contained in various Barks. 323 
when tried by the teft of the folution of glue; but, with the 
folution of fulphat of iron, it gives a {trong black colour, 
(the common black dye,) which differs in denfity according 
to the quality of the bark: this may be further proved, by 
boiling a fkain of worfted in the dye, by which the grada- 
tions of colour will be very perceptibly demonftrated. 
Having thus obtained a point of comparifon; by making 
a fimilar infufion, under fimilar cireumftances, of any bark, 
or vegetable fubftance, and paying ftrict attention to the fpe- 
cific gravity of the infufion, the quantity of precipitate of 
leather, and the denfity of colour produced by given quanti- 
ties of one or the other teft, the refult will be, a comparative 
ftatement of the refpective powers of any bark, or vegetable 
fubftance. This comparative ftatement I conceive to be fuf- 
ficient for all commercial purpofes. 
As oak bark is the ufual fubftance employed in the trade 
of tanning, if a quantity of tanning principle is found to be 
contained in any other bark or vegetable, the commercial 
utility of that bark or vegetable may be determined, by com- 
paring its quantity of tanning principle and price with thofe 
of oak bark, 
For an accurate chemical analyfis, I have tried a variety of 
acids, and fimple and compound affinities; and, having pur- 
fued the above experiments at the fame time that I was em- 
ployed on fome in dyeing, I found the muriat of tin (the 
method of ufing which is defcribed by Mr. Prouft in the 
Annales de Chimie,) very convenient. A folution of it béing 
added to the infufion of bark, forms a precipitate with the 
tanning principle, leaving the gallic acid fufpended: the pre- 
cipitate is of a fawn colour, and is compoled of tanning prin- 
ciple and oxydated tin. 
By thefe means I have been enabled to form a compara- 
tive feale of barks; which, however, I do not produce as 
accurate. Oak bark, in its prefent fiate, as procured for 
commercial purpofes, differs very much in quality, from ac- 
cidental circumftances: the feafon of the year in which it ig 
collected occafions a flill more important difference, confe- 
quently the fcale now produced muft be very imperfect ; 
but [am of opinion, that, by the purfuits of {cientific men, 
faa who 
