INDIA RUBBER, a colloidal substance ob tained from the milky juice of several widely different plants, and otherwise known as caoutchonc or gum-elastic. The most important sources are: Hevea Braziliensis and CastiBoa elastica, two trees native to South America but cultivated in . many other sections; Manihot Glaziovii, Ficus elastica, several vines of genus Landolphia, and the Mexican guayule plant, Parthenium argentatum. Some of the proper ties of india rubber must have been known to the natives of America at a very early period, because balls made by the Haytians of the gum of a tree, bouncing better than the wind-balls of Castile, are mentioned by Herrera in his account of Columbus' second voyage. In a book published in 1615, Juan de Torquemada mentions the tree which yields it in Mexico, describes the mode of collecting the gum and states that it is made into shoes; also that the Spaniards use it for waxing their canvas cloaks to make them resist water. More exact' nformation was furnished by M. de la Condamine in 1735. India rubber was at first known as Elastic Gum, and received its present name from the dis covery (about 1770) of its use for rubbing out black-lead pencil marks, for which purpose it began to be imported into Britain in small quantities about the end of the 18th century. Its application to the manufacture of water proof cloth first gave it commercial importance. About the same time a method was discovered of fabricating articles of various kinds by cast ing india rubber in molds. Until very recent years the india rubber of commerce was ob tained chiefly from South America, but the larger part of the present market supply comes from the cultivated rubber plantations of Brit ish India and the Indian Archipelago, and a considerable and increasing quantity from the west coast of Africa and the Mauritius.
The sap as gathered from the rubber-yield ing plants holds in suspension the globules of rubber, each being surrounded by a protective envelope of a proteid substance. In order to secure the coalescence, of these globules, various methods are employed in different localities and with different rubbers. The most common proc ess is by exposing thin layers of sap to the heat and smoke of an open fire, but a method of chemical coagulation is employed in some localities following a condensation by a centrif ugal treatment of the fresh sap. As it reaches the market, crude rubber is a very uncertain substance, containing from 15 to 50 per cent of impurities, much of which is simply dirt, as sand, bits of wood and clothing, leaves, plant fibres, etc. Other impurities which are normal to the sap are resins, sugars of several kinds, albumen, essential oils and a percentage of water, often very considerable. On some
plantations of cultivated rubber the gum is care fully cleaned and washed before it is marketed, but generally the manufacturer's first operation is to wash the crude material. The lumps of raw rubber are first steeped in warm water until soft and then sliced into thin sections under water and run between deeply corrugated rolls, also under water until they are in thin wavy sheets resembling crepe. They are then dried in the air or in a vacuum chamber, being care fully protected from light which has a tendency to set up injurious chemical changes, espe cially oxidation. By the vacuum process the drying is completed in an hour and a half ; by the air-drying method several days are re quired. As rubber is hygroscopic, the drying operation is one demanding skilled attention, for the vulcanizing of the rubber depends very largely upon the content of moisture.
Pure india rubber has the composition Cain. It is insoluble in water, in the esters, and in the ethers — all of which, however, are readily absorbed by rubber, causing it to swell. The solvents used in the arts are turpentine, dipentine, petroleum spirit, carbon disulphide, benzene and chloroform. The solvent most fre quently used is petroleum spirit, on account of its cheapness. As a matter of fact the usual phenomena of solution are not observed with rubber. The gum absorbs the solvent, becom ing first a jelly, later assumes a viscous condi tion, and as more of the solvent is added it finally takes a freely liquid form. India rubber is highly distensible, and this property increases with the increase of temperature. If the degree of heat is carried to 200° F., the distension be comes permanent. Pure rubber is used to but a limited extent in the arts, but for all general purposes it is first vulcanized. Two grades of vulcanized rubber are prepared, one hard and horny in its texture, the other soft and elastic. In the case of the former the caoutchouc is mixed with about one third of its weight of sulphur and heated for several hours, the temp erature finally rising to fully 300° F. For the soft kind of vulcanized rubber, on the other hand, a much smaller proportion of sulphur is required—viz., from 2% to 10 per cent, and the heat to which it is subjected in the vulcanizing chamber is considerably less. Usually, too, with this latter kind, the articles are made before the rubber is heated. The sulphur is commonly added in the ground state, but sometimes the rubber is treated with some solution containing this element, such as the bisulphide of carbon.