Gum, a general term applied to certain exudations from trees and plants, which are very different in their chemical characters and their general properties. There are, however, three classes of gums which may be more particularly referred to—viz. those containing arabin, those containing bassorin, and gum-resins.
(1) Gums containing arabin are best represented by gum-arabic, the ordinary gum of the shops. This substance is found as an exudation on the bark of the Acacia Senegal, a tree of some 20 feet in height, growing abundantly in western Africa. According to the care taken in collecting it, it ranges from the pure white or colourless gum of Kordofan to the dark-reddish varieties imported from Senegal. Chemically these are absolutely identical, and therefore a single description will suffice. It occurs in irregular lumps, somewhat spherical or vermicular (as in Gum-Senegal). It is brittle, and shows a glassy fracture. It dissolves readily in water, forming a clear, viscid, adhesive solution; but it is insoluble in strong alcohol, glycerine, ether, oils, or chloroform. The addition of alcohol to a watery solution throws down a precipitate of arabin, if a few drops of hydrochloric acid have previously been added.
Medicinally it has very slight remedial powers, but it is largely used in prescriptions for the purpose of suspending insoluble substances in mixtures. The finer varieties, owing to their cost, are rarely found outside the druggist's shop; but in the manufacture of confections and in the arts large quantities of the cheaper kinds are employed. These are known under different names, indicating the district from which they are imported. The chief are: Senegal gum, found in large firm reddish masses; Suakin gum or Talka gum, forming dull opaque-looking tears, colourless or brownish; and Morocco or Barbary gum. Cape gum is derived from the Acacia horrida, a native of Cape Colony, while Wattle gum is a very adhesive variety obtained from Australia. East Indian gum is an African product, being simply imported into Bombay from the Red Sea.
(2) Gums containing Bassorin.—The chief of these, Tragacanth, is obtained from various species of Astragalus, low spiny bushes, natives of Asia Minor and Persia. When the stem of one of these plants is cut transversely it will be found that the space usually occupied by the pith has the appearance of a translucent gummy mass, which the microscope shows to possess the structure of an ordinary pith. If incisions are made in the bark, this semi-solid exudes under pressure, and, according to the nature of the incision forms flattened wing-like masses, nodules or worm-like pieces. The finest variety is known as Flake-Tragacanth, consisting of flakes 1 to 3 inches long by 1 inch in breadth. The surface is marked by wavy lines and the flakes are much contorted. Tragacanth is translucent, white and without lustre, somewhat flexible, and not brittle, and with little taste or smell. When placed in water it swells, absorbing fifty times its weight of that liquid, and form- ing a thick mucilage. It has no active medicinal properties; but it is much used for firming pill masses and lozenges. It enters into many emulsions, for instance, that of cod-liver oil, and it is sometimes employed as a stiffener for the hair. It is used as a stiffening material for various textile fabrics, and is much valued for this purpose, where it is not desired to give gloss to the material.
Besides these true gums, there are (3) the gum-resins. In general terms these consist of certain resins soluble in alcohol, and of the true gum, so that it requires both water and alcohol to dissolve them entirely. They are chiefly used in medicine and perfumery, and may be said to form a connecting link between the true gums and the true resins, commercially speaking. The principal are: (1) Gum-Ammoniacum (see AMMONIACUM), (2) Gum-Asafœtida (see ASAFÆTIDA), (3) Gum-Benzoin (see BENZOIN), (4) Gum-Galbanum (see GALBANUM), (5) Gum-Gamboge (see GAMBOGE), (6) Gum-Myrrh (see MYRRH), (7) Gum-Scammony (see SCAMMONY). There are many other gums known; but these are the ones most used in the arts and medicine. Many also of the true resins, as copal, anise, &c., are called gums, but they are strictly resins. See RESINS.
Gum-substitutes are manufactured from various forms of starch, either by baking, roasting or chemical treatment, so as to convert the starch into Dextrine (q.v.). They are made on a very extensive scale, and are largely employed in dressing calicoes and other fabrics, also as a substitute for the more expensive gums in gumming-paper, as in the case of postage-stamps and labels, which are made adhesive by dextrine. For this and some other purposes, the gum-substitutes are superior to the real gums, as they are easily dissolved, and can be spread more equally over a smooth surface. For the chewing-gum in use in the United States, see CHEWING-GUM; and for gum-trees, see EUCALYPTUS, TUPELO, and LIQUIDAMBAR.