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chara&eriftic properties. Slips of caoutchoucwhen foftened by immerfion for a few minutesin boiling water may be drawn out to feven oreight times their original length, and willafterwards refume very nearly their originaldimenfions. During its exteufion a confiderablequantity of caloric is given out, b which is veryperceivable when the piece is held between thelips; and when it is allowed to contract a de-creafe of temperature will immediately takeplace. By fucceilive extenfions and contractionsin the open air, and efpecially in cold water, itselafticity is much impaired and it refufes toreturn to its former dimenfions; but when inthis ftate it is put into hot water it imbibesagain the heat which it had loft, and by degreesrefumes its original fize. At the temperatureof about 40°, caoutchouc begins to grow rigid,its colour becomes much lighter, and it is nearlyopake; as the cold increales it becomes more{tiff and harder, and in all probability by apretty powerful freezing mixture would be madebrittle. Thefe changes however depend merelyon temperature, for a piece of hard frozencaoutchouc perfectly recovers its elafticity bybeing warmed. The frefh cut furfaces of thisfubftance will unite together by fimple contact,and by a proper degree of preffure may bebrought fo compleatly in union as to be nomore liable to feparate in this part than in anyother. Its fpecific gravity, according to Briffon,js 0.933. ft undergoes no alteration by theaction of the air at the common temperature.When boiled for a long time in water it com-municates to this fluid a peculiar fmell andflavour, and is fo far foftened by it that twopieces thus treated and afterwards ftrongly pref-fed together will form a permanent, adhefion toeach other.

When heated at a temperature nearly equalto that of melting lead, caoutchouc runs into ablack vifcid liquor of the confidence of tar,which does not concrete on cooling, neitherdoes it dry by long expofure to the air. Whenheld to a lighted candle it readily takes fire andburns with a copious white flame, and a largequantity of dark coloured fmoak exhaling at thefame time a peculiar but not unpleafant odour :from its fmoak a confiderable proportion of finelamp-black may be collected. In dry diftillationjt gives out ammonia and carburetted hydrogen.

Concentrated fulphuric acid, efpecially whenheated, acts with confiderable energy on caout-chouc, reducing it to a black, friable, charcoal-hke Jubilance, at the fame time the acid itielf

* Tliompfon’s Chemiilry, iy. p. 355,

is in part decompofed, fulphureous acid gaibeing extricated. When treated with nitricacid, azotic gas is difengaged, mixed with car-bonic and pruffic acids; oxalic acid is left infolution, and the refidue is converted to a yel-low brittle mafs. By digeftion in oxymuriaticacid the colour of caoutchouc is difcharged ; itbecomes opake, is indurated and fomewhatwrinkled, refembling tanned leather; but appear*to undergo no other change. Similar effedtsare produced, though more flowly by muriaticacid.

Ammoniacal gas, according to Dr. Thompfon,is abforbed by caoutchouc, and forms a foft,glutinous and inelaftic compound. The fameable chemift alfo ftates that the fixed alkaliesare capable of combining with and diffolving it;he has however omitted to mention the peculiarcircumftances required for effecting this. Ifflips of caoutchouc are boiled for a long time incauftic potafh, they appear to be flightly har-dened, but undergo no other change; when thealkali is evaporated almoft to drynefs, and thecaoutchouc begins to foften by the heat, a com-bination begins to take place between the two,and an extremely fetid fmell like that of animalputrefaction manifefts itfelf. By repeatedly-waffling the mafs in hot water, its putrid fmellis diminifhed, though by no means got rid of;it is foft, flightly clammy to the feel, and con-fiderably elaltic when extended, it refembles inits fibrous texture, its white colour and Alininggloffmefs, the tendons of animals. c

A fomewhat fimilar effect is produced byliquid ammonia. Some pieces of caoutchoucbeing macerated in exceedingly cauftic ammo-nia for more than a year, exhibited on exami-nation the following properties. Their colourwas changed to a perfectly opake white, theirbulk had not materially increafed although aconfiderable quantity of liquid alkali might befqueezed out of them ; their elafticity was fome-what diminifhed and they were flightly vifcidexternally. On expofure to a melting heat theycrackled, ftirivelled, and became contorted likepieces of animal fibre, and exhaled a faint odour,in which that of burning animal matter wasevidently mixed with that which peculiarly be-longs to caoutchouc. r Ihe inflammability wasremarkably diminifhed; a piece held in the flameof a candle crackled exceedingly, and was eva-porated without combuftion, except a fmallportion that melted and blazed, and appeared tobe the part which had efcaped the action of thealkali. d

« Oyvb Experiments. * Ditto.