170 STUDIES ON FERMENTATION. 



effecting tlie substitution in the following manner, we escape the 

 risk of introducing germs from floating particles of dust, which 

 would nullify all experiments of this kind. We prepare, then, a 

 flask containing sweetened water, free from all foreign germs, 

 which we attach to the other flask \_i.e., two of M. Pasteur's flasks 

 with two necks, one straight and wide, the other bent and narrow 

 (Fig. 8)]. This is done by taking the india-rubber tube off the 

 flask containing j'east, and removing the glass stopper from the 

 other india-rubber tube attached to the flask containing the 

 sweetened water ; then, introducing the right-hand neck of the 

 yeast flask into the india-rubber tube connected with the 

 other, we raise the latter flask so as to pour the sweetened 

 water on to the yeast. At the same time an assistant passes 

 the flame of a spirit-lamp over the bent part of the curved tube 

 attached to the water flask, with the object of destroying the 

 vitality" of the germs in the floating particles of dust, which 

 enter the flask in proportion as it is emptied into the other. 



The sweetened water, which is thus brought into contact 

 with yeast of greater or less freshness, soon begins to ferment. 

 Fermentation accomplished, the vinous liquid is decanted and 

 replaced by fresh sweetened water, which ferments in its turn, 

 although even at this stage with greater difficulty than the 

 first ; this second dose is again decanted, and again replaced by 

 fresh sweetened water, and this process is repeated three or four 

 times. The yeast becomes weaker and weaker, and eventually 

 is unable to cause any fermentation in sweetened water poured 

 on it. 



This exhaustion of yeast in sweetened water may be produced 

 more quickly by the following means : — It is sufficient to sow a 

 mere trace of pure yeast in a large quantity of sweetened water, 

 say 100 c.c. (nearly four fluid ounces), that is, instead of pour- 

 ing the contents of a bottle of sweetened water upon the whole 

 deposit of yeast in the flask which contained the fermented 

 wort, we simply take a little yeast, by means of a fine tube, 

 from the deposit at the bottom of the flask, and introduce it 

 into the flask of sweetened water. This large proportion of 



