liight Absorption Wind Fluorescence. 2'.\ 



trinitrobenzene which is characterized by a frequency in the 

 blue, and the solution is deep red in colour. This second 

 phase, therefore, is favoured by piperidine. If to an alcoholic 

 solution of trinitrobenzene a small quantity of piperidine is 

 added, not more than one molecule of piperidine to 10 mole- 

 cules of trinitrobenzene, the solution remains perfectly 

 colourless. On exposure to light of the frequency character- 

 istic of the phase in ;ilcohol, the solution turns red owing to 

 the formation of the second molecular phase, and the 

 solution slowly becomes colourless again when placed in the 

 dark. 



In order, therefore, to cause a molecule to react it is only 

 necessary in the great majority of cases to bring it into one 

 particular phase, and, in general, this will mean the supply 

 to it of an amount of energy equal to a whole number of 

 quanta at its infra-red fundamental. It is quite possible, 

 of course, since the required reactivity is a function of a 

 particular molecular phase, that it may be necessary to take 

 energy from the molecule, if the phase in which the mole- 

 cules exist initially happens to be less condensed than the 

 required phase. For the present purpose it may be assumed 

 that the initial phase is more con lensed than the reactive 

 phase, and that energy must be given to the molecule in 

 order to enable it to react. Now, the Einstein photochemical 

 law states that in a photochemical reaction a minimum 

 amount of energy equal to one quantum at its absorbing 

 frequency is required for every molecule which reacts. In 

 view of the importance of this law and the very great diver- 

 gencies that have been observed from it which have led to 

 serious criticism of the theorv of discontinuous absorption, 

 it is of some importance to discuss the eneray relations of 

 chemical reaction, since the molecular phase hypothesis 

 seems capable of giving a complete and quantitative basis to 

 these relations. A chemical reaction may be divided for 

 this purpose into two parts, — first, the supply of energy to the 

 molecules to make them reactive, and, second, the reaction 

 itself in which eneray is evolved, the difference between 

 the two being, of course, the observed heat of the reaction. 

 We may deal first with the supply of energy to the molecules 

 in order to initiate the reaction. If this first stage consists 

 in the complete resolution of the molecule into its component 

 atoms, then one quantum of energy must be absorbed by 

 every molecule at its phase frequency, and therefore, as far 

 as this initial stage is concerned, the Einstein law will hold 

 for absorption at the phase frequency. In the more usual 

 case, when the first stage is the change of the molecular 



