26 Prof. E. C. C. Balv 



on 



In actual practice the most probable condition will be one 

 lying between the two extremes — namely, the re-absorption 

 of an aliquot part of the radiated energy. This will result 

 in a large but definite number of molecules reacting for 

 every quantum absorbed, and this doubtless gives an explan- 

 ation of the observations by Henri and Wurmser. The 

 n amber of molecules that react for every quantum of the 

 energy absorbed at the phase frequency will vary directly 

 with the proportion of the internally radiated energy that is 

 re-absorbed by the system. 



In this explanation of the divergence from Einstein's law 

 use is made of two processes — first, when the quantum of 

 energy at the phase frequency is larger than the amount 

 required to activate one molecule, the molecule retains the 

 amount it needs for its conversion into the active phase and 

 radiates the excess ; and, second, the re-absorption of this 

 radiated energy by fresh molecules. Direct evidence has 

 been obtained of the occurrence of both these processes. 

 The conversion of one molecular phase into another phase of 

 higher energy content by the absorption of energy at the 

 frequency of the first phase — a process which involves the 

 absorption of a quantum for each molecule far larger than 

 required for the phase change — was instanced above in the 

 case of trinitrobenzene. Many other instances are known, 

 including the familiar phenomenon of fluorescence. In the 

 latter case one molecule absorbs one quantum at its phase 

 frequency, and is converted into a phase of greater energy 

 content. The excess of energy beyond that necessary for 

 the change of pluise is radiated as one quantum at the 

 frequency of the new phase, this radiation being recognized 

 as fluorescence. 



A characteristic instance of the re-absorption of the 

 radiated excess energy beyond that required for the phase 

 change is afforded by triphenyl-methane in alcoholic solution. 

 The substance absorbs energy at its phase frequency, and is 

 converted into a phase with higher energy content, the 

 excess energy being radiated as one fluorescence quantum 

 at the frequency of the new phase. This radiated energy is 

 partly re-absorbed with the formation of a third phase, the 

 excess again being radiated as one fluorescence quantum at 

 the third phase frequency. Two fluorescence frequencies 

 are therefore observed, the one at the smaller frequency 

 being due to the partial re-absorption of the first. 



It was also stated that the fact that the energy of the 

 reaction proper may be radiated at frequencies characteristic 

 of the products will make no difference as regards its 



