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This word stands for a number of substances which, when dissolved in suitable liquids, possess a powerful adhesive property, and the common and well-known gum-arabic may stand as a type of the class. It is the product of an acacia, and was originally imported into Europe from Barbary and Morocco. In its purest condition, it forms white or rather yellowish masses, which are destitute of any crystalline structure, and break with a shell-like fracture. Its solutions are wrongly called mucilage, which is an entirely different substance. Gum-arabic dissolves in cold water, from which the pure gummy soluble principle can be precipitated by alcohol and by basic acetate of lead. Arabin is composed of 42'1 per cent of carbon, 64 per cent of hydrogen, and 51'5 per cent of oxygen, which, by a curious chemical coincidence, is exactly the composition of crystallized cane sugar, and it illustrates the fact, that among organic bodies, substances of the same ultimate composition may have very dissimilar properties. Another gum is mucilage, very abundant in linseed, in the roots of the mallow, in salep, and in the fleshy roots of the orchis and other plants. It is soluble in cold water, but is less transparent than gum-arabic, and it is precipitated by the neutral acetate or sugar of lead. Gum Tragacanth is chiefly composed of a kind of mucilage to which the name of bassorin has been given, and which does not dissolve in water, but simply assumes a gelatinous aspect. Caustic soda or potash will dissolve it. The principal use to which this gum is put is inthe manufacture of marbled paper, whereit forms the bath en whichthe colors are thrown, and from which they are taken up by the paper. Cerasin is the insoluble portion of the gum of the cherry tree, and is nearly like bassorin. Mr. Schmidt has determined the composition of these various substances, and has found them all more or less allied to starch, invariably containing hydrogen and oxygen, the proportions in which they form water, and all when treated with acids yield grape sugar. The jelly of fruits or pectin is closely related to the gums, but as yet chemists have not paid much attention to it, and consequently much that is said of it is merely conjectural.
