CAO ( 22 7 )

C AO

that Lave as yet been the fubject of any ex-periments, are all decompofable by heat, exceptthe camphorat of ammonia, which fublimes un-altered. When expofed to the flame of a blow-pipe the acid burns off with a blue flame. Thecamphorats are decompofable with abftraclionof the bafe by the mineral acids, and witli ab-ftracfion of the acid by moll of the metallic,barytic, and calcareous falts. The camphoratsof lime, barytes, and magnefia yield their acidto alcohol.

The ufe of camphor in the arts is to a (lift thefolution of relins in various menftrua. It isemployed with fuccefs in cabinets of naturalhiltory to prevent the approach of infects, whichappear to be ftupefied and killed by the pun-gency of its odour. But its principal ufe is asa medicine both externally and internally.CAOUTCHOUC . Elajtic Gum.

This Angular vegetable fubftance was firftbrought to Europe from South America aboutthe beginning of the lift century. But nothingwas known concerning its Natural hiltory tillin 1736 a memoir was prefented to the FrenchAcademy by Condamine, in which it is ftatedthat there grows in the province of Efmeraldas,.in Brafil, a tree called by the natives Hheve,from the bark, of which there flows, on its beingwounded-, a milky juice, which by expofure tothe air is converted into caoutchouc. M. Freneaudifcovered the fame tree in Cayenne, and tranf-mitted an account of it to the Academy fifteenyears after the firft notice of it by Condamine.Later refearches have proved that there are atleaf! two trees natives- of South America fromwhich caoutchouc is- obtained, namely, theHoevea caoutchouc and Jatropha elaflica , and it isnot improbable that it is yielded by other fpeciesof thefe genera. The American caoutchouc isgenerally brought to Europe in the form ofglobular narrow necked bottles like receivers,about an eighth of an inch thick, and capableof holding from half a pint to a quart or more.In its native country it is fabricated by the in-habitants into veffels for containing water andother liquids;, and on account of its ready in-flammability is ufed in Cayenne as the chiefmaterial for torches.

A fubftance pofleffing all the properties ofthe American caoutchouc has alfo lately beenprocured by Mr. Howifon, :l furgeon in Prince°f Wales ’s Ifland, in the Eaft Indies, fromt e juice of a climbing plant, the XJrceola elajli-ea, a native of that fmall ifland, and of thecoalt of Sumatra . The thickcft and oldeft

• Afiat. Refcar

Hems of the urceola yield by far the largeft pro-portion of caoutchouc. If one of thefe is cutinto, a white juice ouzes out, of the confiftenceof cream and flightly pungent to the tafte.When expofed for a time to the action of theair, or more expeditioufly by the addition of afew drops of any acid, a decompofition takesplace; the uniform thick cream-like juice fepa-rates into a thin whitifh liquor refembling whey,and the caoutchouc concretes into a clot or curdcovered fuperficially with a thin coating of abutyraceous matter. If the juice as foon ascollected is carefully excluded from the air, itmay be preferred for fome weeks without anymaterial change, but at length the caoutchoucfeparates from the watery part in the fame man-ner, though not fo perfectly as it does by free'expofure to the air. The proportion of caout-chouc contained in the juice of the oldeft Itemsis nearly equal to two-thirds of its weight; thejuice from the younger trees is more fluid, andcontains a considerably fmaller proportion ofthis fubftance.

According to the experients of Mr. Howifon,cloth of all kinds may be made impenetrable towater by impregnating it with the frefti juice ofthe Urceola, and the pieces thus prepared areexpeditioufly and moil effectually joined to-gether by moiftening the edges with either theentire juice or even the piore watery part, andthen bringing them in contact with each other.Boots, gloves, &c. made of this imperviouscloth are preferable even to thofe formed ofpure caoutchouc, as they are more durable andretain their fbape better. If a fufficient quan-tity of this juice could be obtained, the impor-tant purpofes to which it might be applied arealmoft innumerable.

The colour of frelh caoutchouc is yellowifhwhite, but by expofure to the air it becomes ofa fmoak-grey : American caoutchouc in the ftatein which it is brought to Europe , being formedof a multitude of extremely thin layers, each ofwhich is expofed to the air for fome time, inorder to dry before the next is laid on, is of ayellowifh fmoak-grey colour throughout; butmaffes of Eaft Indian caoutchouc being formedmore expeditioufly, are dark-ccloured only onthe outfide: when cut into they are of a verylight brown, which however foon deepens bythe aftion of the air. Caoutchouc is perfedtlvtaftelefs, and has little or no fmell, except whenit is warmed, it then gives out a faint peculiarodour. The elafticity of this fubftance is veryremarkable, and indeed is one of its molt

»es, v. p. 1^57,

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