BENZOIC ACID, an organic acid present in large quantity in gum benzoin, and found also in dragon’s blood (the resin of Calamus Draco) and some allied substances. It is, besides, prepared by numerous reactions from organic substances, being now largely made from naphthalin, one of the products of the distillation of coal tar. Benzoic acid is extracted from gum benzoin by the process of sublimation. The resin, coarsely powdered, is submitted to a heat of 300° Fahr, in a close vessel, by which the acid is expelled and may be condensed in receivers. By the sublimation process the acid carries away with it a small portion of essential oil, which gives its peculiar sweet odour to sublimed benzoic acid. It may also be separated from gum benzoin by boiling the powdered gum in lime, filtering off the compound of resin and lime, and concentrating the remaining solution of benzoate of calcium, from which benzoic acid is precipitated by hydrochloric acid. The benzoic acid may then be purified by sublimation, but thus prepared it is destitute of odour. It crystallizes into beautiful white silky flexible needles, and yields on heating an acrid, irritating vapour which excites coughing. It is distinguished from the closely allied substance, cinnamic acid, by withstanding the action of boiling dilute nitric acid, which changes the other into bitter almond oil, the hydride of benzoyl. Benzoic acid is rarely employed in medicine alone, but in composition as benzoate of ammonia it acts as a stimulant of mucous membranes, and is occasionally given in chronic bronchial affections. It is an ingredient in some officinal tinctures, such as the compound tincture of camphor, and ammoniated tincture of opium.