TANN1C ACID. 373 



acid is a solid. It mixes with water at low tempera- 

 ture, in all proportions, and is commonly seen in its dis- 

 solved state. Its compounds with metallic oxides are 

 called acetates. The sugar of lead, so called, is an 

 acetate, formed by dissolving litharge in acetic acid. 



938. TANNIC ACID. Tannin, or tannic 

 e acid > exists in nut-galls and in the bark 



properties of and leaves of many trees. It is the prin- 



tannic acid ? . . .... . , . 



ciple which imparts to them their astrin- 

 gent taste, and gives to the tan liquor the property of 

 converting hides into leather. When separated from 

 the other substances with which it is combined in 

 nature, it is a yellowish, gummy mass. It is soluble in 

 water, and possesses the property of precipitating glue 

 or gelatin, and many other metallic oxides. 



939. WRITING INK. Common writing 



What is the . 



coloring mat- ink is prepared from nut-galls and proto- 

 ter^of writing sulphate of iron> W hen first made, it is 



principally a tannate of the protoxide of 

 iron, and forms a very pale solution. Before 

 it is fit for use, it must be exposed for a time 

 to the air, and thereby converted, partially, into 

 tannate of the peroxide. This is a bluish black 

 precipitate, and imparts to it the requisite color. 

 It is essential to the permanence of ink, that 

 the change should take place, in part, in the fibre of 

 the paper itself. Too long exposure should, therefore, 

 be avoided in the manufacture. The pale ink thus 

 produced, which blackens further in using, is much more 

 permanent than a thicker, darker ink, produced when 

 this caution is not observed. 



