Cyanogen (Gr. kvavoς, blue, and yevvaεiv, to produce), a principal ingredient in Prussian blue, being a compound gas consisting of two atoms of carbon and one of nitrogen, and properly designated dicyanogen, the chemical equivalent of which is 60. It is of particular interest, being the first instance known of a compound body performing the part of an element in its combinations. It was discovered by Gay-Lussac in 1814, and may be obtained by decomposing the cyanide mercury in a small glass retort by the heat of a spirit lamp; the mercury sublimes, and the gas passes over. It may also be obtained in combination by heating nitrogenous bodies, as clippings of hides, hoofs, etc, in a close vessel, together with iron and potash or carbonate of potash. The gas as it is produced combines with the potassium and iron to form a ferro-cyanide. Cyanogen is a colorless gas of specific gravity 1 '80, possessing a strong pungent odor similar to that of the kernels of peach stones or of prussic (hydrocyanic) acid. It is inflammable, burning with a blue and purple colored flame, and passing into carbonic acid gas and nitrogen. By the cold of - 22° F. or the pressure of 3'6 atmospheres it may be liquefied, forming a thin colorless fluid; at the freezing point of mercury it becomes solid.

It is absorbed by water, but is soon decomposed in this condition, and forms compounds with the water possessing acid reaction, besides many others of the different elements variously combined. Exposed to a high temperature, the gas is not decomposed; but mixed with two volumes of oxygen, it explodes violently at a red heat, or by the electric spark, separating into carbonic acid and nitrogen. The properties of cyanogen in relation to other bodies are analogous to those of chlorine, bromine, and iodine. It forms an acid with hydrogen (hydrocyanic or prussic acid), and binary compounds with the metals, cyanides, or cyanurets, which readily combine among themselves or with the chlorides and sulphurets, forming double cyanurets, chlorocyanurets, and sulphocyanurets. With oxygen cyanogen unites to form several acids, as cyanic acid, CyHO; fulminic acid, Cy2H2O2; and cyanuric acid, Cy3H3O3. The first is a volatile colorless fluid, with the odor of acetic acid. Its salts are cyanates. Paracyanogen, C3N3, is the brown matter which remains in the retort after the preparation of cyanogen.

It is insoluble in water, is neither volatile nor fusible, and like cyanogen enters into combination with other elementary bodies.