Phosphorescent Glow in Gases. 351 



the phenomenon discovered by Mr. Newall which he has 

 called the " pressure glow.'" 



Prof. A. Smithells, H. M. Dawson, and H. A. Wilson, 

 Phil. Trans. 1899, have shown that in a flame a similar 

 state of things exists, and that the ions in the flame are quite 

 distinct from the molecules that emit light, although these 

 latter are doubtless due to chemical combination of some 

 sort. 



Professor J. J. Thomson has shown quite recently (Proc» 

 Camb. Phil. Soc. vol. xi. part 1), that the ionization produced 

 by Pontgen rays in a mixture of hydrogen and chlorine does 

 not affect the rate of combination in the formation of HC1 

 when it is also exposed to light. 



Thus there appears to be no direct connexion between the 

 luminous or other molecules produced by chemical combi- 

 nation and the ions in the gas. 



Section III. 



(8) The phenomenon of phosphorescence has long been 

 attributed to impurities, and there is strong evidence in 

 favour of the view that the presence of impurities plays an 

 important part in the production of the luminosity. 



Sir George Stokes, in his classical memoir " On the Change 

 of Pefrangibility of Light/' Phil. Trans. 1852, attributes 

 the phenomenon of fluorescence to impurities when the sub- 

 stance is not of complicated molecular structure ; and we 

 know that fluorescence, as Becquerel has shown, is but a 

 form of phosphorescence in which the duration is only a 

 small fraction of a second. Becquerel has also for many 

 years regarded the phenomenon from the point of view that 

 it was due to impurities. 



Dewar, from experiments at low temperatures, concluded 

 that the more complicated the molecular structure of a body 

 the more likely it is to phosphoresce. Pure water is slightly 

 phosphorescent at low temperatures ; but when slightly im- 

 pure it is very phosphorescent. 



The phosphorescence of gases appears likewise to be due 

 to the presence of foreign bodies. Thus I have found it 

 much easier to obtain the after-glow in commercial oxygen 

 than in electrolytic oxygen, in which it was difficult to get the 

 after-glow, and when started it was by no means as brilliant 

 or as persistent as in the commercial oxygen. Again, the 

 colour of the glow is different according to the nature of 

 the foreign bodies present. For instance, as Newall has 



