sometimes called Guiana-elastic, or more commonly, from its use in removing pencil-marks from paper, India-rubber, is a substance *sui generis*, found in the milky juice of a great variety of tropical trees; the most prolific in this remarkable product being *Siphonia Cahuchu* or *Hevea guianensis*, *Ureola elasticia*, and *Ficus elasticia*. The first of these trees, which affords the best commercial caoutchouc, extends over a vast district in central America; the second is abundant in the islands of the Indian Archipelago; and the *Ficus elasticia* abounds in Assam, and is plentifully distributed over some other parts of India. This substance is contained also in small quantity in the milky juices of some European plants.

Though caoutchouc was introduced into Europe early in the last century, its origin was unknown till 1736, when M. de la Condamine communicated to the French academy the discovery that it was the inspissated juice of a Brazilian tree, known by the native name of *Hhev*. The best caoutchouc is still imported from Para, where it is called *borracha* and *seringa*; but the native name is *calucho*—and thence the word caoutchouc is derived. It is procured by piercing the tree, or by making a series of incisions around the trunk and main branches, whence the juice exudes abundantly as a creamy-looking fluid, which gradually becomes firmer and darker by exposure to the air. It is usually collected in clay cups, or in leaves folded in that form, and attached to the tree in the line of incision. It is also spread in successive layers on moulds of clay, which are crushed and shaken out when the process of drying is completed. In this manner the India-rubber bottles and figures of animals are produced. The trees are most prolific during the rainy season. The caoutchouc proper may be separated from the thinner portion of the juice by the application of heat, which coagulates it; and the same result is obtained by means of acids and with alcohol. The Indians have long been acquainted with the fact that cloth imbued with this juice is rendered impervious to water. They also form it into boots; and for this purpose they employ wooden lasts coated with clay, which are dipped from time to time in the fluid caoutchouc until they have received a coating of the thickness desired. To accelerate the process, the caoutchouc is usually dried over wood fires, and thus is blackened by the smoke. While still soft, it is frequently ornamented by indenting patterns on its surface, which its want of elasticity in that state enables it to retain very readily.

Caoutchouc is occasionally imported in its fluid state; but in spite of the utmost care to exclude it from the atmosphere it gradually tends to solidification. Fluid caoutchouc has the consistence and colour of cream, and a specific gravity of 1·012. In some recent experiments on caoutchouc juice, Dr Ure found one sample, with a specific gravity of 1·04125, to yield 20 per cent. of solid caoutchouc; while another of a thicker consistence, and a specific gravity of only 1·0175, yielded no less than 37 per cent. of white, solid, and very elastic caoutchouc. The ordinary commercial caoutchouc is in the form of bottles, solid balls, and crude masses, and varies considerably in quality according to the species of tree from which it is obtained, independently of adulteration by the admixture of earthy and other matters. Previous to being used for manufacturing purposes, the caoutchouc is freed from from its impurities by washing in warm water. Sometimes the masses contain a viscid tarry-looking matter (the result apparently of putrefactive fermentation), which renders them worthless.

Pure caoutchouc is insipid, with little if any perceptible odour, and has a specific gravity of 0·925. Unlike most solid vegetable products, it contains no oxygen. Its constituents, according to Professor Faraday, are carbon 87·2, hydrogen 12·8; while Dr Ure gives the result of his experiments as carbon 90, hydrogen 10, or three atoms of the former to two of the latter. Caoutchouc is insoluble in water, alcohol, and acids; but it dissolves more or less readily in ether, naphtha, coal-tar naphtha, bisulphuret of carbon, and some essential oils, as oil of cassias, lavender, and turpentine. By certain processes it may be dissolved in the fixed oils; but these solutions when spread as varnishes remain in a glutinous state. One of the most perfect solvents for manufacturing purposes is a mixture of 100 parts of sulphuret of carbon and 6 or 8 parts of anhydrous alcohol. Caoutchouc is but slightly affected by the acids at the ordinary temperature; nor will the strongest caustic potash ley, even at a boiling temperature, dissolve it. Sulphuric acid, especially with the aid of heat, decomposes it; and so will nitric acid, if concentrated. Azotic acid changes it into a yellow insoluble substance, with the disengagement of a minute quantity of oxalic acid. From its quality of resisting the action of the most corrosive gases, tubes of this substances are much employed in the chemical laboratory; and it also forms, when fused, a very adhesive and efficient lute. When boiled in water it swells; and in this state it is more readily soluble in its several menstrua. Boiling water also renders it somewhat adhesive; so that a slip coiled about a cylinder, and held in position by a thread, may be formed into a tube. Its solutions in coal-tar naphtha and ether, when dried up, leave the caoutchouc unaltered in its properties. When treated with the first it soon swells to many times its original bulk; and the process of solution may be accelerated by trituration. This solution is much employed for rendering textile fabrics impervious to air and water, and formed the subject of Macintosh's well-known patent. For such purposes the manufacturers usually employ the coarse inelastic lumps and parings, which may be dissolved either in petroleum (coal-tar), naphtha, or oil of turpentine. The solution is effected in a close cast-iron vessel, by the aid of triturating cylinders; the heat generated during the process being alone sufficient to favour the solution. Thirteen cwt. of caoutchouc may thus be prepared at one time in a vessel measuring 4 feet by 4; three days being required to complete the process. The quantity of caoutchouc much exceeds that of the solvent, so as to produce a varnish of considerable consistence. This solution is spread in one or more coats over the surface of the cloth; and in order to ensure its uniform thickness, the varnished cloth is drawn along under an edge or bar, which is placed horizontally to its surface, and almost in contact with it. When it is partially dry, another piece of cloth, similarly prepared, is applied carefully to the varnished surface of the first, and the two are then united firmly together by pressure between rollers. The cloth thus prepared is suspended in a well-ventilated stove-room, where it speedily dries, and loses in a great measure its offensive smell. When required for the purposes of the bookbinder, or any nice operation, this varnish should be well smoothed by rubbing, and then strained. The latter precaution, indeed, should be observed in the preparation of all caoutchouc varnishes, in order to remove the aloetic or other extraneous matter which usually occurs in the caoutchouc of commerce. To obtain the ethereal solution it is necessary that the ether be previously washed with water three or four times; and this purification may be simply effected by agitation with twice its bulk of water in a narrow-necked bottle, which is then to be inverted and allowed to remain in that position until the ether has separated and risen to the surface—when the water is to be drawn off. In this state the ether will completely dissolve it in the course of a few days. The caoutchouc should previously be cut into shreds, and boiled in water for two hours. For nice purposes this solution is particularly well adapted, as when spread it dries very speedily, leaving a coating of caoutchouc unaltered in its properties, and free from any disagreeable smell. It remains somewhat clammy for a while; but this defect may at once be removed by dusting its surface with sulphur or with French chalk. A varnish suitable for aerostatic machines may be obtained by digesting one part of caoutchouc, cut into shreds, in thirty-two parts of rectified oil of turpentine, and then straining the solution through muslin. A good transparent cement, too, for uniting small pieces of glass, &c., is said to be obtained by dissolving from fifteen to twenty grains of caoutchouc in two ounces of chloroform, and afterwards adding half an ounce of mastic; the whole to be allowed to macerate for a week, when it will be ready for use, and may be applied cold with a brush.

The most remarkable property of caoutchouc is its elasticity, which exceeds that of any other known substance. Cold and quiescence render it hard and rigid, but warmth speedily restores its elasticity. If a slip of this substance be softened by immersion in hot water, it may be extended to seven or eight times its length, and will contract very nearly to its original dimensions. Its expansive power may be strikingly exhibited in a very simple manner. If an India-rubber bottle be steeped in well-washed sulphuric ether till it has become quite flaccid, it may be expanded by means of a condensing syringe to a globe five or six feet in diameter. If suffered to dry in this state, its tractile power will be destroyed.

The great variety of purposes to which its elasticity and imperviousness to water and to air have occasioned its application, are too numerous and familiar to be particularized in this place. The extent of its employment in Britain in the several branches of manufacture is shown by the quantity of caoutchouc imported, which for the year ended January 5, 1854 amounted to 17,326 cwt. The total quantity exported from Para alone in 1852 was 32,860 cwt., and 116,465 pairs of shoes. The filature of caoutchouc, for the manufacture of elastic fibres, such as cloth, cord, tape, braces, &c., is an important and increasing branch of our national industry. This operation has been made the subject of various patents, all modifications more or less perfect of our general principle, namely that of reducing it to threads by means of steel edges acting either on the bottle caoutchouc compressed in a mould, on the solid cake, or by cutting it when stretched on a mandrel of wood which is set in rapid revolution. In this operation water is allowed to trickle over the cutting blades in order to prevent the cohesion that would otherwise impede their progress. In this manner, thread of 5000 yards to a pound weight is produced. Previous to its employment in the manufacture of textile fabrics, it is necessary to render the thread elastic; which is effected by winding it tightly on reels, and allowing it to remain in that condition until nearly deprived of its elasticity—which is afterwards restored by exposing it to the action of a heated smoothing-iron. Sheets are sometimes cut from the solid cake; and may likewise be obtained of any size, and of extreme tenacity, by spreading a solution in naphtha upon cloth previously sized, stripping it off when dry. The use of rollers, with a piece of cloth similarly prepared and superimposed on the varnished surface of the first, may also be employed in this operation.

An ingenious application of the elastic force of caoutchouc has been used to raise weights by what are called power purchases; and it has also been used as torsion springs for roller-blinds, for door-springs, as a projectile force, and a great variety of other purposes.

Caoutchouc when impregnated with sulphur—a process technically called vulcanizing—undergoes a very important change. Thus prepared, it becomes more uniformly elastic, always recovering its form after being stretched, and remaining unaffected by climatic temperatures. Various methods for effecting its sulphuration in the most effectual way have been proposed and patented. This may be done, either by adding sulphur to the naphtha or turpentine in which the caoutchouc is to be dissolved; by kneading in by means of a roller-press the flowers of sulphur, in the proportion of ten pounds of sulphur to sixty pounds of caoutchouc; or by impregnating it with the fumes of sulphur or its compounds. Sheets and tubes are well vulcanized by immersion for a few minutes in a mixture of 100 parts of sulphuret of carbon and $2\frac{1}{2}$ parts of protochloride of sulphur, and then rinsing them in water. Other methods also are practised.

Caoutchouc burns with a bright flame, emitting much smoke, and an odour that is scarcely disagreeable. At a temperature of 248° Fahr. it fuses, and remains very long in a glutinous state. When exposed to about 600°, it resolves itself into a vapour which is condensable into a powerfully odorous and extremely volatile amber-coloured oil, which has received the name of caoutchoucine. Caoutchoucine is a powerful chemical solvent; and as when mixed with alcohol it dissolves copal and other resins without the aid of artificial heat, it is peculiarly useful in the preparation of varnishes. It is also a solvent of caoutchouc. By repeated rectification, this dark-coloured fluid becomes bright and transparent; and it may be deprived of its peculiar odour by being mixed and shaken up with nitromuriatic acid, or with chlorine. Though the lightest of known fluids, its vapour is so ponderous that it may be poured like water from one vessel to another. This interesting fluid is the discovery of Mr William Henry Barnard of Greenwich, who obtained a patent for it in 1835.

It is worthy of remark, that in the operation of grinding or kneading the crude masses of caoutchouc for the purpose of reducing the separate fragments into a homogeneous whole, the alternate expansion and contraction of the mass occasions so great an evolution of caloric that the water employed in the process, though cold as it trickles into the drum, is quickly raised to a boiling temperature.

For some very interesting details of the several processes connected with the manufacture of caoutchouc, the reader is referred to the article Caoutchouc in Ur's Dictionary of Arts and Manufactures, from which the leading facts here noticed are derived.