82 VETERIJTARY MATERIA MEDICA. 



The tannin when oxidized being changed into gallic 

 acid makes the difference between them seem simply 

 that of oxidation, while if tannin is considered to be 

 gallic acid anhydride, it would be a matter of hydra- 

 tion. Gallic acid may be prepared from tannin by 

 the action of dilute sulphuric acid, but is usually ob- 

 tained from nutgalls after exposure to artificial heat 

 for a period of one month. Tannic acid is also ob- 

 tained from nutgalls. 



Other vegetable astringents depending on tannic 

 or gallic acid for their therapeutical value are : 



Galla, nutgall, the gall of Quercus lusitanica, 

 caused by the punctures and deposited ova of a cer- 

 tain insect. They are subglobular in shape, about 

 three-fourths of an inch in diameter, somewhat tu- 

 berculated above, otherwise smooth, heavy, and hard ; 

 sometimes showing a circular hole near the middle, 

 communicating with the central cavity; blackish 

 olive-green or grayish; fracture granular, grayish. 

 In the centre a cavity containing either the partly 

 developed insect, or pulverulent remains of it. Nearly 

 inodorous, taste strongly astringent. Contains tan- 

 nin, sixty to seventy per cent. ; gallic acid, three per 

 cent. ; besides pectin, sugar, starch, etc. Dose of the 

 powdered galls twice that of tannic acid. 



Hamamelis, witch-hazel, the leaves of Hamamelis 

 virginica (Linne) of the natural order Hamamelacese, 

 collected in autumn. Contains about eight per cent, 

 of tannic acid, and a bitter principle ; it is best known 

 as the household preparation, Pond's extract or ex- 

 tract of witch-hazel. / 



