270 
THE FUNCTION OF TANNIN. 
and glucose ; those which give the blue-black reaction with 
ferric salts as a rule yield pyrogallol (C 6 H s (OH) 8), which 
gives with ferrous salts a blue-black, with ferric salts a red 
colour; while those which give the iron-green reaction com¬ 
monly yield pyrocatechin (C 6 H 4 (OH) 2), which gives with 
ferric chloride a dark green colour. The blue reaction with 
iron salts, a reaction which lies at the basis of the manufac¬ 
ture of ink, is by far the more prevalent amongst tannins. 
In the tannins known as Kino and Catechu, and in the 
tannins obtained from the leaves of the tea-plant and the 
twigs, &c. of the Sumach, are examples which produce the 
green reaction with the same salts. 
The most important form of tannin, gallotannic acid, is 
especially abundant in the gall-nuts of Qaercus infectorici , and 
other species, in the common oak apple, and many other parts, 
and gives a typical example of the power of resolution into 
glucose and gallic acid, being, however, most probably first 
resolved into glucose and tannic acid, and this latter then 
converted into gallic acid. It is, however, probable that gallo- 
tannic acid, when quite pure, is not a glucoside, but directly 
consists of digallic acid. :;: As gallotannic acid is the only 
form in which tannin has been at all carefully investigated, 
and even this with the difficulties indicated in the foot-note, 
it is clear that nothing of importance can be said of the 
various other forms of tannin recognised, on more or less 
valid grounds, by the chemist. Two conclusions we are 
however justified in drawing. These are (1) that tannin is 
richer in carbon and oxygen than are carbohydrates, and (2) 
* There is marked confusion in the chemical mind as to the exact 
nature of the body in question, and the cause is not difficult to 
recognise. All the solvents used for the extraction of tannin are 
likewise solvents, amongst other things, of glucose, which probably 
in all cases is present in the tissue. Hence, the tannin extracted 
contains free glucose which cannot be separated from the tannin 
itself. It will be seen that this fact likewise affects all experimental 
investigations into the growth of fungi, &c., in solutions of tannin. 
When the chemist has found some way of separating glucose from 
tannin, a more perfect appreciation of the constitution of tannin will 
exist. The ordinarily accepted formula is 
C u H 10 0 9 = C 6 H 2 (OH) 3 -CO O -C 6 H 2 (OH) 2 -C0 2 H. 
Another formula illustrates the relations between tannin and gallic 
acid thus:— 
Ci 4 H 10 0 9 (tannin) + H 2 O = 2 C 7 H 6 0 5 (gallic acid). 
Strecker, who worked very largely at tannin, gave as his earlier 
formula C 27 H 22 Oi 7 . He now accepts the formula C 14 H 10 0 9 for 
tannic acid or tannin, and considers it as probably the first etherial 
anhydride of gallic acid, using the lengthy composition formula given 
above. 
