Resins, a class of proximate principles existing in almost all plants, and appearing upon the external surface of many of them in the form of exudations; also the oxidized and concreted juice of several species of coniferous and other trees. They are produced by certain families in considerable abundance, and in smaller quantities by a very large number of plants. When not exuding spontaneously, they often escape from punctures in the bark made by insects, or may be obtained by making incisions into the wood. They appear in the form of a viscid liquid consisting of the resin in solution in the essential oil of the plant. (See Balsams, and Turpentine.) It is possible that the resins never exist as such in plants, but it is certain that in the majority of cases they are formed by the oxidation of the essential oils contained in the plants. They are sometimes extracted by boiling the sawdust of the wood with alcohol, from which they are precipitated by the addition of water, and as the alcohol is distilled off the particles agglomerate.

Resins are so variously composed of numerous principles, that no little diversity is observed in their general properties, and they are therefore arranged by different authorities under several heads. 1. Resins which exude spontaneously from plants, or from incisions in the stems and branches, and harden on exposure to air; these sometimes contain considerable quantities of gum or mucilage (gum resins), or of volatile oil (balsams). This class includes: a, resins containing benzoic or cin-namic acid, such as benzoin, storax, and balsam of Peru or tolu; and b, resins not containing those acids, such as asafoetida, copaiba, copal, jalap, lac, mastic, and common turpentine. 2. Oxidized fossil resins, such as amber, and others occurring in beds of coal or lignite. 3. Resins extracted from plants by alcohol, such as the resins of cubebs, buchu, and squills. In general the resins are solid bodies of vitreous fracture, and brittle, so as to be readily pulverized when cold. Others are soft and greasy, and some are elastic. They are usually transparent or translucent, rarely colorless, but either brown, red, or green.

Their specific gravity is from 0.92 to 1.2. They occasionally have a decided taste or odor, derived from some essential oil or other foreign substance present; and to the same cause is probably owing the occurrence of some of the resins in a soft state. The solid resins are non-conductors of electricity, and by friction they assume the electric state known as negative or resinous. They melt at a moderate heat, and form a thick viscid liquid; on cooling this becomes a shining solid mass of vitreous fracture, which occasionally, when scratched with a sharp point after sudden cooling, flies off into pieces like Prince Rupert's drops. They readily take fire, and burn with a white or yellow flame and much sooty smoke. Some are soluble in ether, and others in volatile oils, boiling alcohol, or fixed oils with the aid of heat. The alcoholic solutions of some of them possess acid properties; others are neutral. These acid resins combine with the alkalies and form lyes, which when agitated produce a lather like that of soap, differing from it, however, in not being precipitated or becoming hard on addition of common salt. (See Rosin.) Many of the natural resins are mixtures of two or more resins, which may often be separated from each other through their different solvents.

When decomposed at a high heat in close vessels, the resins are resolved into carbonic acid, different gaseous hydrocarbons, empyreumatic oil, a little acidulous water, and a very little shining charcoal. - Chemically the resins consist of carbon, hydrogen, and oxygen, often in such proportions as to indicate a product of the oxidation of a multiple of C5H8. As they slowly absorb oxygen, with or without evolution of carbonic acid or water, or both, they are very unstable. Very few can be crystallized, and hence it is only with extreme difficulty that they can be obtained in a condition of purity. In the case of gamboge, myrrh, and others less frequently met, an atom of oxygen appears to be substituted for two atoms of hydrogen in the essential oil; but mastic, elemi, and others appear not only to exchange hydrogen for oxygen, but also to take up water. Some of them may therefore be considered to be oxides and others hydrates of the essential oils. As the resins have not yet been formed artificially from the essential oils, these views have not been proved correct. - Solutions of resins in alcohol, oil of turpentine, and fixed drying oils form varnishes.

Spirit varnishes are at the same time the most brilliant and the most brittle; their elasticity may be increased by the addition of oil of turpentine. The resins commonly used for varnishes are copal, elemi, lac, mastic, and sandarach.