CYANOGEN, a gas composed of carbon and nitrogen. The name was suggested by Prussian blue (Gr. Kuanos, blue), its earliest known compound. Cyanogen (C.2N2) was first isolated in 1815 by J. Gay-Lussac, who obtained it by heating mercury or silver cyanide ; this discovery is of considerable historical impor tance, since it recorded the isolation of a "compound radical." It may also be prepared by heating ammonium oxalate, by passing induction sparks between carbon points in an atmosphere of nitro gen, or by the addition of a concentrated solution of potassium cyanide to one of copper sulphate, the mixed solutions being then heated. It also occurs in blast-furnace gases. When cyanogen is prepared by heating mercuric cyanide, a brown residue known as para-cyanogen (CN ),, is left ; this is to be regarded as a polymeride of cyanogen. Cyanogen is a colourless gas, possessing a peculiar characteristic smell, and is very poisonous. It burns with a purple flame, forming carbon dioxide and nitrogen ; and may be condensed (by cooling to —25° C.) to a colourless liquid, and further to a solid, which melts at C. (M. Faraday, 1845). It dissolves readily in water and the aqueous solution decomposes on stand ing; a dark-brown flocculent precipitate of azulmic acid, separating, whilst ammonium oxalate, urea and hydrocyanic acid are found in the solution. In many respects it resembles chlorine in its chemical behaviour, a circumstance noted by Gay-Lussac ; it combines directly with hydrogen (at 500° to 55o° C.) to form hydrocyanic acid, and with chlorine, bromine, iodine and sulphur, to form cyanogen chloride, etc. ; it also combines directly with zinc, cadmium and iron to form cyanides of these metals. It combines with sulphuretted hydrogen, in the presence of water, to form the compound and in the presence of alcohol, to form the compound Concentrated hydrochloric acid converts it into oxamide. Potash solution converts it into a mixture of potassium cyanide and cyanate. With tin and hy drochloric acid it yields ethylenediamine.