Soluhle Principles of Oal Bark. 27 J 



coiour, a:i aftrlngeiU taftc, and becomes charged \*lththe mod foluhle fubftances contained 

 in the tan ; that by drawing ofFthe water, and adding a fimilar quantity to the tan repeat- 

 edly, the whole of the foluble parts may be fucceflively extraftcd, the water ceafes to acquire 

 colour, and there remains in the tub a mere fibrous matter, or parenchymatous texture, in- 

 folublc in water, and no longer adapted to promote the operation of tanning. This rcfidue 

 is therefore always rejefted in the manufaftories as ufelefs. It is only ufed by gardeners for 

 their hot-beds, but might probably be advantageoufly applied in the fabrication of coarfe 

 paper. 



It is therefore in the water of infufion, or the lixiviatlons of tan, that we muft feek for the 

 foluble fubftances which alone are efficacious in tanning. 



On examination of the water of the lad filtration, it is found to be not only clearer, lefs 

 impregnated, and lefs acrid than the water of the firft lixiviation, but likewife that it pof- 

 feffes all the properties of the gallic acid. It reddens the infufion of tournfol, arts upon 

 metallic folutions, and more particularly it precipitates a black fecula from fulphate of iron, 

 &:c. And it is alfo found that a piece of frefli Ikin, divefled of its fat and fanguine humours, 

 and macerated in this liquor, inftead of becoming compaft, is foftened and fwells up. 



The liquor of the firft lixiviation exhibits a very different charafler. It is more coloured 

 and aftringent ; it not only exhibits the properties of the gallic acid, by the alterations it 

 caufes in the blue colours of Yegetables, and the black precipitate it forms with the fulphate 

 of iron ; but it likewife pofleffes the remarkable quality of forming, with animal gelatin, or 

 glue, a yellowifh abundant precipitate, infoluble in watjr, not putrefcible, which becomes 

 hard and brittle by drying ; and if a piece of (kin properly prepared be immerfed in this 

 fluid, it becomes gradually more compadl, and is converted into leather. 



There exift, therefore, in the fame fluid, two very different fubflances : the one, which 

 pr(c pitates a black matter from iron, is the gallic acid or principle ; the other, which 

 precipitates animal gelatin or glue, is called the tanning principle, on account of its efficacy 

 in the preparation of leather. 



To leave no doubt on this important point, it was proved, by a number of experiments 

 eafy to be repeated, 



1. That the liquor of the laft lixiviation, though coloured, and of an aftringent tafte, af- 

 fords no precipitate with glue; a faft, which feems to fliew that the gallic acid contained 

 in the bark is lefs foluble than the tanning principle. In fa£l, as has already been remarked, 

 when water is fuccefTively poured on the tan, an infufion is at laft obtained which no longer 

 precipitates glue, though it precipitates fulphate of iron very well. 



2. The liquor of the firft lixiviation, after having been faturated with glue or animal ge- 

 LjtJn, and forming an abundant precipitate with that fubftance, is entirely deprived of the 

 tanning principle. It no longer differs from the liquor of the laft filtrations, and contains 

 merely a portion of the gallic acid. Hence the addition of fulphate of iron aftbrds a new 

 precipitate with this liquor. 



3. As the tanning principle has a ftrong attrai£ilon to the animal gelatin, with which it 

 always forms an infoluble precipitate, this property affords a very convenient re agent to af- 

 ccrtain its prefencc immediately in any fluid, and to determine with precifion its quantity. 

 Accordingly, the infufion of tan poured into milk, whey, fcrum, broth, &c. forms, with 



Vol. 1. — Septemdeh i/yy. Nn thcfc 



