Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/289

Rh in Siberia, and several examples, chiefly immature birds, have been obtained in the British Islands. Both species of Xema are readily distinguished from all other Gulls by their forked tails.  GUM exists in the juices of almost all plants, but is produced in its purest form by various species of Acacia, e.g., Acacia Verek (Guil. and Perrottet), A. horrida, A. arabica (Wildeuow), A. Seyel (Delile), and A. stenocarpa (Hochst.). The name is applied to those exudations from plants, stems, branches, or fruits which are entirely soluble or soften in water, and form with it a thick glutinous liquid or mucilage insoluble in alcohol of 60 per cent., and yield mucic and oxalic acids when treated with nitric acid. In structure gum is quite amorphous, being neither organized like starch nor crystallized like sugar. According to Trecul, the acacias and the Rosacece yield their gums most abundantly when sickly and in an abnormal state, caused by a fulness of sap in the young tissues, whereby the new cells are softened and finally disorganized ; the cavities thus formed fill with liquid, which exudes, dries, and constitutes the gum. The chief varieties have hitherto been divided as follows:—(1)gum arabic, (2) gum tragacanth, (3) cherry tree gum, (4) gum of Bassora, (5) mucilage. The artificial gum dextrin has been already treated of,.

Gum Arabic may be taken as the type of the gums entirely soluble in water. The principal kinds are distinguished as Turkey Picked Gum, Gedda, Amrad, Gheziri, Senegal, Talca, Australian, Barbary, Cape, and East India (from Bombay and Aden). Another variety, spoken of as likely, from its abundance, to be soon in the market, is obtained from the Prosopis dulcis, a leguminous plant, and is called gum mesquite or mezquite ; it comes from Western Texas and Mexico, and is yellowish in colour, very brittle, and quite soluble in water (Pharm. Journ. (3), vol. vi. p. 942).

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The finer varieties are used as an emollient and demulcent in medicine, and in the manufacture of confectionery ; the commoner qualities are used as an adhesive paste, for giving lustre to crape, silk, &c., in cloth finishing to stiffen the fibres, and in calico-printing. For labels, &c., it is usual to mix sugar or glycerin with it to prevent it from cracking. Physiologically nothing is yet definitely known of gum as a food material. Animals fed thereon soon die of inanition. Lehmann says it is not absorbed by the system, but according to Pavy, it is to a slight extent, as shown by the formation of amyloid substance in the liver of subjects fed on gum. Gum Senegal, a variety of gum arabic produced by A. Verek, occurs in pieces generally rounded, of the size of a pigeon s egg, and of a reddish or yellow colour, and specific gravity 1 436. It gives with water a somewhat stronger mucilage than gum arabic, from which it is distinguished by its clear interior, fewer cracks, and greater toughness. It is imported from the river Gambia, and from Senegal and Bathurst, and is collected in December and March yearly.

Chagual gum, a new variety brought from St lago de Chili, resembles gum Senegal. About 75 per cent, is soluble in water. Its solution is not thickened by borax, and is precipitated by neutral lead acetate ; and dilute sul phuric acid converts it into dextro-glucose. The imports of gum arabic into England during 1878 (including all the varieties) maybe estimated as follows:—

Tons. Value. Imports to London, about 1064 40,000 Liverpool, ,, 1554 77,000 2618 117,000

Of this about one-half is exported. These figures are only approximate, as, since the repeal of the duties in 1845, the returns do not discriminate between the various descriptions which are all classed together under the head of gum.

Gum Tragacanth, familiarly called gum dragon, exudes from the stem, the lower part especially, of the various species of Astragalus, and is collected in Asia Minor, the chief port of shipment being Smyrna. Formerly only what exuded spontaneously was gathered ; this was often of a brownish colour; but now the How of the gum is aided by incisions cut near the root, and the product is the fine, white, flaky variety so much valued in com merce. The chief flow of gum takes place during the night, and hot and dry weather is the most favourable for its production.

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