12 PHOTOCHEMISTRY OF VISUAL PURPLE. I 



experiments the rate of intermittent illumination for the human eye 

 is very much greater than those used here. The principle, however, 

 is the same. Parker and Patten (1912-13) have recently shown that 

 a continuous flow of light is a more efficient stimulus for the eye than 

 an intermittent one (1,750 flashes per minute), the difference between 

 the two being of the order of 5 per cent. It is not possible to compare 

 these data with the present results on visual purple, because the ex- 

 periments here reported do not admit of the judgment of differences 

 of the order of 5 per cent in the magnitude of the velocity constant. 



IV. 



From the kinetics of visual purple decomposition it may be con- 

 cluded that this photosensitive material is a chemical entity, and that 

 its bleaching is probably represented by the destruction of a large 

 molecule into smaller ones. In this respect it resembles the hypo- 

 thetical , sensitive substance postulated in the first section of this 

 paper. 



Too much reliance, however, must not be placed on the fact that 

 the course of the reaction is monomolecular. Gross absorption and 

 dift'usion effects are, of course, eliminated by the thinness of the ex- 

 posed solution; they therefore probably do not account for the mono- 

 molecular feature of the curve. But it is frequently true that the 

 stoichiometric equation for a chemical reaction does not correspond 

 to the equation obtained from kinetic data. In such cases (Lewis, 

 1918, p. 395) the stoichiometric equation turns out to be of a higher 

 order. For example, it is conceivable that the bleaching of \dsual 

 purple is a process of hydrolysis, in which case the stoichiometric 

 equation may actually be bimolecular. However, because there is 

 so much water present in the system, the reaction would proceed as 

 if it were monomolecular, visual purple being the only component 

 whose concentration changes to a measurable degree. 



The equation 



S^P + A 



for the hypothetical light reaction is also not to be taken dogmatically. 

 It may well be 



S + X-^P + A 



