LIGHT AND VEGETATION 131 



Analogies with Photosynthesis 



The essential processes in this theory of the photochemical 

 reduction of silver bromide may be divided into three succes- 

 sive phases: 



(1) The absorption of hght, either by the bromine ions or 

 by molecules of the adsorbed colorant, each photon yielding 

 its energy to detach an electron which becomes capable of 

 moving some distance from the ion or from the molecule 

 from which it was extracted ; 



(2) The grouping of these electrons in a few places where 

 they are more or less immobilized in a relatively stable energy 

 state — on a small grain of silver or of sulphide present at the 

 surface of the grain of bromide ; 



(3) An attraction, starting from the electrified centre thus 

 constituted, of the stray AgH- ions which collect there, each 

 taking an electron, becoming neutral atoms and progressively 

 forming a colloidal grain of silver. 



It is probable that three analogous functions exist also in 

 photosynthesis and that this theory will be useful to those 

 investigating it. 



As we have said before, the chemical constitution of silver 

 bromide is much simpler than that of chlorophyll. The theory 

 of Gumey and Mott takes into account the irregularities of 

 crystalline structure caused by thermal agitation, and the 

 Ag+ ions which have left their normal place play an important 

 part in it. Superficial spots of sulphide or silver where the 

 free electrons are captured have also had to be considered. 

 In modern orthochromatic emulsions, it is not generally the 

 bromine which absorbs the luminous energy, but added 

 colorants. Thus, even without mentioning crystalUne defects, 

 edges, cracks, and other factors which have been passed over 

 for simpHfication, we are far from the ideal periodic and 

 monotonous structure of an edifice of Br- and Ag-|- ions 

 alternating in the meshes of a cubic lattice; the lattice is rather 

 a sort of support in which impurities and irregularities play 

 active parts. Even the gelatine in which the grains of bromide 

 are embedded is important in Gurney and Mott's theory. 



