"The cyanide of potassium, so often alluded to while treating
of the metallo-cyanides, may be formed in several ways.
It may be obtained by heating to a dull redness the yellow ferrocyanate
of potash, in a covered vessel, filtering and rapidly evaporating it.
The objection to this method, however, is that without great care
the whole of the ferrocyanate is not decomposed, a circumstance
which much reduces its value for electro-metallurgy. By boiling,
however, the ignited residue with spirits of wine this difficulty
is said to be overcome, as the ferrocyanate is absolutely insoluble
in that menstruum, while the cyanuret, at that heat, freely dissolves,
and is as easily re-deposited on cooling.

"There is, however, a much better process by which this salt may
be formed, viz. by simply transmitting hydrocyanic acid through potassium.
Although the modes of making this acid are very numerous, there is but
one which is likely to be employed on a very large scale, and that is
its formation from the yellow ferrocyanate by means of sulphuric acid.
This process is performed as follows: any given weight of the yellow
salt is taken and dissolved in about five times its weight of water;
this is placed in a retort, or some such analogous vessel, to which is
then added a quantity of strong sulphuric acid, twice the weight of
the salt, and diluted with three or four times its quantity of water.
A pipe is carried from the neck of the retort to the receiving bottle,
which should be kept as cool as possible.

"For small operations, those invaluable vessels, Florence flasks,
answer well: a bent tube being connected at one end to its month,
the other passing into the second vessel; heat should be cautiously
applied by means of an Argand lamp, a little vessel of sand being
placed under the flask, which helps the acid to decompose the salt.