212 BACTERIOLOGY. 



Place some of these pieces (about _o 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 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 

 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 ? 



