EXERCISE 76 (Continued) 



bacteria. (For detailed directions see Caldwell and Eikenberry, 

 " Laboratory Manual for Work in General Science," Rev. Ed., 

 Exercise 43.) 



A substitute for petri dishes may be contrived by inverting 

 a glass sauce dish over a smaller one, but any substitute is 

 unsatisfactory. At least a few petri dishes should be a part of 

 the equipment of every laboratory. A steam sterilizer may be 

 contrived by supporting the dishes above boiling water in a 

 covered pail, but it is impossible to insure that the material will 

 at all times be surrounded by steam at 100 C. A small steam 

 sterilizer is preferable. One or two of the sterilized agar plates 

 should be retained as checks. 



PROBLEMS 



1. Are there bacteria in the air ? Expose one or more of the 

 dishes to the air by uncovering for a measured length of time, as 

 twenty minutes. Replace the cover. Watch for the appearance of 

 colonies of bacteria in two or three days. If dishes are uncovered 

 in different parts of the building, a rough measure of the abundance 

 of bacteria may be secured. 



2. Are bacteria present on one's person, as on the hands ? Draw 

 the fingers lightly across the surface of the agar in a dish. Observe 

 later for the development of colonies of bacteria. (The agar must 

 be well cooled else it will stick to the fingers.) 



3. Repeat above after carefully washing the hands. What conclu- 

 sions may be drawn regarding the sanitary value of washing ? 



4. Is dry sweeping a sanitary procedure ? Expose a dish during 

 the time the janitor is sweeping the laboratory. If possible, expose 

 one just preceding sweeping for comparison. Both should be exposed 

 for the same length of time. What are your conclusions ? 



5. Does a vacuum cleaner have sanitary value ? Expose a dish as 

 in Problem 4 in a room in which a vacuum cleaner is being operated. 



6. Is dusting a sanitary procedure? Expose one plate in a room 

 that is being dusted with a feather duster or a dry cloth. Expose 

 another similarly in a room in which a damp cloth is being used 

 to remove the dust. 



[96] 



