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, vol. vii. p. 146.
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).
Gum arabic occurs in pieces of varying size, and some kinds are full of minute cracks. The specific gravity of Turkey picked gum (the purest variety) is 1 487, or, when dried at 100 C. , 1 525. It is soluble in water to an indefinite extent; boiled with dilute sulphuric acid it is converted first into dextrin, and then into a fermentable variety of sugar. Moderately strong nitric acid changes it into mucic, saccharic, tartaric, and oxalic acids. Under the influence of yeast it does not enter into the alcoholic fermentation, but Berthelot, by digesting with chalk and cheese, obtained from it 12 per cent, of its weight of alcohol, along with lactate of calcium, but no appreciable quantity of sugar. According to Fremy, gum arabic may be regarded as a potassium and calcium salt of gummic or arabic acid, of the formula CaK 3 C 18 H sa 11 .6Ci S H ss O 11 . Graham (Chemical and Physical Re searches] gives dialysis as the simplest and best mode of preparing gummic acid, ami states that the power of gum to penetrate the colloid septum is 400 times less than that of chloride of sodium, and further that by mixing the gum with substances of the crystalloid class_ the diffusibility is lowered, and may be even reduced to nothing. The mucilage must be acidulated with hydrochloric acid before dialysing, to set free the gummic acid. Gummic acid reddens litmus, its reaction being about equal to carbonic acid. When solutions of gum arabic and gelatin are mixed, oily drops of a compound of the two aro precipitated, which on standing form a nearly colourless jelly, melting at 25 C. , or by the heat of the hand. This substance can be washed without decomposition. Gummic acid is soluble in water; when well dried at 100 C., it becomes transformed into metagummic acid, which is insoluble, but swells up in water like gum tragacanth.
Gum arabic, when heated to 150 C. with two parts of acetic anhydride, swells up to a mass which, when washed with boiling water, and then with alcohol, gives a white amorphous insoluble powder called acetyl arabin C 6 H 8 (C 2 H 3 0) 2 5 . It is saponified by alkalies, with reproduction of soluble gum. Gum arabic is not precipitated from solution by alum, stannous chloride, sulphate or nitrate of copper, or neutral lead acetate ; with basic lead acetate it forms a white jelly, with ferric chloride it yields a stiff clear gelatinoid mass, and its solutions are also precipitated by borax.
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:—
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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.[1]
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.
In colour gum tragacanth is of a dull white ; it occurs in horny, flexible, and tough, thin, twisted flakes, translucent, and with peculiar wavy lines on the surface. When dried at temperatures under 100 C. it loses about 14 per cent, of water, and is then easily powdered. Its specific gravity is 1 384. With water it swells by absorption, and with even fifty times its weight of that liquid forms a thick mucilage. Part of it only is soluble in water, and that resembles gummic acid in being precipitated by alcohol and ammonium oxalate, but differs from it in giving a precipitate with neutral lead acetate and none with borax. The insoluble part of the gum is a calcium salt of bassorin (C ]2 H 20 10 ), which is devoid of taste and smell, forms a gelatinoid mass with water, but by con tinued boiling is rendered soluble, owing, according to M. Giraud, to its being changed into pectin. A small quantity of starch is always naturally present in gum tragacanth. The composition of gum tragacanth is generally given as arabin 53 per cent., bassorin and starch 33, water 11 (but M. Giraud considers its composition to be water 20 per cent.), mineral matter 3, a pectinous principle (apparently identical with the pectose of Fremy) 60, soluble gum 10, cellulose 3, starch 3, mineral matter 3, with traces of nitro genous matter. Graham (Chemical and Physical Researches, p. 589) speaks of its being likely that native gums insoluble in water are the pectous form of soluble gum, and Giraud s experiments point to the correctness of that surmise.
- ↑ For varieties of gum arabic, see Phamiacographia, pp. 209, 210 ; Catalogue Museum Pharm. Soc., pp. 37, 38 ; also Pereira, Mat. Med., vol. ii., pt. ii., p. 334 ; for physiological properties, Pavy, Foods and Dietetics, 2d ed., pp. 112, 438 ; and for chemistry, Watts s Dictionary, vol. ii. p. 953.
