The ferrotype and how to make it - The Grian Press

100 THE FEREOTYPE.
composed by its action, and the picture would be
thus lost or rendered useless. It is, therefore,
necessary to remove this salt by applying some
chemical agent capable of dissolving it.
Cyanide of potassium is a compound of cyano-
gen gas with the metal potassium. Cyanogen is
not an elementary body, like chlorin or iodine,
but consists of carbon and nitrogen united in a
peculiar manner. Although a compound sub-
stance, it reacts iu the manner of an element,
and is, therefore, an exception to the ordinary
laws of chemistry.
The cyanide of potassium is the salt most
frequently employed in fixing. It occurs in
commerce in the form of fused lumps of con-
siderable size; in this state, it is usually con-
taminated with a large percentage of carbonate
of potash, amounting in some cases to more than
half its weight. By boiling in proof spirit, the
cyanide may be extracted and crystallized, but
this operation is scarcely required, as far as its
use in photography is concerned.
Cyanide of potassium absorbs moisture on ex-
posure to the air, and is very soluble in w^ater,
but the solution decomposes on keeping, changing
in color, and evolving the odor of prussic acid,
which is a cyan'de of hydrogen. Cyanide of
potassium is highly poisonous, and should, there-
fore, be used with caution.
Solution of cyanide of potassium is a most
energetic agent in dissolving the insoluble silver