940 TANNIC ACID AND BODIES ALLIED TO IT. 



Tannic acid boiled in an excess of caustic alkali, undergoes the 

 same transformation. 



Tannic acid precipitates animal gelatine entirely from solu- 

 tion in thick flocks, which adhere and form a viscid elastic mass 

 when the acid is in excess ; this precipitate dissolves in the 

 supernatant liquid at the boiling point. It is known as tanno- 

 gelatin, and contains about half its weight of tannin. Tannin 

 is also absorbed from solution by the fresh skin of animals, 

 which is then tanned or converted into leather, and ceases to be 

 soluble in water or to be putrescible. Tannic acid also precipi- 

 tates a solution of starch and of albumen, and is capable of 

 combining with animal fibrin. 



Tannates. The neutral tannate of potash or ammonia ap- 

 pears as a thick precipitate, when a moderately dilute solution 

 of tannic acid is treated with that alkali or its carbonate ; the 

 precipitate is very soluble in an excess of alkali. Tannates 

 of barytesj strontian, lime, and magnesia are very sparingly 

 soluble. Salts of protoxide of iron undergo no alteration when 

 mixed with a solution of tannic acid, but if exposed to air 

 become soon of a deep bluish black colour. Tannate of per- 

 oxide of iron is a black pulverulent precipitate, Fe 2 O 3 ,C 18 H 5 O 9 

 H-9HO (Pelouze), formed on adding persulphate of iron to 

 a solution of tannic acid ; it is the basis of writing ink. A good 

 black ink is prepared from bruised Aleppo galls 6 ounces, 

 copperas (sulphate of iron) 4 ounces, gum arabic 4 ounces, 

 water 6 pints. The galls are boiled in the water, the other 

 ingredients then added, and the whole kept in a wooden or glass 

 vessel and occasionally shaken. In two months strain, and 

 pour off the ink into glass bottles to be well corked. To prevent 

 mould, add one grain of corrosive sublimate or three drops of 

 creosote to each pint of ink. (Brande's Manual, p. 1105). Tartar 

 emetic gives a white precipitate with tannic acid, the tannate of 

 antimony, HO 3 + 3C 18 H 5 O 9 . Tannic acid forms sparingly 

 soluble white precipitates with most of the organic bases. 



GALLIC ACID. 



Formula of the crystallized acid, 2HO,C 7 HO 3 -f HO ; of the 

 acid dried at 212 ; 2HO,C 7 HO 3 . Of one gallate of lead dried 



