Microorganisms in Relation to Man 559 



tributed plants and of making their products visible is de- 

 scribed in the following experiment, which is just the familiar 

 way of making " salt-rising " bread. This is an old practice 

 in different countries, and from it have been developed various 

 methods of making leavened or raised bread. 



Exercise. -7- Heat about half a cup of water to a temperature of 

 90 F. In this dissolve one teaspoonful of salt and stir in enough 

 flour, which has been warmed to about the same temperature, to make 

 a thick, smooth batter. (The salt merely prevents the mixture from 

 becoming sour.) Cover the mixture with a piece of glass or other 

 close-fitting covering, and place the vessel containing the batter in a 

 dish of warm water, which should be kept at the same temperature for 

 several hours. Observe it at intervals, but do not let it be chilled 

 by cold air. Note change in volume. When the batter is very light, 

 examine under a magnifying glass a small lump which has been care- 

 fully removed. If a high-power microscope is available, mix a small 

 portion of the light dough with some lukewarm water in a test tube by 

 shaking until the mixture is smooth. Examine a drop on a glass under 

 the microscope. Make a record, with sketches, of what you see. 



280. Making bread. If bread making is done at home, 

 carefully observe the preparation for the next baking. Make 

 a record of the quantities of materials used, method of mixing 

 them, and care of the mixture overnight. You may find that 

 the sponge is a success only if certain conditions are provided, 

 such as a temperature not below 75 F. and sufficient moisture. 



The next morning or several hours after its preparation, the 

 sponge is several times larger than its original volume, is full 

 of pores, and has a peculiar odor. The pores and what they 

 signify will be considered a little later. Examine a small por- 

 tion of the sponge with a magnifying glass. How does it com- 

 pare with the result of your examination of the " salt-rising " 

 sponge? Make a microscopic examination of this as you did 

 of the salt-rising. 



The remaining steps in making bread are interesting. The 

 sponge is stiffened with flour added gradually until the whole 

 mass may be poured upon the kneading board. It is then 



