SECTION V. 



TANNINS. 



THE term Tannin is variously employed by different writers, 

 sometimes to denote a particular substance better described as 

 gallotannic or digallic acid, and sometimes as a collective term 

 for a whole group of substances having certain characteristics 

 in common. In order to prevent confusion it is proposed here 

 to use the word tannin only in the latter sense. 



The properties of the tannins may be summarized as 

 follows : 



1. They are mostly uncrystallizable colloidal substances 

 with astringent properties. 



2. They precipitate gelatine from solution and form in- 

 soluble compounds with gelatine yielding tissues, a property 

 which enables them to convert hide into leather.* 



3. They all give blackish-blue or blackish-green colours 

 with ferric salts, a fact which is made use of in the manufac- 

 ture of ink. 



4. They are precipitated from solution by many metallic 

 salts such as copper or lead acetates or stannous chloride, etc. 



5. They are precipitated from solution by a strong aqueous 

 solution of potassium bichromate or by a I per cent solution 

 of chromic acid. 



* According to some authors this property is not an essential characteristic 

 of tannins ; on the other hand Dekker prefers to regard those substances 

 which do not give this reaction as pseudo-tannins and includes under this head- 

 ing caffetannic acid and the tannins of Portlandia grandiflora, Asperula odo- 

 rata, Rubia tinctorum, Scrophularia nodosa, Humulus Lupulus, etc. Similarly 

 Procter points out that such substances as moringatannic acid, or maclurin, and 

 caffetannic and lupulotannic acids, are more closely related to the colouring 

 matters than to the tannins ; maclurin which is a pentahydroxybenzophenone 

 is the yellow colouring matter occurring in the substance known as fustic, 

 obtained from the wood of Morus tinctoria (see formula on p. 202). 



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