I.] YEAST. 9 



d. Pasteur's fluid with sugar. 



\e. Mayer's pepsin solution 1 ,] 



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, 

 V better still, "</" well, and "e" best of all. Note that 

 bubbles of gas are plentifully evolved from both the so- 

 lutions which contain sugar. 



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 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 "*/"; 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 

 Torula. 



4. Sow some yeast-ceils 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. 



1 Mayer's solution (with pepsin) = 



15 per cent, solution of sugar-candy 20 cc. 



Dihydropotassic phosphate o' I grm. 



Calcic phosphate o* i grm. 



Magnesic sulphate o* igrm. 



Pepsin 0-23 grm. 



