36 SECTIONAL ADDRESSES. 



independently by Perrin and by W. C. McC. Lewis. Briefly stated in an 

 elementary way, this theory postulated that molecules in general have 

 no chemical reactivity, and that they become reactive after they have 

 absorbed energy. In order that a specific reactivity be induced, a definite 

 quantity of energy must be supplied to bring each molecule from its 

 initial stage to its reactive state, this quantity being called the critical 

 increment of energy characteristic of the specific reaction. 



The fundamental basis of the radiation hypothesis was the extension 

 of the Einstein photochemical equivalent law to include thermal radiation 

 as well. The Einstein law states that in a photochemical reaction the 

 absorption of the radiation takes place in the form of quanta, and that 

 each molecule requires for its activation one single quantum hv^, where v,, 

 is the characteristic absorption frequency of the molecule in the visible 

 or ultra-violet region of the spectrum. The conception that a single 

 quantum of energy must be absorbed before a molecule can become 

 activated was not only extended but also intrinsically modified in the 

 radiation hypothesis. W. C. McC. Lewis developed from the Planck 

 radiation formula the expression 



(flog;fc/(^T = NAv/RT^ . . . . (1) 



where k is the velocity constant of the reaction and N is the Avogadro 

 constant. By treating the problem from the point of view of statistical 

 mechanics, J. Rice, following the example of Marcelin, obtained an expres- 

 sion which, with a small simplification, may be written 



(ZlogK/(ZT = E/Rr . . . . (2) 



where E is the amount of energy necessary to bring one gram molecule 

 of a gas into its reactive state. If the like terms in these expressions be 

 equated we have 



E/N = ;iv, 



that is to say the amount of energy that has to be supplied to a single 

 molecule to cause it to react is one single quantum of absorbable radiation. 

 Although Lewis says that this is simply a statement of the Einstein law 

 which is now applied to thermal or infra-red radiation, it is much more 

 than that. The Einstein law merely states that in a photochemical 

 reaction a molecule absorbs one quantum of radiant energy, Avg, and then 

 becomes activated, no assumption being made as to the difference in 

 energy content of the initial and reactive states. The radiation hypothesis 

 states that the difference in energy content of the initial and reactive 

 states, or the critical increment of activation, is a single quantum which 

 can be absorbed from infra-red radiation. The critical increment of 

 energy characteristic of a reaction is neither expressed nor implied in the 

 Einstein law. 



It is a simple matter to calculate the critical increment of a reaction 

 from the observed change of the velocity constant with temperature, and 

 by dividing this quantity, expressed in ergs, by the product N/(, to obtain 

 the critical frequency v. Not only must this frequency be one character- 

 istic of the reactant molecules, that is to say one that can be observed by 

 absorption spectra measurements, but the radiation hypothesis also 



