126 LIGHT, VEGETATION AND CHLOROPHYLL 



crystal at points dispersed at random; the energy of each 

 quantum, necessary for the Hberation of an atom of silver, is 

 available and is effectively used at the point where the 

 absorption has taken place, the absorption of Ught being, by 

 definition, the transformation of luminous energy into a 

 different form of energy. It is therefore at these dispersed 

 points that one would expect to find the atoms of silver, 

 which means that a grain of bromide would become uniformly 



grey. 



By what means of transport, by what directing influence, 

 can this grouping of the atoms of silver in visible grains in the 

 same region of the crystal be explained? The theory of Gurney 

 and Mott makes this clear. 



It may be recalled that in photosynthesis the energy of 

 4 quanta, which were necessarily absorbed at dispersed 

 points, must accumulate for the reduction of a molecule of 

 carbon dioxide, which presupposes a directed transport of the 

 absorbed energy towards a fixed goal. The now famiUar 

 phenomena of the photochemistry of gases did not offer 

 any analogous example. The photochemistry of silver bromide 

 shows such a transport and it can be explained. New pos- 

 sibilites are seen to exist in an ordered sohd structure, like the 

 regular periodic structure of crystals, that may be considered 

 as a rudimentary outhne of the organized structures of living 

 matter. 



Growth of the Grains of Silver in Light 



Before describing Gurney and Mott's conception of the 

 photographic action, let us consider the internal achitecture 

 of a crystal of silver bromide. A crystalhne grain contains an 

 equal number of bromine ions and silver ions (not atoms). 

 The first have one electron more than atoms of bromine and 

 the second one electron less than atoms of silver and are thus 

 denoted by the symbols Br— and Ag+. They are arranged 

 hke a regular pile of cubes and form a three-dimensional 

 rectangular lattice. On all the lines parallel to the edges of 

 these cubes, the two kinds of ions succeed one another 



