BENZENE SERIES. AROMA TIC COMPO UNDS. 379 



Gallic acid, Acidum gallicum, HC 7 H 5 O 5 , or C 6 H 2 (OH) 3 .CO. i H = 

 17O. Obtained by exposing moistened nut-galls to the air for about 

 six weeks, when a peculiar fermentation takes place, during which 

 tannic acid is converted into gallic acid, which is purified by crystal- 

 lization. The crystals contain one molecule of water, which may be 

 expelled at 100 C. (212 F.). It is a white, solid substance, forming 

 long, silky needles ; it has an astringent and slightly acidulous taste, 

 and an acid reaction ; it is soluble in about 100 parts of cold or in 3 

 parts of boiling water, also readily soluble in alcohol, but sparingly 

 in ether and chloroform; it gives a bluish-black precipitate with 

 ferric salts, and does not coagulate albumin, nor precipitate alkaloids, 

 gelatin, or starch. A piece of potassium cyanide added to solution 

 of gallic acid produces a deep rose color. 



Pyrogallol, Pyrog-allic acid, C 6 H 3 .(OH) 3 . When gallic acid is 

 heated to 200 C. (392 F.) it is decomposed into carbon dioxide and 

 pyrogallol, a substance which is not a true acid, but tri-hydroxy- 

 benzene, i. e. y a tri -atomic phenol. Pyrogallol crystallizes in color- 

 less needles, melts at 131 C. (268 F.), is easily soluble in water, 

 ether, and alcohol. In alkaline solution it absorbs oxygen rapidly, 

 assuming a red, then reddish-brown and dark-brown color (Plate 

 VI., 3). Nitric acid also colors it yellow, then brown, and this 

 property is made use of in testing for traces of nitric acid. Solutions 

 of silver, gold, and mercury are reduced by pyrogallol even in the 

 cold. 



Tannic acid, Acidum tannicum, HC 14 H 9 O 9 = 322 (Gallotannic 

 acid, Digallic acid). There are a number of tannic acids, or tannins, 

 found in various parts of different plants (oak-bark, nut-galls, cin- 

 chona, coffee, tea, etc.), the properties of which are not quite identical. 

 All tannins, however, are amorphous, have a faint acid reaction and 

 strongly astringent properties ; they all precipitate albumin and most 

 of the alkaloids ; they give with ferric salts a dark-colored solution 

 or precipitate, the color being dark green or dark blue ; they form 

 with animal substances compounds which do not putrefy. Use is 

 made of this property in the process of tanning i. e , converting 

 hides into leather. 



The official or gallotannic acid is obtained by extracting nut-galls 

 with ether and alcohol, and evaporating the solution ; it forms light- 

 yellowish, amorphous scales, having a faint and characteristic odor, 

 a strongly astringent taste and an acid reaction ; it is easily soluble 

 in water and diluted alcohol. 



