Hydro-cyanic acid exists in a great variety of native combinations in the vegetable kingdom, and imparts to them peculiar qualities, which have been long known. It is, however, only within a few years, that this singular body has been obtained in its separate and independent form[432]; indeed it was not until the publication of the celebrated memoir of _Gay-Lussac_ upon this subject, in the year 1815, that its chemical composition was fully understood. In this memoir, it was clearly shewn to consist of a peculiar, gaseous, and highly inflammable compound of carbon and nitrogen, to which the name of _cyanogene_ has been assigned, and hydrogen; the latter body acting as the acidifying principle; whence the term _hydro-cyanic acid_ is very happily contrived to express its composition. When obtained in its most concentrated form, by the process of _M. Gay Lussac_,[433] it has the following characteristic properties, viz. At ordinary temperatures, it is liquid, colourless, and transparent; possessing an extremely powerful odour, very analogous to that of the blossom of the peach, or bitter almond tree; its taste is, at first, bland and sweetish, but afterwards pungent, bitter, and peculiar. Its extreme volatility is such, that when a drop of it is exposed to the air, on the end of a glass rod, it is rapidly crystallized. The same phenomenon takes place, if a drop be suffered to fall on a sheet of paper. Its specific gravity is ·7055; but, when in a concrete form it is only ·9, while that of its vapour is ·947. If inhaled, it produces almost immediate pain in the head, with deafness, unless very largely diluted with air or water.[434] It is decomposed by a high temperature; and by the action even of light it is, in the course of a very short time, resolved into carbonic acid, ammonia, and carburetted hydrogen, a carbonaceous matter remaining behind.[435] When brought near a body in a state of combustion, it instantly inflames and burns with a blue light. In water it is sparingly soluble; alcohol dissolves it copiously. The “_medicinal Prussic acid_,” as it has been called, as being the preparation lately introduced into medicine,[436] differs only from that we have just described, in its degree of concentration. It is, in fact, the Prussic acid of _Scheele_, and may be considered as equivalent to the preparation of _Gay-Lussac_ diluted with six times its volume, or eight times and a half its weight, of distilled water.