CHAPTER IX 



PHOTOSYNTHESIS AND PHOTOGRAPHY 



Photochemistry in a Solid 



Although for the moment we must abandon the attempt 

 to understand in detail the process of plant photosynthesis, 

 physical science has recently enabled us to describe the 

 sequence of elementary actions by which light decomposes 

 silver bromide in a photographic emulsion. A remarkable 

 theory of the formation of the latent image and of the total 

 decomposition of silver bromide under the prolonged action 

 of light was propounded by two English physicists, 

 R. W. Gurney and N. F. Mott, in 1938. On the basis of 

 modern theories of the ionic structure of chloride, bromide 

 or iodide crystals of monovalent metals, and of the experi- 

 ments of Gudden and Pohl on alkaline salts in this category, 

 they succeeded in giving an almost complete description of 

 the principal photographic phenomena, which then appear 

 as a direct result of the properties of the ionic constituents of 

 silver bromide crystals arranged in a crystal lattice. 



It must be stressed that the chemical decomposition 



AgBr — > Ag+Br 

 is much simpler than the photosynthetic reaction in which 

 carbon dioxide, water and the photo-catalyst, chlorophyll, 

 take part — a reaction which occurs only in the Hving plant. 

 But this photographic theory is one of the first to explain 

 the chemical action of light on solids, which is much more 

 complex than that on gases, for the molecules are no longer 

 isolated from one another, but are well organized with almost 

 perfect regularity in a crystal lattice. The exchanges between 

 molecules are not limited to accidental shocks, but are 

 governed by the structure of the crystal itself. 



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