CHAPTER VII 

 TANNINS 



USING the term in its general application to a group of sub- 

 stances having similar chemical and physical properties, rather 

 than in its limited application to a single definite chemical com- 

 pound known commercially as " tannin," the tannins are a special 

 group of plant substances, mostly glucosides, which have the 

 following characteristic properties. First, they are non-crystal- 

 line * substances, which form colloidal solutions with water, 

 which have an acid reaction and a sharp astringent taste. Second, 

 they form insoluble compounds with gelatine-containing tissues, 

 as shown by the conversion of hide into leather. Third, they form 

 soluble, dark-blue or greenish-black compounds with ferric salts, 

 the common inks. Fourth, they are precipitated from their solutions 

 by many metallic salts, such as lead acetate, stannous chloride, 

 potassium bichromate, etc. Fifth, they precipitate out of solu- 

 tion albumins, alkaloids, and basic organic coloring matters. 

 Finally, most tannins, in alkaline solutions, absorb oxygen from 

 the air and become dark brown or black in color. 



OCCURRENCE 



Tannins occur widely distributed in plants. Practically every 

 group of plants, from the fungi up to the flowering plants, contains 

 many species of plants which show tannin in some of their tissues. 

 Among the higher plants, tannins occur in a great variety of 

 organs. Thus, they are found in the roots of several species of 

 tropical plants; in the stems, both bark and wood, of oaks, pines, 

 hemlock, etc.; in the leaves of sumac, rhododendron, etc.; in 

 many fruits, especially in the green, or immature, stages; and in 



*The needle-like forms, in which commercial "tannin" comes on the 

 market, are not true crystals, but are broken fragments of the threads into 

 which the colloidal tannin is " spun-out " from the syrupy extracts of nut- 

 galls, etc. 



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