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174 THE ADVANCE OF PHOTOGRAPHY
Change in Silver Salts due to Action of Light. — The processes of change which the active salts in the sensitive film undergo when exposed to light in order that a latent image of the object may be produced, have formed the subject for much scientific research.
One theory which was proposed by Fischer in 1814, and extended by Wetzlar in 1834, is that the silver chloride forms a sub-salt, and the reaction taking place can be represented by the equation
4AgCl = 2Ag2Cl +C12
Silver Chloride. SubChloride
of Silver.
The existence of these sub-salts has, however, never been definitely proved.
Others have put forward an oxychloride theory: thus Dr Hodgkinson, who examined the darkened product, found that it consisted of an oxychloride of silver Ag4OCl2.
Abney has also found that silver chloride does not darken in a vacuum after several months if kept dry, and Carey Lea says it does not darken in air or oxygen if these are dry.
On the other hand, silver chloride will darken under petroleum, and in this case no free oxygen or moisture is present.
Then there is the molecular strain theory, which states that no chemical action actually takes place, the action of light being of too short duration. The particles of silver chloride are considered as being made less stable without actually undergoing any decomposition.
An intense strain is set up in the molecules and the amount of this strain is proportional to the intensity of the light received. This strain would render the silver compound much less stable, and in the presence of a reducing agent those molecules in which the strain is greatest would be first reduced to metallic silver,