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64 THE ADVANCE OF PHOTOGRAPHY
with green vitriol, the latter withdraws at once the oxygen from the silver salt, and sets free the silver. Other bodies that readily combine with oxygen operate in like manner, namely, certain organic substances, such as pyrogallic acid and others. It was formerly thought that green vitriol reduced the iodide of silver affected by light. It can be easily proved that this view is false. For, if a wet plate is exposed and the nitrate of silver adhering to it is washed away, and then the developer poured upon it, no picture appears, proving that green vitriol alone is without action on exposed iodide of silver. But if a solution of silver is added, a picture appears immediately.
The solution of silver adhering to the plate plays, however, another part. If a wet plate is washed before it is exposed — that is, if all the nitrate of silver which adheres to it is removed, and it is then exposed — it will be remarked that it is far less sensitive than when the nitrate of silver is present.
This is explained by the peculiar property of many bodies sensitive to light.
There are bodies which in isolation are either not at all, or only very slightly, sensitive to light, but which become so in the presence of substances which are able to unite with one of the constituents liberated during exposure to light ; for example, chloride of iron is not sensitive to light, but chloride of iron dissolved in ether is sensitive, because the liberated chlorine at once unites chemically with the ether.
The same remark applies to iodide of silver. This is, by itself alone, sensitive to light, but only slightly ; in presence of a body which can combine with iodine, it is quickly decomposed in the light. Now, nitrate of silver, which reacts with iodine with the greatest ease, satisfies this condition ; and this explains the greater sensitiveness of iodide of silver in the presence of nitrate of silver.
Other Substances Sensitive to Light. — The number of