354 Mr. J. B. B. Burke on the 



spectrum, the " pressure glow " gives a band spectrum and is 

 the same as that which Schuster had observed in the negative 

 glow of oxygen. 



We are now referring to the spectrum of the glow in gases 

 in which oxygen is the predominant element, but we have 

 reason to regard — after a careful study of the question — the 

 presence of oxygen somehow as a necessary condition for the 

 production of the glow in every case, although, as has been 

 shown, it is not in itself a sufficient one, 



The connexion at once shown, by the identity of the 

 spectrum between the negative glow in oxygen and the 

 " pressure glow " in the same gas, appears to be of the utmost 

 importance in the study of the cause of phosphorescence in 

 gases, since it reveals the operation of a process in the pro- 

 duction of the latter phenomenon such as is known to take 

 place in the neighbourhood of the cathode by the passage of 

 an ordinary discharge through the gas. 



As has been pointed out, the glow in a gas produced by a 

 discharge between electrodes is confined to the neighbourhood 

 of the cathode, but the degree of exhaustion requires to be 

 much higher than in the case of the ring discharge ; the 

 pressure at which this glow is conspicuous is about ^V mm.,, 

 about that at which the resistance of the gas to a discharge 

 between electrodes placed as in the tube is least ; just as the 

 pressure at which the after-glow with the ring-discharge is 

 produced is in the region of that at which the resistance of 

 the gas to the electrodeless discharge is a minimum. 



The after-glow, however, which follows a discharge between 

 electrodes in an ordinary tube is one which I have never 

 observed to be particularly brilliant. Nevertheless, when 

 there is a stream of gas passing through the discharge-tube, 

 the glow, as in the experiment which Professor Dewar 

 showed at the Royal Institution (loc. cit.), is very marked. 

 This is what might have been expected, since the removal of 

 the phosphorescent particles, whose existence has been demon- 

 strated, from the destructive influence of the cathode should 

 greatly increase their persistency, since the experiments 

 described in art. (5) show that cathode-rays destroy the glow 

 whilst the positive discharge may be sent through the glowing 

 gas without materially affecting it. It is to some process 

 occurring at the end of the dark space that the negative glow is 

 obviously due. J. W. Capstick (Proc. Roy. Soc. 1898) has 

 shown that the cathode-fall at the boundary of the dark space is 

 the same in oxygen, air, nitric oxide, and nitrogen with traces 

 of oxvgen ; thus indicating that oxvgen is the carrier of elec- 



