tanning Principle, and gallic Acid, in the Bark of Trees. 261 
FOR THE GALLIC ACID. 
To the pound of bark left in the earthen vessels, and already 
deprived of its tanning principle by these quick infusions, I added 
a given quantity of water, to procure a strong infusion of the 
gallic acid, which requires a longer time, (say 48 hours.) This 
infusion, when obtained pure,* affords little signs of the pre- 
sence of the tanning principle, when tried by the test of the 
solution of glue; but, with the solution of sulphate of iron, it 
gives a strong black colour, (the common black dye,) which 
differs in density, according to the quality of the bark : this may 
be further proved, by boiling a skain of worsted in the dye, by 
which the gradations of colour will be very perceptibly demon- 
strated. 
Having thus obtained a point of comparison ; by making a 
similar infusion, under similar circumstances, of any bark, or 
vegetable substance, and paying strict attention to the specific 
gravity of the infusion, the quantity of precipitate of leather, 
and the density of colour produced by given quantities of one 
or the other test, the result will be, a comparative statement 
of the respective powers of any bark, or vegetable substance. 
This comparative statement, I conceive to be sufficient for all 
commercial purposes. 
As oak bark is the usual substance employed in the trade of 
tanning, if a quantity of tanning principle is found to be con- 
tained in any other bark or vegetable, the commercial utility of 
* It is hardly possible, from the intimate connection of the two principles, to 
separate them entirely by infusion : in the infusion of tanning principle, there will 
always exist a little gallic acid ; and, in an infusion of gallic acid, a little tanning 
principle will commonly be present, unless the infusion of gallic acid is very weak, and 
procured by a third or fourth watering. 
MDCCXCIX. 
M m 
