B.— CHEMISTRY. 37 



demands that exposure of the inactive molecules to radiant energy of 

 that frequency should cause the reaction to take place. As a matter of 

 experimental fact, molecules in their inactive states do not show any 

 evidence of being characterised by frequencies equal to those calculated 

 from the critical increments. This in itself is sulficiently significant to 

 arrest attention, but when it was proved first by Lindemann and then in 

 most elegant fashion by G. N. Lewis that molecules do not react when 

 exposed to radiant energy, not only of the calculated frequency but of a 

 very large range of infra-red frequencies, it was felt on all sides that the 

 radiation hypothesis had been efltectively and completely disproved. 



The situation thus reached is one of considerable interest. There 

 exist on the one hand large and increasing numbers of photochemical 

 reactions which are obviously stimulated by the absorption of radiant 

 energy. If the Planck theory stand fast, the reactant molecules must be 

 activated by the absorption of the energy quanta Avo, since it is well 

 known that the frequency v^ is characteristic of them. On the other 

 hand the radiation hypothesis is based on premises which appear to be 

 theoretically sound ; nevertheless it has been proved to be untenable. As 

 a result the general consensus of opinion has swung over to activation by 

 collision in thermal reactions. It must, however, be confessed that the 

 present position is very far from being a satisfactory one. In the case of 

 true photochemical reactions it is not possible to believe that activation of 

 the reactant molecules is not produced by the direct absorption of radiant 

 energy. In the case of thermal reactions the evidence disproves the 

 activation by the direct absorption of radiant energy, and activation by 

 collision has been substituted. There are, therefore, two accepted methods 

 of activation, but the fact remains that these two have as yet not been 

 properly married together, the general hope apparently being that any 

 offspring will be legitimised when the union has been scientifically 

 canonised. 



When the obsequies of the radiation hypothesis had been sung, it was 

 felt that the corpse had received decent burial. In sympathy with its 

 parents in their bereavement, I venture to point out that this hypothesis 

 may be divided into two parts. The first part is concerned with the 

 critical increment of energy of a reaction, that is to say the minimum 

 quantity of energy, or rather the exact quantity of energy, which is 

 required to bring a molecule from its initial state to its reactive state. 

 Unless the whole conception of different molecular states be dropped, 

 this conception of a critical increment stands on a sure and firm basis. 

 The second part of the hypothesis, namely, that the critical increment 

 can be absorbed as a single quantum of energy by a reactant molecule, is 

 a pure assumption and one that would only be justified by a knowledge 

 that the properties of molecules are in this respect identical with those 

 of elementary atoms. The uncertainty which attaches itself to this 

 assumption impresses me so strongly that I propose to exhume the body 

 in order that the cause of death may be more fully investigated. There 

 exists a considerable amount of evidence which was not before the court 

 and this evidence is worthy of the most serious consideration. 



So far as the phenomena of chemical reaction can help us, our know- 

 ledge of the physical properties of molecules, and in particular their 



