272 Mr. Davy’s Experiments and Observations 
manufacturer, which might have been made to enter into the 
composition of his leather. These observations show, that there 
is some foundation for the vulgar opinion of workmen, con- 
cerning what is technically called the feeding of leather in the 
slow method of tanning ; and, though the processes of the art 
may in some cases be protracted for an unnecessary length of 
time, yet, in general, they appear to. have arrived, in consequence 
of repeated practical experiments, at a degree of perfection 
which cannot be very far extended by means of any elucidations 
of theory that have as yet been made known. 
On the first view it appears singular that, in those cases of 
tanning where extractive matter forms a certain portion oj^he 
leather, the increase of weight is less than when the skin is 
combined with pure tannin ; but the fact is easily accounted for, 
when we consider that the attraction of skin for tannin must be 
probably weakened by its union with extractive matter; and, 
whether we suppose that the tannin and extractive matter enter 
together into combination with the matter of skin, or unite with 
separate portions of it, still, in either case, the primary attraction 
of tannin for skin must be, to a certain extent, diminished. 
In examining astringent vegetables in relation to their powers 
of tanning skin, it is necessary to take into account, not only 
the quantity they contain of the substance precipitable by gela- 
tine, but likewise the quantity, and the nature, of the extractive 
matter ; and, in cases of comparison, it is essential to employ 
infusions of the same degree of concentration. 
It is evident, from the experiments detailed in the Hid section, 
that of all the astringent substances which have been as yet 
examined, catechu is that which contains the largest propor- 
tion of tannin ; and, in supposing, according to the common 
