Phosphorescent Glow in Gases. 345 



in brightness with each successive spark until it reaches a 

 maximum. But prolonged and very rapid sparking fre- 

 quently spoils the glow. It is very remarkable that if a 

 portion of the tube projects into the bulb to the extent of 

 3 or 4 cms., the after-glow only travels down the tube with 

 great difficulty, and even then it does not move farther than 

 20 or 30 cms. from the bulb. This is a point that will be 

 dealt with in the sequel. 



Again, prolonged and rapid sparking very often spoils the 

 glow, and, as further experiments show (see Art. (5)), this is 

 due to the destructive influence of the negative ions on the 

 phosphorescent molecules produced by the discharge. 



(3) It can easily be shown that the glow, which might at 

 first be imagined to be the result of electrostatic effects on the 

 walls of the tube, is not really so, but, as has been indicated, 

 is actually a volume-effect in the gas. 



Thus, if a brass tube which is at zero-potential, by means of 

 an earth-wire soldered to the tube and to a waterpipe, is inter- 

 posed between the bulb, in which the discharge passes, and the 

 long glass tube, which leads to the pump, as already described, 

 it is found that the glow travels down through the brass tube 

 into the glass one, quite unimpeded, in precisely the same 

 manner as at first, when the glass tube was sealed directly to 

 the bulb ; thus proving that the glow cannot be the result of 

 leakage of electricity along the walls of the tube, but that it 

 must actually be a volume-effect in the gas. 



It is known that a phosphorescent after-glow is also pro- 

 duced in any ordinary vacuum-tube with electrodes when 

 a discharge is sent through the gas. In this case the glow 

 makes its appearance first in the immediate neighbourhood of 

 the cathode, especially, as J. J. Thomson has shown, if this 

 electrode is formed of a flat surface of sulphuric acid 

 (J. J. Thomson, k Recent Researches/ p. 184). 



The pressure, however, at which the glow is produced by 

 the ordinary discharge between electrodes is much lower than 

 that at which the phosphorescence from the ring-discharge 

 appears. That the phosphorescence in this case is also a 

 volume-effect follows from experiment (Art. (17)), also from a 

 somewhat similar experiment which has been shown by Dewar 

 (see Roy. Inst. Lect., ' Electrician/ 6th July, 1894). He 

 allowed the gas in the discharge-tube to rush into another 

 tube, the pressure in which was kept lower, by rapid exhaustion, 

 such that a permanent stream of gas was maintained through 

 the discharge-tube. A phosphorescent jet was thus produced ; 

 showing clearly the phosphorescence to be a volume-effect in 

 the gas. 



Phil. Mag. S. 6. Vol. 1. No. 3. March 1901. 2 A 



