YEAST 33 



the yeast has been growing in it for a day or 

 two. Compare with some in which there has 

 been no yeast. What diflference do you find? 

 How do you account for it ? Test the two solu- 

 tions with blue litmus paper (which turns red 

 when placed in an acid). "What result ? Explain. 

 Make a thick syrup of sugar and water, put some 

 into a test-tube, mix a drop of yeast with the 

 syrup, and set the tube in the water-bath for a 

 day. Does the syrup ferment? Can you ex- 

 plain why ? Can you explain why canned fruit 

 frequently " sours " while preserves do not ? 



i. Effect of temperature upon growth. — FiU each of three 

 test-tubes one -third full of Pasteur's solution 

 with sugar, into each put a drop of fluid yeast, 

 and close the mouth of the tube with a plug of 

 cotton-wool. Set the first tube in some place, 

 as in the water -bath, where the temperature 

 can be kept constantly at about 35° C. Eoil the 

 contents of the second tube for two or three 

 minutes, then set this tube with the first. Put 

 the third tube on a block of ice or in a dish of 

 ice-water. Examme each tube two or three 

 times a day for several days and note what 

 takes place. Explain your results. "What is the 

 effect of a very high temperature, i. e., boiling ? 

 Of a very low temperature, i. e., freezing ? 



c. Effect of light upon growth.— FreTpa,re two test-tubes 

 with Pasteur's solution and a drop of yeast as in 

 i. Wrap one tube with thick paper so as to ex- 

 clude the light. Leave the other unwrapped. 

 Set the two tubes side by side in a window where 

 3 



