586 CONSIDERATION OF CARBON COMPOUNDS. 



alkalies. This is due to the fact that the methyl group is combined as in an 

 ether. The ether groups, as OCH 3 , OC 2 H 5 , OC 6 H 5 , etc., are often called methoxy, 

 ethoxy, phenoxy, etc. Anisic acid is formed by the oxidation of anethol, 



OOH 



3 , an ether contained in oil of anise. 



Gallic acid, Acidum gallicum, HC 7 H 5 O 5 -f H 2 O or C 6 H 2 (OH) 3 .- 

 CO 2 H + H 2 O = 186.65. Obtained by exposing moistened nut-galls 

 to the air for about six weeks, when a peculiar fermentation takes 

 place, during which taimic acid is converted into gallic acid, which 

 is purified by crystallization. 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 (difference from tannic 

 acid). A piece of potassium cyanide added to solution of gallic acid 

 produces a deep rose-color. 



Bismuth subgallate, a salt which is somewhat variable in composition, is 

 official. It is a yellow, amorphous, insoluble powder, known as Dermatol. 



Tannic acid, Acidum tannicum, C 13 H 9 O 7 .COOH = 319.66 

 (Gallotannic acid, Digallic acid). There are a number of tannic 

 acids, or tannins, found in various parts of different plants (oak-bark, 

 nut-galls, cinchona, 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 tannic acid is obtained by extracting nut-galls with 

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

 lowish, 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 : 



1. To solution of tannic acid add ferric chloride: a blue-black pre- 

 cipitate falls, soluble in large excess of tannic acid with violet color 



