106 SCIENCE OF HOME AND COMMUNITY 



Bacteria are the smallest plants known and can be seen 

 only with a microscope. They multiply very rapidly, so 

 that a few may develop in a short time into an enormous 

 number. Both molds and bacteria require moisture and 

 warmth in order to grow ; therefore very common means of 

 preserving foods are drying and cold storage. 



DEMONSTRATION 12 



Purpose. To study the activities of bacteria and some means 

 of controlling them. 



Materials. Test tubes, raw meat, beans, boiled potato, 

 white of egg, salt, vinegar, formalin. 



Directions, i. The action of bacteria in causing decay. 



a. Place in a test tube a small piece of raw meat ; half cover 

 with water. Set in a warm place for two or three days. Note 

 the appearance of both meat and water. This change has 

 been caused by bacteria. 



6. With a knife heated in a flame, cut a boiled potato in two 

 parts. On one half of the potato put some dust; leave the 

 other half untouched. Cover them both and allow them to 

 stand for several days. Note any difference. 



2. Effect of drying on their activities. 



In one tube place a bean, in another a bean half covered 

 with water. Allow them to stand for a week and note any 

 difference between the two. 



Effect of freezing and boiling. 



Put a small piece of raw meat in each of three test tubes ; 



nearly cover with warm water. Boil the water in one tube. 



Plug all three tubes with cotton batting. Place one of the tubes 



that has not been heated outdoors where it will freeze, and keep 



the other in the schoolroom by the side of the one that has 



been heated. On the next day bring in the tube from out- 



loors and allow all three tubes to stand in the schoolroom for 



I days. Note any differences. What is shown about 



the effect of high and low temperature on the growth of 



bacteria ? 



