pyrogallol and soda in the development of bromo-gelatine plates.

The ferri or ferrid cyanide of potassium discovered by Gmelin is often,
but improperly, termed red prussiate of potash. It is formed by passing a
current of chlorine gas through a solution of ferrocyanide of potassium
until the liquid ceases to give a precipitate with a salt of sesquioxide
of iron, and acquires a deep, reddish-green color. The solution is then
evaporated, crystallized, and recrystallized. It forms regular prismatic
or tabular crystals, of a beautiful ruby-red tint, permanent in the air,
soluble in four parts of cold water. The crystals burn when introduced
into the flame of a candle, and emit sparks.

The theory of the formation of this salt is, that one eq. of chlorine
withdraws from two eq. of the ferrocyanide of potassium, one eq. of
potassium, forming chloride of potassium, which remains in the mother
liquid. The reaction is explained by the following equation:
2(K_{2}Cfy)+Cl=K_{3}Cfy_{2}+KCl.

The radical ferridcyanogen, isomeric[2] with ferrocyanogen, is supposed
to be formed by the coalescence of two equivalents of ferrocyanogen, and
is represented by the symbol Cfdy; accordingly the formula of
ferridcyanide of potassium is K_{3}Cfdy.

[Footnote 2: Isomeric bodies, or substances different in properties yet
identical in composition, are of constant occurrence in organic
chemistry, and stand among its most peculiar features.]

Ferridcyanide of potassium has found extensive application in
photographic processes for intensifying negatives; those of Eder, in
combination with nitrate of lead, or Selle's, with nitrate of uranium;
Ander's blue intensification of gelatine negatives, Farmer's process of
reducing intensity, the coloring of diapositives, the very important blue
printing, and various others, are daily practiced in our laboratories.

The ferrocyanide of potassium is a chemical reagent of great value,
giving rise to precipitates with the neutral or slightly acid solutions
of metals, like the beautiful brown ferrocyanide of copper, and that of
lead. When a ferrocyanide is added to a solution of a sesquioxide of
iron, Prussian blue or ferrocyanide of iron is produced. The exact
composition of this remarkable substance is not distinctly stated, as
various blue compounds may be precipitated under different circumstances.
Berzelius gives the following account: 3 eq. of ferrocyanide and 2 eq. of
sesquioxide of iron are mutually de