PYROLIGNEOUS ACID. 197 



time, and the liquid should be kept at a temperature 

 of from 70° to nearly 90°. 



487. Very pure vinegar is sometimes made from a 

 mixture of spirit of wine, and water, exposed to the 

 air at a proper temperature. A large cask is filled 

 with wood shavings which have been steeped in vine- 

 gar, and the mixture of alcohol and water is allowed 

 to trickle through it at the same time that a current 

 of air passes through the cask. The large surface 

 which is thus exposed to the air, and the influence of 

 the vinegar contained in the shavings greatly assist 

 the action ; oxygen is absorbed, the mixture becomes 

 warm, and after passing once or twice through the 

 cask is wholly converted into vinegar. 



488. Another source of acetic acid is the destruc- 

 tive distillation of wood. When wood is burnt, only 

 water and carbonic acid are formed ; but when wood 

 is distilled or roasted in close vessels out of the con- 

 tact of air, it is decomposed, and several new sub- 

 stances are formed by the recombination of its ele- 

 ments ; the most important of these substances is 

 vinegar, or, as it is called when thus procured, 

 pyroligneous acid (325). 



489. Acetic acid cannot be obtained perfectly dry. 

 The substance obtained by the above-mentioned pro- 

 cesses, is a solution of acetic acid in water ; this may 

 be concentrated, but if we attempt to obtain the acid 

 free from any water, it is decomposed. Acetic acid 

 forms numerous salts, some of which are important 

 in the arts, such as sugar of lead, which is an acetate 



17* 



