96 THE VEGETATIVE FUNCTIONS OF PLANTS 



yeast, and a fourth tube with yeast but no sugar, no 

 fermentation will take place. In other words, in order 

 to have fermentation three conditions must be reahzed: 

 (i) a ferment, (2) something for the ferment to act upon, 

 (3) suitable external conditions.^ 



94. Products of Alcoholic Fermentation. — If we place 

 a large quantity of the fermenting mixture in a deep 

 glass cylinder, and cover over the top so as to hinder the 

 escape of the gas given off, the gas will collect in quantity 

 in the space above the liquid. If, after fermentation has 

 been allowed to proceed vigorously for a few hours, we 

 insert in the cylinder a lighted splinter of wood or a lighted 

 candle, the flame will at once go out, showing that the 

 oxygen of the air, which supports combustion, has largely 

 disappeared and has been replaced by another gas. 

 The test with the flame should, of course, be made also at 

 the very beginning of the experiment, to show that the air 

 above the Hquid will support combustion before fermenta- 

 tion has begun. 



95. Carbon Dioxide Formed. — In order to ascertain 

 what gas has taken the place of oxygen, we may next 

 insert a fine wire, bent into a small circular loop at the 

 end, and dipped in lime-water. A film of lime-water 

 will extend across the space enclosed by the loop. Lime- 

 water has the characteristic property of turning milky 

 in the presence of carbon dioxide, and in this test the 

 film of lime-water will at once turn white or milky, show- 

 ing that the gas given off by fermentation is carbon dioxide. 

 It is the formation of bubbles of this gas in bread dough 

 that causes the dough to become ''light" and to ''rise." 



^ The student may devise an experiment of his own to ascertain whether 

 or not light is necessary to fermentation. 



