Astringent Substances as Sources of Pyrogallic Acid. 419 
kino, larch, birch and alder barks, &c., are wholly unfit for 
this purpose. Indeed the only salt of iron which gives pretty 
nearly the same coloured precipitates with either species of 
tannin is the acetate, but even these, though at first bluish 
black in the case of the green tannins, become in a day or two 
grayish black, and their reactions with the sulphate, chloride 
and nitrate of iron have no resemblance whatever to those of 
the black tannins. ‘The old distinction, therefore, which di- 
vides the astringents into those which give black, and into 
those which give green precipitates with the inorganic salts of 
iron, appears, so far as it goes, to be perfectly just, as these 
precipitates are not convertible into each other, as affirmed by 
M. Cavallius. There is, however, good reason for believing 
that some of the varieties of tannin, which even agree in their 
reaction upon salts of iron, and in their characters generally, 
are still by no means identical substances. In this respect 
there is considerable analogy between the varieties of tannin 
and the different fatty acids, 
It is much to be regretted that we are unable to procure 
tannin in a state of purity from any other source than nut- 
galls. When pounded galls are treated by Pelouze’s method, 
with hydrated zther, in a displacement apparatus, the liquid 
on standing separates into two strata, the lowest of which 
contains tannic acid in a state of purity. 
When, however, oak bark, valonia, sumach, gum kino, ca- 
techu, &c. are treated with ether in a similar manner, only 
one stratum of liquid is obtained. Pelouze’s process is there- 
fore inapplicable to these substances. This is the more to be 
regretted, as from the extreme facility with which tannin de- 
composes when in contact with moisture, we are unacquainted 
with any good way of obtaining it in a pure state from any of 
the other astringent substances; and consequently, with the 
exception of nut-galls, little progress has been made in the 
investigation of these bodies, 
It has been already mentioned that the tannin of galls and 
gallic acid are the only substances which, when distilled, are 
known to yield pyrogallic acid—a substance whose characters 
are so well marked that it can easily be recognized. It struck 
me, therefore, that this circumstance might be employed as 
an easy test for the presence of gallic acid; and also enable 
us to ascertain when the tannin these substances contained 
was similar or otherwise with that of nut-galls. With this 
view I was induced to subject a number of the astringent 
matters to examination. ‘The first in the order was su- 
mach. 
Sumach, which is so extensively employed in Great Britain 
oF 2 
