A soft, dense, elastic, resinous substance, usually called India rubber, but sometimes very improperly elastic gum. It is obtained from the milky juice of several plants, the chief of which are the Jatropha elastica, and Urceola elastica. The juice, which is obtained by incision, is received in successive coatings on pieces of clay, and dried by the sun, or a fire made from dried leaves: as fast as one layer is dried, another is added, until it obtains the required thickness, when the clay mould is broken, and the tough leather-like substance taken out. The wonderful elasticity of this substance, and its resistance to water, has of late years brought it into extensive use for making water proof garments, besides many other purposes. Its solvents are ether, volatile oils, and petroleum. The ether, however, must be washed with water repeatedly, when it dissolves it completely. Pelletier recommends the caoutchouc to be boiled in water for an hour, then to be cut into thin shreds, then boiled again, and afterwards put into rectified sulphuric ether in a vessel closely stopped. Berneard considers the nitrous ether as preferable.

When this solution is spread upon any object, the ether quickly evaporates, and leaves the surface perfectly covered with the elastic resin.

A solution of caoutchouc in five times its weight of oil of turpentine, and this solution dissolved in eight times its weight of drying linseed oil, is said to form the varnish of air balloons.

Caoutchouc may be formed into various articles without undergoing the process of solution. If it be cut into a uniform slip of a proper thickness, and wound spirally round a glass or metal rod, so that the edges shall be in close contact, and in this state be boiled for some time, the edges will adhere so as to form a tube. Pieces may be firmly united by merely heating their edges and pressing them together. Within these few years Messrs. Hancock and Mackintosh have obtained several patents for various modes of applying this valuable substance, which are daily rising into notice. The patent caoutchouc pipes are formed of alternate layers of solid or dissolved caoutchouc and canvass, or other suitable medium, and by pressure united into very tough pipes of any required bore or strength, without any stitch or seam, the weakest of which it is said is capable of bearing a pressure of 600 lbs. per square inch.