238 5^ GTEBIOL OQY. 



Place some of these pieces (about 5 centimetres long) 

 into a sterilized test-tube, and sterilize them by steam 

 for one hour. 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 the table and then plate it. Sus- 

 pend three or four pieces upon a sterilized wire hook 

 and let them hang for thirty minutes 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 must 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 ? 



