SAPROPHYTES 391 



cloud that forms in the limewater near the open end of the tube. 

 The white substance which forms as the gas from the yeast mixture 

 is forced into the limewater is essentially the same as common 

 chalk. It is formed by the chemical action of carbon dioxide on 

 limewater, and, therefore, its appearance here indicates that the gas 

 which the yeast mixture is giving off is carbon 

 dioxide. You are familiar with the fact that 

 your own breath contains carbon dioxide. Take 

 some fresh clear limewater in a small bottle or 

 test tube and by means of a small glass tube, 

 force your breath through the limewater. Note 

 that the same white substance forms in the 

 limewater. From this, you might conclude that 

 the carbon dioxide which arises from the yeast 

 mixture is formed by the respiratory process of ^^ pr ;J uc * r d j 

 the yeast plants. One trouble with this con- gr0 wing yeast, 

 elusion, however, is the fact that there is en- 

 tirely too much carbon dioxide produced by yeasts for us to explain 

 it as being produced by ordinary respiration. We shall refer to this 

 matter again later. (Review Arts. 92 and 93, especially Ex. 25f.) 



Exercise 90. Growing Yeast also Produces Alcohol 



Permit your original yeast mixture to stand in a warm place for 

 several days until signs of fermentation have about ceased. Now, 

 taste the mixture to see if you can detect the sharp sting and the 

 sweet taste of alcohol. Place the mixture in a distilling flask and 

 distill the alcohol as in Ex. 16, Art. 24. What is the first of the 

 distillate? Do you secure enough alcohol to burn? 



443. Yeasts and Fermentation. All the practical uses which 

 we make of yeasts center in the peculiar relation which they 

 bear to sugar. In the first place, sugar seems to be their natu- 

 ral food. It has been found possible to grow them in certain 

 mixtures which do not contain sugar but in all the practical 

 processes in which we use them, a sugar solution of some kind 

 is used as a CULTURE, or material in which to grow them. 

 In addition to sugar, yeasts must have certain mineral salts 

 which are necessary in the building of protoplasm, but these 

 are always present in sufficient quantities in all natural 

 sugars such as the molasses you used in your culture. 



