COLOUBING MATTERS IN WINE. 205 



cellent in proportion as it is yellow, and a perfect 

 agreement is found to subsist between the colour of 

 the wine and that which it assumes, in consequence 

 of its larger contents of tannic acid, when salt of oxide 

 of iron is added to it. Now, as the tannic acid, the 

 sugar of which has been converted into humic acid, is 

 derived from the grape skins, and not from the juice, 

 we can readily understand that tannic acid, as a 

 colouring matter, may be employed in regulating the 

 colour of liqueur wines, according as many or few 

 grape skins are allowed to ferment in the juice. The 

 colour depends upon the same cause which determines 

 that of Cognac, pure French brandy, exported in oaken 

 casks, whereby a little tannic acid passes from the 

 wood into the wine, and is converted into apothema 

 by the oxygen of the air. This example of Cognac, 

 which is decisive, since it is coloured by nothing else, 

 teaches us that oxidized tannic acid is the principal 

 cause of the dark yellow or even brown yellow colour of 

 wines. "We learn that the same cause makes these 

 wines in bottle gradually become darker. Atmospheric 

 air is dissolved in bottled wine, since alcohol absorbs 

 far more air than water, and hence it is that when 

 alcohol is mixed with water a great deal of atmo- 

 spheric air is expelled. In a mixture of alcohol and 

 water, consequently in wine, more air will be dissolved 

 than in water alone. The oxygen of this air and that 

 which is contained in the upper parts of bottles not 

 quite full of wine, is sufficient to oxidize so much of 



