320 GULL GULL, Sir William Withy, an English physician, born at Thorpe-le-Soken, Essex, Dec. 31, 1816. He graduated M. D. at the university of Lon- don in 1840, was Fullerian professor of physi- ology at the royal institution in 1847-'9, and afterward physician to Guy's hospital till about 1867. He was knighted after his successful at- tendance during a severe illness of the prince of Wales in 1871, and appointed physician extraordinary. He is president of the clinical society. His publications include a lecture on paralysis and treatises on hypochondriasis and on abscess of the brain. GUM, an exudation from certain trees, distin- guished by its either softening or dissolving in water, and not yielding to alcohol ; also by af- fording 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 term- ed gummic or arabic. By boiling down the juices or the infusions of many plants, a sub- stance 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 mat- ter, their elementary composition is usually considered the same, and like that of starch is represented by the formula CeHioOs. 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 dis- solve or swell up in the intestinal fluids, with- out 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 suffi- cient to set aside the data of exact experiment. Mucilaginous fluids are given to invalids as agreeable drinks, and are of benefit in sheath- ing inflamed surfaces and protecting them from irritation. It appears, however, from Dr. Ham- mond'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, especial- ly to infants. Properly given, it is of use in dys- entery, diarrhoea, and inflammation of the bow- els, chest, and bladder. It may be advantage- ously added to diuretics. Several varieties of gum are recognized, most of which may be re- GUM ferred 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, &c. They are in great part composed of arabine, but also contain the prin- ciple bassorine. The third division includes those that are partially soluble in water, and are distinguished by the presence of the prin- ciple 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 particu- larly designated by their localities, as the Tur- key or Arabic gum, the Barbary or Morocco gum, Cape gum, East India gum, gum Senegal, &c. The first named is the product principally of the acacia vera and A. Arabica ; it is col- lected 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. gummife- ra, 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 resem- bling 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 re*d, also white, and of glassy lustre. The gum has a sweetish taste, but no decided odor. Its specific gravity va- ries from 1-3 to about 1'5. It is readily dis- solved in boiling water ; but the solution more slowly effected in cold water keeps better. '. has 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 stiffen- ing 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 vari- ous 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 fermenta- tion under the influence of yeast, but if digest- ed with cheese and chalk it may be made to furnish alcohol. Its nutritive property has al- ready been noticed. The gum is often adulte-
