PHOTOCHEMISTRY OF VISUAL PURPLE. 



II. The Effect of Temperature on the Bleaching of Visual 



Purple by Light. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton University, 



Omaha.) 



(Received for publication, September 23, 1920.) 



1 . The determination of the influence of temperature on the velocity 

 of a photochemical reaction is always of importance. This is partic- 

 ularly true when the mechanism of the reaction is unknown, as in the 

 present instance of the bleaching of visual purple. 



The usefulness of determining the temperature coefficient lies in 

 this. Photochemical reactions have low temperature coefficients, — 

 near unity (Sheppard, p. 304), though exceptions are known. Ordi- 

 nary chemical reactions, however, have temperature coefficients which 

 are much higher, between 2 and 3 or even more. The order of magni- 

 tude of the temperature coefficient may therefore give us some notion 

 of the degree of complexity of a photosensitive reaction. A value of 

 Qio near unity may be taken to indicate that the reaction probably 

 consists of a simple, direct photochemical transformation. On the 

 other hand, a high value for <2io shows most probably that the photo- 

 chemical reaction proper is only one in a series of reactions which 

 makes up the photosensitive process. The associated chemical 

 reactions, because of their high temperature coefficients, raise the 

 temperature coefficient of the reaction as a whole more nearly to their 

 own level. 



An example of just such a condition is the high temperature coeffi- 

 cient (Qio = 2.5) of the photosensory process in Mya. On analysis 

 (Hecht, 1918-19, a; 1919-20, h) a series of at least three reactions 

 has been shown to exist, of which the photochemical proper is only 

 the first. This has a temperature coefficient of 1.06 when investi- 



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