In this paper the author shewed that pure dry bicyanide of mercury, when heated in close vessels, gave off mercury and pure cyanogen only, yet that there remained always in the retort a portion of a black substance of variable density and lustre. He then proceeded, by a new series of analyses, to confirm what he had stated several years ago, that this black substance is identical in composition with cyanogen, being, like that gas, represented by the formula NC2.