138 Miss Wheldale, On the nature of anthocyanin. 



Their characteristics, as given in most text-books, are an astringent 

 taste, the production of a blue or a green colour with iron salts, 

 tanning properties with animal membranes, power to precipitate 

 albumin and gelatine, etc., etc. The exact constitution of the 

 tannins is also rather indefinite though it is obvious that they are 

 aromatic compounds of the benzene series. Broadly speaking 

 tannins include numerous tannic acids, such as gallic, gallotannic, 

 catechinic, caffeic, protocatechuic, diprotocatechuic, ellagic, quer- 

 citannic, etc., usually existing in plants in the form of glucosides, 

 i.e. ethereal compounds with various sugars or with the trihydric 

 phenol, phloroglucin. 



It has been suggested that tannins may be divisible into 

 two groups, derived respectively from a dihydric phenol, such 

 as pyrocatechol, and a trihydric phenol, such as pyrogallol. The 

 kind of relationship might be illustrated as follows : — 



C6H4(OH)2 CeH3(OH)2.COOH C6H3(OH)2 . CO . . CeHg . OH . COOH 



pyrocatechol protocatechuic acid diprotocatechuic acid 



C9H3(OH)3 C6H2(OH)3 . GOGH CoHalOHjs . CO . . C6H2(OH)2 . COOH 



pyrogallol gallic acid tannic acid or digallic acid 



The tannins included in the first group give as a rule an 

 iron-greening reaction (though this is by no means invariably 

 the case), produce pyrocatechol on dry distillation and proto- 

 catechuic acid when fused with caustic alkali. Those of the 

 second group give an iron-blueing reaction, produce pyrogallol 

 on dry distillation and gallic acid on fusion with caustic alkali. 



Waage" and also Nickel^^ are of the opinion that another 

 series of tannic acids arises from the oxidation of phloroglucin, 

 the isomer of pyrogallol : — 



G6H3(OH)3 C6H2{OH)3 . COOH C6H2(OH)3 . CO . . C6H2(OH)2 . COOH 



phloroglucin phloroglucin carbonsaure phloroglucin gerbsaure 



(carboxylic acid) (tannic acid) 



Hence there may well be a complexity of tannins of different 

 natures. 



With a view to finding out whether, and if so, in what way 

 anthocyanin is related to tannin, the extracts from flowers of 

 numerous natural orders, both with and without anthocyanin, 

 were tested for tannin with various reagents. Although the 

 behaviour of the pigments has been diverse in many ways, 

 a certain amount of classification can be based on the results 

 obtained. 



It must at the same time be borne in mind that some of 

 the reagents used in these tests, react not only with the tannins 

 but also with many other bodies, such as phenols and their 

 numerous derivatives. 



