0\' ABSORfllON StECTRA OP ORGANIC COMtOtNDS. £33 



thereby lost their individuality as far as their powers of absorbing or radiating 

 energy are concerned. The conception of the molecular quantum is based on 

 the assumption that the component atoms can gain or lose elementary quanta 

 when in combination. In addition to this, there is definite evidence that the 

 molecule exhibits the specific frequencies of its atoms, since, although these 

 atomic frequencies have not yet been observed in the long-wave infra-red, they 

 are found in combination with the molecular frequencies as subsidiary frequencies 

 within the absorption band groups in the infra-red, visible, and ultra-violet 

 regions. The question then arises as to the course of events when a molecule 

 IS exposed to radiation of a frequency that is the same as one of its characteristic 

 atomic frequencies which may be active in the extreme infra-red. Let it be 

 supposed that the molecule formed by tlie combination of two elementary atoms 

 having the characteristic frequencies 9 X 10'" and 1-5 X 10" is exposed to 

 monochromatic ra/diation of the frequency 9 X 10'". The atom having this 

 frequency will absorb this energy in elementary quanta of 9 X 6-56 X 10"" erg : 

 and further, let it be supposed that this atom absorb five such quanta. Tiie 

 total quantity of energy now absorbed is equal to the minimum quantity of 

 energy which that atom evolves when combining with the atom with characteristic 

 frequency 1"5 x 10", and is equal to one molecular quantum at the true molecular 

 frequency. If the postulate made at the beginning as to the combination of 

 atoms be accepted, then it would seem to follow as a natural consequence that 

 the total energy absorbed by the atom can be transferred to or taken over by the 

 whole molecule as exactly one true molecular quantum. In fact the molecule 

 can obtain one true molecular quantum by the absorption of a whole number of 

 elementary quanta by its atoms, the whole nimiber being of course determined 

 by the frequencies of the other atoms in the molecule and the least common 

 multiple of all the atomic frequencies. Further, there is no reason against 

 this process being continuous in the sense that a molecule will be able to gain 

 more true molecular quanta than the single one by absorption of the specified 

 number of elementary quanta by its atoms. 



Again, this process will be reversible : that is to say, a molecule will be able 

 to radiate one or more true molecular quanta in the form of the specified 

 number of elementary quanta characteristic of one of its atoms. 



It will be seen that this leads to the conception of critical amounts of energy 

 associated with elementary atoms in combination, the critical amount of energy 

 of an atom being a whole number of elementary quanta characteristic of that 

 atom which in their sum equal one true molecular quantum characteristic of the 

 molecule of which that atom forms a part. When an atom is exposed to 

 radiation of a frequency equal to its own frequency, it can absorb its elementary 

 quanta until its critical quantity is reached, when this critical quantity becomes 

 merged into the molecular energy content as one true molecular quantum. 



Amongst the quantitative relationships detailed above was mentioned the fact 

 that the central frequencies of all absorption bands, that is to eay, 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 funda- 

 mental. 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 is the process underlying the phenomena of 

 phosphorescence and fluorescence, and in this particular case the phosphorescence 

 will be in the form of infra-red quanta. Further, it is obvious that the 

 fluorescence 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 

 fundamental or as elementary atomic quanta. For example, if the molecule 

 absorbs one molecular quantum at the frequency 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 quantimi at the infra-red funda- 

 mental itself. In such a case the fluorescence will be in the visible or ultra- 



