the hard fleshy substance which invests the teeth of either jaw. See Anatomy.

(Lat. gumma). This term is applicable solely to those concrete vegetable exudations which soften or dissolve in water, and afford a more or less perfect mucilage, but which are wholly insoluble in alcohol. Gums are thus distinguished from resins—those fusible and combustible vegetable substances which are totally insoluble in water, but which soften and dissolve in ether, essential oils, and alcohol. Gum, properly so called, is used in large quantities for a number of purposes in the arts. There are six varieties of this substance, viz., gum-arabic, gum-senegal, cherry-tree gum, with that of other stone-fruit trees, gum-tragacanth, gum of Bassora, or Bazrah, and that extracted from seeds and roots by boiling water.

Gum-arabic is the purest of these, and consists almost entirely of the principle called arabine. It forms a clear mucilage with water, and is clearer, and keeps better if dissolved in cold water than when prepared with warm water, which is the common method. Gum-arabic is the produce of several species of Acacia; as tortilis, sevot, Ehrenbergii, vera, arabica. One hundred parts of good gum were found to consist of 70-40 of arabine, 17-60 of water, and a few per cent. of saline and earthy matters.

The method of collecting this gum as practised in Morocco may be briefly described:—About the middle of November, that is, after the rainy season, a gummy juice exudes spontaneously from the trunk and principal branches of the acacia-tree. In about fifteen days it thickens in the furrow, down which it ran either in a vermicular form, or, more commonly, in the shape of oval and round tears, about the size of a pigeon's egg, and of different shades of colour as they belong to the white or red gum-tree. About the middle of December the Moors encamp on the border of the forest, and commence the harvest, which lasts six weeks. The gum is packed in very large bags of leather, and brought on the backs of bullocks and camels to certain ports, where it is sold to French and English merchants. It is highly nutritious. During the whole time of harvest, of the journey, and of the fair, the Moors of the desert live almost entirely upon it; and experience proves that six ounces of gum taken during the twenty-four hours are sufficient to support a man for a considerable period.

The quantity of gum-arabic imported into Britain in 1852 amounted to 48,484 cwts.; that of gum-senegal to 4267 cwts.; of tragacanth, 1151 cwts. Previously to the year 1832, the duty on gum-arabic from a British possession was 6s. a cwt., and from other parts, 12s.; but the duty on all gums, from whatever part of the world, was then equalized, being fixed at 6s.; in 1841 it was further reduced to 1s.; and it was finally repealed in 1845. Of the 48,484 cwts. of gum-arabic imported in 1852, Egypt produced 16,414 cwts.; Morocco, 7131; Italy, 3952; East Indies, 16,089; other countries, 4898.

Gum-senegal is also a very pure gum, much resembling gum-arabic, and is applied to many of the same purposes as that of gum. It is also much employed in calico-printing. The tree which yields it is the Acacia senegal, so named from the country of the River Senegal in Africa, whence this gum is procured. Its constituents are arabine 81-10, water 16-10, with 2 or 3 parts of saline matters.

Cherry-tree gum is an inferior and less soluble kind of Gum-Resin gum, containing 54-90 parts of cerasine, 52-10 of arabine, 12 of water, and 1 of saline matter.

Gum-tragacanth, familiarly called gum-dragon, is the produce of several species of Astragalus, but more particularly of A. versus and A. gummifer; the former a native of the north of Persia, Armenia, and Asia Minor, and the latter of Mount Lebanon, Arabia, &c. These at least are the chief sources of the tragacanth met with in commerce. It is likewise yielded, though less abundantly, by A. creticus and A. aristatus, and some other species. It is imported in twisted thread-like pieces, or in flattened cakes, is of a whitish or yellowish colour, devoid of taste and smell, nearly opaque, and a little ductile. It swells in water, and dissolves in part, forming a very thick mucilage. One hundred parts of it consist of 53-30 arabine, 33-30 bassorine and starch, 11-0 water, and from 2 to 3 parts of saline matters. It is used in medicine as a demulcent, and to form lozenges and pills, &c. It is also employed to stiffen and to glaze silks; and the inferior kinds are used by shoemakers to finish off the edges of their work.

Gum of Bassora, or that brought from Bassora in Arabia, possesses most of the properties of tragacanth, and gives its name to the principle called bassorine, which forms a constituent part of this gum and of tragacanth.

Gum from roots and seeds is extracted by boiling water. Linseed, for example, yields, by boiling, a gum consisting of 52-70 arabine, 28-9 insoluble matter, 10-3 water, and 7-11 saline matter.

The substance called British gum, so largely used in calico-printing, is noticed under the head BRITISH GUM.

Gum-Resin. This term is applied to an inspissated juice afforded by many kinds of plants, which combines the properties of gums and resins, being partly soluble in water, partly in alcohol. The principal gum-resins are aloes, ammonia, assafetida, galbanum, gamboge, euphorbium, olibanum, scammony, besides a great variety of other concrete juices. The chief of these are noticed under their respective names, as also the resins properly so called.