26o Mr. Biggin's Experiments to determine the Quantity of 
By dissolving an ounce of common glue in two pounds of 
boiling water, I procured a mucilaginous liquor, which, as it 
contains the matter of skin in solution, is a test for the tanning 
principle. By a saturated solution of sulphate of iron, I obtained 
a test for the gallic acid. 
I then took one pound of the bark I meant to try, ground as 
for the use of tanners, and divided it into five parts, each part be- 
ing put into an earthen vessel. To one part of this bark, I added 
two pounds of water, and infused them for one hour. Thus I 
procured an infusion of bark, which I poured on the second part 
of the bark, and this strengthened infusion again on the third 
part, and so on, to the fifth. But, as a certain ortion of the 
infusion will remain attached to the wood of the bark, after the 
infusion is poured or drawn off, I added a third pound of water 
to the first part, and then followed up the infusion on the several 
parts, till the three pounds of water, or so much of them as could 
be separated from the bark, were united in the fifth vessel; 
from which I generally obtained about one pint of strong infu- 
sion of bark.* 
To a certain quantity of this infusion, I added a given mea- 
sure of the solution of glue ; which formed an immediate preci- 
pitate, that may be separated from the infusion by filtering paper. 
When dried, it is a substance formed by the chemical union of 
the matter of skin with the tanning principle, and is, in fact, a 
powder of leather. By saturating the infusion with the solution 
of glue, the whole of the tanning principle may be separated 
by precipitation. 
* The specific gravity of this infusion was ascertained by an hydrometer whose 
gradations are inverse to those of a spirit hydrometer. 
