Phosphorescent Glow in Gases. 353 



a discharge through them, but which they are not known to 

 do at ordinary atmospheric pressures. 



The phenomenon of the " pressure glow/' loc. cit., dis- 

 covered by Mr. Newall tends to confirm this A'iew. 



He observed that when a ring-discharge is sent through a 

 oas a certain amount of energy is stored up in the gas by the 

 passage of electricity through it, and that this energy which 

 is stored up may — within a certain very narrow range of 

 pressures, lying well within those at which the after-glow we 

 have been discussing makes its appearance — be radiated aw r ay 

 as phosphorescent light. It does not matter whether the 

 pressure at which the spark is sent through the gas is 

 greater or less than that at which the phenomenon of radia- 

 tion takes place ; and the energy thus stored up in the gas 

 becomes latent, until the pressure is adjusted to the proper 

 value at which the phosphorescence occurs. 



I have myself observed a phenomenon which may be 

 attributed to the same cause. When the gas in one of the 

 long tubes was pumped out, down to a pressure of t -J-q 

 millim., and a discharge sent through, then on letting a small 

 quantity of air rush into the tube, a beautiful luminous 

 wave was seen to travel up the tube into the bulb. This 

 phenomenon admits of a ready explanation in the light of 

 Mr. Newall's discovery. 



(10) At the meeting of the British Association at Oxford 

 I read a paper " On the Luminosity produced when a Vacuum 

 Bulb is broken " (see also Phil. Mag. Jan. 1895). 



It was then shown that a luminosity must undoubtedly have 

 been produced by the collision of the particles of glass when 

 the bulb was broken, a view which was verified by breaking 

 a glass plate at the mouth of a tall glass receiver, as a number 

 of luminous spots were distinctly visible at various parts of 

 the receiver. And it was also found that two bits of glass, 

 when struck, produced a bright flash. Nevertheless there 

 was a continuous glow as well to be explained, especially in 

 the case of vacuum-bulbs when broken, and this may easily 

 have been due to the " pressure glow/'' since in a glow-lamp 

 the same change may have been brought about, and energy 

 stored up almost indefinitely in the gas, as by the passage of 

 a spark through it. The spark or flash from the collision of 

 glass may also have produced some ionization. 



(11) Since the pressures at which the after-glow, and the 

 u pressure glow," occur are within the one range of pressures, 

 it suggests at once that the two phenomena can be traced 

 ultimately to the same cause. However, they differ in this 

 one respect, that whereas the after-glow gives a continuous 



