Owing to the fact that, until recently, the constitution of tannin has remained unknown, it is easy to comprehend that the efforts to synthesise the latter substance, or compounds similar to it, have been mainly attempted on similar lines. The oldest investigation in this direction dates from H. Schiff,[Footnote: Liebig's _Ann._, 1873, 43, 170.] who prepared substances similar to tannin by dehydrating hydroxybenzoic acids. By allowing phosphorus oxychloride to interact with phenolsulphonic acid, he obtained a well-defined substance possessing tanning properties, which he considered an esterified phenolsulphonic acid anhydride, the composition of which he determined 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 from the point of view of chemical constitution have nothing in common with the natural tannins. Not only are they of interest to the industry from a practical point of view; they have also been examined very closely from a chemical standpoint.