192 SCIENCE PROGRESS 



equation (2), and the velocity of the back reaction from the 

 collision-frequency method developed by Trautz and others. 



Equation (2) bears some resemblance to the approximate 

 Nernst relation, 



log Kp = — —r^j + Sn 1-7S log T + tnC 



The results of the application of the theory to many chemical 

 reactions indicate that the equation is only approximately true, 

 and that there are other factors (steric) yet to be taken into 

 consideration. The nature of the success that has been achieved 

 by the application of the quantum theory in this field suggests 

 that the problem has been partly solved. 



Photochemistry. — Coehn and Tramm {Ber., 1921, 54, 1148) 

 have investigated the photochemical dissociation of carbon 

 monoxide. The equilibrium position of this reaction is dis- 

 placed by exposure to ultra-violet light. The necessity for the 

 presence of minimal quantities of moisture for the explosion of 

 carbon monoxide and oxygen was recognised by Dixon, and it 

 would be anticipated that water would be necessary for the 

 photochemical reaction. This is not the case neither for the 

 forward nor the back reaction. In fact, CO2 is not even decom- 

 posed by light unless it is strongly dried. Carbon monoxide 

 and oxygen, on the other hand, not only react when so dry that 

 they will not burn, but combine at a rate independent of the 

 moisture content. Coehn gives no explanation of this anoma- 

 lous behaviour. 



Baly and Barker {Trans. Chem. Soc, 1921, 119, 653) have 

 studied the photochemical reaction between hydrogen and 

 chlorine, from the point of view of the Einstein law of photo- 

 chemical equivalence. They find that the deviation from the 

 law increases rapidly with increase in the intensity of the 

 activating light. The observation of Bunsen and Roscoe, that 

 the mixture of hydrogen and chlorine remains activated for 

 many minutes after removal of the activating light, has been 

 confirmed. The peculiar expansion of the gaseous mixture 

 observed on exposure to light (400 /i/i - 2 70 /i/i) is proportional 

 to the rate of reaction. The results obtained are in agreement 

 with the observations of Slade and Higson {Proc. Roy. Soc, 

 1920, [A], 98, 154) in their experiments with the photographic 

 plate, who find that the amount of silver obtained is not pro- 

 portional to the intensity of the light. The divergence from 

 Einstein's law is due to the readsorption, by the unchanged 

 hydrogen and chlorine molecules, of the energy radiated by 

 the combining molecules. The authors suggest a new type 

 of photocatalysis, whereby a photochemical reaction may be 

 brought about by light, which is not absorbed by the reactants 



