After a short statement of the discovery of paracyanogen by Professor Johnston, and of its leading properties, the author proceeded to shew how, with certain precautions, cyanogen may be converted entirely, or nearly so, into the isomeric form paracyanogen. This he accomplished by exposing bicyanide of mercury suddenly to the temperature most favourable to the production of paracyanogen, which he found to be a low red heat, and employing also pressure, by confining the cyanogen gas which is at first expelled. By these means he succeeded in resolving the salt almost entirely into mercury and paracyanogen, the latter of which amounted in some trials to more than nine-tenths of the cyanogen contained in the bicyanide. The pressure required was not quite two atmospheres, namely 1.74.