investigations products containing arsenic (C_7H_7O_8As and
C_14H_11O_12As) are obtained when gallic acid is heated with arsenic
acid.

In his preparation of digallic acid, Iljin [Footnote:
_Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid,
and the ethyl ether of gallic acid showing no characteristics of the
tannins; when, however, he heated gallic acid with arsenic pentoxide, he
obtained bodies exhibiting the reactions given by tannins.

Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at
synthesising tannin; he heated gallic acid or its ethyl ester with
glyoxylic acid or pyroracemic acid, and obtained a substance of the
composition C_14H_10O_9.2H_2O, which certainly showed some of the
characteristics exhibited by tannin, but which by no means was identical
with the latter. Bottinger's preparation is probably identical with
[Greek: b]-digallic acid, one of two dibasic isomers having the
composition--

C_6H_2(OH)_2COOH
|
C_6H(OH)_3COOH

the other possible isomer having the composition

C_6H(OH)_3COOH
CO |
C_6H_2(OH)_3

Fischer [Footnote: _Ber_., 1908, 41, 2875.] obtained a digallic acid