322 On the Tanning Principle and Gallic Acid 
tanning principle. By a faturated folution of fulphat of iron, 
T obtained a teft for the gallic acid. 
I then took one pound of the bark I meant to try, ground 
as for the ufe of tanners, and divided it into five parts, each 
part being put into an earthen veflel. To one part of this 
bark [ added two pounds of water, and infufed them for one 
four, Thus I procured an infufion of bark, which I poured 
on the fecond part of the bark, and this {trengthened infufion 
again on the third part, and fo on to the fifth. But, as a 
ceMtain portion of the infufion will remain attached to the 
wood of the bark after the infufion is poured or drawn off, I 
added a third pound of water to the firft part, and then fol- 
lowed up the infufion on the feveral parts till the three pounds 
of water, or fo much of them as could be {eparated from the 
bark, were united in the fifth veflel; from which I generally 
obtained about one pint of {trong infufion of bark *. 
To a certain quantity of this infufion, 1 added a given 
meafure of the folution of glue, which formed an immediate 
precipitate, that may be feparated frony the infufion by filter-. 
ing paper. When dried, it is a fubftance formed by the che- 
mical union of the matter of {kin with the tanning principle, 
and is, in fact, a powder of leather. By faturating the infufion 
with the folution of glue, the whole of the tanning principle 
may be feparated by precipitation. 
For the Gallic Acid. 
To the pound of bark left in the earthen veffels, and al- 
ready deprived of its tanning principle by thefe gwick infu- 
fions, I added a given quantity of water, to procure a {trong 
infufion of the gallic acid, which requires a longer time, 
(lay forty-eight hours.) This infufion, when obtained puref, 
affords little figns of the prefence of the tanning principle, 
* The fpecific gravity of this infufion was afcertained by; an hydre- 
meter Whole gradations are inverfe to thofe of a f{pirit hydrometer. 
+ Itis hardly poffible, from the intimate connection of the two prin- 
ciples, to feparate them entirely by infufion: in the infufion of tanning 
principle, there will always exift a little gallic acid; and, in an infufion 
of gallic acid, a little tanning principle will commonly be prefent, unlefs 
the infufion of gallic acid is yery weak, and procured by a third or fourth 
wateripg. 
when 
