264 Mr. Davy’s Experiments and Observations 
taken up, the quantities of liquor last obtained, though they did 
not act much upon solution of gelatine, or perceptibly redden 
litmus-paper, produced a dense black with the solution of sul- 
phate of iron : by evaporation, they furnished a brown matter, 
of which a part was rendered insoluble in water by the action of 
the atmosphere ; and the part soluble in water was not in any 
degree taken up by sulphuric ether ; so that, if it contained gallic 
acid, it was in a state of intimate union with extractive matter. 
Two pieces of calf-skin, which weighed when dry 94, grains 
each, were slowly tanned ; one by being exposed to a weak in- 
fusion of the Leicester willow bark, and the other by being acted 
upon by a weak infusion of oak bark. The process was completed 
in about three months ; and it was found, that one piece of skin 
had gained in weight 14 grains, and the other piece about 16-5- 
grains. This increase is proportionally much less than that which 
took place in the experiment on the process of quick tanning. 
The colour of the pieces of leather was deeper than that of the 
pieces which had been quickly tanned ; and, to judge from the 
properties of the residual liquors, more of the extractive matters 
of the barks had been combined with them. 
The experiments of Mr. Biggin* have shown, that similar 
barks, when taken from trees at different seasons, differ as to 
the quantities of tannin they contain : and I have observed, that 
the proportions of the astringent principles in barks, vary con- 
siderably according as their age and size are different ; besides, 
these proportions are often influenced by accidental circum- 
stances, so that it is extremely difficult to ascertain their distinct 
relations to each other. 
In every astringent bark, the interior white bark (that is, the 
• Phil. Trans, for 1799, p. 299. 
