556 PRINCIPLES OF GENERAL PHYSIOLOGY 



nature, since certain reactions can be accelerated by practically any wave length, 

 so long as a dyestuff is present which can absorb these particular rays. 



The most important practical application of optical sensitisers is in the 

 production of photographic plates sensitive to the whole extent of the spectrum. 

 It is to be remembered that the adsorption of the dye by silver bromide does not 

 make this itself more sensitive. It may be, as Weigert suggests (1911, p. 70), that 

 the light absorbed by the dye makes it a better chemical sensitiser than it is in the 

 dark, so that it takes up bromine with great avidity. 



4s. Catalytic Photo-chemical Reactions in which the CataJ.yst remains after the 

 Action of Light. If the catalyst formed is not immediately used up in the 

 reaction, it is clear that its activity may continue. Such a case is that of iodofonn 

 in chloroform ; the iodine set free by light remains active after the light has ceased 

 to act, and continues so for several days. Moreover, if a solution which has been 

 exposed to the light be added to an unexposed one, decomposition of the latter sets in. 



The capacity of being developed at any time after exposure, possessed by 

 photographic plates, is another case. We cannot here discuss the nature of the 

 latent image. The reduction-potential of the developer is not sufficiently high to 

 affect unexposed silver bromide at any considerable rate ; but, where the light has 

 formed a catalyst, metallic silver is produced in development. It appears that 

 the acceleration is due to adsorption of developer on the surface of the 

 heterogeneous catalyst, by which the concentration of the former is raised and, 

 with it, the reduction-potential (see the remarks of Weigert, 1911, p. 74). 



It is clear that, in these cases of catalytic action, if we could add the catalyst 

 in any other way than by the action of light, the result would be the same. This 

 is not so in the three first cases of our list, where the same products of reaction as 

 those produced by light cannot be obtained in the dark, at the same temperature, 

 by other means. 



Electro-chemical analogies for the catalytic action of light may be found in the 

 saponification of an ester in a solution of neutral salt. The catalyst, in this case, 

 is the alkali formed at the cathode, and it disappears by combination with the acid 

 formed from the ester. If we take cane-sugar instead of an ester, the catalyst is 

 the hydrogen ion formed at the cathode and it remains active after cessation of the 

 current, provided that means are taken to prevent diffusion. 



RELATION OF VELOCITY OF REACTION TO INTENSITY OF LIGHT 



Bunsen and Roscoe (1862) showed that in order to produce a definite degree of 

 darkening on silver chloride paper, the time required was inversely proportional to 

 the intensity of the light. That is : 



i t = constant 



where i is the intensity of the light, and t the time of action. This is known as 

 the Bunsen-Roscoe Law. 



When the exposure to light is followed by development, the law does not hold. 

 Schwarzschild (1899) showed that, for silver bromide gelatine plates, the law must 

 be expressed thus : 



it f = constant. 



The value of the exponent/) varies between - 8 and 1, according to the brand 

 of plate used. It seems probable that the exponential form of the equation may 

 depend on the intervention of adsorption in this case, where development is made 

 use of. 



Inertia. There is a certain minimal duration of exposure of a plate to 

 light below which no effect is produced. This is known as the " inertia " of the 

 plate, and appears to be related to the photo-chemical induction already 

 referred to. 



FLUORESCENCE 



There remain to be mentioned some phenomena connected with the absorption 

 of light which are not obviously photo-chemical in nature, that is, chemical 

 changes are not immediately obvious. 



