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 realized: 
(i) a ferment, (2) something for the ferment to act upon, 
(3) suitable external conditions. 1 
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disappeared and has been replaced by another gas.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 liquid 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." 
1 The student may devise an experiment of his own to ascertain whether 
or not light is necessary to fermentation. 
