264 BAOTEBIOLOOY. 



in a sterilized test-tube, and sterilize tliem by streaming 

 steam for one hour or in the autoclave for fifteen min- 

 utes at one atmosphere pressure. At the end of the 

 sterilization remove one piece with sterilized forceps and 

 allow it to brush against your clothing, then make a 

 plate from it ; draw another piece across a dusty table 

 and then plate it. Suspend three or four pieces upon a 

 sterilized wire hook and let them hang for twenty min- 

 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 diiFer 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 ? 



