STUDY OF BACTERIA. 105 



out any further points of structure in the 

 bacteria ? 



E. Sterilization. 



Prepare three dishes of nutrient agar as di- 

 rected above. 



Remove the cover from No. i, and allow it 

 to remain exposed to the air for several min- 

 utes. Label it "Agar No. i." 



Remove the cover from a second dish, ex- 

 pose it as in No. i ; then pour over the surface 

 a thin film of corrosive sublimate (1:1000). 

 This solution is made by dissolving one anti- 

 septic tablet in a pint of water. Replace the 

 cover and label "Agar No. 2 + Poison." 



Expose a third dish of agar to the air for the 

 same length of time as in No. i and No. 2. 

 Heat this dish for a half -hour every 12 hours 

 in a steam sterilizer or over a water-bath. 

 Label "Heated Agar." Keep all three dishes 

 covered, and set them aside where the tem- 

 perature is about 70 F. 



Compare the three dishes at the end of three 

 days. 



1. What differences do you notr between the 



three dishes? 



2. What is the effect of the poison (corrosive 



sublimate) on the growth of bacteria ? 



3. What effect does heating and cooling have 



on the growth of bacteria? 



4. In what two ways can a substance be ster- 



ilized? 



F. From all your experiments state 



