TANNING. 
due is tlierefore always rejected in tlie 
manufactories as useless. It is only used 
by gardeners for their hot-beds, but might 
probably be advantageously applied in the 
tabrication of coar se paper. 
It is therefore in tire water of infusion, 
or the lixiviations of tan, that we must 
seek for the soluble substances which alone 
are efficacious in tanning. On examina- 
tion of the water of the last filtration, it is 
found to be not only clearer, less impreg- 
nated and less acrid than the water of the 
first lixiviation, but likewise that it possesses 
all the properties of the gallic acid. It 
reddens the infusion of tournsol, acts upon 
metallic solutions, and more particularly it 
precipitates a black fecula from sulphate of 
iron, &c. And it is also found that a piece 
of fresh skin, divested of its fat and san- 
guine humours, and macerated in this li- 
<juoi’, instead of becoming compact, is 
softened and swells up. , 
The liquor of the first lixiviation exhibits 
a very different character. It is more co- 
loured and astringent ; it not only exhibits 
the properties of tlie gallic acid, by the al- 
terations it causes in the blue colours of 
vegetables, and the black precipitate it 
forms with the sulphate of iron; but it like- 
wise possesses the remarkable quality of 
forming, with animal gelatine, or glue, a yel- 
lowish abundant precipitate, insoluble in 
water, not putrescible, which becomes hard 
and brittle by drying; and if a piece of 
skin properly prepared be immersed in this 
fluid, it becomes gradually more compact, 
and is converted into leather. 
There exist, therefore, in the same fluid, 
two very different substances : the one, 
which precipitates a black matter from 
iron, is the gallic acid or principle; the 
other, whicli precipitates animal gelatine or 
glue, is called the tanning principle, on ac- 
count of its efficacy in the preparation of 
leather. i 
To leave bo doubt on this important point, 
it was proved, by a number of. experiments 
easy to be repeated. 1. That the liquor of 
the last lixiviation, though coloured, and of 
an astringent taste, affords no precipitate 
with glue; a fact, which seems to show that 
the gallic acid contained in the bark is less 
soluble than the tanning principle. In 
fact, as has already been remarked, when 
water is successively poured on the tan, an 
infusion is at last obtained which no longer 
precipitates glue, though it precipitates sul- 
phate of iron very well. The liquor of 
the first lixiviation, after having been satu- 
rated with glue or animal gelatine, and form- 
ing an abundant precipitate witli that snb- 
stanoe, is entirely deprived of the tanning 
principle. It no longer differs from the li- 
quor of the last filtrations, and contains 
liierely a portion of the gallic acid. Hence 
the addition of sulphate of iron affords a 
new precipitate with this liquor, o. As 
the tannipg principle has a strong attrac- 
tion to the animal gelatine, with wliich it al- 
ways forms an insoluble precipitate, this 
property affords a very convenient re-agent 
to ascertain its presence immediately in 
any fluid, and to determine with precision 
its quantity. Accordingly, the infusion of 
tan poured into milk, whey, serum, broth, 
&c. forms with tliese liquors, a precipitate 
more or less abundant, according to the 
quantity of gelatine they contain. 
This peculiar property of tire tanning 
principle affords an JippUcation which may 
become of great importance in the art of 
treating diseases, to determine the nature 
of urine, and to ascertain some, of its 
changes. In the healthy subject, all whose 
functions are duly exercised, the urine does 
not contain gelatine, nor afford a precipitate 
witli the infusion of tan: on the contrary, 
in all the gastric affections, tire urine is 
more or less cliavged with gelatine, and 
forms, witli the infusion of tan, a precipi- 
tate move or less abundant. The same ob- 
servation is applicable to acute and chroni- 
cal diseases, in which the assimilating or di- 
gestive forces are troubled, deranged, or 
perverted. 4. The gallic acid, or, if other 
terms be preferred, the principle which pre- 
cipitates the sulphate of iron, is often 
found alone, or at least without being ac- 
companied by tire tanning principle. Thus, 
quinquina, crude or torrefied coffee, the 
roots of the strawberry-plant, scrofularia, 
milfoil, arnica, the flowers of Roman camo- 
mile, and all the multitude, of plants va- 
guely comprised under the title of astrin- 
gents, contain the gallic acid only. All 
these form with tire sulphate of iron a pre- 
cipitate more or less coloured and abun- 
dant ; but none of them produce tire slight- 
est change in the solution of animal glue. 
On the contrary, the tanning principle has 
never been found alone, but always united 
or combined with the gallic principle. It 
was long supposed to exist exclusively in 
the oak, the nut-gall, and sumac, the only 
substances used at the tau-works ; but it is 
found more or less abundantly in the sili- 
quastrum, the rose-tree, tire larix, several 
species of pines, the acacias, the lotus, th» 
