ADULTERATION OF WINE. 365 



to have, and yet nothing can be more uncertain. 

 It is everywhere asserted that the precipitate of sugar 

 of lead in red wine, if pure, must be green, whereas it 

 is really dirty pale blue. The mistake seems to have 

 arisen from an alcoholic extract of purple grape skins 

 being used instead of a watery extract. If purple grape 

 skins be extracted with alcohol, a red tincture is 

 obtained, as when water is used. Whether it be that 

 a little tartaric acid clings to it, or that a little acetic 

 acid is formed, it is certain that the colour is not 

 blue, but bright red. 



Sugar of lead gives rise in this alcoholic extract to a 

 green precipitate ; and causes a dirty pale blue precipi- 

 tate in the watery extract. If -J- of alcohol be added to 

 the watery decoction, a blue precipitate is obtained. It 

 must therefore be blue in pure red wine. The reason 

 the precipitate obtained by acetate of lead from the 

 alcoholic solution of purple grape skins is green, is 

 that it is the product of a blue and of a yellow pre- 

 cipitate ; for blue and yellow give green. The cause 

 of this I cannot explain, it does not depend upon the 

 colouring matter of wine, but upon another substance 

 extracted by means of alcohol from the grape skins. 



We generally see that a very red tincture of purple 

 grape skins and alcohol turns pale in a few hours, and 

 acetate of lead then gives rise to a green precipitate 

 in it. Ammonia does not produce a blue, but a green- 

 ish yellow colour in this pale red alcoholic tincture. 



When such changes take place in the reaction of 

 an alcoholic extract from purple grape skins, no con- 



