Elastic Resin, or India Rubber, a substance produced from the syringe-tree of Cayenne and other parts of South America, and possessed of the most singular properties. No substance is yet known which is so pliable, and at the same time so elastic; and it is farther a matter of curiosity, as being capable of resisting the action of very powerful menstrua. From the account of M. de la Condamine, we learn, that this substance oozes out, under the form of a vegetable milk, from incisions made in the tree; and that it is gathered chiefly in time of rain, because, though it may be collected at all times, it flows then most abundantly. The means employed to insipidate and indurate it, M. de la Borde says, are kept a profound secret. M. Bomare, and others, affirm, that it thickens and hardens gradually by being exposed to the air; and as soon as it acquires a solid consistence, it manifests a very extraordinary degree of flexibility and elasticity. Accordingly the Indians make boots of it, which water cannot penetrate, and which, when smoked, have the appearance of real leather. Bottles are also made of it, to the necks of which are fastened hollow reeds, so that the liquor contained in them may be squirted through the reeds or pipes by pressure. One of these filled with water is always presented to each of the guests at their entertainments, who never fail to make use of it before eating. This whimsical custom led the Portuguese in that country to call the tree that produces this resin pao di xirringa, and hence the name of feringa is given both to the tree and to its resinous production. Flambeaux, an inch and a half in diameter, and two feet long, are likewise made of this resin, which give a beautiful light, have no bad smell, and burn twelve hours. A kind of cloth is also prepared from it, which the inhabitants of Quito apply to the same purposes as our oil-cloth and sail-cloth. It is formed, in fine, by means of moulds, into a variety of figures for use and ornament; and the process is said to be thus:—The juice, which is obtained by incision, is spread over pieces of clay formed into the desired shape; and as fast as one layer is dry, another is added, till the vessel be of the proper thickness: the whole is then held over a strong smoke of vegetables on fire, whereby it hardens into the texture and appearance of leather; and before the finishing, while yet soft, is capable of having any impression made on the outside, which remains ever after. When the whole is done, the inside mould is picked out.

Ever since this resin has been known in Europe, its chemical qualities and other interesting properties have been very diligently investigated. In particular, it has been endeavoured to discover some method of dissolving it in such a manner that it would assume different figures with equal ease as when in its original state of milk. In the memoirs of the academy of sciences for 1768, we have an account of several attempts for this purpose, and how it may be effected.

—The state of vegetable milk in which the caoutchouc resin is found when it comes from the tree, led Mr Macquer to imagine that it was composed of an oil and a watery matter. From its wanting aromatic flavour, from its little volatility, and from its being incapable of solution in spirit of wine, he concluded that the oil which entered its composition was not an essential, but a fatty, one. Hence he thought it probable that it passed from a fluid to a solid form by the evaporation of the watery part, and that the oily solvents would reduce it to a soft state. The first trials he made for dissolving it were with linseed oil, essence of turpentine, and several others. But all he could obtain by means of these menstrua was a viscid substance incapable of being hardened, and totally void of elasticity. The rectified essential oil of turpentine was employed seemingly with greater success. To separate from this menstruum the caoutchouc which it had dissolved, Mr Macquer added spirit of wine; but the consequence was, that part only of the oil united with the spirit; the rest remaining obstinately attached to the resin which it had dissolved, and thus preventing it from assuming a solid consistence. The author next endeavoured to dissolve it by means of heat in Papin's digester. But neither water, nor spirit of wine, although in this way capable of dissolving the hardest bones, could produce any other effect upon it than to render it more firm than before. After this, he tried what effect the milky juice of other vegetables would have upon it. He used several kinds, particularly that of the fig. But, in this way, he could obtain no solution. From the great volatility of ether, he was next induced to try it as a menstruum; and, for this purpose, he prepared some with great attention. The caoutchouc, cut into little bits, and put into a proper vessel with as much ether as was sufficient to cover it, was perfectly dissolved without any other heat than that of the atmosphere. This solution was transparent and of an amber colour. It still preserved the smell of ether, but mixed with the disagreeable odour of the caoutchouc, and it was a little less fluid than pure ether. Upon its being thrown into water, no milky liquor was produced; but there arose to the surface a solid membrane which possessed the great elasticity and other peculiar properties of the caoutchouc. He observes, however, that two pints of the best ether, obtained by rectifying eight or ten pints of the common ether by a gentle heat, must be used, in order to the success of the operation.—The distinguishing properties of this substance, viz. its solidity, flexibility, and elasticity, and its quality of resisting the action of aqueous, spiri- tuous, saline, oily, and other common solvents, render it extremely fit for the construction of tubes, catheters, and other instruments, in which these properties are wanted. In order to form this resin into small tubes, M. Macquer prepared a solid cylindrical mould of wax, of the desired size and shape; and then dipping a pencil into the etherial solution of the resin, daubed the mould over with it, till he had covered it with a coat of resin of a sufficient thickness. The whole piece is then thrown into boiling water; by the heat of which the wax is soon melted, and rises to the surface, leaving the resinous tube completely formed behind.

A resin similar to this was some years ago discovered by M. Poivre, in the isle of France; and there are various milky juices extracted from trees in America and elsewhere, which by previous mixtures and preparations are formed into an elastic resin, but of an inferior quality to that of Cayenne: such, for instance, are the juices obtained from the Cecropia pelata, the Ficus religiosa and indica, &c.

Of the genuine trees, those growing along the banks of the river of the Amazons are described by M. Condamin as attaining a very great height, being at the same time perfectly straight, and having no branches except at top, which is but small, covering no more than a circumference of ten feet. Its leaves bear some resemblance to those of the manio: they are green on the upper part, and white beneath. The seeds are three in number, and contained in a pod consisting of three cells, not unlike those of the ricinus or palma Christi; and in each of them there is a kernel, which being stripped and boiled in water produces a thick oil or fat, answering the purposes of butter in the cookery of that country.

A method of dissolving this elastic gum without ether, for the purposes of a varnish or the like, is as follows: Take one pound of the spirit of turpentine, and a pound of the gum cut into very small pieces; pour the turpentine into a long-necked matras, which must be placed in a sand-bath; throw in the gum, not all at once, but by little and little according as it is perceived to dissolve: When it is entirely dissolved, pour into the matras a pint of nut or linseed oil, or oil of poppies, rendered defecative in the usual manner with litharge: Then let the whole boil for a quarter of an hour, and the preparation is finished. This would make an excellent varnish for air-balloonists, were it not so expensive on account of the price of the gum.—Another method, invented by Mr Baldwin, is as follows. Take any quantity of the caoutchouc, as two ounces avoirdupois: cut it into small bits with a pair of scissors. Put a strong iron ladle (such as plumbers or glaziers melt their lead in) over a common pit-coal or other fire. The fire must be gentle, glowing, and without smoke. When the ladle is hot, much be- low a red heat, put a single bit into the ladle. If black smoke issues, it will presently flame and disappear; or it will evaporate without flame: the ladle is then too hot. When the ladle is less hot, put in a second bit, which will produce a white smoke. This white smoke will continue during the operation, and evaporate the caoutchouc: therefore no time is to be lost; but little bits are to be put in, a few at a time, till the whole are melted. It should be continually and gently stirred with an iron or brass spoon. Two pounds, or one quart, of the best drying oil (or of raw linseed oil which, together with a few drops of neats foot oil, has stood a month, or not so long, on a lump of quicklime, to make it more or less drying) is to be put into the melted caoutchouc, and stirred till hot; and the whole poured into a glazed vessel, through a coarse gauze, or fine sieve. When settled and clear, which will be in a few minutes, it is fit for use, either hot or cold.

The Abbé Clavigero informs us, that the elastic gum is called by the Mexicans Olin or Olliz, and by the Spaniards of that kingdom Ule: That it distills from the Olquahuitl, which is a tree of moderate size; the trunk of which is smooth and yellowish, the leaves pretty large, the flowers white, and the fruit yellow and rather round, but angular; within which there are kernels as large as filberts, and white, but covered with a yellowish pellicle: That the kernel has a bitter taste, and the fruit always grows attached to the bark of the tree: That when the trunk is cut, the Ule which distils from it is white, liquid, and viscous; afterwards it becomes yellow; and lastly of a leaden colour, though rather blacker, which it always retains. The tree, he adds, is very common in the kingdom of Guatemala.

As to the genus of this tree, it does not seem to be yet ascertained. Aublet, in his Histoire des Plantes de la Guiane (p. 871.), describes the tree, the fruit, and manner of collecting the juice; but never saw the flower: he calls it, however, Hevea Guianensis. In Jacquin's America, it is called Ecbites corymbosa. The younger Linnæus, in his Supplementum Plantarum (p. 422), names it Jatropha elatior; but acknowledges that he only gives it this name from the structure of the fruit having most resemblance to that genus, his dry species wanting the flowers.

Of the above gum, it is said, the Chinese make elastic rings for lacivious purposes.—Among us it is used by surgeons for injecting liquids, and by painters for rubbing out black-lead pencil marks, &c.