I'Oi ACETIC ACID. 



476. Acetic acid, or vinegar, is formed when a 

 mixture of spirit and water, together with some fer- 

 ment, is exposed to the air ; a portion of oxygen is 

 absorbed, and acetic acid is formed. This change 

 takes place when any liquid containing a portion of 

 spirit and some organic substance, like gluten, is 

 exposed to the air and kept moderately warm ; the 

 acetous fermentation, as it is called, then commences, 

 and vinegar is formed. 



477. When a liquid containing spirit, water, and 

 some decomposable organic matter is exposed to the 

 air, unless the temperature is too low, it soon begins 

 to turn sour, and forms vinegar. When a solution 

 which has fully undergone the alcoholic fermentation 

 is thus kept at a temperature of 70° to 85°, it slowly 

 and -gradually undergoes this change ; but unless 

 care is taken it is very apt to become mouldy and 

 putrefy (380). 



478. The acetous fermentation is facilitated by the 

 addition of a little ferment; hence in the manufacture 

 of vinegar from wine or beer, yeast or ferment of 

 some sort is usually added. When a small quantity 

 of wine which has turned sour during its fermentation, 

 is added to a much larger quantity of properly fer- 

 mented wine, it induces the whole to pass into the 

 acetous fermentation, and good vinegar is formed. 

 For this reason also, better vinegar can be made in 

 an old cask or vat which has been often used, than 

 in a new one. 



479. Wine-vinegar, however, is commonly made in 



