CAOUTCHOUC, a soft, dense, elastic, vegetable substance, well known under the name of elastic gum, and Indian rubber. It is remarkable not only for the above characters, but also for its wonderful inflamma bility; and for some very singular chemical and me chanical properties. In this country it is commonly to be purchased in the form of birds and bottles of a spheri cal shape.
This substance appears to have been introduced into Europe about the beginning of the eighteenth century ; but nothing was known of the mode of its production, until the year 1736, when M. De la Condamine pre sented a paper to the French academy, describing the tree from which it is obtained, and the mode of prepara tion. From De la Condamine's statement, it appeared, that it was obtained from a tree that grows in the pro vince of Esmiralclas, in the Brazils, and which is called by the natives 1 heve. This tree, on being wounded, yields a milky juice, which becomes inspissated on ex posure to the air, and forms caoutchoue. The same tree was afterwards found on the banks oldie Maragnon, by Don Pedro Maldonado ; and in Cayenne, by M. Fres nau. The death of the former of these gentlemen pre ented the publication of his observations respecting it. The latter reach an account of it in the French academy in 1751. More recent observations have now deter mined, that caoutchonc is obtained from two South Ame rican plants, the Thevia caoutchouc, and tee Jatronka elastica. In South America, the juice, which is ob tained by incision, is spread over clay, moulded into the desired form, in successive layers, until the vessel be sufficiently thick. The whole is then suspended over the smoke of burning vegetable matters, which hardens and colours it. Previous to this last mentioned process, the exter.or of ti c vessei is ornamented by various means. When the AdlOie preparation is completed, the clay mould is broken into pieces, and taken out. The vessel is then fit for use.
Besides the No Smith American plants already men tioned, there are several others that yield caoutchouc.
The principal of these arc, the Ficus Indica, the ?rto carpus integrifolia, and the Urccola elastica. The pro perties of the caoutchouc, obtained from the urceok, have been accurately examined by Mr llowison, sur geon at Prince of Wales' Island ; and he has determined the following facts: The thickest and oldest stems of the urceola yield the largest proportion of caoutchouc. On cutting these, a milky, pungent juice exudes. On exposure to the air, a decomposition takes place, and a firm coagulmn, which is the caoutchouc, separates from the thin portion. Tills same effect is produced by time, when the air is carefully excluded. If oxymuriatie acid be poured into the milky juice when first exuded, the caoutchouc is precipitated, and the acid loses its peculiar odour. Fourcroy supposed, that this change was induced by the absorption of oxygen. Should Mr Davy's opi nions be conclusive, sonic other source of the phenome non must be sought for.
The chemical properties of caoutchouc have attract ed the attention of philosophers since the time of Mr .Macquer, in the Memoirs of the French Academy in 1768, an account of his experiments on it. Although this celebrated chemist determined many facts previously unknown, yet lie adopted some erroneous opinions, the fallacy of which was pointed out by Mr Ber niard, whose paper contains the greater proportion of all the facts now known respecting this interesting substance. Fourcroy and Grossart have also contributed to our knowledge of this subject.
Pure caoutchouc, when recent, is of a yellowish white colour, wnich changes to a smoke grey on exposure to the air. Most commonly it is black, or blackish brown, particularly that, which comes from South America : this circumstance is owing to the smoking during the formation of the vessels. Caoutchouc is tasteless and inodorous, except when heated; it then yields a faint peculiar smell. It is soft, pliable, adhesive, wonder fully elastic and tough. Its specific gravity is 0.9335.