Of the three classes of isomeric anhydrides which can be formed from hydroxybenzoic acids, the chemistry of the natural tannins is only concerned with the class comprising the ester anhydrides. If the carboxyl of the first molecule combines with a hydroxyl of the second molecule (ester formation), then a substance possessing character similar to that of a hydroxybenzoic acid is formed, which is capable of combining up with a further molecule in the same way. It is natural to assume that this ester form is much more prevalent in Nature than a combination of two carboxyls by the elimination of water. From the point of view of the chemistry of the tannins, therefore, the starting-point would naturally be that of synthesising the ester anhydrides of hydroxybenzoic acids. Amongst the small number of synthetically prepared ester anhydrides of hydroxybenzoic acids, a few occur exhibiting the properties of the natural tannoids. In order to simplify the terminology of these substances, Fischer [Footnote: Liebig's _Ann_., 1910, 372, 35.] proposed the name "Depsides" from [Greek: depheiv] = to tan. In analogy with peptides and saccharides, the names di-, tri-, and polydepsides of hydroxybenzoic acids would be suitable for these substances. The principles underlying the synthesis of depsides are the following:--If the chlorides of carbomethoxy (or carbethoxy) hydroxybenzoic acids are coupled with the sodium salts of hydroxybenzoic acids, esters are formed, _e.g._, CH_3CO O.O.C_6H_4.CO.Cl + NaO.C_6H_4.COO.Na = NaCl + CH_3.COO.O.C_6H_4.CO.O.C_6H_4.COO.Na On gently saponifying the esters, these are converted into the corresponding hydroxy derivatives-- OH.C_6H_4.CO.O.C_6H_4.COOH According to Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 1909, 372, 32.] this method possesses the following advantages:-- 1. The synthesis takes place at low temperatures, so that any intramolecular rearrangements are improbable. 2. The composition of the substances is controlled by the intermediary compounds, the carboalkyloxy derivatives. 3. The synthesis permits of more definite evidence as regards the structure of the resulting compounds. 4. The substances obtained are easily purified. Depsides produced in this manner are by no means new, and were obtained by Klepl by simply heating _p_-hydroxy-benzoic acid (_cf._ Introduction, p. 4). This simple procedure, however, is not applicable to most other hydroxybenzoic acids which are decomposed at the high temperature necessary to induce reaction. Lowe and Schiff (_loc. cit._) have obtained products similar to tannins, the latter investigator by removing the elements of water from gallic acid, protocatechuic acid, salicylic acid, _m_-hydroxybenzoic acid, cresotinic acid, phloretinic acid, and pyrogallolcarboxylic acid. These depsides, however, are amorphous substances, and it is hence difficult to substantiate their homogeneity. Carbomethoxylation of Hydroxybenzoic Acids