furthermore, other aromatic and aliphatic hydroxy compounds frequently
occur. So far, however, attempts at explaining the constitution of the
complex decomposition products obtained by hydrolysing high molecular
tannoids have not been successful. On the other hand, the constitution
of the simpler natural tannoids is known to a greater or less extent; of
these, lecanoric acid (Lecanorsaure) is the best known, being an ester
anhydride of orsellic acid (a dihydroxytoluylic acid). It combines with
erythrite, forming another tannoid, erythrine. The fact that
hydroxybenzoic acids are constantly encountered together with the
products obtained on hydrolysis of the tannins, seems to point toward
the conclusion that anhydrides of hydroxybenzoic acids are frequent
constituents of the natural tannoid molecules.

The assumption that, for instance, in tannin at least part of the gallic
acid radicals are combined with one another is highly probable, and is
supported by the formation of tri- and dimethylgallic acid from
methylotannin, [Footnote: Herzig, _Monatshefte f. Chemie_, 1909, 30,
343.] and by the formation of ellagic acid when tannin is oxidised.
[Footnote: Nierenstein, _Ber_., 1908, 41, 3015.] Further proof is
brought forward by the existence of the pentacetyl-tannin, [Footnote:
Schiff, _Ann. d. Chem_., 1873, 170, 73.] and by the results of
hydrolysis which has yielded up to 104 per cent. anhydrous gallic acid
fiom tannin [Footnote: Sisley, _Bull. Soc. Chim_. 1909, 5, 727.]

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