AMMONIACUA or AMMONIAC, a gum-resinous exudation from the stem of a perennial herb (Dorema ammonia-cum) belonging to the natural order Umbelliferm. The plant grows to the height of 8 or 9 feet, and its whole stem is pervaded with a milky juice, which oozes out on an incision being made at any part. This juice quickly hardens into round tears, forming the " tear ammonia-cum" of commerce. Lump ammoniacum, the other form in which the substance is imported, consists of aggregations of tears, frequently incorporating large quantities of the fruits of the plant itself, as well as other foreign bodies. In order to free lump ammoniacum from these impurities, it has to be melted and strained, operations which depreciate its therapeutical value. Ammoniacum has a faintly feetid unpleasant odour, which becomes more distinct on heating ; externally it possesses a reddish yellow appearance, and when the tears or lumps are freshly fractured they exhibit an opalescent lustre. It is chiefly collected_ in the province of Irak in Persia; but some quantity is also produced in the Punjab, and comes to the European market by way of Bombay. Its composition, according to Hagen, is - resin, 68.6; gum, 19-3; gluten, 5.4; volatile oil and water, 2.8 ; extractive, &c., 3.9. Ammoniacum is closely related to assafcetida, not only in the plant yielding it, but also in its therapeutical effects. It may be used as a substitute for assafcetida, although, containing a much smaller proportion of volatile oil, its effect is less powerful. Internally it is used in conjunction with squills in bronchial affections ; and in asthma and chronic colds it is found useful. It is, however, more used externally in the form of plasters, as a discutient or resolvent application in indolent tumours, affections of the joints, &c.

African ammoniacum is a totally different substance, though often confounded with the real gum-resin, which is produced only in the East. It is the product of an unknown plant growing in North Africa, and occasionally shipped to our markets from Marocco. It is a dark-coloured gum-resin, possessed of a very weak odour and a persistent acrid taste. A considerable commerce in it is carried on between Mogador and Alexandria, where it is in demand for purposes of fumigation.