QUALITIES. A colourless transparent liquid, although it occasionally exhibits a yellow tinge; _Odour_ like that of bitter almonds; _Taste_ bitterish and peculiar: these properties however are soon lost by exposure to air and light, and the acid undergoes spontaneous decomposition. CHEMICAL COMPOSITION. The true nature of _Prussic_ acid was not ascertained until 1815, when Gay Lussac presented to the Royal Institute of France, a memoir which at once developed its real chemical constitution; and it is now admitted to consist of a peculiar gaseous and highly inflammable compound of carbon and nitrogen, to which the name _Cyanogene_ has been assigned, and of hydrogen; the latter body acting as the acidifying principle, whence the term _Hydro-cyanic_ acid is well contrived to express its composition. The medicinal, or _diluted_ acid, however, contains but a small proportion of this concentrated compound; according to M. Majendie, one part of the acid of Gay Lussac and eight parts and a half of water, by weight, or one part of acid with six times its volume of water, constitute the preparation which should be used in medicine; and which, to avoid the possibility of mistake, ought always to be prescribed as the Acidum Hydro-cyanicum _dilutum_, and is, in fact, the _Prussic acid_ of Scheele. Dr. Ure, who has lately taken considerable pains upon this subject, has constructed a table exhibiting the relations between the specific gravities, and quantities of real acid, in preparations of different strength; from these experiments it would appear that an acid of specific gravity 0·996 or 0·997 is such as is usually prescribed in medicine.[342] MEDICAL USES. In a sufficient dose, hydro-cyanic acid instantly destroys life by extinguishing the nervous energy of the body;[343] but it has at the same time been observed that animals submitted to its action would often continue to breathe for several hours freely, and to circulate their blood, although no trace of sensibility or muscular contractility could be found after its application. This remarkable property of extinguishing the general sensibility, without any ostensible injury to respiration and circulation, naturally led to a belief that the hydro-cyanic acid, or prussic acid, might be advantageously used in cases of excessive sensibility and irritation, particularly when these two morbid states are likely to affect either the respiratory organs or the circulation generally. This kind of analogical reasoning, it is said, induced Professor Brera, ten years ago, to administer it in cases of high pulmonary and other inflammations, in doses of four drops twice a day; when, as we are told, the violence of the disease was quickly subdued. The remedy, however, does not appear to have excited much attention, until after the first essay of Dr. Majendie, who deserves whatever credit may belong to its introduction. Seven years of trial have elapsed, and the general sense of the medical profession with