1883.] of 'Tannins in the vegetable cell. 389 



first regarded as identical with the particular form occurring in 

 galls, but later investigators have distinguished many varieties, 

 such as Gallotannic, Quercitannic, Cinchona-tannic, Catechutannic, 

 Moritannic and Oaffetannic acids. Many of these are obviously 

 related to one another, but since any thorough investigation has 

 as yet been limited to gallotannic acid it is impossible to make a 

 definite statement with regard to them. 



According to their behaviour with Ferric chloride they can be 

 well separated into two classes, viz. (1) those which give a blue 

 black and (2) those which give a green precipitate. The blue 

 black tannins and the green tannin of willow-bark were shown 

 by Stenhouse 1 to be glucosides, while many of the others appear 

 to yield phloroglucin (the isomer of pyrogallol) in place of glucose. 

 It was Strecker* who pointed out that the tannin of oak-galls 

 was in reality a glucoside, but it is to Schiff 3 that we are indebted 

 for any really definite knowledge as to the formula and probable 

 relations of gallotannic acid. He first prepared tannic acid free 

 from sugar by the action of phosphorus oxy chloride on tannin, 

 and gave for its formula C 34 H O8 2 „, thus properly regarding it as 

 a glucoside of tannic acid, and not as Strecker had done as a 

 glucoside of gallic acid which necessitated for its formula C 27 H 22 17 . 

 According to Schifif tannin is decomposed by the action of ferments 

 or dilute acids according to the following equations. 



(glucose) (dipallic or tannic acid) 



(«) C 34 H 28 22 + 2H 2 = C 6 H 12 6 + 20 14 H 10 O 9 



(gallic acid) 



(&) C ]4 H 10 O 9 + H 2 O = 2C 7 H 6 O 5 . 



On boiling an aqueous solution of gallic acid to which a small 

 quantity of arsenic acid is added, although the arsenic acid 

 remains unaltered, the gallic acid loses water and tannic acid is 

 produced. 



Thus 2C 7 H 6 O 5 -H 2 O = C 14 H 10 O 9 . 



Thus tannic acid appears to be an anhydride of gallic acid. 

 Furthermore by treating tannic acid with acetic anhydride Schiff 

 obtained what was first supposed to be a tetracetyl, but which 

 afterwards proved to be a pentacetyl tannic acid. There are thus 

 five replaceable hydroxy Is and we may regard tannic acid as an 

 etherated anhydride of gallic acid — gallic acid being as we know 

 trioxybenzoic acid. Thus 



2 (Gallic acid) - (water) = (Tannic acid) 



OH HOOCH [OH HOOCH 



pu OH HOl pTT TTn _ r u J OH H ^ 



°6 H * 1 OH HO f °c H 2 - H 2° - <- A QH H0 > C G H 2 



CO[OH H]Oj [CO 



1 Stenhouse. See Watts, Chem. Die. Vol. v. p. CGO. 

 - Strecker. Watts, Chem. Die. v. 660. 

 s Schiff. Deut, Ges. Bcr. iv. 231, 967. 



