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Encyclopedia Britannica - Main :: GUI-HAN |
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GUM (Fr. gomme, Lat. gommi, Gr. K1, possibly a Coptic word; distinguish " gum," the fleshy covering of the base of a tooth, in O. Eng. gbma, palate, cf. Ger. Gaumen, roof of the mouth; the ultimate origin is probably the root gha, to open wide, seen in Gr. xaivety, to gape, cf. "yawn"), the generic name given to a group of amorphous carbo-hydrates of the general formula
starch
Gum arabic may be taken as the type of the gums entirely soluble in water. Another variety, obtained from the Prosopis dulcis, a leguminous plant, is called gum mesquite or mezquite; it comes from western Texas
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 too C., 1.525. It is soluble in water to an indefinite extent; boiled with dilute sulphuric acid it is converted into the sugar galactose. 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 M. P. E. Berthelot, by digesting with chalk and cheese, obtained from it 12 % of its weight of alcohol, along with calcium lactate, but no appreciable quantity of sugar. Gum arabic may be regarded as a potassium and calcium salt of gummic or arabic acid. T. Graham (Chemical and Physical Researches) recommended dialysis as the best mode of preparing gummic acid, and stated that the power of gum to penetrate the parchment septum is 400 times less than that of sodium chloride, 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. By adding alcohol to the solution, the acid is precipitated as a white amorphous mass, which becomes glassy at too'. Its formula
standing
Gum arabic, when heated to 15o' 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 C6H8(C2H30)205. 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. Gum senegal
Senegal
Chagual gum, a variety brought from Santiago, Chile, resembles gum senegal. About 75% is soluble in water. Its solution is not thickened by borax, and is precipitated by neutral lead acetate; and dilute sulphuric acid converts it into d-glucose. Gum tragacanth, familiarly called gum dragon , exudes from the stem, the lower part especially, of the various species of Astragalus, especially A. gummifer, and is collected in Asia Minor, the chief
chief
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 too C. it loses about 14% of water, and is then easily powdered. Its specific gravity is 1.384. With water it swells by absorption, and 716 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 (C12H200,o), which is devoid of taste and smell, forms a gelatinoid mass with water, but by continued boiling is rendered soluble. Gum tragacanth is used in calico-printing as a thickener of colours and mordants; in medicine as a demulcent and vehicle for insoluble powders, and as an excipient in pills; and for setting and mending beetles and other insect specimens. It is medicinally superior to gum acacia, as it does not undergo acetous fermentation. The best pharmacopeial preparation is the Mucilago Tragacanthae. The compound powder is a useless preparation, as the starch
Gum kuteera resembles in appearance gum tragacanth, for which the attempt has occasionally been made to substitute it. It is said to be the product of Sterculia urens, a plant of the natural order Sterculiaceae. Cherry tree gum is an exudation from trees of the genera Prunus and Cerasus. It occurs in shiny reddish lumps, resembling the commoner kinds of gum arabic. With water, in which it is only partially soluble, it forms a thick mucilage. Sulphuric acid converts it into l-arabinose; and nitric acid oxidizes it to oxalic acid (without the intermediate formation of mucic acid as in the case of gum arabic). Gum of Bassora, from Bassora or Bussorah in Asia, is some-times imported into the London market under the name of the hog tragacanth. It is insipid, crackles between the teeth, occurs in variable-sized pieces, is tough, of a yellowish-white colour, and opaque, and has properties similar to gum tragacanth. Its specific gravity is 1.36. It contains only 1% of soluble gum or arabin. Under the name of Caramania gum it is mixed with inferior kinds of gum tragacanth before exportation. Mucilage.Very many seeds, roots, &c., when infused in boiling water, yield mucilages which, for the most part, consist of bassorin. Linseed, quince seed and marshmallow root yield it in large quantity. In their reactions the different kinds of mucilage present differences; e.g. quince seed yields only oxalic acid when treated with nitric acid, and with a solution of iodine in zinc iodide it gives, after some time, a beautiful red tint. Linseed does not give the latter reaction; by treatment with boiling nitric acid it yields mucic and oxalic acids. Gum Resins.This term is applied to the inspissated milky juices of certain plants. which consist of gum soluble in water, resin and essential oil soluble in alcohol, other vegetable matter and a small amount of mineral
scammony
End of Article: GUM (Fr. gomme, Lat. gommi, Gr. K1, possibly a Coptic word; distinguish " gum," the fleshy covering of the base of a tooth, in O. Eng. gbma, palate, cf. Ger. Gaumen, roof of the mouth; the ultimate origin is probably the root gha, to open wide, seen in If you wish, you can link directly to this article.
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