246 BACTERIOLOGY. 



utes free in the air, being sure that they touch nothing 

 but the hook ; then plate them separately. 



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 min- 

 utes in a room where no one is at work. Treat a sec- 

 ond 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 rea- 

 son for this difference ? 



Which predominate colonies resulting from the 

 growth of bacteria, or those from common moulds ? 



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-par- 

 ticles 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 bacte- 

 ria more or less detailed directions will be found for the 

 discovery and isolation of each of the pathogenic bacteria. 



