1020] on Low Temperature Studies 250 



all this is utilised by the charcoal as latent heat of evaporation of 

 the absorbed air (approx. 8 cc. evaporated per calorie) the volume of 

 gas evolved would be 0*256 cc. 



In the same way the theoretical amount from the same black 

 cube at 100° C. would be 38 calories per minute, entering the neck, 

 of which the cell could absorb 0*001 calorie, equivalent to a gas 

 evolution of * 784 cc. In both cases the absorption in the 

 membrane over the charcoal reduces this to approximately 80 per 

 cent. The maximum gas evolution would therefore be of the order 

 of 0*63 cc. per minute with the black cube at 100° C, and 0*22 cc. 

 with the cube at 15° C. The observed values in one case were * 34 cc. 

 at 12° C. and # 956 cc. at 100° C. There was, however, some 

 additional radiation from the upper region of the walls of the vacuum 

 vessel which the cooling liquid did not reach. 



AVhen, instead of a black lined vacuum vessel, a polished 

 lining was used, practically all the radiation should reach the cell 

 — namely, 2*68 calories per minute from a cube at 15° C, and 

 7*58 calories from a cube at 100° C, equivalent respectively to 

 21*44 cc. and 60*65 cc. of evaporated oxygen per minute. This 

 however should be reduced, not only by the 20 per cent, absorption 

 of the membrane, but also by at least 15 per cent, loss on reflection 

 from the metal (assuming only single reflection, whereas much of the 

 radiation will be several times reflected before reaching the lower 

 end of the cylinder). This would give a maximum theoretical 

 evolution of 14*6 cc. per minute from the cube at 15° C, and 

 41 2 cc. per minute from the cube at 100° C. This evolution would 

 disturb the gas equilibrium in the cell too much, but 10 seconds' 

 exposure gave measures at the rate of 3 * 8 cc. per minute for the 

 15° C. exposure and 0*0 cc. per minute for the 100° C. exposure— 

 i.e. about one-fourth of the theoretical. This sufficiently indicates 

 the increasing lag with increasing intensity. 



The response of the cell to known small increments of tempera- 

 ture afforded another means of studying this question. A simple 

 method was to increase by a small measured amount the pressure 

 under which the liquid air was evaporating from the bath in which 

 the cell was immersed. A manometer, similar to that employed for 

 the radiation measurements, was connected to the closed vacuum 

 vessel, and an outlet tube was arranged at different measured depths 

 in an open vessel of mercury, sulphuric acid, or alcohol, according to 

 the plus pressure required. In this way an increase of temperature 

 of the liquid air amounting only to a few hundreths of a degree was 

 obtained, and the consequent rise of temperature of the cell was 

 observed by the movements of the manometer attached to it. The 

 equilibrium of different gases in the charcoal was also studied, and 

 information obtained as to the application of oxygen, nitrogen, 

 hydrogen, helium, etc., to the purposes of a low temperature charcoal 

 thermoscope. 



s 2 



