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THE CHEMICAL ACTION OF LIGHT 61
By the light of a gas lamp in a dark room the chloride of silver appears perfectly white, but it quickly takes a violet tint in the daylight. It is often said that it becomes black ; this, however, is not the case. This change of colour is the result of a chemical decomposition. The chlorine is in part set free, and may be detected by its odour if large quantities of chloride of silver be employed. The violet powder which remains behind was formerly thought to be metallic silver.
Metallic silver does, it is true, under certain circumstances, present itself in the form of a grey or violet powder. The violet-coloured body formed on exposing chloride of silver to light is not, however, metallic silver ; it is only a combination of silver with chlorine, which contains half as much chlorine as chloride of silver. Silver and chlorine form two compounds — one white and rich in chlorine, the other violet and with little chlorine, named subchloride of silver. In the same manner, silver forms two compounds with bromine — one light yellow, rich in bromine, named bromide of silver ; and a yellowish-grey compound, less rich in bromine, named subbromide of silver. Further, analogous to these there exist a yellow iodide of silver, and a green subiodide of silver, less rich in iodine. Subbromide and subiodide of silver are produced exactly in the same manner as the subchloride, by the operation of light. The chemist says, therefore, that bromide, chloride, and iodide of silver are reduced by the action of light to the corresponding subchloride, subbromide, and subiodide.
The change of colour by which this chemical change is accompanied is most striking with chloride of silver, less with bromide of silver, and least so with the iodide.
It would appear from this that chloride of silver is the most useful to photography. But this is not the case. Plates sensitized by means of bromide or bromide and