(coloration with iron salts), but they cannot be regarded as tannins
from either a technical or a physiological standpoint.

As regards their chemical constitution, the natural (true) tannins
probably belong to different groups of organic compounds, and with our
present-day scant knowledge of their chemistry, it is impossible to
classify them. One is, however, justified in assuming that both the
natural tannins and the related humic acids are ester-derivatives of
hydroxybenzoic acids. [Footnote: E. Fischer, _Ber._, 1913, 46, 3253.]

The production of synthetic tannins employs two quite distinct
methods; one is to synthesise the most simple tannin, viz., the
tannic acid contained in galls (tannin), or to build up substances
similar in character to the tannins, from hydroxybenzoic acids. The
other, entirely new way, is to produce chemical substances, which
certainly have nothing in common with the constitution of the natural
tannins, but which behave like true tannins in contact with animal pelt,
and in addition, since they can be manufactured on a commercial scale,
are of practical value.

Owing to the fact that, until recently, the constitution of tannin has
remained unknown, it is easy to comprehend that the efforts to
synthesise the latter substance, or compounds similar to it, have been
mainly attempted on similar lines. The oldest investigation in this
direction dates from H. Schiff,[Footnote: Liebig's _Ann._, 1873, 43,
170.] who prepared substances similar to tannin by dehydrating
hydroxybenzoic acids. By allowing phosphorus oxychloride to interact
with phenolsulphonic acid, he obtained a well-defined substance
possessing tanning properties, which he considered an esterified
phenolsulphonic acid anhydride, the composition of which he determined