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