1920] Low Temperature Studies 245 



WEEKLY EVENING MEETING, 



Friday, January 16, 1920. 



Sir J. J. Thomson, O.M. M.A. LL.B. D.Sc. F.R.S. M.R.I., 



Professor of Natural Philosophy, R.I., in the Chair. 



Sir James Dewar, M.A. LL.D. D.Sc. F.R.S. M.R.L, 

 Fullerian Professor of Chemistry. 



Low Temperature Studies. 



In 1906" it was pointed out that a workable thermoscope could be 

 constructed by taking advantage of the fact that charcoal saturated 

 with air or other gases at a low temperature is very sensitive to 

 radiant energy, and that the small increase of temperature produced 

 in a 1 gramme bulb of charcoal by the approach of a candle flame 

 expels sufficient gas to move a suitable index. As the response was 

 obtained even though, in the forms of apparatus then employed 

 (Fig. 1), the radiation had to traverse several thicknesses of glass 

 before reaching the charcoal, it was inferred that a still more sensi- 

 tive thermoscope could be constructed if the receptacle containing 

 the charcoal were closed by a thin membrane of stretched india- 

 rubber, which is more transparent to heat than glass. Such a 

 membrane being practically impervious to most gases at low temper- 

 atures,! it appeared possible to make the receptacle air-tight. 



This arrangement was tried with the charcoal in a metal capsule 

 covered with a very thin clear membrane, as described in the diffu- 

 sion experiments of 19154 The tube from this cell led to a 

 small liquid manometer. The cell was immersed in liquid air with 

 the clear membrane directed vertically upwards, and the charcoal 

 was allowed to saturate with clean air up to atmospheric pressure. 

 Covering the membrane was a light metal shutter. When every- 

 thing had become equilibrated, the shutter was lifted, thus exposing 

 the cell to radiation from any source above and in line with the neck 

 of the silvered vacuum vessel. With a Leslie cube at room tempera- 

 ture as source, an immediate response was obtained, sufficient to 

 displace the index of the manometer to the limit of its scale in a few 

 minutes. On replacing the shutter the opposite effect was obtained, 

 as the charcoal re-absorbed the gas displaced by the dark radiation. 



In addition to rubber membranes, plates of rolled silver chloride 



* Proc. Roy. Inst., xviii. p. 445. 



t Proc. Boy. Inst., xvii. p. 424 ; xxi. pp. 558, 816. 



X Proc. Roy. Inst., xxi. p. 558. 



