296 Mr Newall, On Luminosity attending [Apr. 2G, 



the tube, in some cases quite slowly (about 1 foot per second), 

 in other cases too quickly for any estimate to be made without 

 special apparatus. The rate varies with the pressure and with 

 the mixture used. 



4. The colour of the phosphorescence is very varied ; I find 

 among my notes that at various times the following colours have 

 been observed : — white, silvery-grey, yellow, yellow-green, old gold, 

 ruddy yellow, brilliant blue. The phosphorescence is undoubtedly 

 diffused throughout the whole volume of the gas enclosed in the 

 tube. There is an appearance of semi-opacity in the phos- 

 phorescent gas, except in the case of a certain brilliant blue 

 phosphorescence. The contrast in this respect between the 

 yellow and the blue afterglows is as marked as that between 

 a clear solution of copper sulphate and a liquid rendered turbid 

 by a slight yellow precipitate in suspension. The blue phos- 

 phorescence is marked, in my mind, as of an exceptional nature ; 

 I have only seen it four or five times, but it has generally been 

 of so short duration, that I have not been able to study it fully, 

 nor can I say what conditions are necessary for its appearance, 

 except that I connect it with oxygen and sulphur. 



Spectrum of the phosphorescent gas. 



5. A careful and long continued spectroscopic investigation 

 shows that the spectrum of the phosphorescent glows that I have 

 examined is a continuous spectrum. Prof. Thomson records the 

 observation of a few bright lines, if very high dispersion is used, 

 but I have been unable to corroborate this observation. (In 

 view of what follows in this paper, I should explicitly state that I 

 refer here to the spectrum of phosphorescence which has been 

 excited electrically in the rarified gas.) 



6. An arrangement somewhat similar to a phosphoroscope 

 was used. 



Between the spectroscope and the tube in which the phos- 

 phorescence was produced, a sector or sectors of a circle were 

 rotated by clockwork, which also drove a contact-break in the 

 circuit that produced the bright discharge in the tube. It was 

 arranged that the sector intercepted the light of the bright 

 discbarge, and allowed only the light from the phosphorescent 

 gas to pass into the spectroscope. The spectra were observed 

 visually and were also photographed. Both methods showed a 

 continuous spectrum without any trace of bright lines, except on 

 one occasion to which I refer later (§ 27). 



