INDIA-KUBBEK ordinary temperature, say 15 C., tlie primary effect of heat is to increase its flexibility and elasticity. This is well illustrated by the fact that a strip of rubber stretched by a weight contracts when it is heated to a temperature of about 40 C. This diminution as regards length is, however, accompanied by a more than corresponding increase in thickness, on account of the expansion in volume due to an elevated temperature. When caoutchouc is exposed to a temperature ranging between 100 and 120 C., it becomes considerably softened, and almost entirely loses its elasticity; but, if of good quality, it slowly recovers its former condition under the influence of a moderate degree of cold. When, however, the heat is pushed to 150, it becomes viscous, and at 200 it fairly melts, forming a thick liquid which possesses the same composition as ordinary caoutchouc, but has no tendency to resume its original condition even when exposed to cold for a prolonged period. At a still higher temperature, caoutchouc yields a variety of volatile hydrocarbons ; and, on subjection to dry distillation in a retort, its conversion into these bodies is tolerably complete, only a trifling carbonaceous residue remaining behind. Among the most notable volatile products resulting from the dry distillation of caoutchouc may be mentioned caoutchin, an oil-like body having a composition and vapour volume corresponding to the formula C 10 H 8 , and boiling at 171 C.; and isoprene, another hydrocarbon oil identical in composition with caoutchiu and with caoutchouc itself, and boiling at 38 C. Other hydrocarbon oils are also formed, as, for example, heveene and caoutcheiie, these being members of the C H tL ra series. The former boils at 228, and the latter at 14 0- 5. The mixed products of the dry distillation of caoutchouc, often described under the name caoutchoucin, form an excellent but rather expensive solvent of this body. When exposed to the air, caoutchouc gradually oxidizes and undergoes deterioration ; the oxidation is often much favoured by exposure to sunlight or to alternate conditions of dampness and dryness. The deteriorated caoutchouc is either somewhat soft and deficient in tensile strength, or brittle and resinous in its nature. Spiller found 2 7 3 per cent, of oxygen in a resinous product resulting from the decay of caoutchouc. Ozone rapidly attacks and destroys the substance. Dilute acids or alkalies have little or no action on caoutchouc, but strong and hot sulphuric acid chars, and concentrated nitric acid rapidly oxidizes and destroys it. The moderate action of either chlorine, bromine, or iodine hardens or vulcanizes it ; but, if allowed to act freely, they completely destroy it, The action of sulphur will be con sidered below. Caoutchouc, when pure, is odourless and nearly white, and possesses a specific gravity of 915. It is porous and cellular in texture, and absorbs from 10 to 25 per cent, by weight of water when long soaked in it. Alcohol is similarl) r taken up. Up to this point caoutchouc has been referred to as if it consisted of one substance only ; but as a matter of fact all ordinary samples contain two distinct modifications, viz., the hard or fibrous and the soft or viscous. These two caoutchoucs are identical in com position, and similar as regards general properties and reactions. On subjecting a piece of raw caoutchouc, however, to the action of such a solvent as cold benzol, the essential difference between the two forms manifests itself. The fibrous or hard constituent merely swells up to many times its original bulk, but the viscous yields a true solu tion. In a high class rubber, such as that imported from the province of Para, the former modification is the prin cipal factor ; in a caoutchouc of low quality, such as " African tongue," the latter. Freshly cut surfaces of caoutchouc unite together firmly, and this circumstance is duu to the presence of the viscous variety ; vulcanization, by hardening this, destroys the adhesive property. Certain liquids, such as benzol and its homologues, car bon disulphide, petroleum, ether, volatile oils, chloroform, and melted naphthalene, dissolve caoutchouc more or less perfectly ; but unless the substance has been subjected to the process of mastication, its fibrous constituent appears, not to dissolve in the strict sense of the term, but rather to swell up, forming a paste analogous to starch which has been acted on by hot water. Carbon disulphide and chloroform, however, exercise a more powerful solvent action on the fibrous parts of india-rubber than benzol or essential oils ; and Payen has found that carbon disulphide to which 5 per cent, of absolute alcohol has been added forms one of the best solvents. One part of masticated caoutchouc dissolved in thirty parts of this solvent^forms a liquid which can be filtered through paper, and which leaves a film of exquisite tenuity and purity when allowed to dry on a level glass plate. Most fatty matters exercise a remarkable destructive action on caoutchouc, causing it to become first soft, and afterwards hard and brittle. It has often happened that traces of fatty oils in the liquids employed for dissolving india-rubber, or fatty matters in the textile basis, have led to the destruction of waterproof goods. A like cause has in many cases led to the rapid detsrioration of the caoutchouc threads in elastic webbing. In the industrial working of india-rubber, the first matter to be attended to is the removal of the various impurities present in the crude material. These are in some cases natural products which have originated with the caoutchouc, while in other cases they owe their presence to careless collection or to adulteration. Among the impurities of the former class may be mentioned various gum-like or mucilaginous matters, and acid products arising from their decay or oxidation. A remarkable volatile body, which is probably of the nature of a polyatomic alcohol, has been discovered by Gerard 1 in the crude caoutchouc from the Gaboon. This substance, called by the discoverer dambonite, has a composition corresponding to the formula C 4 H 8 O 3 , is sweetish to the taste and soluble in water, and crystallizes in needles which melt at 190 C. and volatilize between 200 and 210. The admixtures may range from fragments of bark or wood to stones or large lumps of clay, such as are sometimes introduced into negrohead rubber, hay or a similar substance being also placed inside to make the mass about equal in specific gravity to the genuine article. Alum and sulphuric acid are often employed to effect the coagulation of the juice; and traces of the latter remain ing in the rubber appear, in some instances, to work mischief. All the above-mentioned impurities are in actual practice very efficiently removed by the following process. The lumps of crude caoutchouc are first softened by the prolonged action of hot water, and then cut into slices by means of a sharp knife, generally by hand, as thus any large stones or other foreign substances can be removed. The softened slices are now repeatedly passed between grooved rollers, known as the washing rollers (fig. 7), a supply of hot or cold water being made to flow over them. Solid impurities speedily become crushed, and are carried away by the water, while the rubber takes the form of an irregular sheet perforated by numerous holes. The washed product contains in its pores a notable proportion of water, which is removed by hanging the rubber for some days in a warm room. It is now ready either for incorporation 1 Compt, Rend., Ixvii. p. 820, and Zeitschrift, fur Chem,, 1869,
p. 66.
