CAOUTCHOUC, or, as it is usually, though improperly named, elastic gum, is a vegetable matter, which, in several of its physical qualities, as well as in its chemical relations, has some similarity to vegetable gluten, and which so far agrees both with it and albumen, as to approach in the nature of its composition to animal matter.
The substance to which the name of caoutchouc, or elastic gum, has been more particularly given, was brought from Spanish America, in the form of hollow spheres or bottles, In which state it is still imported into Europe ; it was evident, therefore, that it had undergone some artificial preparation. Condamine gave the information, that it is the inspis sated juice of a tree belonging to the family of the Euphorbia, which has since received the botanical name of Havea guianensis, or Havea caoutchouc. Inci sions are made in the bark of this tree : a milky juice exudes, which is collected. It is applied in successive coatings over a mould of clay ; is dried up by exposure to the sun, andafterwards by being placed in the smoke from burning fuel ; when dry, the clay mould is crushed, and the fragments extracted, and in this manner the spherical bottles are formed. It has since been discovered, that caoutchouc is not exclusively the produce of this vege table ; but that it is furnished likewise by other plants, either perfectly the same, or with very slight variations ofproperties. It is obtained in large quantity from the Jatropha elastica, a native likewise of different provinces of South America. Fourcroy procured specimens of the juice of the caoutchonc, in the state in which it exists previous to its inspissa tion, from the Island of Bourbon, from Cayenne, and the Brazils, and examined its properties. From experiments he con cluded, that caoutchouc exists ready form ed in the juice of the tree, and is capable of being separated in the concrete form ; but that a portion also exists, not suffici ently perfect to be deposited with its elastic property ; that it acquires this, to gether with its inspissation, from the ac tion of oxygen; and that, by this opera tion, exposure to the atmosphere influ ences the concretion of caoutchouc in the usual process in which it is brought to the solid form.
The purest caoutchouc Fourcroy sup poses to be that which separates sponta neously from the juice in close vessels. It is white, or of a slight fawn colour. The properties of caoutchouc have been de termined, principally, from the state in which it exists in the elastic bottles which are imported to Europe ; and in this state its properties do not seem to have under gone any important change, or to be dif ferent from those of the pure caoutchouc. Its colour is a dark brown ; its external surface is smooth ; its internal texture is rough, and presents a fibrous appear ance. Its specific gravity is nearly the same with water, being from 9.3 to 10.0. It is inodorous, and is also destitute of taste.
The most remarkable physical property of which this substance is possessed, and which eminently distinguishes it, is its high elasticity. It can be stretched out to a great length, and when the force that has been applied to it is withdrawn, it instantly returns to its former dimen sions. Its pliancy is increased by heat, while it is rendered more rigid by cold ; and its softness, which is connected with the former quality, is so much increased by warmth, that it can be moulded into any form, and two parts newly cut may even be pressed together, so as to be in timately united.
Caoutchouc, exposed in a dry state to a high temperature, softens, swells up, and emits a fetid odour, similar to that of animal' substances : as the heat is in creased, it melts into a viscid matter, and remains in this state when cold. If heated sufficiently high, it takes fire, and burns with a vivid light and dense smoke ; in the countries in which it is produced it has been used for torches. It is perfectly insoluble in water and alcohol, but is dis solved in either.
This substance is capable of being ap plied to important purposes, from its softness and flexibility, its elasticity, and particularly its indestructibility, and not being affected by air, water, or indeed the'greater number of chemical agents. Tubes for conveying gases, and other chemical instruments, are accordingly prepared from it ; and boogies, catheters, and similar surgical instruments of ca outchouc, are much preferable to what can be prepared from any other sub stance.
The solution of caoutchouc in some of the oils has been used as a varnish, to render flexible substances, as silk, &c. impermeable to water or air. It has the advantage of being perfectly flexible; but it is long before it dries, and is liable to be softened by a very moderate heat. To render it less viscid, it is generally prepared from a mixture of volatile and expressed oils.
CAP of maintenance, one of the rega ha, or ornaments of state, belonging to the kings of England, before whom it was carried at the coronation, and other great solemnities. Caps of maintenance are also carried before the mayors of several cities in England.
It is of crimson velvet, faced with er mine, and was formerly esteemed a badge and symbol of dignity, and suitable to a prince of the blood, being worn by King Edward III. and succeeding sovereigns down to Edward VI. but of late it has been granted to private families. It is frequently to be met with above the hel met, instead of a wreath, under gentle men's crests.
CAP, in a ship, a square piece of tim ber put over the head or upper end of any mast, having a round hole to receive the mast. By means of these caps the top-masts and top-gallant-masts are kept steady and firm in the tressel-trees where their feet stand.
CAP of a gun, a piece of lead which is put over a touch-hole of a gun, to keep the priming from being wasted or spoiled.