714 Dr. N. Campbell and Prof. E. C. C. Baly on the 



and secondly on the density of the radiated energy and there- 

 fore on the intensity of the activating light. In other words* 

 the number of molecules reacting above and beyond that 

 demanded by Einstein's law will vary, firstly with the con- 

 centration of the reactant molecules when the intensity of 

 the activating light is kept constant, and secondly with the 

 intensity of the activating light when the concentration of the 

 reactant molecules is kept constant. 



The fact that the deviation from Einstein's law varies 

 with the concentration of the reactant molecules has already 

 been proved, and this was pointed out in my paper. Further, 

 Bodenstein * has found that the amount of hydrogen chloride 

 formed from a mixture of hydrogen and chlorine in unit 

 time under constant illumination varies as the square of 

 the concentration of the chlorine. Clearly, therefore, the 

 first condition as regards the re-absorption is established. 



The second condition, namely the variation in the intensity 

 of the light with a constant concentration of the reactant 

 molecules, has been investigated by Mr. W. Barker and 

 myself. The photochemical union of hydrogen and chlorine 

 under constant pressure in the presence of water has been 

 studied, the concentration of the chlorine and hydrogen 

 being kept constant by the dissolution of the hydrogen 

 chloride in water as fast as it is formed. The detailed 

 results will be published elsewhere, but a brief statement as 

 to their nature may now be made. In the first place, it is 

 obvious that, if re-absorption by the chlorine of the energy 

 radiated by the hydrogen chloride takes place, the amount 

 of hydrogen chloride formed in unit time with constant 

 intensity of the activating light will at first conform with 

 Einstein's law and then steadily increase up to a constant 

 maximum. This increase in the rate of the reaction up to a 

 constant maximum was first noted by Bunsen and Roscoe f 

 and now has been amply confirmed by Mr. Barker and 

 myself. 



In the second place, we have found that these maximum 

 rates are not proportional to the intensity of the incident 

 light but that they increase far more rapidly than the in- 

 tensity. In other words, the deviation from Einstein's law 

 increases with the intensity of the incident light exactly as 

 foretold from the theory. It may also be pointed out that 

 the velocity curves indicate that at a finite intensity the 

 number of molecules reacting for every quantum absorbed 



* Zeit. Phi/s. Chem. lxxxv. p. 297 (1915). 

 f Fogg. Ann. c. p. 481 (1855). 



