Acacia arabica. (Acacia vera.) Acacia.
Nat. Ord. — Fabaceae, or Leguminosae. Tribe. — Mimoseae. Sex. Syst. — Polygamia Monoecia.
The Concrete Juice. Gum Arabic.
Description. — Acacia Arabica, also known by the name of Egyptian Thorn, or Egyptian Gum Arabic is a small tree or shrub, but which sometimes attains the hight of forty feet, with a trunk from twelve to sixteen inches in diameter. The leaves are alternate and doubly pinnate, with four or six pairs of pinnae, each being composed of from ten to twenty pairs of minute, smooth, oblong, linear leaflets, with a gland on the common petiole, below the first, and generally one between the last pair of pinnae. Spines straight and in pairs. Flowers yellow, in globose, pedunculated, axillary, subternate heads. The Legume is moniliform.
Acacia Vera is a tree of middling size, with a crooked stem covered with a smooth gray bark, numerous scattered branches covered with a yellowish-green or purplish bark. The leaves are alternate, bipinnate, and composed of two pairs of opposite pinnae, with numerous, small, oblong, linear, and smooth leaflets, supported on very short footstalks ; on the common petiole, and between each pair of pinnae, is a gland. The flowers are bright yellow, inodorous, small, and collected in globular heads about two together, and supported on slender, axillary peduncles, and furnished with two small bracts. The branches and petioles are glabrous ; the spines are in pairs, sharp, and from three to six lines long, and are situated at the insertion of each leaf, being united at their base. The legume is four or five inches long, moniliform, smooth, fiat, of a pale-brown color, and divided into several orbicular portions, in each of which is contained a single, flattish seed. The best quality of Gum Arabic is obtained from this tree.
History. — These trees grow in upper and lower Egypt, Senegal, and other parts of Africa, also flourish in Arabia, and in Hindostan, where their gum is used for food by the natives. The A. Arabica is the most widely diffused of the gum-bearing trees. Gum Arabic is likewise obtained from several other species of Acacia, but not so largely as from the two above-named. The gum of the Acacias exudes spontaneously from the bark of the trunk and branches, and hardens on exposure ; but incisions are sometimes made in order to facilitate the exudation. It exudes from the trees in the form of a thick and somewhat frothy juice, soon after the rainy season has softened their bark, and rendered it apt to split during the hot weather that succeeds. It is secreted in greatest abundance by old stunted trees, and in dry, hot seasons, and is thought by some to be the result of disease.
The best quality of Gum Arabic has a very pale, straw color, breaks with a vitreous fracture, is transparent, inodorous, insipid, and feels quite viscid in the mouth. It is generally in small, round, irregular lumps, of easy fracture. Its colored varieties are bleached by exposure to the light of the sun. Its specific gravity varies from 1.3 to 1.4. In powder it is always white.
Gum Arabic is soluble in cold or hot water, forming a viscid solution called mucilage, which, when evaporated, yields the gum unchanged. It is insoluble in alcohol, ether, and the oils, and is precipitated from its aqueous solution by alcohol. Concentrated acids decompose it. A solution of borax coagulates it. It unites with sugar in solution, which when evaporated yields an uncrystallizable, transparent, solid substance. It is also soluble in dilute acids, solutions of the pure alkalies, and lime-water.
A concentrated aqueous solution of Gum Arabic may be kept a long while, unless the weather be very hot, in which event it will ferment. A weak solution ferments speedily, and acetous acid is developed. Nitric acid changes pure gum into mucic or saccho-lactic acid. Analysis shows it to contain bi-malate, and muriate of lime, muriate and acetate of potassa, and some other matters.
Properties and Uses. — Nutritive and demulcent. Used in irritations or inflammations of mucous surfaces ; as, hoarseness, sore-throat, cough, gonorrhea, catarrh of the urinary bladder, dysentery, diarrhea, strangury, and tenesmus. It may be given, ad libitum, in the form of solution or lozenge; as an article of diet in cases requiring a rigid regimen, as in fevers, it is superior to any other substance ; it may be used for this purpose by dissolving the gum in powder, half an ounce, in five ounces of water, and sweetening with loaf-sugar, of which a tablespoonful may be given every two or three hours ; in low stages of fever, in typhoid fever, and wherever a mild stimulant is required, one ounce of a saturated solution of camphor in sulphuric ether may be added to the above, and administered in the same way ; it is diuretic, promotes the action of the absorbents, and does not materially increase arterial action. Equal parts of pulverized alum and Gum Arabic form a good preparation to check hemorrhages from small cuts, wounds, etc.
Externally, the application of its solution to burns and scalds has proved serviceable, repeating it until a complete coating is secured. It is likewise much used for compounding pills, lozenges, mixtures and emulsions ; also for administering insoluble substances in water, as oils, resins, balsams, camphor, musk, etc.
Mucilage of Gum Arabic. — To four ounces of finely pulverized Gum Arabic, add, very gradually, a pint of boiling water, and rub the whole until perfectly blended. Dose, ad libitum. When Gum Arabic is adulterated with cherry gum, it is not easy to form a good mucilage ; the cerasin of the cherry gum will cause it to be ropy.