136 CONSTITUENTS OE WINE. 



acid is evolved to saturate the wine, and produce 

 tension enough to eject the cork the moment the 

 string is cut, and cause the wine, which contains 

 several times its own volume of carbonic acid, to pour 

 foaming out. 



Effervescing wine may be prepared from every good 

 kind of grape juice. 



An approximation to the composition of wine may 

 be deduced from the general statement of the com- 

 position of grape juice given (p. 5), and from the 

 change which it undergoes during fermentation, whe- 

 ther or no the skins are allowed to ferment. The 

 composition differs according as the wine is red, or not 

 red. In the last-mentioned, no particular colouring 

 matters are found, and only a trace of tannic acid ; in 

 the former both are present. Alcohol and water are 

 also among the principal ingredients, then sugar, gum, 

 extractive and albuminous matters; then free acids, 

 such as tartaric, racemic, malic and acetic acid, tartrate 

 of potash, of lime, and of magnesia, sulphate of potash, 

 common salt, and traces of phosphate of lime ; also, 

 and especially in cellared wines, substances which im- 

 part aroma, as oenanthic and acetic ether, in variable 

 proportions, and other volatile matters of which we 

 shall hereafter speak more particularly. In red wines 

 and in many others, a little iron, and according to the 

 statement given (p. 26), some alumina may also be 

 detected. Lastly, the best wines contain, according to 

 Eaure, a peculiar substance which he calls osnanthine. 



