FERMENTATION. 157 



proportions: the properties of these three substances 

 are as opposite as they well can be, and yet, the 

 whole difference is in the proportions of their ele- 

 ments. It is well known that, when a solution of 

 sugar ferments, it is changed into spirit ; it parts 

 with a portion of carbon and oxygen, and the ele- 

 ments left constitute spirit ; hence, during ferment- 

 ation, carbonic acid is given off. Again, when 

 spirit and water is exposed to the air and moderate 

 warmth, it soon changes into vinegar : this change is 

 wholly effected by the absorption of a little oxygen 

 from the air. 



365. Fermentation is a very singular process, and 

 a knowledge of the effects which it produces, enables 

 us to understand many changes which would other- 

 wise appear incomprehensible. Under ordinary cir- 

 cumstances, pure sugar, dry or dissolved in water, 

 may be kept for a long time without its undergoing 

 any change ; when, however, it is mixed with a small 

 quantity of certain decomposing matters containing 

 nitrogen, it ferments, and is changed into spirit. 

 The substance added, does not combine with the 

 sugar or its elements, but, whilst itself decomposing, 

 it causes the sugar also to change. 



366. Fermentation, then, is the spontaneous de- 

 composition of a substance, occasioned by the presence 

 of a small quantity of decomposing matter. The 

 yeast, or ferment of beer, possesses the power of in- 

 ducing the decomposition of sugar and similar sub- 

 stances when mixed with them. Common yeast, and 



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