Mr Gough took thin slips of caoutchouc, previously rendered pliable by being immersed in water, observed the temperature, by applying these slips slightly ex tended to to e lips, and suddenly stretched them, when a very sensible increase of temperature took place. When allowed to relax to the original degree of extension, the temperature was suddenly returned. The same gentle man also ascertained, that if these slips be plunged into cold water at the time that they are considerably extend ed, that they lose their contractile power, and retain nearly the same degree of extension ; but if plunged into warm water, or if kept warm for sonic time in the hand, they become again elastic. The specific gravity of caoutchouc is considerably increased, by being weigh ed when in a state of tension under water. These facts are valuable, not only as they give nen characters of this substance, but as they are beautiful illustrations of the theory of latent heat. See Caoutchouc is variously acted upon by different agents. When exposed to a high temperature, it is softened, becomes viscid, swells, emits a peculiar odour, and burns with a very bright flame. If a more moderate heat be applied, it retains its u iscidity, never resuming Its elastic state.
The atmosphere affects the colour ; but we arc not aware of any alteration which it prodm es on the other properties of caoutchone.
Water was supposed, by Grossart, to be a sok ern of this substance, merely because it is softened, and be comes somewhat transparent on the edges, on being im mersed for some time in boiling water. It is ver, clear, however, that this effect depends on the increased tem perature, rather than on the water itself. When boiled for a %cry considerable time, no alteration takes plat ., except in a way that is decidedly referable to the agency of heat. By this heating, the caoutchouc is rendered so adhesive, as to admit of being united very closely by simple pressure.
It was very soon determined, that alcohol does not dis solve it. It, however, renders it colourless. M. i\lacquer confirmed these observations.
The same chemist also found, that sulphuric ether was a solvent of caoutchouc ; but Berniard afterwards repeated the experiment without success. Ile also tried the nitric ether, and found it to be a very imperfect solvent. :%Ir Cavallo was the first that determined the causes of this discordance, between two chemists re markable for their accuracy. This philosopher found, that ether is a solvent, only after having been w ell washed with water. Subsequent experience has shewn this statement to be correct. It is evident, that Macquer and Berniard had used the same kind of ether in two different states. On evaporating the ether, the caout chouc remains unaltered ; hence, from the solution, various instruments may be made with facility. Pelletier
found, that the solution is considerably facilitated, by boiling the caoutchouc for some time before it is put into the ether. I)r Roxburgh did not find that the nitric acid dissolved the caoutchouc, which he obtained from the Urccola clastica.
The volatile oils dissolve caontchoue ; but on evapo ration, it remains clammy and unfit for any use. The fixed oils have a partial effect when aided by heat ; and melted wax dissolves a N cry minute portion.
I)r Thomson discovered, that ammoniacal gas dis solves a very minute proportion of caoutehoue, although Berniard had denied the action of the alkalies. The Doctor has further determined, that all the alkalies pro duce similar effects.
The acids decompose caoutchouc very partially.— Mr Ilatchett obtained about 12 per cent. of charcoal from caoutchouc that had been charred by sulphuric acid. When acted upon by nitric acid, azote, carbonic acid gas, prussic acid gas, and oxalic acid, are said to have been formed.
Petroleum dissolves it, and it remains unaltered after evaporation.
When distilled, it gives out ammonia, a watery liquid, empyremnatic oil, and charcoal.
Such are the leading properties that have been as certained of this singular substance. It exists in many plants, in which it has usually been confounded with other matters. In the fluid state in which it is found in plants, it unites readily with water, by which means it may be easily separated from the stalks. This fact is a valua ble one. as it may hereafter be employed in the manu facture of instruments made of caoutchouc. When combined with resin, it may be separated by alcohol ; and, for its separation from gum, or extractive. the fol lowing formula is given by Hermbstadt "Digest a part of the plant containing it (caoutchouc), first in water, and then in alcohol, till all the substances soluble in these liquids are extracted. Dry the residuum, and digest it in four times its weight of rectified petroleum. Ex press the liquid part, by squeezing the substance in a linen cloth. Let this liquid remain several days to settle, then decant off the clear liquid part, mix it with a third part of water, and distil ; the caoutchouc remains behind.
Opium is said to contain caoutchouc, and the insoluble part of mastic possesses some of its properties.
Caoutchouc is used for various purposes. In South America, whence it was first brought to Europe, it is formed into solid masses, with which the natives play at a game somewhat resembling that of fives. It is used for bottles and boots, for which it answers extremely well. It is also formed into torches, which burn with great splendour.
In Europe, it is commonly used to take out the marks of blacklead pencils; for syringes, bougics, catheters, elastic tubes, and varnish.
_ .