T A N

TAN

( 400 )

two fubftances was effected by tbe acid, andthat the brown folution contained only the un-oombined carbon diffolt'ed in nitric acid, whilftthe orange-coloured mafs was furnilhed by therefin. This was confirmed by treating in thefame way amber, afphaltum, different fpeciesof pit-coal, and laftly pure charcoal, all ofwhich yielded the brown folution in abun-dance, particularly the charcoal, but only thofefubiianecs that contained bitumen depofitedany of the orange-coloured mafs. The charcoaltherefore yielded none of this latter, but diffolvedcampleatly in the acid, making a dark reddifh-brown liquid. This liquid was llowly evapor-ated to drynefs, and left a brown glofly fub-ftance, with a refinous fradlure, the properties, of which are the following:

1. It is fpeedily diffolved by cold water andby alcohol.

2. Its tafte is highly aftringent.

3. Expoled to heat it fmokes but little,fwells much, and affords a bulky coal.

.4. The watery folution reddens litmus paper.

5. The fame folution copioufly decompofesthe metallic falts, efpecially muriat of tin, ace-tite of lead, and red fuiphat of iron, formingbrown or chocolate-coloured precipitates.

6. It precipitates gold from its folution, inthe metallic date.

7. It alfo precipitates the earthy falts.

8. The fixed alkalies at firfl only deepen thecolour of the folution, but after a time renderit turbid.

9. Glue or ifinglafs diffolved in water andadded to tlie above folution forms an immedi-ate precipitate move or lefs brown accordingto the degree of concentration, which is info-luble in cold or boiling water, and appearsclofely to referable the precipitate formed byadding gelatin to the infufion of galls or othervegetable aftringents.

From thefe fatls it appears that whenevercarbon, either as fimple charcoal or in naturalmixture with refins in the form of bitumen orcoal, is digefted in nitric acid, it is fpeedily dif-folved therein, and is then converted into a fub-ftance clofely refembling the natural Tannin,from which mod of the excels of acid may befeparated by cautious drying, and re-folutionin water.

This artificial tannin is probably purer thantannin procured in any way from vegetable in-fufions, being entirely free from gallic acid,though fomething refembling extract is pro-duced along with the tannin.

Further experiments confirm the ftrong re-

femblance of the artificial with the naturaltannin. When fulphuric acid is added to theartificial folution it becomes turbid, and a co-pious brown precipitate fubPules, which is fo-luble in boiling water, and then precipitatesgelatin. When carbonat of potaih is added,the colour is deepened, and a brown flocculentfubftance fubfides.

Pure ammonia diffolves the artificial tan, andthis folution gives no precipitate with gelatintill neutralized by an acid. In all thefe cir-cumftances it agrees cloiely with the naturaltannin.

When a folution of artificial tannin, made bypure charcoal and nitric acid, is precipitated asmuch as poflible by gelatin, the lupernatantliquor yields by flow evaporation to drynefs, alight brittle cinnamon-brown fubftance, with aftrong odour refembling that of oak-bark, foiu-ble in water, fcarcely added on by gelatin, butprecipitating the folutions of iron, tin, andlead, and which in fome degree refembles ex-tract; fo that this latter fubftance appears alfoto be an artificial as well as a natural produc-tion.

The chief, if not the only material, differ-ence which has hitherto been found betweenthe natural and the artificial tannin is, that theformer has its diftinguifhing properties, parti-cularly that of precipitating gelatin, entirelydeftroyed by digeftion with nitric acid; whereasthe artificial kind is made by means of thisacid, and refifts all further change by it, in anytemperature, at leaft for a confiderable lengthof time. ■

It has been mentioned that when nitric acidis digefted with any fpecies of refin, if thelatter contains no uncombined carbon, it is re-folved into an orange-coloured vifeid mafs, ofitfelf infoluble in the liquid, but which maybe diffolved by a further affufion of acid. Inthis ftate it gives no precipitate with gelatin,and therefore contains no tannin fully formed;but Mr. Hatchett found that on repeatedly ab-ftrafting the acid to drynefs, the refidue be-came foluble in water, forming a clear yellowliquid, in which the prefence of tannin wasthen detected by the folution of ifinglafs. Anykind of refin will yield tannin in this manner,as will fome of the gum refins that have beenexamined. This therefore is a fecond mode offorming the.artificial tannin. The fimple nitrou*folution of indigo alfo gives a portion of tannin,together with an intenfely bitter cryftallizablefubftance.

There is ftill another method of producing