on a Substance possessing the Properties of Tannin. 289 
turbid, a tenacious film was deposited, which was insoluble in 
boiling water, and was evidently composed of gelatine and 
tannin. 
15. Twenty grains of prepared oak bark, by the like treat- 
ment, afforded a solution in water, which still acted in some 
measure upon gelatine, as it caused a solution of isinglass to 
become slightly turbid, and a film completely insoluble in 
boiling water was, as in experiment 14, deposited on the sides 
and bottom of the vessel. 
16. Infusions were prepared as nearly as possible of equal 
strength from galls, sumach, shavings of oakwood, oak bark, 
and the artificial tanning substance ; half an ounce in measure 
of each was then put into separate glasses, and one drachm in 
measure of the strong nitric acid was added. 
The different infusions were then examined by solution of 
isinglass, and I found that those of galls, sumach, and oak wood, 
were not rendered turbid, whilst the contrary happened to the 
infusions of oak bark, and of the artificial substance ; for these 
continued to precipitate gelatine, until four drachms or half an 
ounce of the nitric acid had been added to each half ounce of 
the infusion. 
When the results of these experiments are compared, they 
seem to establish, that although the artificial product is by much 
the most indestructible of all the tanning substances, yet there 
is some difference in this respect even between the varieties of 
natural tannin ; and that common catechu, and the tannin of oak 
bark, resist the effects of nitric acid much longer than galls, 
sumach, kascutti, and Pegu cutch. The last, as I have ob- 
served, is replete with mucilage, and by nitric acid yields a 
large quantity of oxalic acid ; it also appears to be the most 
MDCCCV. P p 
