All the natural tannins with which we are acquainted, are derived from, and yield on decomposition either catechol, phloroglucol, or pyrogallol, and sometimes more than one of these. Artificial products, however, with many of the reactions of tannins have been obtained from other members of the group, and most phenols and their derived acids give either purplish or greenish black with ferric salts. Several classifications of the tannins have been suggested. The division most obvious to the tanner is into those tannins which yield the whitish deposit in the surface of the leather, called "bloom," and those which do not. Stenhouse, some years since, divided tannins into 2 classes, one of which gives a bluish, and the other a greenish-black with ferric salts. In the main these 2 classes correspond to the 2 former, as most tannins which yield a blue-black with iron acetate also give bloom to the leather. In some cases, however, the difference of tint is due to accidental impurities, and even gallotannic acid will give a decided green with strongly acid ferric chloride. These classifications both correspond to well-defined differences of constitution, and it is obviously more scientific to arrange tannins according to the products which they yield on decomposition, and which indicate their ultimate structure, rather than on any less essential point. If those tannins which give bloom to leather are cautiously heated to about 392° F. (200° C.), they are decomposed, and a substance is volatilised which condenses in feathery crystals, and which on examination turns out to be pyrogallol. Those tannins, on the other hand, which yield no bloom, but red deposits, produce a somewhat similar sublimate of catechol. From oak-bark and valonia, which yield both bloom and red colouring matters, both catechol and pyrogallol have been obtained. We may, therefore, divide tannins broadly into derivatives of catechol, which yield no bloom, and usually give greenish-blacks with iron acetate, and which include hemlock, mimosa, cutch, gambier, quebracho, &c; derivatives of pyrogallol, which give bluish-blacks with iron, deposit bloom in leather, and embrace galls, sumach, divi-divi, myrobalans, pomegranate rind, &c., and tannins which contain both pyrogallol and catechol, such as oak-bark and valonia, and which, as is well known, yield bloom, and give blue-blacks with iron.