IX.] YEAST. 387 



That any growth at all takes place, in the case of 

 experiments a and b, is due to the fact that the drop of 

 yeast added contains nutritious material sufficient to pro- 

 vide for that amount of growth. 



2. Prepare two more specimens of "^" and keep one 

 in a cold — the other in a warm (35" C.) place, but 

 otherwise under like conditions. Compare the growth 

 of the yeast in the two cases ; it is much greater in 

 the specimen kept warm. 



3. Prepare two more specimens of "^"; keep both 

 warm, but one in darkness, the other exposed to the 

 light: that in the dark will grow as well as the other; 

 sunlight is therefore not essential to the growth of 

 Toriila. 



4. Sow some yeast-cells in Pasteur's solution in a flask, 

 the neck of which is closed by a plug of cotton 

 wool, and boil for five minutes ; then set it aside ; 

 no signs of vitality will afterwards be manifested by 

 the yeast in the flask ; it is killed by exposure to this 

 temperature. 



5. [Take two test tubes; in one place some yeast, with 

 Pasteur's solution containing sugar; in the other place 

 baryta water, and then connect the two test tubes by 

 tightly fitting perforated corks and a bent tube passing 

 from above the surface of the fluid in the first tube to 

 the bottom of the baryta water in the second ; pass a 

 narrow bent tube, open at both ends, through the cork 

 of the baryta water tube, so that its outer end dips just 

 below the surface of some solution of potash ^ All gas 

 formed in the first tube will now bubble through the 

 baryta water in the second, and, from thence, any that 



^ The object of the potash is to shield the baryta water from any 

 carbonic anhydride that may be in the atmosphere. 



25—2 



