TA N

TAN

( 399 )

and fome carbonat of lime. It alfo affordsgallic acid by diftillation.

From thefe and other fa£ls it appears thatthis precipitate is not pure tannin, but is a verycompounded fubftance containing tannin, gallicacid, alkali, and lime, and perhaps extraft.The prefence of alkali is peculiarly indicated,by the circumftance of its affording no precipi-tate with gelatine till faturated with an acid,which is precifely the effect produced in theinfufion of galls by adding the cauftic alkalies,as mentioned under the article Galls.

A compound of acid and tannin is producedoil adding an acid to the infufion of galls ashas alfo been mentioned under that article.This precipitate is tenacious like pitch, andwhen it is waflied with cold water, rediflblvedto laturation in boiling water, and a little.car-bonat of potafli added to faturate the acid,another precipitate takes place, which now nolonger contains any acid, and is fuppofed byProud: to be pure tannin. To obtain the wholeof it, the remaining liquor fhould be evaporatedand then allowed to cool, and the precipitatewaflied with cold water.

This fubftance certainly contains the tanninmore approaching to purity than in the formercafe, but as extract is alfo known to be preci-pitated by acids, it is probable that fome of itis alfo contained in this cafe, and befides itftiews traces of gallic acid.

From all that has been done on this fubject,it appears therefore that we cannot be certainthat we have ever obtained pure tannin, freefrom all other admixture; and it is equally cer-tain that none of thefe methods can extraft thewhole of this fubftance from vegetable infufi-ons; fo that it is fliil the beft method, in ex-periments where the quantity of tan alone isrequired, to make ufe of the infufion of animalgelatin, and eftimate the quantity of tan fromthe weight of the compound of tan and gelatinthus produced.

With proper precautions, the gelatin onlyfeparates the tannin from the vegetable infufi-ons; and this compound appears tolerably uni-form in its nature, and in the proportion of itsconftituent parts, though we have not yet beenable to feparate them without deltroying thecharadferiltic properties of each.

In applying tnc folution of gelatin to infufi-ens containing tannin, fevered- circumftancesrauft be noted. d It appears, in the lirft place,that the mere dilution of the liquids- influencesthe quantity of precipitate, this being the

• Davy. < ph

greateft, in proportion as the folution of tanninis moft concentrated.

This circumftance introduces an unavoidablefource of error, for though an uniform ftand-ard of ftrength may be adopted for the gelatin,by diflolving a given quantity of ifmglafs(which is nearly pure gelatin) in a known bulkof water, it is impoffible to make an equallyuniform folution of tannin from a compoundvegetable fubftance.

The proportions of ifmglafs and water em-ployed by Mr. Davy are 6 grains of the formerto an ounce of liquid, which is nearly as ftrongas can be made at a moderate temperature,without being inconveniently' ftiff and gelatin-ous. Care muft be taken not to add an overproportion of gelatin to the vegetable infufion,for when this happens, it appears that fome ofthe compound is difl'olved in the mixture, aslefs of it is precipitated. Probably thereforegelatin unites with tannin in different proporti-ons, and the compound is only infoluble whenthe gelatin is in the inferior proportion. Theprecipitate fhould always be dried at a tolerablyuniform temperature fomewhat higher thanthat of the atmofphere. This precipitate iscompofed, according to Mr. Davy, on an ave-rage, of about 54 per cent , of gelatin, and 46of tannin. There is much more tannin, how-ever, in this compound, than in tanned leather,or fkin faturated with tannin, though the con-ftituent parts are nearly the fame.

It remains to mention a very curious produc-tion of tannin by artificial means from fub-ftances which do not naturally contain a particleof it, lately difeovered by Mr. Hatchett, 0 andpartially noticed in this volume under- the articleReft ft.

The ingenious author was led to the difeo-

,0

very by purfumg his former experiments onBitumens and Bovey Coal, and in particular ontheir habitudes with nitric acid. When a purerefin is digefted with this acid it is convertedinto an orange-coloured viieid fubftance,, whichat firft feparates, but by a further affufion ofacid is rendered foiuble in water and alcohol.On the other hand when a bitumen is treatedin this manner, the firft effect of the acid is tofeparate this orange-yellow fubftance, and atthe fame time to produce a very dark bnnvnfolution. Now as the bitumens were fhewn byprevious experiments to confift of a refinholding a portion of uncombined enrhen^Mr. H. conceived that a reparation of thelie

il. Tranf. for 1805 6.