 



Vol. XXIII. (No. 114) s 



