Whereas, until recently, tannin had been considered to be gallic acid
anhydride, or digallic acid, closer investigations have revealed that
neither is tannin digallic acid nor is the synthetically prepared
digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871,
231 and 967.] prepared digallic acid by the interaction of phosphorus
oxychloride and gallic acid, and believed the product obtained to be
identical with tannin; to this latter he first ascribed an ether formula
(I.), later an ester formula (II.)--

(OH)_2 (OH)_2
|| ||
C_6H_2---0---C_6H_2
| |
COOH COOH
(I.)

(OH)_2
||
C_6H_2(OH)_3--C--O.C_6H_2
|| |
O COOH
(II.)

Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's
condensation product contained phosphorus or arsenic acid and ascribed
its tanning properties to the latter; according to this investigator,
digallic acid, when completely freed from arsenic acid, does not react
with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39,
ii. 268 and 283.] did not consider the action of arsenic acid that of a
catalyst, but held that it entered into reaction; according to his