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.