138 ILLINOIS STATE ACADEMY OF SCIENCE 



cules in the lattice by definite forces. A certain amount 

 of energy must be added, therefore, in addition to the 

 energy necessary to decompose a given molecule, in 

 order to separate it from its fellows. Moreover, if pres- 

 ent ideas of molecular structure are accepted, the action 

 of light may be to set up vibrations of the atoms with 

 reference to each other, or to cause a separation to a 

 given distance of an electron. According to Perrin's 

 theory the molecule would either decompose or return 

 to its initial state. It would seem that the probability 

 of decomposition would depend on the extent of the 

 separation. Bearing this in mind, a formula analagons 

 to that for the photoelectric effect would be obtained in 

 which the rate of reaction is a linear function of the 

 frequency 



dx 



= k I h(v — v ) 



dt 

 In this formula the rate is assumed to be proportional 

 to the intensity of the radiation, I, in the same manner 

 as in most of the other theories. 



The most general fallacy in reasoning connected with 

 photochemical processes seems to be in making the as- 

 sumption that one general theory can be found which 

 will account for all reactions affected by radiation. It 

 is well known that a catalyst will not cause a reaction to 

 take place unless the reaction has a tendency to take 

 place without the catalyst. In other words if a reaction 

 leads to a decrease in free energy, as in the combination 

 of hydrogen and nitrogen to form ammonia, a proper 

 catalyst should greatly increase the rate of reaction, 

 even though the rate of reaction is immeasurably slow 

 under ordinary conditions. On the other hand if a re- 

 action involves an increase in free energy, as in the com- 

 bination of nitrogen and oxygen to form nitrogen diox- 

 ide, the mere use of a catalyst will not cause the reac- 

 tion to take place. In the case of photochemical react- 

 ions, it seems that a similar distinction should be made. 



Bodenstein 8 has classified photochemical reactions as 



8 Bodenstein, Zeit. phys. Chem., 85, 333 (1913). For a good summary see 

 Lind, The Chemical Effects of Alpha Particles and Electrons, The Chemical 

 Catalog Company, 1921. 



