GENERAL BACTERIOLOGY 91 



6, Note the conditions of these ten tubes after 2 or 

 3 days. Compare the results, and tabulate them. 

 Note especially coagulation, time elapsed before coagu- 

 lation sets in, gas formation, condition of whey, film, 

 and odor. 



Plates in lactose litmus gelatin should be made from 

 each of these tubes, and the colonies studied and 

 counted. Subcultures on agar slants may also be made 

 and the usual media inoculated from these, if the indi- 

 vidual species are to be studied. 



SECTION 5 



EXERCISES ON INFECTION AND STERILIZATION 



EXERCISE I. PHENOMENA OF INFECTION 



1. Prepare three agar plates. 



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. 



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

 the jelly of the third plate. 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 2. PHENOMENA OF STERILIZATION 



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

 stopper. (See p. 41.) 



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

 and observe the results. 



