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, 

 "c" better still, "d" 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 (35C.) 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 "d"; 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-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. 



1 Mayer's solution (with pepsin) = 



1 5 per cent, solution of sugar- candy 20 cc. 



Dihydropotassic phosphate o* i grm. 



Calcic phosphate o' i grm. 



Magnesic sulphate o' i grm. 



Pepsin 0-23 grm. 



