292 BIOLOGICAL EFFECTS OF RADIATION 



after many alternate exposures to light the sensitivity to this phototropy 

 becomes less. 



Fumaric and Maleic Acid Isomerization. — The well-known isomeriza- 

 tion reactions of fumaric and maleic acids may be represented by the 

 equation : 



H— C— COOH H— C— COOH 



HOOC— C— H ^ H— C— COOH 



Fumaric acid Maleic acid 



With ultra-violet light between 2000 and 2800 A, fumaric acid is con- 

 verted into maleic acid with a quantum efficiency of about 0.1. Maleic 

 acid is converted into fumaric with still less efficiency — about 0.03 mole- 

 cule per quantum on the average (53). 



The low quantum yield in both reactions requires further explanation. 

 The energy available in these short wave-lengths is considerably greater 

 than that necessary to bring about the simple isomeric change. The 

 reaction should be reinvestigated, particularly with reference to the 

 possibility of a mechanism involving the formation of free radicals. 



On the basis of these quantum yields, it can be calculated that the 

 ratio of fumaric to maleic acid should be about 1:3 when the reactions 

 have reached a steady state after long continued illumination at a con- 

 stant light intensity. This value agrees closely with the value obtained 

 experimentally. 



This isomerization can be effected also by visible light in the presence 

 of bromine. Very likely the mechanism is different. In carbon tetra- 

 chloride solution the di-ethyl ester of maleic acid is converted into the 

 fumarate ester, with a quantum yield of about 8 at 4360 A and 4 at 

 5460 A (16). 



Hydrogen Peroxide. — Hydrogen peroxide is readily decomposed by 

 ultra-violet light at 3130 A and below, producing water and free oxygen. 

 The course of the reaction may be followed readily either by the volume 

 of oxygen liberated or by titration with potassium permanganate. The 

 photochemical reaction is a chain reaction and quantum yields from 7 to 

 80 were obtained by Kornfeld (29) and from 4 to 100 by AUmand and 

 Style (1). These investigators reported a dependence of the chain length 

 on concentration and wave-length and particularly on the light intensity. 

 They reported that the rate of decomposition varies inversely as the 

 square root of the light intensity. The photolysis probably involves 

 dissociation into ions according to the reaction : 



H2O2 ^ H+ + OOH- 

 OOH- = OH- + O 



The marked influence of pH on this photolysis supports the view that 

 the OOH- ion plays an important part in the photolysis. F. O. Rice 



