178 Mr. G. J. Stoney. Note on the [May 30, 



the spectrum where the phosphorescent bands occur, are as bright as 

 these latter. Then, and not till then, will an equivalent exchange 

 take place of the energy passing in and out of the closed chamber, 

 and only then will the augmentation of the temperature of the black 

 body cease. 



Accordingly, if we regarded only the temperature to which by 

 radiation it can raise a neighbouring body, the phosphorescent sub- 

 stance would appear to be at a white heat. 



This remarkable behaviour of the bodies that are conspicuously 

 phosphorescent is, of course, consequent upon the molecular events 

 that go on within them. We have accordingly next to inquire what 

 their internal dynamical state must be to have such results. 



We may distinguish the movements of molecules into three kinds 

 of events. 



1 . A events ; by which are to be understood motions of the centres 

 of mass of the molecules relatively to one another. This is a kind of 

 motion which is probably inconspicuous in solids, while considerable 

 in liquids ; and Avheii Ave come to gases it becomes so much the pre- 

 dominant event that it usually engrosses about two-thirds of the 

 energy of all the molecular events that are going on. 



But besides these there may be motions of the parts of each mole- 

 cule relatively to its centre of mass. These internal events, as they 

 may be called, are of at least two kinds, as follows 



2. Ba events ; i.e., events within a molecule which act on and are 

 reacted on by a neighbouring molecule if sufficiently near. Accord- 

 ingly, whenever the Ba events within a molecule happen at any time 

 to have more than average activity owing to the fluctuations in the 

 distribution of energy that are always taking place, this excess of 

 energy tends to be ti-ansferred over wholly or in part to the neigh- 

 bouring molecules in consequence of the interaction between mole- 

 cules. This interaction seems to proceed continuously in solids and 

 liquids, but in gases only during a portion of the whole time, viz., 

 only on the occasions when encounters take place, which is perhaps 

 something like one-seventieth of the whole time in the more perfect 

 gases at atmospheric temperatures and pressures. 



3. Bb events. But, besides, there may be more isolated events going 

 on within molecules, events of the kind that manifest themselves con- 

 spicuously in phosphorescent bodies, events which but slowly part 

 with any excess of energy they may possess to the other events going 

 on within the molecule, or to neighbouring molecules. 



It is to these Bb events that our attention has at present to be 

 chiefly directed. A conspicuously phosphorescent substance was 

 selected above in order most easily to demonstrate their existence. 

 But what has been said of persistently phosphorescent bodies applies 

 equally to many of the bodies about us which become visible when 



