Brazilian C. is the product of several species of siphonia (natural order euphorldacetr), but chiefly siphonia elastica. Bates says that " this tree is not remarkable in appear ance; in bark and foliage it is not unlike the European ash, but the trunk, like that of all forest trees, shoots up to an immense height before oil branches." The C. of New Granada, Ecuador, and Central America is obtained from castilloa elastica (nat. ord. artocarpacea), that of East India from the beautiful glossy-leaved Pus elastica (nat. ord. moracce), now so common as au ornamental plant in our conservatories, that of Borneo from urceola elastica, and that of western Africa from several species of landolphia, and also flews. Species of vahea, zcillughbcia, euphorbia, and other genera likewise yield useful varieties of C., and the sources of some kinds are unknown.
C. is sometimes collected by cutting the trees down, but much more usually by mak ing simple incisions in the trunks. The method of collecting and preparing the liquid C. is thus described in a work recently published at Rio Janeiro. In a few hours, the juice which flows out fills the basins, made of large leaves and plastic clay, which are adapted to the lower part of the tree. It is then poured into other vessels of various shapes; in a short time it becomes thickened, and solidifies in consequence of the evapo ration of the liquid part, In order to dry it completely, the practice is to expose it to a gentle heat; for this purpose it is suspended over a brazier lighted with wood, and the flame maintained with the fruits of auricuri, in such a manner that it may receive the smoke, hence the blackish color which the C. of commerce generally presents. Whilst it is liquid, it is fashioned by means of molds, according to the purposes to which it is destined. An attempt has recently been made to import the juice of the tree, and sub ject it to the drying process in this country, but little has as yet been imported into Britain. The characters of the juice are, that it possesses the consistence of cream, has a yellow color,.is miscible with water, but not with naphtha or other of the solvents of ordinary C., and its specific gravity varies from 1.02 to 1.41—ordinary C. being 930. The juice contains about 30 per cent of caoutchouc. When heated, it coagulates (as the glaire of egg does), owing to the presence of albumen; and exposed to the air, it dries up and leaves a film of caoutchouc. In the preparation of pure C., the natural juice is mixed with five or six times its bulk of water, and then either heated or mixed with common salt or hydrochloric acid, when the pure C. separates as a white opaque sub stance, which becomes transparent when dry. Pure C. is a carbo-hydrogcu, its compo sition being carbon 87.5 and hydrogen 12.5.
Para 0. is the best, and commands the highest price in the market. The other
South American kinds are of medium quality. East Indian rubber—naturally a fine quality—is too often injured by adulteration and careless collecting. The poorest kind is the w. African, being clammy, offensive in its odor, and only slightly elastic.
Commercial C. is a tough fibrous substance, possessing elastic properties in the high est degree. Reduced to the temperature of freezing water (32° F.), it hardens, and in greater part, if not entirely, loses its elasticity, but does not become brittle. When heated, as by placing in boiling water, it softens, and becomes very much more elastic than at ordinary temperatures, though it does not in any degree dissolve in the water. If suddenly stretched to seven or eigbt times its original length, it becomes warm; and if kept in this outstretched form for several weeks, it appears to lose, in great part, its elastic properties, and in this condition is readily cut into those thin threads which are used in the elastic put in gloves, bonnets, etc., and the elasticity of which is readily renewed by the application of gentle heat. Of late years, however, elastic thread is usually prepared with vulcanized rubber. Commercial C. is insoluble in water and alcohol. is not acted upon by alkalies or acids, except when the latter are concentrated, and heat is applied; but is soluble in ether, chloroform, bisulphide of carbon, naphtha, petroleum, benzol, and the essential oils of turpentine, lavender, and sassafras. Many other essential and fixed oils, when heated with C., cause it to soften, and produce thick glutinous compounds, especially linseed oil, which, in the proportion of 1t lb. of the oil to 4 ozs. C. in thin strips or films, yields a solution which, when strained, is of great use in rendering shoes, cloth, etc , water-proof. When heated to 248' F., 0. fuses; and at 600° it is volatilized, at the same time undergoing decomposition, and yields a liquid called caoutchoucine or caoutchisine, with the specific gravity 680, and possessing great solvent powers over C. and other substances. Caoutchoucine is neces sarily very expensive, and hence its use is limited; but cordage steeped in it and dried acquires great supple and tenacious properties, and cloth saturated with it, and dried by exposure to the air, becomes watertight, In the employment of C. as a branch of manufacture, the first operation is the puri fication of the crude material as it comes from abroad. The crude material is cut into minute shreds, and washed by powerful machinery, immersed in water, which releases the solid impurities, and the pure C. being removed, is placed on iron trays, and dried, in a rootn heated by steam. The material then undergoes a process of kneading under very heavy rollers, which causes the adhesion of the varions pieces Of C. to each other.