TANNIC ACID IN WINE. 197 



"We shall be able to speak still more clearly, and 

 in detail, about this matter, when we have considered 

 the colouring matter of wine. 



The preservation of the wine is in a great measure to 

 be ascribed to tannic acid, for the albuminous matters, 

 which are always combined with tannic acid in wine, 

 are thus prevented from decomposition, and the prin- 

 cipal cause of the wine spoiling is thereby checked. 

 Wines therefore which are intended for exportation, or 

 to be long cellared, must not be deprived of too much 

 tannic acid by means of albumen or gelatine (p. 98). 



It has been observed that if red wine be put into 

 well-corked glass bottles, and exposed to full daylight, 

 a flocculent precipitate is obtained, and the colouring 

 matter in this manner withdrawn from the wine. The 

 flavour and aroma of the wine do not suffer. The 

 first phenomenon is easily explained. Light promotes 

 the production of apothema from tannic, especially if 

 sufficient air be admitted. The apothema thus pro- 

 duced is insoluble, and withdraws the colouring matter 

 from the wine by virtue of the so-called surface 

 attraction. 



It would be impossible for light to act in this matter 

 unless a sufficiency of air were dissolved in the wine, 

 but a large quantity is unnecessary, and some air is 

 always present in all wine. 



