392 



CHEMISTRY. 



ence of unstable organic substances, the conversion may be 

 regarded as a sort of fermentation. 



Actually the change is not so simple as represented in the equation just 

 given. It has two stages. As when starch is converted into sugar there 

 is an intermediate substance dextrin into which it is changed prepara- 

 tory to its conversion into sugar, so also, in the formation of acetic acid, 

 the alcohol is first changed into an intermediate substance called aldehyde. 

 The two changes may be expressed thus : 



Alcohol. Oxygen. Aldehyde. Water. 



C 3 H 6 O + O = C 2 H 4 O + H 2 O 



Aldehyde. 



Oxygen. 

 *0 



Acetic acid. 

 C a H 4 2 



Compare the equations in 537 explaining alcoholic fermentation. 



555. Quick Mode of Making Vinegar. Alcohol can be 

 converted into acetic acid in a very short time by provid- 

 ing for a very free exposure of it to the air, so that the ox- 

 ygen may act upon every 

 drop of it at once. This is 

 done in the manner repre- 

 sented in Fig. 118. A bar- 

 rel is filled with shavings 

 which have been steeped in 

 vinegar. Near the top of 

 the barrel is a shelf, perfo- 

 rated with holes, in which 

 there are fastened either 

 bits of string or straw, that 

 the liquid poured in, which 



is alcohol and water with a little yeast, may trickle down 

 upon the shavings. A free access of air is secured to the 

 whole surface of the shavings by holes made in the side 

 of the barrel, and some holes in the perforated shelf large 

 enough to admit glass tubes of considerable size. Now 

 as the fermentation creates heat, the cold air admitted in 



