Land. Ferrocyanide of Potassium.

This salt is prepared by calcining at a red heat a mixture of hoofs, horns, or blood, and impure potassa, in an iron vessel: a black carbonaceous mass is procured. The soluble parts are dissolved in water, and sulphate of protoxide of iron is added, until the Prussian blue which is formed ceases to be decomposed by the free potassa in the solution. The ferrocyanide is then crystallized, and procured free from sulphate of potassa, by repeated solution and crystallization.

Qualities.-Ferrocyanide of potassium is procured in large, translucent, four-sided tabular crystals, of a fine yellow colour, permanent in the air. It is inodorous; has a saline taste; water at 60° dissolves about one third, at 212° its own weight of the salt: it is insoluble in alcohol. When heated at a temperature of 212°, it loses its water of crystallization, 12.82 per cent., and becomes white; but recovers it in a moist temperature. When strongly heated in a retort, it is decomposed, giving off ammonia and hydrocyanic acid, whilst carbonate of potassa and sesquioxide of iron remain as a residue. It is a compound of 2 eq. cyanide of potassium = 131.08 + 1 eq. cyanide of iron = 54.39 + 3 eq. water = 27, making the equivalent 212.47. It is incompatible with proto and sesqui salts of iron, and salts of iron.

Medical properties and uses.-Ferrocyanide of potassium operates as a sedative, an astringent, and a diuretic. It is rarely employed in this country: but in America it is prescribed to reduce the pulse and allay pain. Dr. Smart1 prescribed it successfully in a case of chronic bronchitis in a child; it lowered the pulse, relieved the dyspnoea, and diminished the frequency and hardness of the cough. Dr. Smart has also found it useful in hooping-cough. Its astringent influence is manifested in checking sweats, the colliquative in chronic bronchitis and phthisis, and in lessening the discharge of leucorrhoea.

This salt may be administered in solution (3 ij. to f j. of water) in doses of xxx. to xl. to an adult, every four or five hours. When ove dosed it causes vertigo, coldness, numbness, and a sensation of sinking.

Officinal preparation.-Acidum Hydrocyanicum dilutum, L.