Disappearance of Gas in the Electric Discharge. 919 



allowed for true equilibrium to be obtained, there will be 

 a large amount of phosphorus absorbed on the glass walls 

 in addition to that present as free vapour. However, by 

 choosing suitably the time of exposure, so that the equi- 

 librium vapour-pressure is closely approached without 

 considerable absorption on the walls, the amount of phos- 

 phorus introduced can be regulated with the accuracy 

 required for these experiments. The greatest amount that 

 could be introduced by this method, unle>s the whole system 

 was artificially maintained at above the room temperature, 

 which never exceeded 25° C, was 0*06 mgm. : by a combina- 

 tion of the two methods the whole range up to about 1 mgm. 

 could be covered sufficiently. 



4. Since the contamination of the gas with a continually 

 renewed supply of mercury or grease vapour prevents all 

 absorption, and since access of the gas to cooled traps would 

 have removed all the phosphorus, it was necessary to conduct 

 the absorption in a vessel completely sealed off. Two methods 

 have been used for estimating the decrease of pressure, which 

 alone can be used as a measure of the absorption that 

 occurs. The first involved the use of the lamp filament as the 

 hot wire of a Pirani gauge according to the method recently 

 described by us *. The second depends upon a distinction 

 between pressures which are above and those which are below 

 the value at which the applied potential is equal to the glow 

 potential. (I. 590 ; III. 228.) 



The glow potential in a vessel, such as a lamp, having 

 electrodes of which the area is very small compared with 

 that of the walls, is the potential at which the current 

 between those electrodes changes very suddenly from a small 

 value, limited by the space charge and almost independent of 

 the thermionic emission, to the value corresponding to the 

 saturated thermionic emission, and therefore dependent on 

 the temperature of the cathode. Suppose, then, that a lamp 

 in which the pressure can be varied is connected in series 

 with a similar but perfectly evacuated lamp in a Wheatstone 

 bridge, the other arms being constant resistance coils. 

 Whatever the pressure in the lamp, the bridge can be 

 balanced for any constant applied voltage by adjusting the 

 coils ; but if the pressure in the first lamp is above that 

 corresponding to the glow potential, the balance will change 

 if the current through the bridge (and therefore the temper- 

 ature of the filaments) is changed ; for an increase of space 



* Research Staff of the G. E. C, London. Phys. Soc. Proc. xxxiii. 

 p. 287 (1921). 



