FERMENTATION. 393 



the holes becomes expanded by the heat, and so rises to 

 pass out through the glass tubes ; and in this way quite 

 a brisk circulation of air is kept up, bringing, therefore, 

 oxygen to the whole surface of the shavings with con- 

 siderable rapidity. The fluid as it runs out below passes 

 into the receiver. Of course the barrel is whole as used ; 

 in the figure a portion of it is represented as cut away, 

 merely that you may see the interior arrangement. 



556. Adulteration of Vinegar. Even vinegar is some- 

 times adulterated. When a manufacturer desires to sell 

 poor vinegar as good he gives the requisite sharpness to it 

 by adding some substance, as, for example, sulphuric acid. 

 Adulteration with this article can be very easily detected. 

 Fill a jar or mug half full of water, and set upon it a cup 

 containing some of the vinegar with grape-sugar in it. If 

 you set the mug upon a hot stove the vinegar in a little 

 time will be all evaporated. If now what is left in the 

 cup be of a black color, there is proof of the presence 

 of free sulphuric acid. The explanation is this : As the 

 vinegar evaporates, the sulphuric acid, not being volatile, 

 remains in the cup, and at length, when all the water is 

 gone, is so concentrated that it carbonizes the non-volatile 

 organic matter. (See 244.) 



Great care must be taken not to heat the contents of 

 the mug too hot, or the organic matter may be charred 

 by the heat alone. 



QUESTIONS. 



535. What is said of different kinds of fermentation ? What is the dif- 

 ference between fermentation and putrefaction ? What are antiseptics ? 

 536. Explain the nature and action of ferments. What is said of the 

 growth of organized bodies? 537. Explain the chemical change in alco- 

 holic fermentation. Give the equation. 538. What is yeast ? 539. What 

 is said of the fermentation of wines ? When are wines dry ? 540. What 



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