FERMENTATION 227 



hydrogen sulphide (H 2 S), a compound that is found in 

 large amounts in eggs that have lost their usefulness. 



Decay is a process of decomposition which goes on 

 slowly and quietly. Its causes are not well understood 

 in all cases, but it is thought to be sometimes due to 

 very tiny vegetable forms called bac- 

 teria. These tiny bodies are too small 

 to be seen without a powerful micro- 

 scope ; they are common in the air, 

 the soil, and in water, as well as in 

 various other substances. Heat gen- 

 erally kills them, and most kinds 

 seem to work best in a good supply 

 of oxygen. Fruits are often put up in jars while hot 

 and at once covered tightly; in this way they may be 

 kept for a long time without decaying. Fig. 148 shows 

 several bacteria, greatly magnified. There are of course 

 other causes of decay. 



267. Fermentation. If apple juice is allowed to 

 stand for a time, we know that alcohol may form in 

 it and the juice becomes cider; similarly, grape juice 

 may become wine, containing alcohol. Clearly a chem- 

 ical change goes on in the liquid, and this change is 

 called fermentation. It is caused by something that is 

 present in the fruit juice, or that gets into it from the 

 air. These things that may cause fermentation are 

 called ferments. 



Many different ferments are known, and they act 

 upon many substances. One of the most common of 

 ferments is yeast; it acts upon dextrose or grape sugar 



