2o8 TANNINS 



acacias, mimosas, oak barks (but not oak wood, fruits or galls), 

 quebracho wood, cassia and mangrove barks, canaigre, cutch 

 and gambier. 



The tannins of this class are characterized by the following 

 properties : — 



1. They give with iron alum a greenish-black colour, 

 though the reaction is liable to be rendered uncertain by the 

 presence of other colouring matters. 



2. When treated with bromine water, until the solution 

 smells strongly of it, they give a yellowish or brown precipi- 

 tate ; in weak solutions the precipitate may form slowly. 



3. The addition of concentrated sulphuric acid to a drop 

 of the infusion produces a dark red or crimson ring at the 

 junction of the two liquids ; on dilution the liquid turns pink. 



4. These tannins deposit no " bloom," but when boiled 

 with acids deposit red insoluble colouring matters known as 

 phlobaphenes (see p. 205). 



Some of the tannins belonging to this group, notably 

 gambier and cutch, contain phloroglucinol as one of their 

 constituents ; this substance may be tested for by moistening 

 a pine wood shaving with a little of the infusion and then 

 adding a little concentrated hydrochloric acid; the formation, 

 after a short time, of a bright red or purple stain indicates the 

 presence of phloroglucinol. 



PROPERTIES AND DESCRIPTION OF INDIVIDUAL TANNINS. 



As already stated the term Tannin is merely a generic 

 name for the whole group of substances, though it has been, 

 and still is, frequently used to mean a particular tannin, namely 

 that contained in oak galls. This substance is, however, 

 better named gallotannic acid, as it is customary to name the 

 tannins after the source from which they are obtained ; thus 

 quercitannic acid indicates the tannin of oak bark, sumac- 

 tannin that derived from sumac, and so on. 



PYROGALLOL TANNINS. 



GALLOTANNIC ACID. 

 (Syn. Digallic acid, or Tannic acid, or merely " Tannin".) 



The two chief commercial sources of gallotannic acid are : — 



