Disappearance of Gas in the Electric Discharge. 929 

 Absorption without the Glow. 



14. In (I.) and (III.) it was stated that (in the absence of 

 phosphorus) no absorption took place unless the potential 

 driving the discharge was greater than the glow potential. 

 Of course this statement, interpreted literally, is not true ; 

 for it is well known that absorption occurs in X-ray tubes 

 and other high-vacuum apparatus at pressures far below 

 that at which the applied potential will maintain a visible 

 glow. But it is true that, at the pressures of a few 

 hundredths of a millimetre with which we were there 

 concerned, the absorption without the glow is so slow that 

 it cannot be detected by the method employed, and that, 

 when the glow appears, the rate of absorption increases 

 enormously. The explanation of the glow potential given 

 in (III.), p. 33, shows that the attainment of the glow 

 potential and the appearance of the glow are accompanied 

 by an enormous increase in ionization, and that the doubt 

 on this point expressed in (I.) was unjustified. Accordingly 

 if, as all our experiments indicate, absorption and ionization 

 are intimately connected, it is to be expected that the 

 absorption which accompanies the glow will also occur, 

 though at a much less rate, with potentials less than the 

 glow potential, so long as they are above the ionization 

 potential. 



The ratio of absorption to ionization is very much greater 

 in the presence than in the absence of phosphorus vapour*; 

 and it seemed possible that perceptible absorption might 

 occur in its presence, even with potentials too small to 

 cause the glow. The expectation has been fulfilled. The 

 experiments were conducted partly with a vessel like that 

 of fig. 1 in (I.), partly" with lamps designed for very low 

 voltages (less than 30). It was found, as was to be 

 anticipated, that the first stage of the absorption discussed 

 in Par. 1 3 was little affected by the potential between the 

 electrodes, and that a notable absorption always occurred at 

 the moment of lighting the filament, if it had been coated 

 with phosphorus or the gas mixed with phosphorus vapour. 

 But the second stage occurred also, and a perceptible 

 absorption would continue after the first stage was ended, 

 although no glow was visible and electrical measurements 

 showed that the current was limited by the space charge 



* This statement is not certainly established. It is certain that the 

 absorption corresponding to a given electron current from the cathode 

 is much greater if phosphorus is present ; but the phosphorus appears to 

 produce some increase of ionization as well as of absorption. 



Phil. Mag. S. 6. Vol. 43. No. 257. May 1922. 3 



