36 STUDIES IN PHYSIOLOGY 



If the yeast mixture is set aside in a warm place (70 to 90 

 degrees Fahrenheit) for a short time, it begins to " work," 

 and bubbles of gas rise to the surface. At the end of several 

 hours, we notice that the sweetness of the molasses is dis- 

 appearing, that the mixture begins to smell sour, and that a 

 sharp, biting taste is becoming evident. All these changes 

 are caused by the growth of living yeast cells. 



Now, what is the gas that is formed in this process, and 

 what causes the changes in taste and odor? To answer 

 these questions we must carry our experiments still further. 

 When the mixture is "working" well, the bottle should be 

 tightly closed with a rubber stopper, through which extends 

 one arm of an inverted U-shaped tube. The other end of this 

 tube should run over to the bottom of a test tube half-filled 

 with limewater. The gas that has been rising through the 

 yeast mixture, now passes through the U-tube, and as it comes 

 in contact with the limewater, the latter changes to a milky- 

 white color. This proves, as we have seen on p. 8, that the 

 gas formed during the growth of yeast is carbon dioxid. 



Distillation. After " working " a day or two, the yeast 

 mixture will have a strong taste and odor. A part of it 

 should then be poured into a glass Florence flask (commonly 

 used in the chemical laboratory for boiling liquids), and the 

 mouth should be closed by a rubber stopper. The short 

 arm of a long delivery tube should be passed through this 

 stopper. When the flask is heated gently, some of the liquid 

 is changed to a vapor. If the delivery tube is cooled by cov- 

 ering it with cloths wet in cold water, the vapor condenses 

 into a liquid, which comes from the end of the tube in drops. 

 This operation we have been describing is known as dis-til- 

 la'tion. In distilling a liquid, we first convert it into a vapor, 

 and then condense this vapor into a liquid. After collecting a 

 few spoonfuls, the liquid should be slowly distilled a 

 second time. Then we obtain a colorless fluid that has the 

 distinct smell and taste of alcohol. It burns, too, with 

 a pale blue flame. 



