206 TANNINS 



this reason it was regarded by Schiff* as being identical with 

 gallotannic acid. This view has been proved by Waldenf to 

 be untenable, since the physical properties of the two sub- 

 stances are quite different (see p. 212). 



Reactions. 



1. Gallic acid is precipitated from solution by lead acetate ; 

 on adding caustic potash a carmine-coloured precipitate is 

 formed, which dissolves in excess to a raspberry-red solution. 



2. Ferric chloride produces a blue-black colour or pre- 

 cipitate according to the strength of the solution ; excess of 

 ferric salt changes the colour to green, while excess of gallic 

 acid reduces the ferric salt to ferrous and destroys the colour. 



3. Iodine solution produces a transient red colour. 



4. Gallic acid does not precipitate gelatine from solution. 

 (Distinction from tannic acid.) 



5. When heated with concentrated sulphuric acid it turns 

 green and then purple, being converted into rufigallic acid, 

 C 14 H 8 O 8 , a substance used in dyeing. 



6. Potassium cyanide gives a pink colour which disappears 

 on standing, but returns again on shaking with air. 



7. Lime water gives a blue coloration or precipitate ; in 

 very dilute solutions a reddish colour is produced. 



ELLAGIC ACID. C 14 H 8 O 8 . 



Closely related to gallic acid is the substance known as 

 Ellagic acid, its name being obtained by the inversion of the 

 word gallic. 



Whether or not this substance occurs free in nature is not 

 definitely established ; certain it is, however, that ellagic acid 

 can be readily obtained by the hydrolysis of ellagitannic acid, 

 a substance which almost invariably accompanies gallotannic 

 acid in the numerous vegetable products in which this latter 

 occurs ; it also occurs in conjunction with tannins of the pyro- 

 gallol class, and constitutes the bloom which is produced on 

 leather by this type of tannin. 



* Schiff: " Ber. deut. chem. Gesells.," 1871, 4, 232, 967; 1879, 12, 33; 

 "Annalen," 1873, 170, 143. 



f Walden : " Ber. deut. chem. Gesells.," 1897, 30, 3153 ; 1898, 31, 3167. 



