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.

The properties of the alkaline liquid "Ammonia" are given in Part III. Ammonia dissolves Chloride of Silver readily, but not Iodide of Silver: hence its use is necessarily confined to the paper proofs upon Chloride of Silver. Even these however cannot advantageously be fixed in Ammonia unless a deposit of Gold has been previously produced upon the surface by a process of "toning," presently to be explained: a peculiar and unpleasant red tint is always caused by Ammonia acting upon the darkened material of a sun picture as it comes from the printing-frame: but this is obviated by the employment of the Gold.

ALKALINE CHLORIDES, IODIDES, AND BROMIDES.

The Chlorides of Potassium, Ammonium, and Sodium possess the property of dissolving a small portion of Chloride of Silver. In the act of solution a double salt is formed; that is, a compound of Chloride of Sodium with Chloride of Silver, which may be crystallized out by allowing the liquid to evaporate spontaneously.

The earlier Photographers employed a saturated solution of common Salt for fixing paper prints; but the fixing action of the Alkaline Chlorides is slow and imperfect, and their use may now be said to be obsolete.

The Iodide and Bromide of Potassium have both been used as fixing agents. They dissolve Iodide of Silver, forming with it a double salt in the manner before described.

It is important to remark in the solution of the insoluble Silver Salts by Alkaline Chlorides, Iodides, etc., that the amount dissolved is not in proportion to the _quantity_ of the solvent, but to the degree of concentration of its aqueous solution. This is not usual with solvents which act by entering into chemical combination with the substance dissolved. Commonly a given weight of the one salt dissolves a given weight of the other, independent of the amount of water present. The peculiarity in the case before us depends upon the fact that the double salt formed is _decomposed_ by a large quantity of water. Hence it is a _saturated_ solution of Chloride of Sodium which possesses the greatest power of fixing paper prints; and with the Bromide or Iodide of Potassium the same rule holds good--the stronger the solution the more Iodide of Silver will be taken up. The addition of water produces milkiness and a deposit of the silver Salt previously dissolved.

ALKALINE HYPOSULPHITES.

Hyposulphurous Acid is one of the Oxides of Sulphur. It is, as its name implies, of an acid nature, and takes its place upon the list immediately below Sulphurous Acid ("υρο," under).

The Hyposulphite of Soda commonly employed by Photographers is a neutral combination of Hyposulphurous Acid and the alkali Soda. It is selected as being more economical in preparation than any other Hyposulphite adapted for fixing.

Hyposulphite of Soda occurs in the form of large translucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold; they have a nauseous and bitter taste.

In the solution of Silver compounds by Hyposulphite of Soda a _double decomposition_ always takes place; thus:--

Hyposulphite of Soda + Chloride of Silver = Hyposulphite of Silver + Chloride of Sodium.

The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms a soluble double salt, which may be crystallized out by evaporating the solution. It possesses an intensely sweet taste, and contains one atom of Hyposulphite of Silver, chemically combined with two of Hyposulphite of Soda. In addition to this there is a second double Salt, differing from the first in being _very sparingly_ soluble in water. It is formed by acting upon Chloride of Silver with a solution of Hyposulphite of Soda already saturated, or nearly so, with Silver Salts; and contains single atoms of each constituent.

The fact that the Silver contained in an ordinary fixing Bath is present in the state of _Hyposulphite_ must be borne in mind, because this salt is liable to undergo peculiar chemical changes, as will be better shown in