on the constituent Parts of astringent Vegetables .. 237 
acting upon an astringent infusion, is compared with the quan- 
tity of the precipitate obtained, the difference between them will 
indicate the proportion of tannin, as it exists in the combination. 
After the tannin has been separated from an astringent in- 
fusion, for the purpose of ascertaining its other component parts, 
I have been accustomed to evaporate the residual liquor very 
slowly, at a temperature below 200 0 .* In this process, if it 
contains extractive matter, that substance is in part rendered 
insoluble, so as to fall to the bottom of the vessel. When the 
fluid is reduced to a thick consistence, I pour alcohol upon it. 
If any gallic acid or soluble extractive matter be present, they 
will be dissolved, after a little agitation, in the alcohol ; whilst 
the mucilage, if any exist, will remain unaltered, and may be 
separated from the insoluble extract, by lixiviation with water. 
I have made many experiments, with the hope of discovering 
a method by which the respective quantities of gallic acid and 
extractive matter, when they exist in solution in the alcohol, 
may be ascertained ; but without obtaining success in the results. 
It is impossible to render the whole of any quantity of extrac- 
tive matter insoluble by exposure to heat and air, without at the 
same time decomposing a portion of the gallic acid. That acid 
cannot be sublimed, without being in part destroyed ; and, at 
the temperature of its sublimation, extractive matter is wholly 
converted into new products. 
Ether dissolves gallic acid; but it has comparatively little 
* M. Deyeux has shewn, ( Annales de Chimie, Tome XVII. page 36,) that in 
the process of evaporating solutions of galls, no gallic acid is carried over by the 
water, at a temperature below that of ebullition. Many astringent infusions, however, 
lose a portion of their aromatic principle, even in cases when they are not made to 
bod; but this substance, though evident to the smell, in the water that comes over, 
cannot be detected by chemical reagents. 
