RESINS, a large class of substances, existing chiefly in the vegetable kingdom, and of which common resin, rosin, or colophony, is the type. They are generally obtained by incising the bark of trees ; oleo-resin, or a mixture of a volatile oil and resin, then exudes, and gradually hardens. It is possible that the resins do not exist as such in plants, but that they are produced by the oxidation of essential oils.
The chief properties of resins are : insolubility in water ; non volatility ; solubility in alcohol, ether, benzole, essential oils, and by the aid of heat in fixed oils ; are insulators of electricity ; become negatively electric by friction ; fuse when heated ; and, in contact with air, burn with a bright, but very smoky, flame.
But little is known of the constitution of resins. Constantly, but slowly, absorbing oxygen, with or without evolution of carbonic acid or water, or both, it is almost impossible to obtain them in anything like a definite state. Again, with one or two exceptions, they do not crystallise, and this greatly increases the difficulty of determining their individuality. Some—such as santonin, eugenin, gamboge, cube bin, myrrh, and chrysophanic acid—seem to be produced by the simple replacement of a number of equivalents of oxygen for an equal number of equivalenta of the hydrogen in the essential oil. Others, for example, etyracin, anime, red resin of rhubarb, mastic, amber, elemi, and aloetin ; not only take up oxygen in the place of hydrogen, but also assimilate water'. Many are probably mere oxides of the volatile oils; some we know to be simply hydrates. As yet, the resins have not been artificially formed from the pure essential oils : could this be accomplished, an important insight into their constitution would, no doubt, be obtained.
The solutions of several of the resins iu alcohol redden litmus-paper, indicating that they are acid bodies. Such resins combine with alkalies to form soaps, differing but little from ordinary soaps. Common rosin, for example, contains vide acid, which crystallises in prisms ; piniarie add, which is obscurely crystalline ; and pinic acid, which is amorphous. These acids, as well as many other solid and liquid bodies
existing in or obtained from resins, will be found described somewhat more in detail tinder TURPENTINE.
Many of the resins are used in medicine; several have considerable commercial value, being used for the preparation of varnishes by dissolving in turpentine, wood naphtha, or spirit of wine.
The more important resins are treated of in this division of the ENGLISH CYCLOPEDIA under their respective names, or in the NATURAL HISTORY Divisiox either individually or under the name of the plant from which they are obtained.
Gum-resins, as the name implies, are mixtures of Gun and resin.
For fricane, and resin ofileica, see Tux PENTLNE.
The resins of commerce are in some cases natural exudations, while others are obtained from vegetable compounds by the action of alcohol. They are for the most part brittle, tasteless or insipid, and fusible at a moderate heat ; they seldom have any smell; they generally burn with a strong yellow flame, emitting a large quantity of smoke.
The ordinary resin of the shops is colophony, obtained as a residue after the distillation of oil or spirit of turpentine from common tur pentine. The black resin is the cooled brittle mass in the state in which it leaves the still; whereas the yellow resin is the black modified by the action of water. Resin-oil is made in London, Liverpool, Bristol, Hull, and Glasgow, as an ingredient in lubricating grease for railway axles, and for the bearings of heavy machinery. It is also used in France as an ingredient in printers' ink, thereby giving the unpleasant odour which is often observable in newly-printed French newspapers. Most of the common resin comes from North America, to the extent of 20,000 tons or more annually. Resin-gas is largely made in America ; but in England it does not successfully compete with coal-gas. The chief uses of the various resins in England are for varnish, lacquer, sealing-wax, and dyeing.