ON ABSORPTION SPECTRA OF ORGANIC COMPOUNDS. ^05 



of 17 molecular quanta is radiated to the surroundings. This excess energy 

 may be radiated either as 17 quanta at the molecular frequency or as one quan- 

 tum characteristic of the new phase. This second alternative, which is of 

 frequent occurrence, is the familiar phenomenon of fluorescence, and it is 

 important to note that the frequency of tlie fluorescence is always characteristic 

 of the molecular phase into which the molecules are changed by the exciting 

 light. To be strictly accurate, it should be stated that fluorescence always 

 occurs when a molecule is exposed to light of its phase frequency, but when 

 the emission of energy takes place in the infra-red this energy merely evidences 

 itself as heat. 



The phenomenon of phosphorescence is exactly analogous, but in this case 

 a reservation must be made. In phosphorescence the emission of energy persists 

 after the exciting light has been removed, and consequently the phosphorescence 

 must be the energy evolved when the newly produced phase reverts to the 

 normal phase. It follows from this that the f rec[uency of pliosphorescent emis- 

 sion can only be a sub-multiple of the activating frequency, as otherwise the 

 phosphorescence fretjuency cannot be characteristic of the newly produced phase. 

 Thus, if a phase quantum of 20 molecular quanta is absorbed and the phase is 

 produced which is characterised by the quantum of 10 molecular quanta, then 

 on reversion to the original phase this new phase can emit its own quantum 

 of 10 molecular quanta. Again, if the newly produced phase is characterised 

 by a quantum of five molecular quanta, it can emit during reversion to its original 

 phase three of its characteristic Cjuanta. An important confirmation of this 

 explanation of phosphorescence is to be found in the well-known fact that an 

 activated substance very rapidly loses its energy, as phosphorescence when ex- 

 posed to infra-red radiation. The activated substance is in a metastable condi- 

 tion, and this condition is disturbed by the absorption of infra-red quanta, with 

 tlie result that the svstem rapidly reverts to its norinal phase ecjuilibrium. 



The change in phase of molecules on exposure to radiation of their phase 

 frequency can readily be observed by absorption spectra methods, but, since 

 these are only anplicable to gases and liquids, it is necessary that the newly 

 produced phase be stabilised in some way, as it otherwise reverts instantly to 

 the original phase. A typical example is afforded by an alcoholic solution of 

 trinitrobenzene, containing a trace of piperidine. On exposure to ultra-violet 

 light of the phase frequency of trinitrobenzene the solution turns red, owing to 

 the conversion of the trinitrobenzene into a phase of greater energy content. 

 On remaining in the dark this new phase reverts to the original phase, and the 

 solution again becomes colourless. The phase change of solid substances on 

 exposure to light of their phase frequency is a familiar phenomenon as exempli- 

 fied by photography, and by the change in colour of many inorganic salts on 

 exposure to rays of very short wave-length. 



Up to the present the phases in which molecules exist have only been con- 

 sidered for substances in the free state, when the phase equilibrium is deter- 

 mined only by the relation between the atomic force fields and by the tempera- 

 ture. There yet remains to be discussed the influence of a solvent which is 

 one of great importance, because a solvent not only can alter the equilibrium 

 between the phases in which a substance normally exists, but also can cause 

 the molecules to pass into new phases. From the point of view of ordinary 

 absorption spectra observations the latter property of a solvent is of greater 

 interest than the influence of radiant energy in the form of heat or light, since 

 it is these phase changes which for so long have been subpoenaed as evidence 

 in favour of the structure-absorption theories. The explanation of the action 

 of a solvent follows very readily from what was stated above about the forma- 

 tion of the molecular force field. When the atomic force fields of a freshly 

 synthesised molecule are unequally balanced, the condensation between them 

 with the evolution of molecular quanta to form the molecular force field cannot 

 proceed very far, since sooner or later a balance of one or other type of affinity 

 will remain uncompensated, and this will arrest the condensation process. If 

 this uncompensated balance be removed in some w-ay there will be nothing to 

 prevent the force field condensation from proceeding further with the evolution 

 of more molecular quanta. The extent to which this proceeds will depend on 

 the atomic fields themselves. Since the uncompensated balance of affinity endows 



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