[CHAPTER IV.]
ON FIXING THE PHOTOGRAPHIC IMAGE.
A sensitive layer of Chloride or Iodide of Silver on which an image has been formed, either with or without the aid of a developing agent, must pass through further treatment in order to render it indestructible by diffused light.
It is true that the image itself is sufficiently permanent, and cannot be said, in correct language, to need fixing; but the unchanged Silver Salt which surrounds it, being still sensitive to light, tends to be decomposed in its turn, and so the picture is lost. It is therefore necessary to remove this salt by applying some chemical agent capable of dissolving it. The list of solvents of Chloride and Iodide of Silver has been given in Chapter II., but some are better adapted for fixing than others. In order that any body may be employed with success as a fixing agent, it is required not only that it should dissolve unchanged Chloride or Iodide of Silver, but that it should produce no injurious effect upon the same salts reduced by light.
This solvent action upon the image, as well as upon the parts which surround it, is most liable to happen when the agency of light alone, without a developer, has been employed. In that case the darkened surface, not being reduced perfectly to the metallic state, remains soluble to a certain extent in the fixing liquid.
CHEMISTRY OF THE VARIOUS FIXING AGENTS.
The following will be mentioned:—Ammonia—Alkaline Chlorides—Alkaline Iodides—Alkaline Hyposulphite—Alkaline Cyanides.
AMMONIA.