TANNIN. 



rated to dryiicfs, which left the tannin in "a (tatc of eon- 

 fidcrablc purity. 



He ftill, however, found that it contained a portion of 

 fulphate of lime. In order to feparate this fait, he diflblved 

 ihe tannin in water, to which he added carbonate of potafs : 

 this caufed a precipitation, which has been already noticed. 

 The dear liquor being feparatcd, a foiution of acctite of lead 

 was added to it. A precipitate fell down, which confifted 

 of tannin, combined with ox yd of lead, and probably ful- 

 phate of lead. The hmc alfo combined with the tannm, 

 forming an infoluble compound. He then fcparated this 

 precipitate, mixed it with water, and pafTcd a ftrcam 

 of fulphurctted hydrogen gas through it. The lead and 

 falphur became feparatcd, and the combination of lime 

 and tannin unchanged, while the pure tannin remained in 

 foiution, which was obtained by evaporating the feparatcd 

 liquid to drynefs. The tannin thus obtained, approaches 

 much nearer to purity than that obtained by any of the 

 former proceffes. But we (hall (hew further on, that, even 

 in this (late, its purity is doubtful. 



Tannin obtained by the above procefs does not differ 

 much from that formed by the other proceffes. We' have 

 already ilated it to be foluble in alcohol : when, however, 

 bioth the tannin and alcohol are pure, the tannin does not 

 diffolve. 



Moll of the metallic oxyds form infoluble compounds 

 with tannin. There is, however, great reafon to believe 

 that the gallic acid, which is difficult to feparate from it, 

 has a much greater " effect upon thefe bodies. It is fup- 

 pofed that when the metallic oxyds are precipitated by tan- 

 nin, the latter combines with the oxygen, and, in feme 

 inftances, converts the tannin into extraft. 



The common method of detaching the prefence of tannin 

 is by a foiution of gelatine in water. Ifinglafs is moftly 

 ufcd for this purpofe. The folutions both of the tannin and 

 the gelatine (hould be in a coniiderably concentrated ilate ; 

 as weak folutions of either rediffolve, to a certain extent, 

 the precipitate which the tannin forms with the gelatine. 



The gelatine fhould be quite frefh, as the precipitate is 

 imperfecl when it has the lead figns of putridity. 



Sir Humphrey Davy ftates that the proportion of the 

 gelalme to the water (hould be 1 20 grains of the former to 

 20 ounces of the latter. 



According to the authority of the fame chemift, the com- 

 pound formed by the tannin and gelatine dried at 1 50°, is 

 compofed of 



Gelatine . . . . ^^ 



Tannin - - - - 46 



100 



Potafh, foda, and ammonia, combine with tannin, forming 

 compounds which are lefs foluble in water than pure tannin. 

 Thefe idkalies have a ftronger attraftion than gelatine for 

 tannin; as the alkaline folutions do not precipitate gelatine 

 till the alkali is faturated by an acid. 



The combinations of the earths vpith tannin are moftly 

 infoluble. Thofe with barytes and lime are (lightly fo, and 

 do not precipitate gelatine till an acid is added. 



Moft of the acids form infoluble compounds with tannin ; 

 but v.hen extraft is prefent, it is alfo precipitated : hence 

 the imperfetlion of the procefs for feparating tannin by 

 acids. 



When perfectly free from gallic acid it has no aftion 

 upon fulphate of iron ; but it produces a deep bhie pre- 

 cipitate from the oxyfnlphate : hence its efieft in commoD 



5 



writing-ink and black dye. The black is not complete 

 without expofure to the air. The precipitate which the 

 tannm forms is very heavy, and alniolt immediately feparates 

 from the water ; while that formed by the gallic acid re- 

 mains longer fufpendcd, ajid certainly, on that account 

 alone, is an cflTential ingredient in writing-ink. 



Taimin exifts in a great number of vegetables in fome 

 proportion, but is the moft abundant in nutgalls ; and of 

 them, the Aleppo galls afford the moft. Sir Humphrey 

 Davy has given the following analyiis of the Aleppo galls. 

 He extrafted, by infufion with water, all the foluble part 

 from 500 grains of powdered galls. .This foiution he fub- 

 mitted to (low evaporation, from which he obtained, in folid 

 matter, 1 85 grains. Thefe he found to confift of 



Tannin ...... 



GaUic acid, with extractive matter 

 Mucilage and matter rendered infoluble"} 

 by evaporation . - . .j 



Calcareous earth and (aline matter 



130 

 3' 

 12 



12 



185 



We are indebted to the fame diftlngui(hed chefijift for 

 bringing into notice a new fubftance, which contains a large 

 proportion of tannin. This fubftance is brought from the 

 Eail Indies, and is known by the names of catechu, or terra 

 Japonica. It is produced by the evaporation of a vegetable 

 fufion from the wood of a fpecies of mimofa, which grows 

 in India. 



There are two varieties of this fubftance, one brought: 

 from Bengal, and the other from Bombay. The former is 

 of a chocolate colour, of the fpecific gravity I-.28 ; the 

 latter of a lighter colour, of the fpecific gravity 1.39- 

 Both have an aftringent tafte, leaving an impreffion of fweet- 

 nefs. They are not changed by expofure to the air. 



Sir Hiimphrey procured an infufion from this fubftance 

 by long decoftion, the fpecific gravity of which was 1.102 : 

 500 grains of this infufion yielded, by evaporation, 41 grains 

 of folid matter ; 34 of which were tannin, and 7 of a pe- 

 culiar extraftive matter. 



This fubftance, in its original ftate, feems to contain a 

 very large proportion of tannin. The above chemift found 

 that 100 grains of the powdered catechu required 18 ounces 

 of water for its infufion. The refiduum, or undiffclvcd 

 part, is feldom more than -rT-th of the original weight, and 

 condfts of calcareous and aluminous earth, with a little fine 

 fand : 200 grains of the Bombay catechu gave 



Tannin -.-... jop 



Peculiar extraft . - . . . gg 



Mucilage - - - - - - 13 



Of the refiduum above-mentioned - lo 



. 200 



The catechu from Bengal gave, in 200 grains, 



Tannin - - . - - . py 



Peculiar extraftive matter ... yj 



Mucilage -..-.. i5 



Refidual matter, confifting of hme, :du-7 

 mine, and fand - - ■ "i 



»4 



The 



