Light Absorption and Fluorescence. 7 



Amongst the quantitative relationships detailed at the 

 beginning of this paper was mentioned the fact that the 

 central frequencies of all absorption-bands — that is to saj-, 

 all molecular frequencies — exhibited by a molecule in the 

 visible and ultra-violet are exact multiples of the infra-red 

 fundamental . It is therefore evident that one molecular 

 quantum absorbed at one of the molecular frequencies in 

 the visible or ultra-violet is equal to an exact number 

 of quanta at the infra-red fundamental- If a molecule 

 absorbs one quantum at one of these higher frequencies, 

 this amount of energy can be radiated again as a whole 

 number of quanta at the infra-red fundamental, or partly 

 as quanta at this frequency and partly as elementary atomic 

 quanta. This, as I have shown in previous papers, is the 

 process underlying the phenomenon of phosphorescence or 

 fluorescence, and in this particular case the phosphorescence 

 emission will be in the form of infra-red quanta. Further, 

 it is obvious that the phosphorescence emission need not 

 of necessity be evolved as a whole number of molecular 

 quanta at the infra-red fundamental, but may be radiated 

 as one molecular quantum at a molecular frequency which 

 is a multiple of the infra-red fundamental, the remainder 

 being radiated as molecular quanta at the infra-red funda- 

 mental or as elementary atomic quanta. For example, if 

 the molecule absorbs one molecular quantum at the fre- 

 quency which is ten times the infra-red fundamental, this 

 energy may be evolved as one quantum at the frequency 

 which is nine times the infra-red fundamental and one 

 quantum at the infra-red fundamental itself. In such a 

 case the phosphorescence will be in the visible or ultra- 

 violet region of the spectrum. The factors governing 

 these various alternatives are determined by the conditions 

 under which the molecules exist, but the consideration of 

 these must be postponed. It will be seen from this that 

 a molecule can acquire one or more molecular quanta at 

 the infra-red fundamental in three different ways : by 

 exposure to radiation equal to its atomic frequencies, 

 by exposure to radiation of frequency equal to the infra- 

 red fundamental, or by exposure to radiation of a frequency 

 which is an exact multiple of the infra-red fundamental. 

 Great stress must be laid on this deduction, for it is of 

 paramount importance in connexion with the chemical 

 properties of the molecule. 



The next point to be considered is the structure of the 

 absorption-bands — that is to say, the system of subsidiary 

 frequencies which, are always found associated with the 



