CHAPTER VII 



EXERCISES ON INFECTION AND STERILIZATION 

 EXERCISE I. PHENOMENA OE INFECTION 



1. Prepare three agar-plates as directed in Chap. VI. 



2. Touch the surface of the jelly in one plate with 

 the tips of the fingers. 



3. Touch the surface of the jelly of another plate with 

 the tips of the fingers, after washing the hands carefully. 



4. Catch a fly and allow it to walk on the surface of 

 the jelly of the third plate for a few seconds. Release 

 the fly and replace the cover. 



5. Place these three plates in a locker or thermostat 

 for 24 hours. Observe and describe the results. Make 

 hanging-drop and stained preparations of some of the 

 colonies formed. 



EXERCISE n. PHENOMENA OF STERILIZATION 



1. Make an infusion of hay in a flask with cotton 

 stopper. (See Appendix I.) 



2. Set the flask aside for 24 hours in a warm place, 

 and observe the results. To what are these results due ? 



3. Expose a tube of unsterilized broth to steam in 

 the "Arnold," another to steam in the autoclav, at 120° 

 for 5 minutes each. 



4. Set aside in a thermostat, and observe the results. 



EXERCISE III. PHENOMENA OF STERILIZATION (CON- 

 TINUED) 



Action oj Berkefeld and cotton filters. — Berkefeld filters 

 are made of diatomaceous earth, and are porous so as 



45 



