TANNINCJ MATERIALS. 633 



with a scent of turpentine, which may be increased by pouring 

 turpentine on it, is used for catching bark-beetles ; birch-bark, 

 owing to its white colour, toughness, durability and heating- 

 power, is used extensively in the north of Europe, America 

 and Asia ; small artistic articles, also useful boxes and vessels 

 are made of bark ; cherry bark is especially ornamental. 



SECTION III. PROPERTIES, ETC., OF THE CONSTITUENT 

 PARTS OF BARK.* 



A. Tanning Materials. 



Tanning materials in the form of weak acids of various 

 composition are widespread in all parts of plants throughout 

 the vegetable kingdom ; they contain less carbon and oxygen 

 than other carbo-hydrates. Gallic acid or true tannin comes 

 from the galls on our oaks, as well as on (jHcirn* in/cctoria, 

 and from other galls. 



Tannin is an amorphous substance with an astringent 

 taste, soluble in alcohol and water ; according to their origin, 

 tannins from oak-bark, from oakwood, from spruce-bark, from 

 catechu, etc., may be distinguished : they form salts with 

 inorganic or organic bases (alkaloids) ; solutions containing 

 iron are coloured green or blue by these different tannins, so 

 that this serves as a test for the nature of the tannin. The 

 most important property of tannin is, that, when it acts on 

 gelatine, a substance contained in the skin of animals that 

 swells in water, it converts these skins into a connected, 

 strong, tough and durable material, leather.! When leather 

 is made by treating skins with tannin, the process is called 

 tanning. 



Leather, however, may be formed in other ways from skins ; 

 viz., by salts of alumina, especially chloride of alumina with 

 common salt ; this process is termed tawing and produces 

 white kid for gloves, etc. Shamoying employs fats or oils 

 and produces soft wash-leather. Tanning also may be done 

 with salts of iron, of chromium (sulphate or chloride of 

 chromium), of nickel, or by utilizing the electric current. 



* A. Mayer, " Lchrbucli der Agrikultur-Chimie." 5th ed., 11)01. 

 | Krn-yc. IJrit., Vol. X I V.. Leather." 



