CAOUTCHOUC, Elastic Resin, or India Rubber, a substance produced from the syringe tree of Cayenne and other parts of South America, and possessed of singular properties. No substance is yet known which is so pliable, and at the same time so elastic; and it is further 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 maybe collected at all times, it then flows most abundantly. The means employed to inspissate and indurate it, M. de Laborde says, are kept a profound secret. M. Bomare and others affirm that it thickens and hardens gradually by being exposed to the air; and that 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 fails to make use of it before eating. This whimsical custom led the Portuguese in that country to call the tree that produced the resin pao di xirrin-ga ; and hence the name of seringat is given both to the tree and to its resinous production. There are likewise made of this resin, flambeaux, an inch and a half in diameter, and two feet long, 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 purpose as our oil-cloth and sailcloth. Lastly, it is formed, by means of moulds, into a variety of figures for use or ornament. The process is said to be this: 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, by which means it hardens into the texture and appearance of leather; and before the finishing, while yet soft, is capable of receiving any impression, which remains for ever afterwards. When the whole is finished, the inside mould is picked out.

Since this resin has been known in Europe, its chemical qualities and other interesting properties have been very diligently investigated. In particular, chemists have 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 fluid state. In the memoirs of the Academy of Sciences for 1768, we have an account of several attempts of this kind, with information how the object desired may be effected. The state of vegetable milk in which the caoutchouc resin is found when it comes from the tree, led M. Macquer to imagine that it was composed of an oil and a watery matter. From its wanting aromatic flavour, from its having little volatility, and from its being incapable of solution in spirits of wine, he concluded that the oil which entered into 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, apparently with greater success. To separate from this menstruum the caoutchouc which it had dissolved, M. Macquer added spirits of wine; but the consequence was, that part only of the oil united with the spirit, whilst the rest remained obstinately attached to the resin which it had dissolved, and thus prevented 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 spirits 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 produce 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 accordingly 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 [7:6:107]smell of ether, but mixed with the disagreeable odour of the caoutchouc, and 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 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 insure the success of the operation. The distinguishing properties of this substance, its solidity, flexibility, and elasticity, and its quality of resisting the action of aqueous, spirituous, 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 ethereal solution of the resin, daubed the mould with it, till he had covered it over with a coat of resin of a sufficient thickness. The whole piece was then thrown into boiling water, by the heat of which the wax was soon melted, and rose to the surface, leaving the resinous tube completely formed behind.

Grossart informs us that he succeeded very well in employing the essential oils of turpentine and lavender as a solvent for the elastic gum, and thus forming it into tubes, or giving it any shape that was wanted. When the elastic tube is prepared with oil of lavender, the latter may be separated by immersing the tube in alcohol, which charges itself with oil, and becomes a good lavender water. Alcohol serves another purpose besides taking up the essential oil. It accelerates very much the drying of caoutchouc instruments which are thus formed. Oil of turpentine appeared always to have a kind of stickiness; and the smell, which could not be got rid of by any means yet discovered, was another inconvenience.

Grossart proposes another solvent, which is easily procured, and is not liable to the inconvenience just mentioned. This solvent is water. “I conceive,” says he, “it will appear strange to mention water as a solvent of elastic gum, that liquid having been always supposed to have no action upon it. I myself resisted the idea; but reflecting that ether, by being saturated with water, is the better enabled to act on caoutchouc, and that this gum, when plunged into boiling water, becomes more transparent at the edges, I presumed that this effect was not due simply to the dilatation of its volume by the heat. I thought that at that temperature some action might take place, and that a long-continued ebullition might produce more sensible effects. I was not disappointed in my expectations, and one of those tubes was prepared without any other solvent than water and heat. I proceeded in the same manner as with ether. The elastic gum dilates but very little in boiling water; it becomes whitish, but recovers its colour again by drying it in the air and light. It is sufficiently prepared for use when it has been a quarter of an hour in boiling water: by this time its edges are somewhat transparent. It is to be turned spirally round the mould, in the manner we described before, and replunged frequently into the boiling water during the time that is employed in forming the tube, to the end that the edges may be disposed to unite together. When the whole is bound with packthread, it is to be kept some hours in boiling water; after which it is to be dried, still keeping on the binding.

“If we wish to be more certain that the connection is perfect, the spiral may be doubled; but we must always avoid placing the exterior surfaces of the slips one upon the other, as those surfaces are the parts which most resist the action of solvents. This precaution is less necessary when ether is employed, on account of its great action upon the caoutchouc.

“It might be feared that the action of water upon caoutchouc would deprive us of the advantages which might otherwise be expected; but these fears will be removed if we consider that the affinities differ according to the temperatures; that it is only at a very high temperature that water exercises any sensible action upon caoutchouc. I can affirm that at 120 degrees of Reaumur’s thermometer (302 degrees of Fahrenheit) this affinity is not such as that the water can give a liquid form to caoutchouc; and it does not appear that we have any thing to fear in practice from a combination between these two bodies, which, though it really is a true solution, does not take place in any sensible degree but at a high temperature. It is therefore at present easy to make of caoutchouc whatever instruments it maybe advantageous to have of a flexible, supple, and elastic substance, which is impermeable to water at the temperature of our atmosphere, and resists the action of acids as well as that of most other solvents. As to the durability of these instruments, few substances promise more than this, because it may be soldered afresh in a damaged part. Any woven substance may be covered with it; it is only ʹ required that the substance should be of a nature not to be acted upon during the preparation, either by ether or by boiling water; for these two agents are those which appear to me to merit the preference. Artists will frequently find an advantage in employing ether, as it requires less time; so that a person may make in a single day any tube he may have occasion for. The expense of ether is very little, since it is needful only to dispose the caoutchouc to adhere; and being brought into that state, the caoutchouc may be kept in a vessel perfectly well closed. It would also diminish the expense of the ether, if, instead of washing it with a large quantity of water, there should be added to it only as much water as it can take up.” (^Annales de Chimie, vol. xi. p. 149.)

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 peltata, the Picus religiosa and Indica, and from other trees.

Of the genuine trees, those growing along the banks of the river of the Amazons are described by M. Condamine 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 manioc; they are green on the upper part, and white beneath. The seeds are three in number, being contained in a pod consisting of three cells, not unlike those of the ricinus or ρalma Christi ; and in each of these there is a kernal, which being stripped and boiled in water, produces a thick oil or fat, answering the purpose 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 matrass, 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 matrass a pint of nut or linseed oil, or oil of poppies, rendered desiccative 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 balloons, were it [7:6:108] 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 caoutchoue, as two ounces avoirdupois; cut it into small bits with a pair of scissors. Put a strong iron ladle, such as plumbers and 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 below 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 neat’s 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 it 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 Ollin or Olli, and by the Spaniards of that kingdom Ule ; that it distils from the olqua-huitl, wbich 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, and 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; and that when the trunk is cut, the ule which distils from it is white, liquid, and discous, but afterwards 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 Guatimala.

Different trees, it would appear, yield the elastic gum. Aublet, in his Histoire des Plantes de la Guiane (p∙ 871), describes the tree, the fruit, and manner of collecting the juice; but he never saw the flower. He calls it, however, Hevea Gaianensis. In Jaequin’s America it is called Echites Corymbosa. The younger Linnaeus, in his Supplementum Plantarum (p. 422), names it Jatropha Elastica ; 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.