The [Greek: a]- and [Greek: b]-derivatives thus obtained differ in their behaviour towards polarised light, and are, again, probably mixtures of two stereoisomers, _i.e._, mixtures of derivatives of [Greek: a]- and [Greek: b]-glucose. Compared to methylotannin, these preparations exhibit very close resemblance to the former, from which it may be concluded that they are closely related to this substance, and probably possess the same or a very similar structure; the result of the above experiments has, therefore, brought us at least in close proximity to the structure of tannin. It must, however, be borne in mind that the analysis and hydrolysis of tannin does not afford an explanation of the question as to whether tannin is a compound of glucose and 10, 9, or 11 molecules of gallic acid; it is also possible, though not probable, that tannin would contain a polysaccharide instead of glucose itself. Similarly to sugar, the true glucosides can be coupled with hydroxybenzoic acids, which is proved by the preparation of tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also, exhibits tannoid character. 2. DIGALLIC ACID Whereas, until recently, tannin had been considered to be gallic acid anhydride, or digallic acid, closer investigations have revealed that neither is tannin digallic acid nor is the synthetically prepared digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871, 231 and 967.] prepared digallic acid by the interaction of phosphorus oxychloride and gallic acid, and believed the product obtained to be identical with tannin; to this latter he first ascribed an ether formula (I.), later an ester formula (II.)-- (OH)_2 (OH)_2 ¦¦ ¦¦ C_6H_2---0---C_6H_2 ¦ ¦ COOH COOH (I.) (OH)_2 ¦¦ C_6H_2(OH)_3--C--O.C_6H_2 ¦¦ ¦ O COOH (II.) Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's condensation product contained phosphorus or arsenic acid and ascribed its tanning properties to the latter; according to this investigator, digallic acid, when completely freed from arsenic acid, does not react with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39, ii. 268 and 283.] did not consider the action of arsenic acid that of a catalyst, but held that it entered into reaction; according to his investigations products containing arsenic (C_7H_7O_8As and C_14H_11O_12As) are obtained when gallic acid is heated with arsenic acid. In his preparation of digallic acid, Iljin [Footnote: _Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid, and the ethyl ether of gallic acid showing no characteristics of the tannins; when, however, he heated gallic acid with arsenic pentoxide, he obtained bodies exhibiting the reactions given by tannins.