Their chemical constitution is complex and in most cases imperfectly understood, but all the natural tannins which have been investigated prove to be derivatives of the trihydric phenol, pyrogallol, or of the dihydric phenol, catechol, the latter of which is often accompanied by a trihydric phenol, phloroglucol, which is isomeric with pyrogallol. The phenols are, themselves, a class of derivatives of benzene, C₆H₆, in which one or more of the hydrogen atoms are replaced by OH groups. Common phenol or “carbolic acid” is their simplest representative. Many of them, including pyrogallol and catechol, are used as photographic “developers.” The phenols on replacing another hydrogen by carboxyl (CO.OH) form true acids, of which salicylic corresponds to common phenol, protocatechuic to catechol, and gallic to pyrogallol; and the tannins are apparently complicated acids, in which one of the two latter acids is linked to a second molecule of the same or another acid as an anhydride, in some cases possibly with the addition of phenols or other organic groups. For more detailed information, see L.I.L.B., p. 45. Gallotannic acid is apparently digallic acid, in which two molecules of gallic acid are linked together after giving up the elements of a molecule of water. Natural gallotannic acid and many other tannins are glucosides, or at least contain glucose, which in many cases can be removed by purification. From what has just been said, it is obvious that a classification of the tannins according to constitution, is at present impracticable, not only from our imperfect knowledge, but from the difficulty of separating and determining the products of their decomposition. It is not, however, difficult to distinguish the catechol- from the pyrogallol-tannins by their chemical characteristics, apart from actual separation of the phenols, and the division is important as it is marked by certain broad differences in their properties which affect their use in tanning.