TRANSACTIONS OF SECTION B. 395 



photochemical or short-wave reactions. Later Kriiger, in 1911, has suggested that 

 infra-red radiation is the cause of such effects as the solubility of a substance in a 

 solvent, electrolytic dissociation and solution pressure. The suggestion made by 

 Trautz has, however, not led to any quantitative results in the field of chemical 

 kinetics, due, no doubt, to the fact that at the time the suggestion was made the ideas 

 of Planck had not received the attention they subsequently did after Einstein had 

 shown that they might be applied to the problem of atomic heats. Aia important 

 step may, however, be taken by introducing the theory of quanta and applying certain 

 of Planck's relations to the problem of thermal reactivity. I propose to show as 

 briefly as possible that this method of treatment leads to a relation identical in form 

 with that of Marcelin for the variation of the velocity constant with temperature, and 

 if then one equates the two expressions, it will be seen that Marcelin's critical 

 increment under the simplest conditions is identical with one quantum of the infra- 

 red radiation of the type which the substance is capable of absorbing. In other 

 words, we arrive at Einstein's law of the photochemical equivalent for this type of 

 radiation. The application of this view to homogeneous catalysis rests on the 

 assumption that the catalyst is the source of the infra-red radiation which the 

 substance absorbs, thereby becoming reactive in the Marcelin sense. It does not 

 preclude the formation of complexes between the catalyst and the reacting substance, 

 and should apply, not only to the effect of added catalyst (such as the effect of acid 

 on esterification, hydrolysis, inversion, cfec), but likewise to the relative catalytic 

 effect of different solvents when alone and when mixed upon one and the same 

 reaction, and may possibly be extended to cases of chemical reactivity in which 

 catalytic effects as such have not hitherto been recognised. 



In the first place, in order that any of Planck's relations may be applied, it is 

 essential that the temperature of the radiation and of the material system shall be 

 the same as that of the surroundings— a condition which can be realised approxi- 

 mately by the use of a thermostat of sufficient heat capacity. If chemical reactivity 

 be dependent on thermal radiation it is evident that the factor of greatest significance 

 will be represented by the amount of radiation of the absorbable or useful type 

 (frequency v) present per unit volume of the system, in other words, the energy density 

 which is usually denoted by the symbol U„. For the sake of simplicity, let us con- 

 sider a monomolecular reaction whose rate according to the ordinary mass action 

 principle may be expressed 



dx , , 



, oc (a — x). 



dt ^ ' 



If further a catalyst be present, then, in the simplest case in which the catalytic 

 effect is simply proportional to the concentration of the catalyst (Cs), we can write 

 the velocity of decomposition 



- - ex: a (a — x) Gs. 

 dt 



Such an expression has been found to give results in good agreement with experi- 

 ment, though it obviously gives no information regarding the mechanism of the 

 energy transfer involved, nor does it predict how the velocity will be affected by a 

 change in temperature. It is therefore incomplete, and it is evident that the intro- 

 duction of some additional factor is required. On the hypothesis that molecular 

 changes are due to the transfer of radiant energy in quanta, the simplest assumption 

 is to regard the energy density as the significant factor. From this standpoint, 

 therefore, the chemical reactivity or active mass of a substance reacting in the 

 presence of a catalyst depends upon the concentration of the substance, the concen- 

 tration of the catalyst, and upon the radiation density of the useful frequency v. 

 The previous equation, therefore, takes the form 



^^^ = A(a - a;) X C, X U, (1) 



As a matter of fact, this mode of representing active mass is not new except in so 

 far as it is applied to thermal reactions. Einstein, in deducing the law of the photo- 

 chemical equivalent, makes ihree assumptions, of which one is that the rate at 

 which the light sensitive reactant decomposes depends upon the radiation density. 

 Einstein does not consider the case when a catalyst is present, and further has in 

 view a photochemical reaction in the ordinary sense. It may also be pointed out 



