2 66 Mr. Davy’s Experiments and Observations 
and internal layers, and which were about the average thickness 
of the barks commonly used in tanning, namely, \ an inch. 
Of these barks, the oak produced, in the quantity of an ounce, 
6 i grains of matter dissolved by water, of which 29 grains were 
tannin. 
The Spanish chesnut 53 grains, of which 21 were tannin. 
And the Leicester willow' 71 grains, of which 33 were tannin. 
The proportions of these quantities, in respect to the tanning 
principle, are not very different from those estimated in Mr. 
Biggin’s table.* 
The residual substances obtained in the different experiments, 
differed considerably in their properties ; but certain portions of 
them were, in all instances, rendered insoluble during the process 
of evaporation. The residuum of the chesnut bark, as in the 
instance of the strongest infusion, possessed slightly acid pro- 
perties ; but more than f of its weight consisted of extractive 
matter. All the residuums in solution, as in the other cases, 
were precipitated by muriate of tin ; and, after this precipitation, 
the clear fluids acted much more feebly than before on the salts 
of iron ; so that there is great reason for believing, that the 
power of astringent infusions to precipitate the salts of iron 
black, or dark coloured, depends partly upon the agency of 
the extractive matters they contain, as well as upon that of the 
tanning principle and gallic acid. 
In pursuing the experiments upon the different astringent 
infusions, I examined the infusions of the bark of the elm and 
of the common willow. These infusions were acted on by re- 
agents, in a manner exactly similar to the infusions of the other 
barks : they were precipitated by the acids, by solutions of the 
* Phil. Trans, for 1799, p. 263. 
