96 
THE TANNINS. 
“ Tannic, acid is an alcoholic anhydride of gallic acid, 
very probably digallic acid.” 
This structural formula was at first suggested by 
Schiff: 
Tannic Acid. 
Gallic Acid. 
He subsequently changed this formula to that of an 
ethereal anhydride, as follows : 
OH 
OH 
OH 
CO\ 
O / 
OH 
OH 
CO.OH 
Schiff* also investigated the reaction, first described 
by Lowe, of arsenic acid and gallic acid, which the 
latter considered to be an oxidation. Schilf, however, 
obtained similar results as with phosphorus oxychlo¬ 
ride, and considered it an easier method for obtaining 
digallic acid, since heating the gallic acid with a dilute 
solution of arsenic acid was sufficient to effect the con¬ 
version. There is, however, considerable difficulty in 
removing the arsenic from the tannic acid, a portion of 
the latter being reconverted into gallic acid. 
