PRUSSIC acid, in chemistry and the arts, is one of the most important of the acids. It was discovered by accident, about the beginning of the last century, by Diesbach, a chemist of Berlin. This gentleman, wishing to precipitate a decoc tion of cochineal with an alkali, got some potash, on which he had distilled several times his animal oil, and as there was some sulphate ofiron in the decoction, the liquor instantly exhibited a beautiful blue in the place of a red precipitate. Hence he saw the method of producing the same sub stance at pleasure, and it soon became an object of commerce, and obtained the name of Prussian blue, from the place where it was discovered. This substance is now formed, chiefly, during the decom position of animal substances in high tem peratures. Three parts of blood, evapo rated to dryness in an iron dish, are to be mixed with one part of subcarbonate of potash, (common pearlash) and calcined in a crucible, which should be only two thirds filled by the materials, and covered with a lid. The calcination must be con tinued, with a moderate heat, as long as a blue flame issues from the crucible ; and when it becomes faint, and likely to be extinguished, the process must be stopped. Throw the mass, when cold, into ten or twelve parts of water ; allow it to soak a few hours, and then boil them together in an iron kettle. Filter the li quor, and continue pouring hot water on the mass as long as it acquires any taste. To this solution, add one composed of two-parts of alum and one of sulphate of iron, in eight or ten of boiling water, and continue the mixture as long as any effer vescence and precipitation ensues. Wash the precipitate several times with boiling water. It will have a green colour ; but, on the addition of a quantity of muriatic acid, equal to twice that of the sulphate of iron which has been used, it will assume a beautiful blue colour. Wash it again
with water, and dry it in a gentle heat. In this state it is the pigment, called Prus sian blue, which consists of a mixture of prussiate of iron with alumine. From prussiate of iron, the prussic acid may be separated by the following process : mix two ounces of red oxide of mercury, pre pared by nitric acid, with four ounces of finely powdered Prussian blue, and boil the mixture with twelve ounces of water in a glass vessel, shaking frequently.— Filter the solution, which is a prussiate of mercury, while hot, and when cool, add to it, in a bottle, two ounces of iron fil ings, and six or seven drachms of sulphu ric acid ; shake these together, .decant the clear liquor into a retort, and distil off one-fourth of the liquor. The distilled liquor is the prussic acid, which combines with alkalies and earths, and has many of the properties belonging to the other acids. It has a sweetish taste, and a smell resembling that of bitter almonds ; it does not redden blue vegetable colours. It precipitates sulphurets, and curdles soap. It separates alumine from nitric acid. Oxygenized muriatic acid entirely decomposes it. It does not appear to have a strong affinity for alkalies, nor does it take them from carbonic acid, for no ef. fervescence arises on adding it to a solu tion of alkaline carbonates ; on the con trary, its combinations with alkalies and earths are decomposed by exposure to carbonic acid, even when highly diluted, as in atmospheric air. It readily com bines, however, with pure alkalies, de stroys their alkaline properties, and forms crystallizable salts. It does not precipi tate iron blue, but green, and this green precipitate is soluble in acids The rays of light render the green precipitate blue, as does also the addition of metallic iron, or sulphurous acid.