38 SECTIONAL ADDRESSES. 



change from one to otlier state of energy content, is singularly meagre, 

 and it would seem that little more can be gained in this direction even 

 by the most intensive study of purely chemical processes. I venture to 

 stress this point of view because I believe that the necessary evidence can 

 only be gained from sources of information which are independent of the 

 processes we wish to explain. Such independent sources of information 

 may be found in the phenomena of phosphorescence, fluorescence and 

 absorption spectra of compounds. Observations in these three fields are 

 sufficiently differentiated from those of chemical reaction to be trusted 

 to give evidence which is free from any bias. I myself believe that these 

 observations when interpreted on the energy quantum theory constitute 

 a mine of information which can render signal service in the quest for a 

 comprehensive theory of chemical reaction. 



The term phosphorescence is a broad one and includes both photo- 

 luminescence and cathodoluminescence, together with certain subsidiary 

 phenomena. The only one of these that can serve our present purpose is 

 photoluminescence, since a knowledge is essential of the frequency of the 

 activating radiation as well as that of the emitted radiation. It is not 

 possible to give here any detailed account of the many observations, 

 both qualitative and quantitative, of the phenomenon of photoluminescence, 

 but particular attention may be directed to one or two of these which have 

 a special significance in the present connection. 



It would perhaps be advisable first to describe very briefly the principal 

 facts which have been established. In the first place the molecules of the 

 phosphore are brought into a state of higher energy content, or the 

 activated state, by the absorption of radiant energy. The phosphorescent 

 emission is the radiation of energy during the change of the molecules 

 from the activated state to the original state, and this energy is equal in 

 amoimt to that gained during the activation. The persistence of the 

 phosphorescence, that is to say the period of the time during which the 

 luminescence persists, is a measure of the stability of the activated state. 

 The more stable is the activated state, the longer is the persistence, and 

 vice versa. The intensity of the luminescence is in inverse ratio to the 

 persistence. After a definite quantity of energy has been absorbed by 

 the phosphore, then in the radiation of that quantity in the form of 

 phosphorescence the velocity must affect the persistence and intensity in 

 opposite senses. Since the phosphorescent emission is the integration of 

 the individual radiation of a number of molecules, the intensity decreases 

 with time as the number of molecules in the activated state becomes smaller. 

 If the intensity at any time t be measured in relation to the initial intensity 

 (t=0) then 



l—^=a-\-bt 



and in the majority of cases n=2. 



The stability of the activated state is determined both by the tem- 

 perature and by the concentration of the phosphorogen in solid solution 

 in the diluent. The higher is the temperature, the less stable is the activated 

 state, and there always exists an upper temperature limit, characteristic 

 of every phosphore, above which no phosphorescence can be observed. 

 The stability of the activated state is the greatest with a pure substance, 



