Phosphorescent Glow in Gases. 355 



tricity in each case, just as we have seen that it is the active 

 and necessary agent in the production of the phosphorescent 

 pressure-glow, which by its spectrum may be identified with 

 the spectrum of the negative glow in oxygen. 



The Hon. R. J. Strutt (Phil. Trans, vol. cxciii. p. 393) has 

 shown that the minimum spark-potential, or cathode-fall — 

 which he has proved to be of the same magnitude — is in atmo- 

 spheric air 341 volts and in ordinary nitrogen between 347 

 and 388 volts, but that in nitrogen specially freed from all 

 traces of oxygen it was as low as 251 volts. 



A gas appears to have the capacity to store up energy when 

 a spark is sent through it, just as a certain amount of energy 

 is necessary before a discharge can be made to pass ; and this 

 extra amount appears to be about the same in all gases in 

 which there is a trace of oxygen. This minimum amount of 

 work which has to be performed is done in the boundary of 

 the dark space, near the region also of the negative glow. 



It is most probable that the effect of cathode-rays is to 

 increase suddenly the brilliancy of the phosphorescence to- 

 such an extent as to reduce its duration. As in the destruc- 

 tive effect of infra-red rays on phosphorescent substances, 

 discovered by Becquerel, which has been explained in this 

 manner as a particular kind of thermo-luminescence. 



This is what might be expected from the fact that the 

 negative glow is at all possible in the presence of the cathode- 

 rays when a discharge passes ; since, as we have seen, the glow 

 lasts much longer when it is removed from the influence of 

 the cathode. 



(12) Returning to the tube, fig. 3, another observation. 

 of some importance may be described in connexion with 

 the negative glow. Thus if either of the bulbs A or B 

 had been standing for some time unused (say A), and a 

 ring-discharge was sent through it, the pressure in the 

 tube being such that the after-glow was produced round the 

 cathode when a discharge was sent between the electrodes 

 c d ; it was found that there was no visible effect accom- 

 panying the ring-discharge in the neighbourhood of the 

 electrode c, except when a discharge had previously been sent 

 between the electrodes c ■ d, c being cathode: then a brilliant 

 fluorescence accompanied the ring-discharge just round the 

 curved portion of the end of the side tube at c; and this 

 effect w T as obtained a considerable time after the passage of 

 the discharge between c d — on one occasion, fully an hour 

 after. When c is anode the phenomenon does not occur.. 

 Philipps (' Electrician/ Sept. 1900, and B. A.Bradford, 1900) 



