200 TANNINS 



2. The dihydroxy acid Protocatechuic acid. 



OH 



IOH 



COOH 



Protocatechuic acid 



3. The Trihydric phenols Pyrogallol and phloroglucinol. 



OH OH 



IOH 

 OH 



Pyrogallol Phloroglucinol * 



4. The trihydroxy acid Gallic acid. 



OH 



Gallic acid 



The above substances occur in varying proportions among 

 the products obtained by subjecting different tannins to fusion 

 with caustic potash or other chemical treatment ; and upon 

 their occurrence is based the chemical classification of the 

 tannins. 



PYROCATECHOL, CATECHOL OR PYROCATECHIN. C 6 H 4 (OH) 2 . 



This substance is so called from the fact that it is obtained 

 by the destructive distillation of catechu, an extract of the 

 bark of Mimosa Catechu ; it is also obtained by the fusion 

 with potash of other tanno-resins such as kino, the sap ot 

 various species of Pterocarpus, Butea or Eucalyptus ; also it 

 occurs in small quantities combined with sulphuric acid in the 

 urine of horses and of human beings. It crystallizes from 

 benzene in colourless glistening plates and melts at 140. 



* Although behaving normally as a trihydric phenol, phloroglucinol reacts 

 with certain ketonic reagents as though it had the following constitution : 



CO 



CH 2 CH a 

 CO CO 

 , * CH, 



I 



