442 Our Surroundings 



one tube without removing the plug, to serve as a check, or con- 

 trol. Label it 1. 



Expose the contents of the second tube for several minutes 

 to the air of an empty classroom. Plug the tube, and label it 2. 

 Similarly, expose the contents of the third tube to the air of the 

 classroom at the close of a recitation, and expose the contents 

 of the fourth tube to the air of the school yard. Label these 

 tubes 3 and 4. 



Keep all these tubes in a warm, dark place for two or three 

 days. At the end of this period count the colonies of bacteria 

 and record the number observed in each tube. Also notice 

 whether molds have formed. Compare the numbers of colonies 

 in the different tubes. State your conclusions. 



To Determine the Presence of Molds in Bread. Place a 

 piece of fresh bread in a can or box, and leave it for three or four 

 days in a warm, dark, moist place, where there is little circulation 

 of air. At the end of this time examine the bread. What do you 

 observe? How should bread be kept to prevent molds forming 

 on it? 



To Determine the Presence of Bacteria in Milk. Using 



three prepared Petri dishes, set 

 one aside without uncovering, 

 for a control. Label it 1. Uncov- 

 ering the others, spread on the 

 medium of one a drop of fresh 

 milk, and on the second a drop 

 of milk a day old. Label them 



BACTERIA IN PETRI DJSHES 2 and 3 ' Leave the dishes in 



Each white spot is a colony of many a warm, dark place for two or 



three days, and then examine 



the contents of each. Record the results, stating the number of 

 colonies observed in each case. What is your .conclusion? 



To Determine the Presence of Bacteria and Molds in 

 Water. Using three prepared Petri dishes, set one aside with- 

 out uncovering, for a check or control. Label it 1. Uncovering 

 the others, spread on the medium of one a drop of ordinary 

 drinking water, and on the second a drop of rain water. Label 



