AROMATIC ACIDS AND THEIR DERIVATIVES. 301 



2(XaC 7 H 5 3 ).H 2 0, and lithium salicylate, 2(LiC 7 H 5 O 3 VH 2 0, both 

 of which are white, soluble salts. 



Gallic acid, Acidum gallicum, HC.H 5 5 or C 6 H 2 (HO) 3 .C0 2 H = 188. 

 Obtained by exposing moistened gall-nuts 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 crystallization. The crystals contain one molecule of water, 

 which may be expelled at 100. It is a white, solid substance, 

 forming long, silky needles; it has an astringent and slightly 

 acidulous taste, and an acid reaction ; it gives a bluish-black 

 color with ferric salts, and does not coagulate albumen; by 

 heating, it is decomposed into carbon dioxide and pyrogallic acid, 



Tannic acid, Acidum tannicum, C U H 10 9 = 322. There are a 

 number of tannic acids, or tannins, found in various parts of 

 different plants (oak-bark, gall-nuts, cinchona, coffee, tea, etc.), 

 the composition of which is not identical with the above formula. 

 All tannins, however, are amorphous, soluble substances, having 

 a faint acid reaction and astringent properties; they all precipi- 

 tate albumen, alkaloids, ferric salts (bluish-black), and form 

 with animal substances compounds which do not putrefy. 



The officinal tannic acid is obtained by extracting nut-galls 

 with ether and alcohol, and evaporating the solution. 



Tannic acid 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. 



Analytical reactions. Tannic acid forms with ferric salts a 

 bluish-black precipitate (ink), and also precipitates gelatine, 

 alkaloids, albumen, gelatinized starch, and solution of tartar 

 emetic. 



QUESTIONS. 



431. What is the difference between aromatic and fatty compounds, 

 and from which hydrocarbons are aromatic compounds derived ? 



432. State the composition of carbolic acid. In what respects does it 

 differ from true acids ? 



433. Mention properties of and tests for carbolic acid. 



434. What is the difference between creasote and carbolic acid ; from 

 what are the two substances obtained ? 



