Hydrocyanic or prussic acid (C_{2} N, H,) a very volatile liquid, soluble in water, is one of those mysterious vegetable compounds which, containing no chemical element that is not also contained in animal structure, are yet gifted with a wonderful power over the vital forces. One drop of the pure acid is sufficient to cause death. But the officinal acid is an aqueous solution, containing only 2 per cent. of the other. It is a powerful Sedative to the nerves generally. Its action is direct, and is not preceded by any primary stimulation. Diminishing sensibility, it is therefore used as an anodyne. Controlling muscular action, it is employed as an anti-spasmodic. But it appears especially to influence the reflex nervous actions. It is therefore of especial use in the paroxysmal coughs of Hooping-cough and of Phthisis, which have a reflex origin in the irritation of the mucous membrane. It acts on nerves as well as on nerve-centres. Thus it is particularly useful in neuralgic forms of Gastrodynia; and it seems to allay the pain in the stomach by diffusing itself directly it reaches it, so passing through the mucous membrane at all points, and coming in contact every where with the irritated sensory nerves. It is sometimes useful in cases of vomiting, or pyrosis. If applied to the surface of the skin, it numbs the superficial nerves; but on account of its rapid volatility, and the dangerous results that would follow the inhalation of its vapour, it is not appropriate as an external anaesthetic. A poisonous dose produces first convulsions of the whole frame, then syncope, and death. Strychnia produces convulsions, but they are not caused in the same way as those of Hydrocyanic acid. For the two medicines are obviously opposed in their action, Strychnia being an exalter of sensation and of reflex action, and Hydrocyanic acid a depresser of the same. The convulsion caused by Strychnia is tonic; that by Hydrocyanic acid, asthenic. The one, being a special Stimulant, causes convulsions by an excitement of the spinal cord; the other, being a Sedative, does it by deranging the function of that centre. (_Vide_ pp. 245, 243.) ACONITE. Class II. Div. III. Ord. I. SEDANTIA GENERALIA. This medicine, like the last, is a powerful General Sedative, and a dangerous poison. It depresses the influence of the brain, and paralyzes all the nervous functions. It tends at length to kill by producing syncope.