Of the former type, two compounds are known, _i.e._, di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid and vanilloyl-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid-- HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COOH CH_3O The first named of these two compounds was obtained by Klepl, in addition to the didepside, by heating _p_-hydroxybenzoic acid. Fischer and Freudenberg obtained a beautifully crystalline form in the following way: carbethoxyhydroxy-benzoyl chloride was coupled with _p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution, the compound dissolved in a mixture of pyridine and acetone, and ammonia added for the purpose of removing the carbethoxy group. The tridepside was then obtained as long needles by re-dissolving in acetone. Both tridepsides melt well above 200° C., are practically insoluble in water, and are but sparingly soluble in practically all organic solvents. In alcoholic solution they give colour reaction with ferric chloride similar to those given by _p_-hydroxybenzoic acids. Preparation of Tetradepsides [Footnote: Fischer and Freudenberg, Liebig's _Ann._, 1910, 372, 32.] Here, again, two forms are known, _e.g._, tri-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid-- HO.C_5H_4.COO.C_6H_4.COO.C_6H_4COO.C_6H_4 COOH and vanilloyl-di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid-- HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COO.C_6H_4.COOH CH_3O The former has been prepared from carbethoxyhydroxy-benzoyl-_p_-hydroxybenzoyl chloride and _p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution; the second tetradepside was prepared from carbomethoxyvanilloyl-_p_-hydroxybenzoyl chloride and _p_-hydroxybenzoyl-_p_-hydroxy-benzoic acid. The preparation of these compounds is rendered difficult by the slight solubility of the substances and their slight affinities for entering into reaction. Both tetradepsides were obtained in crystalline form, and are but very little soluble in most organic solvents. They decompose on being fused. Tannoid Substances of the Tannin Type