326 APPLICATION OF METHODS OF BACTERIOLOGY 



Note the results. 



In what way do these experiments differ and how can 

 the differences be explained? 



Expose to the air six Petri dishes into which either sterilized 

 gelatin or agar-agar has been poured and allowed to solidify; 

 allow them to remain exposed for five, ten, fifteen, twenty, 

 twenty-five, and thirty minutes in a room where no one is 

 at work. Treat a second set in the same way in a room where 

 several persons are moving about. Be careful that nothing 

 touches them, and that they are exposed only to the air. 

 Each dish should be carefully labelled with the time of its 

 exposure. 



Do they present different results? What is the reason for 

 this difference? 



Which predominate colonies resulting from the growth 

 of bacteria, or those from common molds? 



How do you account for this condition? 



Sprinkle a little fine dust over the surface of a plate of 

 sterile gelatin or agar-agar; examine the dust-particles with 

 the microscope immediately after depositing them on the 

 medium, and again after eighteen to twenty-four hours. 

 What differences do you detect? What information of 

 sanitary importance does this give? 



Under the description of each of the pathogenic bacteria 

 more or less detailed directions will be found for the dis- 

 covery and isolation of each of the pathogenic bacteria. 



