B.— CHEMISTRY. 39 



and in order to observe phosphorescence at temperatures below the upper 

 limit, it is necessary that the phosphorogen be in dilute solid solution in 

 some diluent. This was first observed by Lenard and Klatt with their 

 alkaline earth sulphide phosphores, and more strongly emphasised by 

 Urbain and Bruninghaus in the case of the rare earths. 



The foregoing is a brief account of the characteristics of photo- 

 luminescence, and we may now consider in detail one or two of these, 

 selecting as the first the relation between the frequencies of the exciting 

 radiation and the emitted radiation. In the alkaline earth sulphide 

 phosphores the phosphorescent radiation is very often complex in the 

 sense that it consists of several separate emission bands. Lenard and 

 Klatt, however, satisfied themselves that each emission band is character- 

 istic of a single activated state, since each has its own frequency of activa- 

 tion and its own upper temperature limit. The relation between the 

 absorption band at which activation takes place and the emission band 

 after activation is an intimate one, and it has been shown by later work 

 on less complex phosphores that the absorption and emission bands have 

 structures which are analogous. 



Now Lenard and Klatt established the very important fact that 

 phosphorescent emission is not a truly reversible process. It is not in 

 any way possible to activate a phosphore by exposing it to radiation of 

 the same frequency as that which it emits when it has been activated. 

 It is only possible to activate a phosphore by means of radiant energy 

 of the same frequency as that of its characteristic absorption band which 

 lies on the short wave-length side of the characteristic emission band. 

 In short, these investigators proved the complete validity of Stokes' law, 

 and as the result of later work on true phosphorescence this law has been 

 proved invariably to hold. 



The importance of this may at once be recognised if the facts be stated 

 in more scientific phraseology. When an activated phosphore is emitting 

 its characteristic luminescence each activated molecule radiates a single 

 quantum of energy in passing from the higher energy state to the lower 

 energy state, the total luminescence being the sum of all these radiated 

 quanta. In the process of activation the change from the lower to the 

 higher state is caused by the absorption of that same quantity of energy 

 by each molecule, and in view of the radiation as a single quantum it is 

 legitimate to assume that it is absorbed as a single quantum, nothing 

 being expressed or implied as to the mechanism of the absorption. Each 

 molecule, therefore, requires for its activation a critical quantum of energy 

 Avj, and the value of Vj may be directly obtained from the measurement 

 of the luminescence. The proof given by Lenard and Klatt and by others 

 that Stokes' law is valid indicates that it is impossible to activate a 

 phosphore by means of radiant energy of the frequency Vj, and that the 

 critical quantum of activation cannot be supplied to a molecule by a 

 singular absorption process. There exists, therefore, in this respect a 

 sharp difterentiation between the physical properties of molecules and 

 atoms. 



The lethal dose of criticism which killed the radiation hypothesis was 

 based on the experimental proof that molecules are not able to do this 

 very same thing, namely, absorb their critical quanta of activation Av, 



