as HO.C_6H_4.SO_2.O.C_6H_4HSO_3. It is, however, probable that this
substance is not homogeneous, but consists of a mixture of higher
condensation products.

Klepl [Footnote: _Jour. pr. Chem._, 1883, 28, 208.] obtained--by simply
heating _p_-hydroxybenzoic acid--a so-called di- and tridepside, but
this simple method is not applicable to many other hydroxybenzoic acids,
since these are decomposed by the high temperature required to induce
reaction.

Amongst other attempts to produce condensation products with
characteristics similar to those possessed by the tannins, those by
Gerhardt [Footnote: Liebig's _Ann_, 1853, 87, 159.] and Loewe [Footnote:
_Jahresh. f. Chem._, 1868, 559.] must be especially noted; they treated
gallic acid with phosphorus oxychloride or arsenic acid, and thereby
obtained amorphous compounds, exhibiting the reactions characteristic of
tanning substances. E. Fischer and Freudenberg, [Footnote: Liebig's
_Ann._, 372, 45.] by treating _p_-hydroxybenzoic acid in the same way,
succeeded in obtaining a didepside, and during the last years
practically only these two investigators have demonstrated the syntheses
of these depsides and produced high-molecular polydepsides.

At the same time researches were instituted with the object of
determining the constitution of tannin, and E. Fischer succeeded in
demonstrating its probable composition as being that of a glucoside
containing 5 molecules of digallic acid per 1 molecule of glucose.

This last-named class of synthetic tannins--which may be properly termed
"tanning matters" in contradistinction to the true tannins--exhibit very
distinct tanning character when brought in contact with animal hide, but