I.] YEAST. 9 



Keep all at about 35 C., and compare the growth of the 

 yeast, as measured by the increase of the turbidity of the 

 fluid, in each case, "a" will hardly grow at all, "b" better, 

 "c" better still, "d" well, and V best of all. Note that 

 bubbles of gas are plentifully evolved from both the solutions 

 which contain sugar. 



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

 ments a and b, is due to the fact that the drop of yeast added 

 contains nutritious material sufficient to provide for that 

 amount of growth. 



2. Prepare two more specimens of "d" 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 "<Z"; keep both warm, 

 but one in darkness, the other exposed to the light : 

 that in the dark will grow as well as the other; sun- 

 light is therefore not essential for the growth of 

 Torula. 



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 heat to 100 C. for five minutes ; then set 

 it aside ; no signs of vitality will afterwards be mani- 

 fested by the yeast in the flask ; it is killed by ex- 

 posure 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 



