Gum , an exudation from certain trees, distinguished by its either softening or dissolving in water, and not yielding to alcohol; also by affording mucic acid, when acted upon by nitric acid. The resins, which resemble the gums in origin and appearance, are insoluble in water, but dissolve in alcohol, ether, and the essential oils, and are moreover distinguished from the gums by their inflammability. The gums, as they issue through the punctured bark of trees, are held in solution in the vegetable juices; and as these evaporate on exposure they form a thick adhesive substance, which by further exsiccation may become dry, hard, and pulver-izable. They are mixtures of the calcium and potassium salts of a feeble vegetable acid termed gummic or arabic. By boiling down the juices or the infusions of many plants, a substance of this nature is often obtained, even when the plants are not known to produce it naturally. Flaxseed may thus yield a product, called bassorine, which when dried is like gum arabic. The gummy substances obtained in this way are generally distinguished by the name of mucilage.

Though the gums differ in their chemical reactions from amylaceous matter, their elementary composition is usually considered the same, and like that of starch is represented by the formula C6H10O5. Starch acquires the properties of gum when dried at a temperature of 266° F. (See Dextrine.) Gums possess little if any dietetic value, since experiment has shown that they merely dissolve or swell up in the intestinal fluids, without being changed to sugar, and are absorbed, if at all, only in an exceedingly small quantity. It is said, however, that gum arabic is used as food by some savage and semi-barbarous tribes; but reports of this character are hardly sufficient to set aside the data of exact experiment. Mucilaginous fluids are given to invalids as agreeable drinks, and are of benefit in sheathing inflamed surfaces and protecting them from irritation. It appears, however, from Dr. Hammond's experiments, that the insertion of large quantities of gum may irritate the mucous membrane of the bowels, and therefore it should not be administered too freely, especially to infants. Properly given, it is of use in dysentery, diarrhoea, and inflammation of the bowels, chest, and bladder.

It may bo advantageously added to diuretics. - Several varieties of gum are recognized, most of which may be referred to one or other of three divisions. Those of the first, distinguished by their solubility in water, are chiefly composed of the peculiar principle arabine, and are represented by gum arabic, the purest of the gums. The second division comprises those which soften and swell in water without dissolving, as gum tragacanth, Bassorah gum, etc. They are in great part composed of arabine, but also contain the principle bassorine. The third division includes those that are partially soluble in water, and are distinguished by the presence of the principle cerasine. The gums of the cherry and many other fruit trees belong to this group. - Gum Arabic may be used as a generic name for the various gums produced by the several species of acacia, many of which are particularly designated by their localities, as the Turkey or Arabic gum, the Barbary or Morocco gum, Cape gum, East India gum, gum Senegal, etc.

The first named is the product principally of the acacia vera and A. Arabica; it is collected mostly in Upper Egypt, Nubia, Kordo-fan, and Darfoor, and is brought from different ports of the Mediterranean. The second is supposed to be obtained from the A. gummifera, and is brought from Mogadore and Maza-gan. The Cape gum is from the Cape of Good Hope, the product of a species of acacia resembling the vera, and named by Burchell the capensis. The East India gum, mostly from Bombay, is the product of different species of acacia. Gum Senegal, from Senegambia, is supposed to be obtained from A. Senegal, A. vera, A. Seyal, and A. Adansonii. The first named of these is a small tree which forms vast forests in the hottest regions of Africa. These varieties of gum arabic differ somewhat in purity, hardness, and color. The best real gum arabic is in rounded or amorphous tears, some of which are as large as nutmegs, some transparent, and some opaque, of light shades of yellow, sometimes red, also white, and of glassy lustre. The gum has a sweetish taste, but no decided odor. Its specific gravity varies from 1.3 to about 1.5. It is readily dissolved in boiling water; but the solution more slowly effected in cold water keeps better.

It ha3 an acid reaction, and alcohol added to it causes the gum to fall as a white precipitate. Gum arabic is used in the arts for producing a glazed surface upon fabrics, as also for stiffening or giving to them a body, answering the same purpose as starch. By increasing the density of liquids in which it dissolves, these are made to retain substances in suspension which would otherwise subside. It is for this purpose introduced into writing ink and various preparations of pigments and of medicines. It is also administered for its own medicinal qualities, which are chiefly of a demulcent character. It is not susceptible of fermentation under the influence of yeast, but if digested with cheese and chalk it may be made to furnish alcohol. Its nutritive property has already been noticed. The gum is often adulterated with inferior gums, and also with starch and Hour. Iodine readily detects the presence of starch or of flour in solutions of gum containing them by the blue color produced. - Gum Tragacanth, also called Gum Dragon, is the product of various species of astragalus, especially the A. verus of the north of Persia, Armenia, and Asia Minor, and the A. gummi-fer of Arabia, Mt. Lebanon, etc.

It is collected in the months of July and August from the natural exudations of the shrubs, and from those resulting from incisions made in the stem near the root. It is a hard, tough substance, more or less white according to its purity, in very irregular flattened shapes, and in tortuous vermicular filaments. It may be pulverized after drying at a temperature of 212°. It has no taste or smell. Its specific gravity is 1.384. It swells slowly in water, partially dissolving, and forms a thick adhesive paste. By boiling with sufficient water, a solution is obtained of similar appearance, and Brande thinks of the composition, of that of gum ara-bic. Different analyses are given of it, but that of Guerin-Varry is generally adopted; it is as follows : arabine, 53.3 per cent.; bassorine or tragacanthine, 33.1; water, 11.1; inorganic matter, 2.5. Starch is detected in very small quantity in the bassorine. From the ultimate analysis the same chemist deduced the formula C5II10O5. The uses of gum tragacanth are similar to those of gum arabic. Apothecaries employ it to give adhesiveness to the ingredients of pills, and confectioners apply it with a similar object to the materials of lozenges.

Gum of Bassorah, from Bassorah near the head of the Persian gulf, is of this class of gums, and furnishes the name for the peculiar principle they contain. - Cherry-tree Gum, including in this name the exudations of the peach, plum, and other kindred trees, is an inferior quality of gum, somewhat like gum arabic, and consisting of a portion soluble in cold water, which is arabine, and a portion insoluble, which is the principle named by M. Guerin-Varry cerasine. (For gum kino, see Kino.) - Gum Mezquite is the product of the mezquite tree (prosopis glandulosa) of Texas and New Mexico, brought to notice in 1854 by Dr. Shumard of the United States army. It is described as similar in its properties to gum arabic, exuding spontaneously from the tree, and concreting into tears and lumps of variable sizes, semi-transparent, and of lemon white to dark amber shades of color. It is brittle and easily pulverized, and the fractured surfaces are brilliant. From an ounce to 3 lbs. has been obtained from a single tree, and more, no doubt, by making incisions in the bark. The branches furnish a purer quality than the trunk.

The best time for collecting it is the latter part of August. The trees abound upon the plains over regions thousands of miles in extent, and flourish luxuriantly in dry and elevated situations. If the gum could be easily obtained in large quantities, it would become an important commercial article; but an abundant exudation takes place only in unusually dry seasons after an interval of several years. Specimens have been analyzed by Dr. Campbell Morfit with the following results : arabine, 84.967; bassorine, 0.206; water, 11.640; inorganic matter, 3.000; impurities, 0.236; total, 100.049. Elementary composition: carbon, 44.706 per cent.; oxygen, 48794; hydrogen, 6.500. It thus shows a close resemblance to gum arabic and gum Senegal in composition as well as in its physical and chemical properties. It is kept in the drug stores of the Mexican cities, and considerable quantities have been sent to San Francisco from the Mexican ports on the Pacific. - This country receives its supplies of gums in great part through England.