PLANTS IN THEIR RELATION TO HUMAN WELFARE 147 



156. Changes caused by yeast. A yeast mixture may be easily 

 prepared for experimentation by pouring into a jar a cup of water, 

 adding a spoonful of molasses, and a spoonful of the milky fluid 

 made as described in 154. 



If the jar with its contents is set aside in a warm place (70 to 

 90 F.) 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 disappearing, that the mixture 

 begins to smell sour, and that a sharp, biting taste is becoming evi- 

 dent. 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 that the gas formed during the 

 growth of yeast is carbon dioxid. 



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 opera- 

 tion 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 



