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
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
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
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.
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
Hyposulphite of Soda.
Symbol, N4 0, S2
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.
Fixing Solution with Cyanide of Potassium.
|Cyanide of potassium,
Fixing Solution with Hyposulphite of Soda.
|Hyposulphite of soda,
Fixing Solution with Sulphocyanide of Ammonium.
|Sulphocyanide of ammonium,