Caoutchouc. Gum Elastic. Indian Rubber.
Nat. Ord. — Euphorbiaceae. Urticaceae. Sex. Syst. — Monoecia Syngenesia.
Description. — The tree (Ficus Elastica) from which Caoutchouc is obtained has a trunk from two feet to two-and-a-half in diameter, and from forty to sixty feet high. The leaves are alternate, approximated, three foliolate, articulated at the top of a long slender stalk, convex below, furrowed above, and swelled at its base ; leaflets smooth, oval, acute, green above, cinereous beneath. Flowers monoecious. Calyx five-cleft. Fruit oblong, greenish, three-cornered, broadest at base, tricoccous, each coccus opening with two valves. Seed ovate, brownish variegated with black, with a thin, brittle testa, and a sweet, nut-like pleasant kernel.
History. — This substance is the coagulated or inspissated juice of several tropical trees, the Siphonia Cahuchu, Siphonia Elastica, Jatropha Elastica, and Hevea Guianensis, all of which are supposed to be identical. On being wounded, the juice flows out, which is dried on molds of clay, and which comes to us in various shapes. It is generally blackened by smoke, but when pure is nearly colorless ; in thin layers, transparent, highly elastic, lighter than water, without taste or smell, insoluble in water, alcohol, weak acids and alkaline solutions. It dissolves in pure ether, oil of turpentine, naphtha, coal-tar naphtha, bisulphuret of carbon, and volatile oils. Its solutions in ether, oil of turpentine, and coal-tar naphtha, when dried up, leave the gum in an elastic state, and on this principle water-proof cloth is made ; the same is said to be the case with its solutions in the oils of lavender, sassafras and cajuput. Under exposure to heat, caoutchouc first melts, and then distils, yielding a mixture of several oily liquids, all of which, as well as pure caoutchouc itself, are carbo-hydrogens.
Caoutchouc is not affected by atmospheric air, chlorine, muriatic or sulphurous acid gas, or ammonia. It consists of 87.2 parts of carbon, and 12.8 of hydrogen.
Caoutchoucine is said to be the lightest fluid known, and yet its vapor is denser than the heaviest of the gases. It is prepared by cutting India rubber into small pieces containing about two cubic inches each, placing them into a cast-iron still connected with a well-cooled worm-tub, or any flat vessel with a large evaporating surface, the entire top of which can be removed for the purpose of cleaning it out. Heat is to be applied in the usual way, until the thermometer rang-es at about 600° F., when as it progresses upward to this temperature, a dark-colored oil or liquid is distilled over. When the thermometer reaches 600° or thereabouts, nothing is left in the still but dirt and charcoal. This oil is to be rectified, and thereby obtaining fluids varying in specific gravity, the lightest of which has not been under .670. At each rectification the color becomes brighter and paler, until at about specific gravity .680 it is colorless and highly volatile. It must be rectified with one-third its weight of water. To enable the dirt to be removed from the bottom of the still with greater ease, throw in common solder to the depth of about half an inch ; when this becomes fused the dirt is easily taken off. The disagreeable smell of this liquid may be removed by shaking it up with nitro-muriatic acid, in the proportion of four fluidounces of the acid to one gallon of the liquid. Mixed with alcohol, caoutchoucine dissolves all the resins, especially copal and india rubber, at the common temperature of the atmosphere, and it speedily evaporates, leaving them again in the solid state. It mixes with oils in all proportions. It promises to be a very valuable article for the solution of resins in the manufacture of varnishes, and for liquefying oil-paints with instead of turpentine. Being very volatile it requires to be kept in close vessels.
"When caoutchouc is immersed in a bath of fused sulphur, heated to various temperatures, by absorbing the sulphur, it assumes a carbonized appearance, and finally acquires the consistency of horn ; this is termed Vulcanized caoutchouc. The same vulcanized condition can also be produced either by kneading the india rubber with sulphur, and then exposing it to a temperature of 190°,— or by dissolving the india rubber in any known solvent, as turpentine, previously charged with sulphur. Thus treated, caoutchouc remains elastic at all temperatures ; in its ordinary state it is quite rigid at a temperature of 40° ; it is not affected by any known solvents, nor by heat short of the vulcanizing point, and acquires extraordinary powers of resisting compression. A cannon hall was broken to pieces by being driven through a mass of vulcanized caoutchouc, which exhibited no other trace of its passage than a scarcely perceptible rent. This article may be used for various useful purposes, as springs for locks, ornaments, bottles for volatile fluids, as a covering to protect wires from corrosion in sea or on land, life-boats, etc.
Properties and Uses. — Caoutchouc is much used for erasing pencil marks ; in forming flexible tubes for chemical purposes, and catheters and bougies for surgical uses. Melted, it is applied as a luting to the joints of chemical apparatus ; in the shape of thin layers, for covering the mouths of bottles, and for other purposes in which the exclusion of air and moisture is requisite. Heated and softened by the flame of a taper, it may be applied, but not while hot, to the cavity of a decayed tooth to relieve toothache, also over leech-bites, to suppress hemorrhage. It has been given in doses of one or two grains, gradually increased, in phthisis, but is seldom or never employed internally. Externally, it has been used as an ingredient of sticking plasters and liniments. Caoutchouc dissolved in oil of origanum or cajuput, and spread, upon oil-silk or cloth, and allowed to dry, forms an excellent stimulating "plaster for many local difficulties.