s62 Mr. Biggin’s Experiments to determine the Quantity of 
that bark or vegetable may be determined, by comparing its 
quantity of tanning principle and price with those of oak bark. 
For an accurate chemical analysis, I have tried a variety of 
acids, and simple and compound affinities ; and, having pursued 
the above experiments, at the same time that I was employed 
on some in dying, I found the muriate of tin (the method of 
using which is described by Mr. Proust in the Annales de Chi- 
mu') very convenient. A solution of it, being added to the 
infusion of bark, forms a precipitate with the tanning principle, 
leaving the gallic acid suspended : the precipitate is of a fawn 
colour, and is composed of tanning principle and oxidated tin. 
By these means, I have been enabled to form a comparative 
scale of barks ; which, however, I do not produce as accurate. 
Oak bark, in its present state, as procured for commercial pur- 
poses, differs very much in quality, from accidental circumstances: 
the season of the year in which it is collected occasions a still 
more important difference, consequently, the scale now pro- 
duced must be very imperfect ; but I am of opinion, that by 
the pursuits of scientific men who may be inclined to investi- 
gate this subject more fully, a very accurate scale may hereafter 
be formed. 
In the following scale, I have taken Sumach as the most pow- 
erful in the comparative statement; leaving, however, a few de- 
grees, for a supposed maximum of tanning principle , which I 
reckon 20. 
