Pure anhydrous hydrocyanic acid is so powerful, and so extremely dangerous if abused, and withal is so liable to spontaneous change, and therefore to uncertainty of strength, that it has been discarded from the officinal lists, and should not be kept in the shops. The medicine should be used only in the dilute form, either as nature offers it to us in various vegetable products, or as prepared according to the direction of the officinal codes. Numerous plants belonging to the genera Cerasus, Prunus, and Amygdalus - the cherry, plum, almond, peach, etc. - yield hydrocyanic acid in connection with a peculiar volatile oil, when treated with water. The bark, leaves, blossoms, and kernel of the fruit are the parts from which these products may be most abundantly obtained. All these parts have a peculiar aromatic odour, and a not unpleasant bitterness, which they owe in part at least to the associated acid and oil alluded to, and which are quite characteristic. Neither the hydrocyanic acid nor the oil exists, in any considerable proportion, in the parts of the plants mentioned; but both are the product of a reaction between a principle contained in the plant, and water. The principle referred to is a bitterish substance named amygdalin, which, through the agency of another substance, of nitrogenous composition and albuminous character, denominated emulsin, also contained in the plant, and with the presence of water, is converted into hydrocyanic acid and a volatile oil, which come over together in distillation, and which, thus obtained, are identical with the well and long-known essential oil of bitter almonds. The emulsin acts merely as a kind of ferment; the real change taking place between the amygdalin and the water. Hence it is that the various vegetable products mentioned, when quite dry, are destitute of the characteristic odour; and they have the characteristic taste and effects on the system, when swallowed, only because they find, in the saliva and the gastric liquids, the water necessary to the formation of the new bodies. The following are the parts of these plants officinally recognized, along with their several preparations; the wild cherry bark, which has already been treated of among the tonics, being omitted. After these will come an account of the several officinal preparations of hydrocyanic acid itself.