264 Mr. Biggin’s Experimetits , &c. 
It is to be observed, that the barks do not keep any respec- 
tive proportion in the quantity of gallic acid and tanning prin- 
ciple contained in each; which is an evidence of the distinctness 
of principle, and may perhaps open a new field for saving oak- 
bark in dying, as the willows, sallow, ash, and others, produce 
a very fine black. It is also worthy of observation, that the 
quantities of gallic acid and tanning principle do not differ in 
equal proportions, between the winter and spring felled oaks. 
This fact may lead to the discrimination of the proper time for 
cutting ; which is, probably, when the sap has completely filled 
and dilated that part of the vegetable intended for use. This will 
make a difference in the season of cutting oak, elm, and other 
trees, shrubs, &c. Leaves should be taken when arrived at 
their full size, and then dried under cover; for, as the tanning 
principle is so soluble, and the substance that contains it so thin, 
(in a leaf,) the dew alone might dissolve it. 
Finally, as the gallic acid does not seem to combine with the 
matter of skin, and as its astringency will corrugate the surface, 
we may, I think, conclude, that its presence in tanning is not 
only useless, but detrimental. 
