268 



BACTERIA, YEASTS, AND MOLDS 



Method of Experimenting. The order in which the experiments 

 are given is the one which most naturally follows the subjects treated 

 in the body of the text, and should be followed as closely as possible. 



Where possible each 

 scholar should perform 

 the experiments, but 

 this will be found imprac- 

 ticable in most cases. In 

 such cases the experi- 

 ment must be performed 

 by the teacher in the 

 presence of the class. 



Most experiments with 

 microorganisms require 

 two or three days for the 

 bacteria to grow, and 

 the observations must 

 therefore be made some 

 time after the prepara- 

 tion is made. Hence it 

 is especially important 



FIG. 82. Hot-air sterilizing apparatus. 



that everything should 

 be carefully and intel- 

 ligibly labeled and that the scholars understand the meaning of the 

 labels. When the teacher performs the experiments the scholars 

 should see the preparation as well as the final results, and each 

 scholar should make careful notes. 



Sterilizing. All glassware must be 

 sterilized before it is used. This is abso- 

 lutely necessary and the success of the 

 experiments will depend upon it. The 

 glassware should be first washed clean. 

 Then all test tubes, flasks, and fermenta- 

 tion tubes should be tightly plugged with 

 cotton, as shown in Figs. 38 and 64, and then placed, with all 

 other glass apparatus, in the dry sterilizer. By means of a Bunsen 

 flame the sterilizer should then be heated to a temperature of about 



FIG. 83. A petri dish for 

 plate cultures. 



