EXERCISE 137 



Problem. Do commercial mouth washes, as ordinarily used, destroy bacteria ? 



What to use. Three squads of experimenters ; four sets of sterilized culture dishes ; commercial 

 mouth washes ; water. 



What to do. Leave one set of culture dishes unexposed for control (a). One set of pupils rinse 

 mouth with a teaspoonful of water, and each throw the rinsing into a separate culture dish (b) ; the 

 second set of pupils wash mouth with plain water, then rinse with a teaspoonful of water and save rins- 

 ing in culture dishes (c) ; the third set wash mouth with commercial mouth wash, then rinse with a 

 teaspoonful of water and throw rinsing into culture dishes (d] ; label all dishes names, dates, wash 

 used, etc. and set aside in same temperature, illumination, etc. 



Record. Report average number of colonies, or spots, in each set of dishes. Compare results and 

 record conclusions. 



Questions. 1. In what way are commercial mouth washes supposed to produce their results? 

 2. In what other ways can the desired results be produced ? 



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