CHAPTER IX. 



TANNIC ACID IN WINE. 



IT has already been observed that not a trace of 

 tannic acid is to be detected even in the juice of the 

 purple grape, that this acid is principally found in 

 coloured wines, and its amount is in proportion to the 

 depth of the colour, so that light-coloured liqueur wines 

 contain very little. This fact may be easily ascertained 

 by adding a clear solution of isinglass to the wine ; in 

 the coloured wines it forms a strong precipitate, in 

 the others is usually a scarcely perceptible one. The 

 addition of salt of oxide of iron makes the precipitate 

 more evident, thus a trace of tannic acid can be always 

 detected in old and colourless wines. 



I know nothing about the existence of gallic acid 

 in wine, though it is so easily produced from tannic acid 

 Its absence need not, however, surprise us, since free 

 admission of air is necessary to the conversion of 

 tannic into gallic acid, and such admission is not 

 allowed in wine. If the tannic acid of red and other 

 wine be precipitated by means of isinglass, and filtered, 

 the addition of salt of oxide of iron does not produce 

 a dark colour, a proof that gallic acid does not exist. 



