300 Mr Newall, On Luminosity attending [Apr. 26, 



18. With reference to the amount of compression necessary 

 for maximum brightness in the pressure-glow, a number of 

 experiments were made under varied conditions and the results 

 were found consistent with the idea that the maximum brightness 

 of the compressed gas occurred at a pressure between mm '5 and 

 mm '3, fairly definite for a given mixture of gas. If the initial 

 pressure of the rarified gas in the pump-bulb were small (e.g. 

 mm- 01) the pressure-glow did not begin to take jjlace with any 

 brilliance until such a diminution of volume (e.g. about 20 or 30 

 fold) had been brought about as would increase the pressure to 

 within the limits above stated. If, on the other hand, the initial 

 pressure was comparatively high (e.g. mm 'l), then a much smaller 

 diminution (two or three fold) of volume was required to produce 

 the afterglow. 



19. By regulating the rate at which compression was produced 

 the brightness and duration of the glow could be controlled to a 

 certain extent. The phenomena observed in this connection were 

 all in conformity with the idea that in a given mixture and 

 quantity of gas in the pump-bulb a definite amount of energy was 

 to be got rid of by radiation for a given alteration of pressure, and 

 that the time-integral of the intensity of the pressure-glow was 

 approximately constant. 



20. Attempts were made to obtain the pressure-glow without 

 any electrical excitation like that described in § 16. The appa- 

 ratus was left in complete working order one evening, and the 

 next morning (i.e. after an interval of 15 or 16 hours), the rarified 

 gas was compressed, but there was no glow: — that is to say, the 

 energy requisite for the formation of the low-pressure-compound 

 had not passed into the gas. 



A strong beam of light was passed through the gas, in the hope 

 that this would communicate the requisite energy. But there 

 was no glow. 



Electrical excitation was applied (as described in § 16), and 

 the pressure-glow was as bright as before. 



21. The actual distance between the tube for the electrode- 

 less discharge and the pump-bulb was not more than 18 inches, 

 but the connecting tubes were about 6 — 7 feet long. Very few 

 electrodeless discharges in the tube were enough to put the gas in 

 the pump-bulb into the state requisite for the production of the 

 pressure-glow. The same gas was used again and again with 

 apparently no uncertainty in the phenomena exhibited. 



22. In order to make the conditions more simple, I sur- 

 rounded the pump-bulb itself with a coil of thick wire and 

 attempted to produce the electrodeless discharges in the pump- 

 bulb : but though I have generally found it easier to produce the 



