This classification is as yet very incomplete, and there are many tannins of which the decomposition products have not been examined, while our knowledge of the differences between the tannins which are classed together is extremely limited. In order to make the information which has been given practically available for further research, the characteristics and mode of recognition of the different products will be given, and as simple a scheme as possible of treatment of the tannin to be examined will be described; but the recognition of such products in a state of mixture presents great practical difficulties, and the tanner will usually be compelled to confine his attention to simpler, though less conclusive tests, based on the work of chemical specialists. Such tests will be described later (p. 111). [Illustration: Pl. III. _E & F N. Spon, London & New York_ "MARS PHOTO" SPRAGUE & CO. LONDON ARTIFICIAL FERMENTATION.] General Methods of Examination of Tannins. _Decomposition by Heat._--The ordinary method is to distil the tannin or dried extract in a small retort, and examine the distillate for catechol and pyrogallol. Unless the heat be very carefully regulated, much loss is caused by the destruction of the catechol and pyrogallol with formation of metagallic acid, &c., and their detection is greatly complicated by the presence of secondary products. This difficulty is somewhat lessened by passing a stream of carbon dioxide through the retort, which carries the products quickly out of the heated portion. A better method is to heat the tannin in glycerin (Thorpe, Chem. Soc. Abstr., 1881, 663; Allen, 'Commercial Organic Analysis,' 2nd ed.). About 1 _grm._ of the sample is heated with 5 _c.c._ of pure glycerin to 392°-410° F. (200°-210° C.) for 20 minutes. After cooling, about 20 _c.c._ of water is added, and the liquid is shaken with an equal volume of ether, without previous filtration. The ethereous layer, which contains the pyrogallol and catechol, is separated from the aqueous portion, evaporated to dryness, and dissolved in 50 _c.c._ of water. The filtered solution is divided into several portions and tested with lime-water, ferric chloride, and ferric acetate (see pp. 66-7); by these means it is easy to distinguish between catechol and pyrogallol; and either may be detected in presence of a small portion of the other; but if in nearly equal quantities, their recognition is difficult. Catechol may be derived from catechin, &c., and pyrogallol from gallic acid, and it is therefore necessary in some cases to remove these bodies from the tannin before treatment. As a general rule, however, catechins and catechol derivatives are only present in any quantity with catechol-tannins, and the same is true of gallic acid with regard to pyrogallol. (For methods of separation see pp. 69, 71, 80). Catechol has been formed by long continued heating of cellulose, starch, and other carbohydrates with water under pressure (see p. 67).