Photochemistry of Visual Purple. Ii. the Effect of Temperature on the Bleaching of Visual

1. The determination of the influence of temperature on the velocity of a photochemical reaction is always of importance. This is particularly 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 lles in this. Photochemical reactions have low temperature coefficients.near unity (Sheppard, p. 304), though exceptions are known. Ordl~ary chemical reactions, however, have temperature coefficients which are much higher, between 2 and 3 or even more. The order of magnitude of the temperature coefficient may therefore give us some notion of the degree of complexity of a photosensitive reaction. A value of Q10 near unity may be taken to indicate that the reaction probably consists of a simple, direct photochemical transformation. On the other ban.d, a high value for Q10 shows most probably that the photochemical 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 coefficient (Q,0 = 2.5) of the photosensory process in Mya. On analysis (Hecht, 1918-19, a; 1919-20, b) 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 investi285