286 BIOLOGICAL EFFECTS OF RADIATION 



give one excited molecule for each photon absorbed and accordingly 

 one molecule of di-anthracene. The quantum yield should be unity, 

 except for the slight loss of activation through fluorescence. At the 

 stationary state, when the di-anthracene has accumulated to such an 

 extent that the decomposition rate is equal to the rate of production, there 

 will be no apparent change while the beam of light is being absorbed. In 

 other words, the quantum yield will appear to be zero. Between these 

 limits of no di-anthracene and equilibrium quantities of di-anthracene, 

 one may obtain any apparent quantum yield ranging from 1 to 0. In 

 this case the significant photochemical reaction is that taking place at 

 the beginning of the reaction. Quite frequently, when complicating 

 reactions follow the primary reactions, one can obtain a knowledge of 

 the primary process by working with short exposures. 



Bromination of Cinnamic Acid. — When cinnamic acid and bromine 

 are dissolved in carbon tetrachloride, the bromine very slowly adds to 

 the cinnamic acid, forming di-bromocinnamic acid and producing a 

 decrease in the concentration of the remaining bromine. The con- 

 centration of the bromine is easily determined by pouring the solution 

 into an aqueous solution of potassium iodide in water and shaking 

 thoroughly. Bromine from the carbon tetrachloride enters the aqueous 

 solution and liberates iodine from the potassium iodide, which is accu- 

 rately titrated with sodium thiosulphate. 



This reaction has been studied by many investigators. It is con- 

 venient to carry out with visible light, and is easily measured by standard 

 titrations. In a recent investigation (5), it was found that the quantum 

 yield decreased as the bromine in solution decreased. Extrapolation 

 to infinite dilution of bromine gave a quantum yield of one molecule of 

 bromine reacting per quantum absorbed. These results were obtained 

 at 30°C. At 20° the quantum yields were about one-third as much, and 

 at 10° roughly one-ninth as much. The interesting point is that all of 

 these quantum yields at the different temperatures gave extrapolated 

 values of unity at infinite dilution, as would be expected from the Einstein 

 law, which applies to the primary photochemical process. Apparently, 

 then, in this case it is possible to separate the primary photoprocess from 

 the secondary thermal reaction. The secondary thermal reactions are 

 subject to the large temperature effect which ordinarily applies to thermal 

 reactions. 



It has been shown recently (6) that all these photochemical measure- 

 ments on the photobromination of cinnamic acid really involve an oxygen- 

 inhibited reaction. It has been found that, when the oxygen is boiled 

 out of the carbon tetrachloride solution by evaporating part of the carbon 

 tetrachloride with a water aspirator, the reaction goes so rapidly in weak 

 light that it cannot be readily followed. This inhibition by oxygen of 

 the addition of halogen to a double bond is probably quite general. 



