280 THE AMERICAN MONTHLY [Ang. 



that most of the cells iu the yeast cake are iu a similar condition at the 

 outset. It is of course necessary in examining different cultures of 

 yeast to make sure that there is no mixing of diff'erent lots, and that 

 enough different slides are examined to eliminate exceptional cases. 

 It is important that all cultures be made under conditions that are uni- 

 form except as to the one condition which is being investigated, and iu 

 every case a standard control culture under the most favorable condi- 

 tions should be made and examined as the basis of comparison. 



26. Food of yeast. — Cultivate at 32 C. for 12 hours 

 equal amounts of yeast in : (a) distilled or hydrant water; 

 (b) Pasteur's solution without sugar; (c) sugar without the 

 rest of Pasteur's solution ; (d) Pasteur's solution.* Make 

 carefnl examinations of all four and determine by means 

 of the growth of the colonies which is the best food. 

 Carefully study the composition of Pasteur's solution 

 and consider the inference that can be drawn from 

 this experiment with reference to the nutrition of a non- 

 chlorophyll-containing cell. Could Amoeba thrive in 

 Pasteur's solution ? 



2G b. Gas Produced by Growth of Yeast. — Culti- 

 vate yeast in closed flask and collect the gases from it in 

 ajar of water — test the gas thus obtained: first by low- 

 ering alighted match or candle in it, noting that it will 

 not support combustion ; and then prove by means of 

 baryta water that the gas is carbon -dioxyd. 



27. Temperature. — Cultivate for eight, hours in Pas- 

 teur's solution equal amounts of yeast, at the following 

 different temperatures, viz. : (a) 18 C. ; (b) 32 C. ; (c) 40 

 C. ; compare these and determine which is the 

 most favorable temperature ; (d) place a portion of 

 yeast in Pasteur's solution and heat slowly to boiling, 

 then cool to 32 and keep at that temperature for eight 

 hours and then examine to determine the effect, by 

 comparison, with the best of the three preceding ; (e) 



*For the formula for making Pasteur's solution, See Parker's Ele- 

 mentary Biology. 



