196 TANNINS 



For the inner bark of the American oak, Quercus Prinus, 

 Trimble * found the following seasonal variation : 



December . . . . . 9-33 per cent 



March . .... . . 10-63 



June ...... ii'22 



July . . . . . . 1170 



September 6-66 



As a general rule the barks collected in May and June are 

 the richest in tannin, but this does not hold for all parts of 

 plants. Thus, Levi and Wilmer f found that in the case of 

 the horse-chestnut, Aesculus Hippocastanum^ the youngest 

 leaves were richest in tannin, the minimum amount obtained 

 in June, whilst in August the quantity rapidly rose until the 

 original value was reached ; finally a diminution of tannin oc- 

 curred just before leaf-fall. Weekly analyses of leaves were 

 made from the opening of the buds to the fall of the leaves in 

 September. The obtained percentages of tannin were : 6'5, 

 3*3, 3'5, 2-8, 37, 3-2, 1-9, 2-8, 3-5, 3-6, 3-4, 5-1, 3-1, 5-3, 4-4, 

 4-3, 3-4, 6-2, 6-6, 5-2, 6-1, 6-5, 4-5 per cent 



These variations in the tannin-content of parts of plants 

 are of great interest ; the value, however, of such estimations 

 would be greatly enhanced if estimations were carried out at 

 the same time to see whether, for instance, there is any obvi- 

 ous relationship between the tannin-content of leaves and of 

 other parts of the plants such as the periderm. 



MICROCHEMICAL REACTIONS OF TANNINS. 



Before passing on to the detailed examination of the vari- 

 ous tannins, the following microchemical tests may be men- 

 tioned, but it must be borne in mind that these reactions do 

 not enable one to distinguish between the various tannins. 



1. Tannins reduce Fehling's solution. 



2. They are precipitated by basic lead acetate and the 

 salts of many other metals ; thus uranium acetate gives a 

 brown precipitate or a brown or brown-red coloration, and an 

 aqueous solution of copper acetate gives a brown precipitate. 



3. Potassium bichromate in a strong aqueous solution or 



* Trimble: "The Tannins," Philadelphia, 1892, 1894. 

 + Levi and Wilmer : ' Hide and Leather," 1905. 



