Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XVII.
FIXING SOLUTIONS.

Fixing solutions consist of chemical substances that dissolve the sensitized salts of silver on plates or paper, on which photographic images have been developed. The parts which form the image are covered with reduced silver, or an altered iodide or chloride of silver, which is insoluble in the fixers; whereas those parts which have not been impressed by the actinic rays are made transparent with the fixing solutions, which dissolve the opaline silver compounds, and cause the picture afterward to be unchangeable when exposed to light. The fixing solutions at present in use are: Cyanide of potassium, Hyposulphite of soda, and Sulphocyanide of ammonium.

Cyanogen.

Symbol, C₂N, or Cy. Combining Proportion, 26. Spec. grav. 1.819.

This substance is properly a Bicarbide of Nitrogen; it is a very important material, as being the type of what are denominated compound salt-radicals; it was the first of this class of bodies discovered. Cyanogen is always produced in combination when an alkaline carbonate is heated with organic matter containing nitrogen. It does not exist either in a free or combined state in nature; it is a production of decomposition, in which the elements contained in it are brought together in the nascent state, in connection with some metallic base.

Preparation of Cyanogen.

This compound radical is obtained by heating either a cyanide of silver or of mercury in a flask of hard glass; a gas, the substance in question, is produced, which may be collected, by reason of its greater specific gravity than air, in a tall glass jar, by directing the outlet tube to the bottom; or it may be collected over mercury. It is colorless, but its odor is quite peculiar and characteristic. It barns with a peach-colored flame, yielding carbonic acid and nitrogen. Water dissolves four volumes of this gas, and alcohol as much as twenty-five volumes. An aqueous solution is decomposed when exposed to light into a variety of ammoniacal compounds. By the pressure of four atmospheres it is reduced to the liquid state. It combines with alkaline solutions precisely in the same way as chlorine, iodine and bromine, and gives rise to salts denominated cyanides.

Hydrocyanic Acid-Prussic Acid.

Symbol, H Cy.

This acid is obtained from the cyanides or the ferrocyanides by the superior affinity of the mineral acids for their bases in a manner similar to that by which the other hydracids are obtained. Take, for instance, three parts of the yellow prussiate of potash (ferrocyanide of potassium) in fine powder, two parts of sulphuric acid, and two of water, and distill the mixture in a flask or retort; the vapor which passes over is condensed in a receiver surrounded by ice. Prussic acid is a colorless liquid of the specific gravity of 0.6969. It is exceedingly poisonous.

Cyanide of Potassium.

Symbol, K Cy.

This substance, so exceedingly useful to the photographer, might be formed by passing the vapor of hydrocyanic acid through a solution of potassa to saturation, and then evaporating to dryness without access of air. It is formed, however, by heating ferrocyanide of potassium in an iron bottle to an intense red heat; the tube of the bottle dips into water to conduct away the gases. The cyanide of iron becomes decomposed into carbide of iron and charcoal, and its nitrogen is given off, whilst the cyanide of potassium remains undecomposed, and when melted swims on the surface of the porous black mass below. It is afterward pulverized and dissolved in boiling weak alcohol, from which it crystallizes as the alcohol cools; or whilst in a fused condition it is poured upon marble slabs and afterward broken up and bottled. This substance is almost as poisonous as hydrocyanic acid, but being a fixed salt it is easily detected in the stomach; whereas hydrocyanic acid, by reason of its volatility, seldom leaves any trace behind by which the cause of death can be recognized. This salt is decomposed by the red oxide of mercury into cyanide of mercury and potassa, showing the superior affinity of cyanogen for mercury. On this account the ordinary tests for mercury do not act on cyanide of mercury, with the exception of hydrosulphuric acid; analogous to hyposulphite of silver in which hydrochloric acid or a soluble chloride does not precipitate the chloride of silver, hydrosulphuric acid alone being capable of forming a precipitate.

Sulphocyanide of Potassium.

Symbol, Cy S₂ K.

This salt is obtained by a process similar to the last with an addition of sulphur to the amount of half the weight of the ferrocyanide of potassium used. It is an excellent test of the persalts of iron, with which it produces blood-red precipitates. I do not see why this salt may not be used instead of the following as a fixer; it certainly can be more easily procured, and is no doubt just as poisonous.

Sulphocyanide of Ammonium.

Symbol, Cy S₂ NH₄.

This is the new fixing salt of Meynier which is said to be endowed with properties for photographic purposes as powerful as those of cyanide of potassium, without having the poisonous and otherwise deleterious properties of this salt. Meynier, I think, must have made a mistake as to this latter property. Sulphocyanide of ammonium may be formed by distilling the vapor of hydrocyanic acid into a solution of sulphide of ammonium and evaporating the solution at a very gentle heat; or still better by neutralizing hydrosulphocyanic acid by means of potassa.

Hydrosulphocyanic Acid.

Symbol, Cy S₂ H.

This acid is analogous with the hydracids; it is obtained as a colorless liquid by decomposing sulphocyanide of lead by means of dilute sulphuric acid; and sulphocyanide of lead results from the decomposition of sulphocyanide of potassium with acetate of lead.

Hyposulphite of Soda.

Symbol, N₄ 0, S₂ O².

This very important salt is obtained by digesting sulphur in a solution of sulphite of soda, which dissolves a portion of sulphur. By slow evaporation the salt crystallizes. Hyposulphurous acid can not be isolated from any of its combinations. When this salt is pure it produces no precipitate with nitrate of baryta. The crystals contain five equivalents of water, and are soluble in a very high degree in this menstruum. Its taste is nauseous and bitter.

The photographic properties of the three salts, whose preparations have been just indicated, are to dissolve the chloride, iodide, and bromide of silver in their recently formed state, without acting as solvents on the altered chloride, iodide, and bromide, after decomposition by light and developers. In all cases of solution they form cyanide, sulphocyanide, or hyposulphite of silver, which frequently enters into combination with the solvent and gives rise to a double salt, as the hyposulphite of silver and the hyposulphite of soda, together with either chloride, bromide, or iodide of sodium. Chloride and bromide of silver are soluble to a greater extent than iodide of silver in hyposulphite of soda. Cyanide of potassium is not only a solvent of the silver salts above mentioned, but also a reducing agent; it thus produces in the ambrotype and the melainotype a whiteness in the silver film which can not be effected with hyposulphite of silver. For this reason it is regarded by many photographers as the fixing agent peculiarly adapted for collodion positives by reflected light; whereas in the negative, where the whiteness of the silver film is of little or no consequence, hyposulphite of soda is regarded as the proper fixer. Many photographers disregard these refined distinctions, and use, in consequence of the superior solvent properties of cyanide of potassium, this substance as a fixing agent indifferently for negatives and positives. But because cyanide of potassium dissolves the silver salts so easily, it has to be used in a dilute condition, and to be watched very closely, otherwise it will dissolve at the same time the fine parts of the image. Another reason why cyanide of potassium is preferred in all collodion operations, arises from the difficulty of washing the hyposulphite of soda and of silver from the collodion film; for if any trace of these salts be left, the collodion film will eventually be destroyed by crystallization taking place on its surface, accompanied with a decoloration and soiling of the image.