TANNIN. 



Die great iincertjiinty which has prevailed refpefting the 

 definite nature of tannin, feems to have been completely 

 removed by the very important difcovery made by Mr. 

 Hatchett, who has fucceeded in forming artificial tan- 

 nin. An account of his experiments may be found in 

 three papers in the Philofophical Tranfaftions for 1805 

 ;uid 1806. 



The moft direfl procefs which is given for obtaining it, 

 is by pouring an ounce of nitric acid, diluted with two 

 parts of water, upon 100 grains of charcoal, in a matrafs. 

 This is to be placed in a fand-heat. Great effervefccnce 

 takes place, and much nitrous gas is difcngaged. At the 

 end of two days he added a fecond ounce of acid, and 

 fometimes even a third. The digelUon is continued till 

 the whole is difTolved. This folution is of a reddifli-brown 

 colour. It is then to be (lowly evaporated to drynefs, 

 which produces a brown glolTy fubftance, exhibiting a refi- 

 no js frafture. 



This fubftance has the following properties : 



1. It diflTolves in cold water and alcohol. 



2. The flavour is highly allringent. 



3. Expofed to heat, it fmokes a little, fwells up, and af- 

 fords a bulky coal. 



4. The folution in water reddens litmus paper. 



5. It copioudy precipitates the metallic falts, efpecially 

 the muriate of tin, acetite of lead, and oxyfulphate of iron. 

 The precipitates are commonly of a brown colour. 



6. It precipitates gold in the metallic flate. 



7. It precipitates the earthy falts ; fuch as the nitrates of 

 lime, barytes, &c. 



8. When the ahkalies are added to this folution, the 

 colour becomes deeper, and ultimately turbid. 



9. A folution of ifinglafs added to the fame folution jjro- 

 duces a precipitate, which is infoluble in boiling water, re- 

 lembling in its cffential properties the precipitate formed by 

 the natural tannin. . 



Mr. Hatchett produced the fame fubftance by treating 

 various kinds of coal in the fame way, fuch as pit-coal, coke, 

 and animal charcoal. What may feem very curious, he 

 formed it from the coal ot one portion of ifinglafs to pre- 

 cipitate another portion difTolved in water ; and hence aflferts 

 that one portion of the Ikin of an animal may be employed 

 to convert the other into leather. 



Of the different fubftances employed, he found thofe the 

 beft which confifted of carbon unmixed with other vege- 

 table matter, which always reduced the quantity ; and 

 that thofe. vegetable fubftances which contained gum or 

 mucilage, produced the leaft tannin. 



When he fucceeded in producing tannin from other vege- 

 table fubftances, fuch as indigo, refin, lac, and many 

 other bodies, it was by repeatedly adding frefh nitric acid ; 

 by which he \erf properly fuppofes that the carbon be- 

 comes feparated, fo as to put it under fimilar circumftances 

 to the charcoal itfelf. Indigo produced the moft in this 

 way. At the commencement of his paper he mentions the 

 faft of Mr. Chenevix having found that coffee-berries ac- 

 quired, by roafting, a portion of tannin. He made fome . 

 experiments which, although not very fuccefsful, convinced 

 him that the chief charafteriftic properties of tannin may be 

 formed or developed at fome particular temperature and 

 under favourable circumftances, by very fimple means. 



He ingenioufly conjeftures, that the tannin found in fome 

 varieties of peat has been produced in this way. 



In making ufe of fulphuric acid to char various fubftances, 

 he found that, in ^me inftances, the artificial tannin, or a 

 fubftance nearly rcfembhng it, was formed. He firll difTolved 



too grains of camphor in an ounce of concentrated fulphuric 

 acid. The camphor firft difTolved, without producing mucli 

 change of colour. In a little time it became brown, and 

 ultimately black. During this cliange, fulphurous acid gas 

 was dilengaged. After two days, during which time the 

 alembic had not been heated, the difengagement of gas dimi- 

 nifhed, and the veflel was placed in a moderately liealed fand- 

 bath. This increafed the aftion. At the end of two days, 

 fix ounces of cold water were added ; the liquid ciianged to 

 a reddifli-brown colour ; the difengagement of gas ceal'ed, and 

 was fucceeded by a fmell refembling a mixture of tlie oils 

 of lavender and peppermint. By gradual diftillation, the 

 water came over impregnated ftrongly with the above 

 odour, and accompanied by an cffential oil, which weighed 

 tlnee grains. 



When the whole water had come over, two ounces more 

 were added. The fmell before mentioned did not return, and 

 the evaporation was continued to drynefs. The blackifli- 

 brown refiduum was not afted upon by water, but by feveral 

 digeftions with alcohol, leaving behind a compaft coaly re- 

 fiduum, which, when dried and heated to a red heat in a 

 clofe vefTel, weighed 53 grains. The alcohol was then drawn 

 ofT from tlie folution by diftillation in a water-bath, leaving 

 a blackifli-brown mat's, of the appearance of a gum refin, 

 and the fmell of calomel. Its weight was 49 grains. The 

 whole of this therefore confifted of 



The effential oil above-mentioned - - - . - 3 



A compaft hard coal in fmall fragments - - J3 



And the b lac kifti -brown mafs above-mentioned - 49 



105 



•This increafe 'of five grains Mr. Hatchett attributes lo 

 the oxygen united to the carbon by the acid, or the water 

 combined with the blackifh -brown fubftance. 



This latter fubftance had the following properties : 



1. It had an aftringent tafte, and when difTolved in cold 

 water, formed a d;u-k-brown folution. 



2. It yielded a dark-brown precipitate with fulphate of 

 iron, acetite of lead, muriate of tin, and nitrate of lime. 



3. It precipated gold in its metallic ftate. 



4. It formed fo complete a precipitate with a folution 

 of ifinglafs, thatthe liquid became colourlcfs as water. 



The precipitate was nearly black, and was, hke the 

 other compounds of tannin and gelatine, infoluble in hot 

 water. 



Mr. Hatchett obferves that although this fubftance pof- 

 feffcs the general charafter of that obtained by the nitric acid 

 with charcoal, yet it feems to aft lefs powerfully upon Ikin. 

 The precipitate, at the time of its formation, is more floc- 

 culent and lefs tenacious than that produced by the other 

 procefs. 



Mr. Hatchett fuppofes this difference may arife from the 

 want of azote, which feems to exift in that produced by 

 nitric acid and charcoal. 



He afcertained this by fubjefting to analyfis a portion 

 which had been prepared from vegetable charcoal. He ex- 

 pofed in a retort fome of this tannin in the dry ftate, to the 

 heat of a lamp connefted with a jar, to obtain the gafeouB 

 produft. 



Firft a fmall quantity of water rofe ; thin a little niuic 

 acid, which had not been expelled in th-- evaporation. Next 

 a fmall quantity of yellowilh liquor, whicli fta.:.ed the neck 

 of the retort. Tlie fire was liien raifed, when a quantity 

 of gas was explofively d^fengaged, and upfet the jar. 

 This gas he judged from the fmcllto be ammonia. He .illo 



obfcrved 



