An unfortunate peculiarity, apparently common to all catechol-tannins, is that, however light-coloured the leather produced by them, it darkens and reddens rapidly by exposure to strong light, and ultimately becomes quite friable and rotten.[152] Cp. pp. 234, 272. [152] Cp. Report of Committee on Leathers for Bookbinding, Journ. Soc. of Arts, 1901, p. 14. Wagner, a German chemist, attempted to classify the tannins into “physiological” tannins, which were produced in the natural growth of the plant, and “pathological” which were caused by the attack of insects such as the gall-wasps, and he further ventured the assertion that only the former class were capable of producing leather. It has since been shown that the tannins produced in galls are identical with some of those found in healthy plants, and galls themselves have been used in tanning from very ancient times. It is only necessary to remind the reader of the use of Turkish gall-nuts, in place of sumach, which was common in the East in the tannage of moroccos, and of the “Knoppern,” or oak-galls formerly so largely used in Austria as a tanning material for sole leather. It is true that the tannin of galls is not very suitable for the latter purpose, consisting as it does mostly of gallotannic acid, which, giving no solid deposit of bloom or reds, is incapable of making a heavy or solid leather. Pure gallotannic acid itself produces a very white and soft leather. The class to which the tannins of the different tanning materials belong is mostly mentioned in the Botanical List (Chap. XVIII.), but it may be well here to specify a few of the most common. Galls and sumach contain gallotannic acid with a little ellagitannic; myrobalans, valonia, divi-divi, algarobilla, oakwood and chestnut are all pyrogallol-tannins giving ellagic and gallic acid among their decomposition products. All the pine barks, including the American hemlock, and the larch, all the acacias and mimosas, including the Indian Babul (_Acacia arabica_), the oak barks (though not the oak wood, fruits, or galls), quebracho wood, cassia[153] and mangrove barks, canaigre, cutch and gambier are catechol-tannins, and the two last contain phloroglucol, of which minute traces are also present in many other catechol-tannins (p. 297). [153] _Cassia auriculata_, or “turwar” bark, is the ordinary tannage of the East Indian or “Persian” sheep- and goat-skins, largely used in bookbinding, but which redden and decay very rapidly. Gallotannic acid, and several artificial tannins with the characteristic reactions of the class have been produced in the laboratory, but there is no present prospect of their manufacture at prices which can in the faintest way compete with those of natural production.