CHAPTER XIII. 

 ALCOHOL. 



Fermentation. If a glass of sweet cider is set in a 

 warm place for a day or two, it will probably be observed 

 that bubbles of gas are given off and it will now have a 

 sharp, pungent taste. The gas is carbon dioxid, and the 

 new taste is chiefly due to alcohol, though attributable in 

 part to other substances that have been produced and also 

 to the loss of sugar. The same change would be likely 

 to occur in a moderately strong solution of sugar, and in 

 many fruit juices, especially if sweet. Sweet liquids under- 

 going this change usually become frothy, or "work," as 

 we say, and at the same time acquire a sharp taste. This 

 change is due to a process called fermentation. 



Yeast. Any substance in which alcohol is produced in 

 this way is found to contain yeast. Yeast is a microscopic, 

 one-celled plant, oval or elliptical in outline, which acts as 

 a ferment. Common baker's yeast represents one group 

 of these ferments. 



It has been clearly proved that yeast is the cause of 

 the above changes, some of the more manifest evidence 

 being as follows : (i) Yeast may always be found in liquids 

 undergoing alcoholic fermentation. (2) Yeast is killed 

 by boiling. If such liquids as have been mentioned are 

 thoroughly boiled and placed, while boiling hot, in a 

 perfectly clean jar and sealed air-tight, they will keep 

 indefinitely without any fermentation of this kind. (3^ 



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