Characters. - Colourless or white prismatic crystals, becoming dark-coloured on exposure to light; odourless, having a bitter metallic taste, and a neutral reaction.

Reactions. - When slowly heated the salt decomposes into metallic mercury and cyanogen gas, which is inflammable, burning with a purplish flame. On further heating, the blackish residue containing globules of metallic mercury is wholly dissipated. On adding hydrochloric acid to the aqueous solution, hydrocyanic acid vapour is evolved.

Tests. - A 5 per cent, aqueous solution of the salt, when mixed with a dilute aqueous solution of iodide of potassium, should not yield a red or reddish precipitate soluble in excess of either liquid (absence of mercuric chloride).

Dose. - 1/16 to 1/8 grain

Uses. - It may be given in syphilis. A solution of 5-10 grains in an ounce of water, painted on with a camel's-hair brush, is a useful application to syphilitic sores of the tongue or mouth.