and ultimately yields a mass or block of C. in which the condensation is so perfect that all air-holes, and other cells and interstices, disappear. The block of C. is then cut under water by powerful knives or shears into sheets, from which the pieces sold by stationers may be shaped out, or from which C. bands or thread may be obtained. In the manufacture of square threads, mere cutting is had recourse to; and the delicacy of the operation may be understood when it is stated that 1 lb. of C. will yield 32,000 yds. of thread. The round thread elastic is prepared from C. which has been treated with about double its weight of bisulphide of carbon, containing about 5 per cent of alcohol, which yields a soft material resembling in consistence bread dough or putty; and this being squeezed through a series of small holes, is obtained in minute round threads, which are first received on an endless piece of velvet and ultimately on an endless web of common cloth 500 to 600 yds. long, during the transit of the threads across which, the solvent or bisulphide of carbon evaporates, and leaves the caoutchouc. When it is wished to weave these threads into cloth, they are wound upon bobbins, taking care to stretch the C. as much as possible, so as to deprive it, for the time being, of its elasticity; and after it has been woven into the cloth, a hot iron is passed over the fabric, and immediately the C. resumes its elasticity.
In the manufacture of water-proof clothing, or Mackintoshes (see MAckENrosu), which was the first application of rubber on a large scale, the C. is made into a solu tion with spirits of turpentine, or other solvent, and spread upon the cloth; when thus coated, the fabric is pressed between heavy rollers. This variety of water-proof cloth has now, however, been almost entirely superseded by another kind made with vulcanized rubber, which We shall notice presently.
Vu/canized Caoutchouc.—Pure india-rubber is now used only to a limited extent in the arts, but it is applied in the vulcanized state to an almost endless variety of pur poses. The remarkable change which C undergoes when mixed with sulphur and heated, according to circumstances, from 240° to 310° F., was discovered by Charles Goodyear, in America, in 1843, and independently, about the same time, by Mr. Thomas Hancock, in England. In the process of vulcanizing, the rubber, as a preliminary step, is either torn into shreds or crushed into thin pieces by machinery, and afterwards washed. There are two principal kinds of vulcanized rubber, one hard and horny in its texture, the other soft and elastic. In the case of the former, the C. is mixed with about one third of its weight of sulphur, and heated for several hours, the tem perature 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—namely, from 2f to 10 per cent, and the heat to which it is subjected in the vulcanizing chamber is con siderably 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 Although sulphur is the only essential ingredient required for vulcanizing rubber, yet other substances are usually added. Thus, in the case of machinery belting, pipes, and some other articles, the silicate of magnesia (French chalk) is used to prevent adhe siveness. Litharge, or carbonate of lead, again, is frequently mixed with the rubber and sulphur for certain purposes; but there is really a long list of materials more or less used in preparing different qualities of vulcanized C., each manufacturer using mixtures, the exact nature of which he is careful not to divulge. Asphalte, tar, lamp black, whiting, rosin, sulphide of antimony, and ground cork are some of the ingredients most commonly employed in this way. Belting for machinery, and some kinds of tubing, are formed of alternate layers of canvas and vulcanized rubber Natural C., as already stated, is elastic, cohesive, impervious to gases, insolu ble in water, and resists many chemical re-agents; but it loses its elasticity by cold, softens by heat, and is destroyed by many fixed oils. After being vulcanized, C. has its elasticity greatly increased, is not hardened by cold, and does not soften or become viscid at any temperature short of its absolute decomposition. Besides, it is barely soluble in turpentine, naphtha, and the other solvents of pure C.; nor does oil readily penetrate or soften it.
It would be a hopeless task to attempt to specify the many useful purposes to which vulcanized C. is applied, even if we had the space to spare. From the year 1843, when it was first made, to the present time, the various patented applications of it must be two or three thousand in number. The mere abridgments of the specifications connected with this material, issued by the English patent office, form a thick volume. Under the head GoLositEs, will be found a brief description of the process of making india-rubber shoes. Water-proof coats are now made in a similar way, the mixture of rubber and vulcanizing materials being pressed on the surface of any suitably woven fabric by heated iron rollers in a calender. The coats are then cut out and the various pieces put together, without sewing, by some solvent, such as turpentine, which makes the edges adhere. They are afterwards heated in the vulcanizing chamber. Both coats and shoes of this material have, however, the objectionable property of preventing the escape of moisture from the skin. Belting, buffers, wheel tires, washers, valves, pipes, fire-hose, and other engineering appliances, form a large branch of the rubber trade. For medical and surgical purposes, many, articles are made of this material. Of such an apparently trivial matter as vulcanized rubber threact, one English firm turns out aboir's. -3,000 lbs. per day, and another single small article—namely, tobacco pouches—is mad3 in another factory at the rate of 3,000 per diem.