Origin

This is the concrete exuded juice of different species of Acacia, thorny shrubs or trees, growing in various parts of Africa and Arabia. Acacia vera and A. Arabica have been usually considered as the most abundant sources of it. The gum-producing species generally grow in the deserts, and yield their product during the dry season, whin the juice exudes through fissures in the bark, and hardens on the surface. The finest is brought from Upper Egypt and Nubia, and, being exported from Alexandria, is usually called Turkey gum. Other varieties are brought into the market, under the designations of Senegal, India, and Barbary gums, etc. Though portions of these are of excellent quality, yet, on the whole, the Turkey gum is the purest and best for medical use, and should, I think, be exclusively kept for such purposes in the shops.

Properties

When unbroken, gum arabic is in pieces of irregular size and shape, usually roundish, and either translucent, or of a somewhat opaque appearance, in consequence of innumerable minute cracks or fissures with which they are pervaded. Each separate fragment, however, of such pieces is transparent. The best variety generally comes to us already broken into small angular fragments, which are beautifully shining, of a whitish colour, sometimes tinged with yellowish or reddish-brown, hard, brittle, and of a glossy fracture. The powder is purely white. Gum arabic is inodorous, of a very feeble, slightly sweetish taste, very soluble in water both hot and cold, and insoluble in alcohol, ether, or the oils. Alcohol precipitates it from a strong aqueous solution. Pieces of it held in the mouth slowly dissolve in the saliva. A solution of it precipitates a solution of Subacetate, but not of acetate of lead, is gelatinized by sesquichloride of iron, and precipitated by nitrate of mercury.

Composition, etc

Gum arabic consists of arabin or pure gum, with some uncombined water, and a little saline matter, yielding, when the gum is burned, about three per cent. of ashes. Arabin or pure gum is composed of carbon, hydrogen, and oxygen, the two latter ingredients being in mutually saturating proportions, so that the gum may be said to be a compound of carbon and water.

if perfectly dry, gum arabic may be kept any length of time unchanged. its solution, which is usually called mucilage, speedily becomes sour on exposure, especially in warm weather, in consequence of the production of acetic acid.

Effects and Uses. As regards any dynamic operation on the system, gum arabic is probably quite inert. it is simply a bland article of food, of comparatively slight nutritious power. As a demulcent, it has the properties belonging to the class; but retaining water with less tenacity than some other gummy substances, and becoming hard and stiff upon the drying of its solution, it is not well adapted for application to the exterior surface of the body. For internal use it is one of the best, perhaps, upon the whole, the best of the demulcents, and is among those which are most used. A lump of it held in the mouth, and allowed slowly to dissolve, answers an excellent purpose in allaying coughs dependent on irritability of the fauces, and in soothing the inflamed membrane in angina. The solution may be used in all acute febrile and inflammatory cases, and is particularly adapted to inflammation of the mucous membrane of the stomach and bowels. To cases in which acrid poisons have been swallowed, it is highly appropriate. it is also used freely in inflammatory conditions of the bronchial and urinary passages; but I am not confident that, in these affections, it operates in any other way than by pure dilution, in other words, by the quantity of water taken with it. it not only acts as a demulcent, but affords also the kind of diet adapted to the highest stage of febrile and inflammatory excitement. To children it is peculiarly well suited, in both these capacities, in consequence of its want of taste.

For ordinary use, an ounce of the gum may be dissolved in a pint of water. if preferred, the solution may generally be flavoured with sugar and lemon-juice. in almost all cases, it should be taken cold.

Gum arabic is also much used for suspending insoluble substances, whether solid or liquid, in water, for the convenience of administration; and in the preparation of pills. Two drachms of it may be used in a mixture of six fluidounces.

The Mucilage of Gum Arabic (Mucilago Acact;e, U. S., Br.; Mistura Acaciae, Lond,; Mucilago, Ed.) is an officinal preparation, made, according to the U. S. Pharmacopoeia, by dissolving four troyounces of the gum in half a pint of boiling water. Each fluidounce contains half an ounce of the gum; and half a fluidounce may, therefore, be prescribed in preparing a mixture or emulsion of six fluidounces. When the powder to be suspended is heavy, a larger proportion may be used.

A Syrup of Gum Arabic (Syrupus AcacIae, U. S.) is now directed in the U. S. Pharmacopoeia, made by dissolving in half a pint of water, first two troyounces of gum arabic without heat, then fourteen troyounces of sugar with a gentle heat, and straining. The syrup is used in the preparation of liquid mixtures, pills, and lozenges.

Under the name of gum pectoral, a demulcent compound is prepared by dissolving equal parts of gum and sugar, and evaporating the solution. it dissolves more slowly than gum, and on this account, as well as for its flavour, it is often used for the purposes of a demulcent lozenge in catarrh and sore-throat.