286 TANNINS 



of these fruits on long standing frequently deposit ellagic acid, 

 most probably by the action of a ferment contained in the 

 plant ; it is, however, prepared by pouring a hot concentrated 

 alcoholic extract of divi-divi into cold water ; the acid is 

 thereby precipitated, and may be filtered, and purified. 



Properties and Reactions. 



Ellagic acid is a yellow microcrystalline soHd which is 

 very slightly soluble in water, and therefore readily separates 

 from aqueous solutions in which it is formed ; it is also very 

 slightly soluble in alcohol or ether, but dissolves somewhat 

 readily in boiling pyridine. 



The dried substance treated with 1-2 drops of nitric acid 

 gives, on dilution with 10-20 drops of water, a blood-red colour 

 (Griessmayer's reaction). 



Catellagic, Metellagic, and Flavellagic acids are the names 

 given by Perkin to artificially synthesized acids obtained by 

 him. They are closely related to ellagic acid, but have not, 

 as yet, been found to occur naturally. 



THE CLASSIFICATION OF TANNINS. 



With the present incomplete state of our knowledge con- 

 cerning the chemical constitution of the tannins, it is difficult 

 to make a proper chemical classification of these substances. 



While a number of different classifications have been sug- 

 gested * the one due to Procter f is perhaps the most generally 

 useful, but it must be understood that it is not a rigid classifi- 

 cation since some tannins are known which possess certain of 

 the characteristics of each of the two groups into which he 

 divides tannins. 



Procter's classification is based upon the fact that tannins 

 when heated to 180-200° C. yield as a general rule either 

 pyrogallol or catechol, for which reason he has adopted the 

 following classification :— 



(A) Pyrogallol tannins, including oak gall tannin, oak wood 



* Trimble : " The Tannins." Philadelphia, 1894, Vol. II., p. 132. 

 Dekker : " De Looistoffen," Amsterdam, 1906. 



t Procter : " The Principles of Leather Manufacture," London, 1903. 



