TAN 
TAN 
TAN 
7&J 
the peroxide of tin changes it also into an ex- 
tract, perhaps by communicating oxygen. 
The action of the metals upon tan does not 
seem to be great, but almost all the metallic 
oxides have an affinity for it, and are capable 
of combining with it ; the compound is usu- 
ally nearly insoluble in water. Hence the 
reason why tiie infusion of nutgalls precipi- 
tates metallic solutions so readily. These 
compounds have been hitherto in a great 
measure overlooked by chemists. The fol- 
lowing observations contain the facts at pre- 
sent known. 
l When' the peroxide of tin or zinc is boiled 
in the infusion of galls, it acquires a dull yel- 
low colour, and abstracts all the constituents 
from the infusion" leaving behind only pure 
water. The oxides thus combined with tan, 
&c. are partly soluble in muriatic acid, and 
the solution indicates the presence of tan and 
gallic acid. When the peroxide of tin is 
allowed to act upon the cold infusion, it abs- 
tracts all its constituents in a few days. But 
Mr. Proust affirms, that in that case the gallic 
acid is mostly destroyed, and a portion of the 
tan brought to the state of extract. 
When the metallic salts are mixed with 
the infusion of galls, the precipitate consists 
of the metallic oxide combined with the tan, 
the extract, and the acid of the infusion ; and, 
according to Davy, it contains also a portion 
of the acid of the metallic salt. 
Tan produces no change upon the solution 
ofsulphat of iron ; but when it is mixed with 
a solution of the oxysulphat of iron, a deep 
blue coloured precpitate immediately ap- 
pears, consisting of the tan combined with 
the oxide. This precipitate, when dried* as- 
sumes a black colour. It is decomposed by 
acids. 
When too great a proportion of oxysul- 
phat of iron is poured into a solution of tan, 
the sulphuric acid, set at liberty by the com- 
bination of the iron and tan, is sufficient to re- 
dissolve the precipitate as it appears ; but the 
precipitate may easily be obtained by cau- 
tiously saturating this excess of acid with 
potass. When the experiment is performed 
in this manner, all the oxysulphat of iron 
which remains in the solution undecomposed 
is converted into sulphat. Mr. Proust sup- 
poses that this change is produced by the tan 
absorbing oxygen from the iron. The same 
change takes place if oxide is mixed with a 
considerable excess of 'sulphuric acid, and 
diluted wjth water. Common writing-ink is 
a combination of gallat of iron and tannat of 
iron. 
The alkalies combine readily with tan, and 
form with it a compound soluble in water. 
This was first observed by Deyeux, whose 
experiments have been verified by Mr. Davy. 
When potass or soda is added to the infusion 
of nutgalls, the liquid assumes a reddish- 
brown colour, and loses the property of pre- 
cipitating gelatine, till alkali is saturated with 
an acid. When the alkalized infusion is eva- 
porated to dryness, an olive-coloured mass 
remains of a faint alkaline taste, which deli- 
quesces in the air. Ammonia produces the 
same effect upon the infusion of galls; but 
when the mixture is exposed to the heat of 
boiling water, part of the ammonia flies off, a 
precipitate falls, consisting of most of the tan 
and gallic acid, while the extract remains in 
Solution. 
Vol. II. 
All the earths hitherto tried have a strong 
affinity for tan, and form with it compounds 
tor the most part insoluble in water, the pro- 
perties of which have scarcely been examined 
by chemists. 
One of the most important properties of 
tan is the insoluble compound which it forms 
with glue or gelatine, as this substance is 
termed by chemists. It is therefore em- 
ployed to detect the presence of gelatine in 
animal fluids': and, on the other hand, solu- 
tions of gelatine are employed to detect the 
presence of tan in vegetable fluids, and to as- 
certain its quantity. Now, although the 
compound of gelatine and tan is insoluble in 
water, it is soluble both in the solution of tan 
and of gelatine when sufficiently diluted. It 
is necessary, therefore, that the solution of 
gelatine, used to detect tan, should be as con- 
centrated as is consistent with its perfect 
fluidity; for glue, when gelatinous, does not 
act upon tan. It is necessary also that it 
should be employed quite fresh; for when in 
a state of putrefaction, it loses its property of 
precipitating tan. Mr. Davy has ascertained 
that the best proportion for use is a solution 
of ICO grains of isinglass in 20 ounces of 
water, ('are must be taken not to add an 
excess of the solution to the liquid from which 
the tan is to be separated ; because the com- 
pound of tan and gelatine is re-d'issolved by 
the solution of gelatine. According to the 
analysis of Mr. Davy, this compound, when 
dried in the temperature of 150°, is com- 
posed of 
54 gelatine 
4G tan 
100 . 
It appears, from the experiments of Mr. 
Davy and Mr. Chenevix, that tan is some- 
times formed in vegetables by the action of 
heat. Thus no tan can be detected in the 
decoction of coffee-beans, unless they havo 
been roasted ; but in that case their decoction 
precipitates gelatine. 
From the experiments of Mr. Davy, we 
learn that the affinities of the different classes 
of bodies capable of combining with tan are 
nearly in the following order : 
Earths, Acids, 
Alkalies, Neutral salts. 
Gelatine, 
But the order of the individual substances be- 
longing to each of these classes remains still to 
be ascertained. 
'Ban affects particularly the bark of trees : 
but it exists also in the sap and in the wood 
of a considerable number, and even in the 
leaves of many. It is very seldom that it 
exudes spontaneously ; yet this seems to be 
the case with a variety of kino. 
It has been ascertained by Mr. Biggin, 
that when the barks of trees are examined at 
different seasons, they vary in the quantity 
of tan. The quantity varies also with the age 
and size of the trees. The greatest propor- 
tion of tan is contained in the inner barks. 
The epidermis usually contains none. 
The following table exhibits the proportion 
of solid matter extracted by water from dif- 
ferent vegetable substances, and the quantity 
of tan contained in that solid matter, as ascer- 
tained by the experiments of Mr. Davy. 
One Ounce of 
White inner bark of old oak 
Solid 
Matter. 
Grains. 
108 
Tan. 
Grains. 
young oak 
111 
77 
nut 
G p tlilioll L 11 vu 
89 
63 
low 
Coloured or 
middle bark of oak 
117 
43 
72 
19 
chesnut 
41 
14 
willow - 
Entire bark of oak - 
Entire bark of Spanish chesnut 
Leicester willow 
34 
61 
53 
71 
16 
29 
2 1 
33 
elm 
common vvi! 
Sicilian sumach 
Malaga sumach 
Souchong tea 
Green tea - 
Bombay catechu 
Bengal catechu 
Nutgalls - 
165 
156 
180 
13 
1 1 
78 
79 
48 
41 
261 
231 
127 
TANACETUM, tansy, a genus of plants 
of the class of syngenesia, and order of poly- 
garnia superflua, and in the natural system 
ranging under the 49th order, composite. 
The receptacle is naked ; the pappus some- 
what emarginated ; the cal 3 - x imbricated and 
hemispherical ; the florets of the radius are 
trifid, and scarcely distinguishable. There are 
nine species ; of which one only is a native 
of Britain, the vulgare, or common tansy. 
Of this species there is a variety with curled 
leaves, which is therefore called curled tansy. 
The tansy has a bitter taste, and an aromatic 
smell, disagreeable to many people. Jt is 
esteemed good for warming and strengthen- 
ing the stomach ; for which reason the young 
leaves have obtained a place among the culi- 
nary herbs, their juice being an ingredient in 
puddings, &c. It is rarely used in medicine, 
though extolled as a good emmenagogue. A 
drachm of the dried flowers has been found 
very beneficial in hysteric disorders arising 
from suppression. The seeds and leaves 
were formerlv in considerable esteem for de- 
stroying worms in children, and are reckoned 
good in eholics and flatulencies. 
TANiECIUM, a genus of the angiospermia 
order, in the didynamia. class of plants, and in 
the natural method ranking under the 25th 
order, putaminese. The calyx is mcrnophyl- 
lous, tubulated, truncated, and entire: the 
corolla long, monopetalous, and white ; the 
tube cylindrical ; the lymbi erect, spreading, 
and nearly equal ; the fruit a berry, covered 
with a thick bark, large, oblong, internally 
divided into two parts ; in the pulp are con- 
tained a number of seeds. r l here are only 
two species of this genus, the jaroba and pa- 
rasiticum, both natives of Jamaica. They 
grow by the sides of rivers, and climb on trees 
and bushes. 
TANAGRA, tanager, in ornithology, 3 
genus of birds belonging to the order of pas- 
seres. The beak is conical, acuminated, 
emarginated, almost triangular at the base, 
and inclining a little towards the point. Dr. 
Latham has described 44 species, all of which 
are of foreign extraction. See Plate Nat, 
Hist. tig. 390. 
