Prussiate of potash is the commercial name given to what chemists
call “ferrocyanide of potassium,” it is used largely for the
production of Prussian blue, and also in the art of electro-plating,
which, of late, has greatly increased the demand for its manufacture,
and many patents have been taken out for various modifications in the
form of production, the result, however, is the same, and it occurs
in commerce in magnificent crystals of a square form, with bevelled
edges, and of a yellow color; there is, however, another kind called
the “red prussiate,” which is much used in the process of calico
printing. Prussiate of potash is made by heating together carbonate
of potash, scraps of iron, and any kind of animal refuse, as blood,
horns and hoofs, clippings of skin or old woollen rags; these are
stirred together, and the heat raised, till they form a sort of paste
of a black color, which, when cold, is thrown into water and all the
soluble parts washed out; this fluid is filtered till quite clear,
evaporated, and set aside that crystals may form; these are dissolved
and again crystallized. The red prussiate is made by passing chlorine
over the yellow prussiate in powder dissolving and crystallizing.
The most deadly of all poisons, prussic or hydrocyanic acid, is
obtained from the ferrocyanide of potassium, or prussiate of potash,
by adding diluted sulphuric acid and distilling the vapour into an
ice-cold receiver. This experiment should never be made by any but
those well conversant with such matters.