The elimination of the carbomethoxy group may be brought about by dilute alkaline solutions in the cold, or by aqueous ammonia. If the depside formed is so stable as to resist the action of alkali for several hours, the use of the latter is very convenient for the purpose required. The substance is dissolved directly in sufficient normal alkali to neutralise the carboxyl group and a further 2 molecules of caustic soda for each carbomethoxy group to be eliminated are added. The temperature should be about 20° C., when the reaction as a rule is completed after one-half to three-quarters of an hour. It is usual, however, to use an aqueous ammonia solution in considerable excess, whereby the temperature should again be about 20° C., and the solution of ammonia normal or half normal. The didepsides so far investigated are crystalline bodies, sparingly soluble in cold water; they--as a rule--decompose when fused, possess acid reaction, and are dissolved by bicarbonates. On account of the presence of a free phenolic group they give a coloration with ferric chloride; if the phenolic group occupies the _o_-position to carboxyl, the coloration with ferric chloride is red or bluish-violet Excess of dilute alkali resolved all didepsides into their components at ordinary temperatures. The didepsides of gallic, proto-catechuic, gentisinic, and [Greek: b]-resorcylic acids precipitate gelatine and quinine acetate, and in this respect approach the natural tannins. The following summary gives an account of depsides which have been prepared synthetically or which occur naturally:--[Footnote 1: _Ber._, 1908, 41, 2888; 1909, 42, 217; 1913, 45, 2718; 1913, 46, 1130, 2396, 1141, 1143; Liebig's _Ann._, 384, 230, 233, 238; 391, 356, 362.] Di-_p_-hydroxybenzoic acid. Di-_m_-hydroxybenzoic acid. Disalicylic acid. Diprotocatechuic acid. Digentisinic acid. Di-[Greek: b]-resorcylic acid. _p_-Diorsellic acid. _o_-Diorsellic acid. _m_-Digallic acid. Disyringic acid. Di-_o_-cumaric acid. Diferulic acid. Di-[Greek: b]-hydroxynaphthoic acid. _p_-Hydroxybenzoyl-_m_-hydroxybenzoic acid. _m_-Hydroxybenzoyl-_p_-hydroxybenzoic acid. Salicyl-_p_-hydroxybenzoic acid, Vanilloyl-_p_-hydroxybenzoic acid. Feruloyl-_p_-hydroxybenzoic acid. [Greek: a]-Hydroxynaphthoyl-_p_-hydroxybenzoic acid. Orsellinoyl-_p_-hydroxybenzoic acid. Protocatechuyl-_p_-hydroxybenzoic acid. Galloyl-_p_-hydroxybenzoic acid. Pyrogallolcarboy _p_-hydroxybenzoic acid. Syringoyl-_p_-hydroxybenzoic acid. _p_-Hydroxybenzoyl-syringic acid. Pentamethyl-_m_-digallic acid. Pentamethyl-_p_-digallic acid. Vanilloyl vanillin. Preparation of Tridepsides Monohydroxybenzoic acids allow theoretically of tri-depsides of the type HO.C_6H_4COO.C_6H_4.COO.C_6H_4.COOH only; if, on the other hand, di- or trihydroxybenzoic acids are dealt with, two formulae are possible, viz.:-- HO.C_6H_4.COO } C_6H_3.COOH HO.C_6H_4.COO