128 LIGHT, VEGETATION AND CHLOROPHYLL 



to move and, attracted by the negative charge of the grain 

 of silver, attach themselves to it and there neutralize an 

 electron by becoming atoms. This explains both the discharge 

 of the grain of silver, which will be able to collect new 

 electrons, and its increase. 



To sum up, in a grain of bromide, already containing 

 some grouped atoms of free silver, the electrons snatched 

 from the bromine ions by Ught are captured by the silver, 

 and this negative charge assures the increase of the grain 

 of silver by attraction of the Ag+ ions wandering in the 

 crystal. 



Such, in outline, is the process of the decomposition of 

 silver bromide under the action of light. It still does not show 

 how the grain of silver comes to be formed, but only how it 

 grows. It grows because it is a trap for electrons ; by assuming 

 that such traps, of another nature, are present at the surface 

 of the grains of bromide, we can understand that a grain of 

 silver will be formed there during irradiation. 



Sheppard had already shown, before this theory was 

 formulated, that photographic sensitivity seemed to be 

 localized at certain points of the grains of bromide at which 

 small spots of silver sulphide had formed; it was known, in 

 fact, that silver salts easily become sulphurated when the 

 atmosphere contains traces of hydrogen sulphide. These 

 traces of silver sulphide must act as traps for electrons at the 

 surface of the grains of bromide, but only on condition that 

 the energy levels for the electrons are lower there than in the 

 bromide, which is very probable. 



It therefore appears easy to explain the origin of the 

 grains of silver at certain superficial points, and then their 

 growth, by processes of the same nature. 



The Influence of Temperature 



The authors of this theory also explain why, in strong 

 illuminations, the sensitivity of photographic plates diminishes 

 when they are cooled (Webb's experiments on this subject 

 extend over the temperature range +50° to - 75° C), while, on 



