on the constituent Parts of astringent Vegetables. 269 
of acting upon reagents, but all precipitable by gelatine. This 
opinion is sufficiently conformable to the facts generally known 
concerning the nature of the substances which are produced in 
organised matter ; but it cannot be considered as proved, till the 
tannin in different vegetables has been examined in its pure or 
insulated state. In all the vegetable infusions which have been 
subjected to experiment, it exists in a state of union with other 
principles ; and its properties must necessarily be modified by 
the peculiar circumstances of its combination.. 
From the experiments that have been detailed it appears, that 
the specific agencies of tannin in all the different astringent 
infusions are the same. In every instance, it is capable of en- 
tering into union with the acids, alkalis, and earths ; and of 
forming insoluble compounds with gelatine, and with skin. The 
infusions of the barks affect the greater number of reagents in a 
manner similar to the infusion of galls ; and, that this last fluid 
is rendered green by the carbonated alkalis, evidently depends 
upon the large proportion of gallic acid it contains. The infu- 
sion of sumach owes its characteristic property, of being pre- 
cipitated by the caustic alkalis, to the presence of sulphate of 
lime ; and, that the solutions of catechu do not copiously pre- 
cipitate the carbonated alkalis, appears to depend upon their 
containing tannin in a peculiar state of union with extractive 
matter, and uncombined with gallic acid or earthy salts. 
In making some experiments upon the affinities of the tanning 
principle, I found that all the earths were capable of attracting 
it from the alkalis : and, so great is their tendency to combine 
with it, that, by means of them, the compound of tannin and 
gelatine may be decomposed without much difficulty; for, 
after pure magnesia had been boiled for a few hours with 
