Gum; a concrete vegetable juice; of no particular smell or tafte; becoming viscous and tenacious when moistened with water, totally diffolving in water into a liquid more or less glutinous in proportion to the quantity of the gum; not dissolving in vinous spirits or in oils; burning in the fire to a black coal, without melting or catching flame; suffering no dissipa-tion in the heat of boiling water.

1. Gummiarabicum Pharm. Lond. & Edinb, Gummi acanthinum & thebaicum quibusdam. Gum-arabic: the gum, exuding from the Egyptian acacia tree, (Mimofa nilotica Linn.) whose fruit affords the infpiffated juice of that name; brought to us from Turkey in small irregular masses, of a clear whitish or very pale yellowish colour.

The medical character of gum-arabic is its glutinous quality; in consequence of which, it serves to incrassate and obtund acrimonious thin humours, in tickling coughs, alvine fluxes, and other like disorders: Prosper Alpinus says, it is often used successfully by the Egyptians for restraining hemorrhagies. It is given chiefly in the form of powder, from a scruple to a dram or two; and sometimes dissolved in water, in such proportion as not to make the liquor disagreeably slimy. An ounce renders a pint of water considerably glutinous: four ounces give a thick syrupy consistence. The solutions mingle equally with vegetable and with mineral acids, and with neutral saline mixtures; but on the addition of alkalies, fist or volatile, the liquor grows turbid, and the dissolved gum separates.

Though the gum in its dry state is not affected by oily liquids, yet when softened with water into the consistence of a thick mucilage, it unites, by trituration, both with the fluid oils and the thicker balsams, so intimately, that the whole compound dissolves in water, without se-paration, into an emulsion or milky liquor; one part of gum-arabic, softened with an equal weight of water, is sufficient for rendering four parts or more of oil or balsam diffoluble(a). The solid resins may in like manner be reduced into emulsions, by grinding them thoroughly with powdered gum, and adding the water by degrees. By these means, all resinous and oily bodies may be dissolved in watery liquors, and thus excellently sitted for being taken in a liquid form, without any alteration in their smell, taste, or virtue. These emulsions, like the solutions of the gum itself, mingle uniformly with acids and neutral salts; but on the mixture of any alkali, they suffer immediately a separation of their parts.

2. Gummi senegalense Pharm. Paris. Gum-mi senega vel senica. Gum-senegal or senica: a gum brought'from the island of Senegal on the coast of Africa, said to be the produce of a tree of the same genus with that which affords the gum-arabic, acacia siliquis compressis Pharm. Paris. Mimosa senegal Linn. Greatest part of this gum is in larger and darker coloured masses than the arabic, and not smooth like it, but rough on the outside. In quality, the two sorts are scarcely different from one another, or from that which exudes from plum, cherry, and other trees among ourselves: in the shops, the clearer pieces of the gum-senegal generally sup-ply the place of the more costly gum-arabic. It is supposed that the Senegal gum is the strong-eft and mod substantial, and the Arabic the purest and fined.

(a) See on this subjeft the medical obsernations and inquiries published by asociety ofphysicians in London, voL i. art. 28./. 358.

3. Tragacantha Pharm. Lond. Gummi tragacantha Pharm. Edinb. Gummi tragacantha & dragacanthe. Gum-tragacanth or dragant: the gum exuding from a prickly bush of the same name, (tragacantha C. B. goats thorn; tragacantha cretica incana flore parvo lineis purpureis striato Tour. Astragalus Tragacantha Linn.) which grows wild in the warmer climates, and endures the cold of our own, but does not here yield any gum. This commodity is brought chiefly from Turkey, in irregular lumps, or long vermicular pieces bent into a variety of shapes; the bed sort is white, semitransparent, dry, yet some-what loft to the touch.

Gum-tragacanth differs from all the other known gums, in giving a thick consistence to a much larger quantity of water; and in being much more difficultly dissoluble, or rather dis-solving only imperfectly. Put into watar, it slowly imbibes a great quantity of the liquid, swells into a large volume, and forms a soft but not fluid mucilage: if more water be added, a fluid solution may be obtained by agitation, but the liquor looks turbid and wheyish, and on standing the mucilage subsides, the limpid water on the surface retaining little of the gum. Nor does the admixture of the preceding more soluble gums promote its union with the water, or render its dissolution more durable: when gum-tragacanth and gum-arabic are dissolved together in water, the tragacanth seems to sepa-rate from the mixture more speedily than when dissolved by itself.

Tragacanth is usually preferred to the other gums for making up troches, and other like purposes, and is supposed likewise to be the most effectual as a medicine; but on account of its imperfect solubility is unfit for liquid forms. It is commonly given in powder with the addition of other materials of similar intention: thus to one part of gum-tragacanth, arc added one of gum-arabic, one of starch, and six of sugar.