130 LIGHT, VEGETATION AND CHLOROPHYLL 



easily at a low temperature, but at a higher temperature the 

 electrons tend, at the same time, to escape from the metal. 

 This is the classic thermionic effect— a sort of evaporation of 

 electrons. The grain of silver remains negatively charged only 

 if it is surrounded with an "atmosphere of electrons" the 

 pressure of which is sufficient to prevent this evaporation. 



Therefore, in a poor light and at a high temperature, the 

 light is badly used ; the electrons that it liberates in the 

 bromide do not succeed in fixing themselves on the grain of 

 silver, which does not increase in size in default of a negative 

 charge capable of attracting to it the available stray silver ions. 



The different properties of silver bromide emulsions 

 according to temperature and intensity of illumination are 

 thus explained. 



Put in this way, the theory offers an analogy with the 

 "law of the minimum" often referred to in photosynthesis; the 

 least active of the factors which take part in the accompUsh- 

 ment of a photochemical transformation slows down all the 

 operations and determines the speed of the reaction. 



The merit of the theory of photographic blackening is 

 that it gives in detail a description of the phenomena within 

 the framework of the conceptions of atomic physics and 

 enables us to understand in a more concrete fashion how 

 analogous processes may be performed in the photosynthetic 

 organ of the plant. 



The Action of Sensitizers 



Gurney and Mott also explain the action of sensitizers. 

 The first photographic emulsions were sensitive only to the 

 blue, the violet and the ultra-violet, but not to the green, the 

 yellow, or the red. The incorporation of certain colorants in 

 the emulsion enabled the sensitivity to be extended to the 

 whole of the visible spectrum and even to the near infra- 

 red. 



These colorants are adsorbed at the surface of the grains 

 of bromide. They, and not the bromine ions of the crystal 

 lattice, absorb the hght and provide the free electrons. 



