_THE CHEMISTRY OF THE TANNINS._ The essential constituents of tanning materials are members of a large group of organic compounds known as “tannins” or “tannic acids,” which are widely distributed throughout the vegetable kingdom, and said to have one representative among animals, in the body of the corn-weevil. Their use in vegetable physiology is as yet uncertain, and indeed they appear in some cases to be waste products of organic change. The tannins, though varying considerably in their chemical constitution, and in many important characteristics, are all marked by the power of precipitating gelatine and some allied bodies from their solutions, of converting animal skin into the imputrescible material known as leather, and of forming dark-coloured compounds with ferric salts which are often utilised as inks. They are also precipitated by lead and copper acetates, stannous chloride, and many other metallic salts, and form insoluble compounds with many organic bases, such as quinine, and with the basic aniline colours. They are possessed of feeble acid character. All tannins are soluble in water to a greater or less degree; they are also soluble in alcohol, in mixtures of alcohol and ether, in ethyl acetate, acetone, and a few similar solvents, but are not dissolved by dry ether alone, nor by chloroform, petroleum spirit, carbon disulphide, nor benzine. As the tannins are uncrystallisable, and incapable of being distilled without decomposition, they are exceedingly difficult to obtain in a state of purity, and, owing to the considerable differences in their character, no one method is equally applicable to the whole group. As their successful separation requires considerable chemical training, and experience, detailed description is outside the scope of the present work, but some particulars of the more important methods employed are given on p. 43, L.I.L.B.