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acid which comes over is diminifhed, which(hews that this change depends chiefly, if notentirely upon the aftion of the oxygen of the de-compofed acid. This deep yellow folution pro-duces no effect on nitrat of lime thus {hewingthe abfence of oxalic acid. With acetited lead itgives a copious yellow precipitate, and with afolution of ifinglafs or any other animal gelatinit produces a denfe yellow precipitate, infolublein boiling water; and refembling in every otherproperty the precipitate produced by galls, ca-techu, and other native vegetable tannins. Thislaid effe£t however is not produced unlefs therefin has been long and repeatedly treated withnitrous acid, whereas the refinous propertiesare loft at an earlier period of the action ofthis acid. On the other hand, if the refin hasbeen previoufly charred to a flight degreeeither by heat or by fulphuric acid, the artificialtannin is produced much more fpeediiy. HenceMr. Hatchett fuppofes that this tanning fub-ftance is formed in confequence of part ofthe oxygen of the nitric acid combining withthe hydrogen of the refill fo as to produce•water, and a proportionate quantity of carbonbeing thus denuded it is capable of yielding thetannin by the fubfequent action of the acid.It ftiould be added tco, that the fame fubftanceis produced in much greater abundance by theaction of nitric acid on charcoal alone, whichthus may be made to yield more than its ownweight of tannin.

It has been mentioned that no oxalic acid isformed by the action of the nitric acid on thepure vefins, and this appears to be a very -roodcharafteriftic mark of thefe bodies, fince moftother vegetable principles yield it in greater orlefs abundance. This is particularly the cafewith mucilage and extract of every kind, andhence the natural Gum-refms when treated withnitric acid afford a fmall quantity of oxalic acid,which may ferve to diftinguifh them from thetrue refins. This production of oxalic acidappears abfolutely to prevent the formation ofthe artificial tannin, fo that none of the lattercould be obtained from gum-arabic, tragacanth,and other mucilages. The prefence of thebenzoic acid alfo in fome degree counteractsthe formation of tannin, and hence lefs of it isproduced from gum benzoin, and balfam oftolu, than from the Ampler refins

When concentrated fulphuric acid is pouredupon any of the folid refins reduced to finepowder, it diflolves them in a few minutes andforms a tranfparent yellowifh brown folution ofthe confiftency of a vifcid oil. This procefs

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feems at firft to be a Ample folution, but in alittle time (efpecially if warmth be ufed) thecolour of the folution progreffively deepens,fulphureous acid gas exhales, and at length thewhole becomes a thick fluid, of an inky black-nefs. A fimilar effedt is produced by the adtionof this acid upon the liquid refins. If the acidis poured on common turpentine it diflolves italmoft immediately, and if a portion of this fo-lution be poured into cold water, the turpentineis precipitated in a folid brittle ftate like com-mon yellow rofin. But if a fecond portion isthus decompofed after the interval of an houror two the precipitated refin is of a dark brown;and a third portion after the interval of four orfive hours is quite black. If the digeftion becontinued for feveral days, the production offulphureous gas ceafes, and the turpentine (orwhatever refin be ufed) is converted into ablack porous coal, which yields no refin to al-cohol, and amounts in general to from a thirdand more to a fifth of the refin employed.The quantity of coal produced by fulphuricacid far exceeds that which is afforded by theAmple application of heat, fince thefe refinswhen heated ftrongly per fe in clofe veffels leavebehind fcarcely a hundreth of their weight ofcoal. If feparate portions of the refin procuredby dropping the fulphuric folution in water afterdifferent periods of digeftion are merely diflolvedin nitric acid, the artificial tannin is only foundin thofe folutions in which the refin is adluallyor nearly charred. The coal obtained from therefin by means of fulphuric acid is not only inmuch greater quantity than that produced byAmple heat, but differs from it confiderably intexture and appearance, being when obtainedby the humid procefs, Alining, hard, and fome-times iridefcent; whereas that produced by cal-cining the fame refins in clofe veffels is fpongyand light. The former kind burns like fomeof the mineral coals with great difficulty, butthe latter like common charcoal, i he greatdifference in the proportional quantity of coal,is fhewn by the following ftatement: ioo grs.of common refin afford by fulphuiic acid 43 ofcoal, of which after expofure to a red heat 30ftill remain, but the fame quantity yields by cal-cination in clofe veffels only f of a grain.

100 grains of maftich afford 6 6 grs. of coal,by the firft procefs, hut only 4L by the fecond.

100 grains of amber yield 56 grs. of coal byfulphuric acid, but oi ly 3 '- by calcination.

On the other hand the difference betweenthefe refpedtive quantities of coal from themucilages is much lefs.

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