TANNIC AND GALLIC ACIDS 645 



yielded by the Rhus semialata ; large Mecca galls, called 

 Dead Sea apples, are imported from Bussorah. 



Galls vary from the size of a bean to that of a hazel-nut, 

 are round, hard, and studded with tubercles ; of a bluish- 

 grey colour externally, and yellow within. An inferior 

 variety, from which the larva has escaped, are smoother, of 

 lighter colour, lower density, and less astringency. Galls 

 are easily reduced to a yellow-grey powder, devoid of odour, 

 but having an intensely astringent taste. The active 

 principles are dissolved by forty parts of boiling water and 

 still less of diluted alcohol. Ferric salts, added to a watery 

 solution, slowly precipitate the dark-blue or black iron 

 tannate, the basis of writing ink. An aqueous solution of 

 gelatin throws down a grey flocculent precipitate of tanno- 

 gelatin. These reactions, and other important properties, 

 depend on the presence of tannin or tannic acid, which, 

 according to the quality of the galls, ranges from 15 to 70 

 per cent., and is associated with about 3 per cent, of gallic 

 acid. 



TANNIC ACID, or tannin (C 14 H 10 9 .2H 2 0), is the compound 

 to which oak bark, galls, logwood, and many vegetable 

 astringents owe their properties. The tannic acid from these 

 several sources has, however, somewhat different charac- 

 teristics, and generally receives such special designations as 

 gallo-tannic, cincho-tannic, catechu-tannic acids. Gallo- 

 tannic acid is prepared by softening powdered galls by 

 keeping them for two days in a damp place, digesting them 

 for several hours simultaneously with water, which dissolves 

 the tannic acid, and with ether, which dissolves colouring 

 matter and gallic acid. The mixture, filtered and allowed 

 to stand, forms into two layers, and the lower, carefully 

 evaporated, yields tannic acid. It occurs in pale yellow 

 masses, or thin glistening scales ; has a strongly astringent 

 taste and an acid reaction ; is readily soluble in water, 

 dilute alcohol, and slowly in glycerin ; very sparingly 

 soluble in ether. 



GALLIC Aero, or tri-hydroxy-benzoic acid (C 6 H 2 (OH) 3 , 

 C0 2 H,H 2 0), may be prepared by the action of diluted sul- 

 phuric acid on tannic acid or powdered galls. It occurs in 

 acicular prisms, or silky needles, which are colourless or 



