MODELS OF POSITIVE ION 325 



action of Becquerel rays, the pressure at which the 

 ordinary after-glow appears may be much greater 

 than in ordinary circumstances, that is, it may be 

 produced at a pressure of 2 mm. instead of 

 0*5 mm. Here, of course, there is greater ionisa- 

 tion, and, as we shall show, the large molecules are 

 much more easily formed ; for the forms of 

 radiation which excite phosphorescence in every 

 case produce ionisation. The emanation from 

 radium bromides glows more intensely when the 

 gas in a vacuum is compressed (see Ramsay, 

 Nature, August 13, 1903). 



When a spark is taken in a dense liquid 

 medium there is again a displacement of the 

 spectrum, which, as Wisling and others have shown, 

 is extensive and indefinable. 



The model of fluorescence, consisting of a system 



of pellets, attached to a weightless string at equal 



intervals, suggested by Stokes, illustrates the change 



of refrangibUity, but does not explain the fact 



that the fluorescent light is independent of the 



exciting light, and that fluorescence does not 



always occur at the border of an absorption band, 



as the model would require.^ 



1 If we have a number of particles attached along a string 

 and let them assume a form as in this figure, 



TO = mass of each particle. 



