Microorganisms in Relation to Man 561 



of the rising of the sponge and dough. In the experiment you 

 found that a mere mixture of flour and water, when kept at a 

 fairly constant temperature, acted just as the sponge to which 

 yeast was added which you observed at home. What causes 

 the batter to rise ? Or what causes the sponge, to which a small 

 cake of yeast is added, to rise? Is it just a physical or chemical 

 change brought about by the combination of the different sub- 

 stances, such as the milk, potato, flour, moisture, and salt, and 

 by keeping the mixture at a certain temperature ? Or do these 

 conditions make possible a different kind of change, which is 

 not merely physical or chemical, but a living process also? In 

 order to observe just what takes place and what causes the 

 changes, it will be necessary to make use of experiments such 

 as the following. 



Exercise. Prepare a simple yeast mixture by dissolving a small 

 cake of compressed or dried yeast in a pint of warm water and adding 

 one ounce of molasses or an equivalent amount of sugar. Mix these 

 thoroughly in a large, wide-mouth bottle and place it where the tem- 

 perature will not fall below 75 F. In the course of an hour or two 

 bubbles will be formed. Taste the mixture at the beginning and then 

 at intervals of an hour for several hours. When it is working well, 

 close the bottle with a stopper through which you have inserted a 

 glass tube. Attach to the tube a piece of rubber tubing and run it 

 into a test tube half full of limewater. What does the change in the 

 limewater indicate as to the kind of gas produced by the yeast? 

 (See page 124.) 



At the same time you prepare the yeast mixture, arrange the follow- 

 ing for later observation and use. Prepare three test tubes by 

 thoroughly cleansing and sterilizing them. Sterilize also one rubber 

 stopper. Label the tubes. Fill one about two thirds full of fresh 

 cider or grape juice, and close loosely with a clean cotton stopper. 

 Fill the others with about the same quantity and heat directly over a 

 flame or indirectly in boiling water to a temperature of 176 F. to 180 

 F. Close one at once with the sterilized stopper. Let the other re- 

 main open. Put them all where they may be observed easily. Keep 

 a record of the procedure and of the facts observed in the behavior 

 of each, until other tests are made. What gas is being produced, as 

 shown by the bubbles rising from the bottom? 

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