BACTERIA 103 



carefully by the edge, drain off most of the water, but do 

 not wipei Lay a piece of round filter paper in the bottom of 

 each dish, placing the cover over it at once. It is better that 

 the paper be sterilized by heating in a hot-air chamber at 

 a temperature of 150° C. for half an hour, but this is not 

 necessary if a good quality of filter paper be used. There 

 have now been prepared six moist chambers, relatively 

 free from germs. Why ? 



3. Cut the boiled potatoes into thin slices with a knife 

 that has been thoroughly cleaned and heated in a flame. 

 Place a slice or two on the filter paper in each Petri dish. 

 Be careful not to touch the cut surface with the fingers or 

 any object save the heated knife blade. Lift the cover of 

 the Petri dish carefully, handling only the outer edge, and 

 replace quickly. The culture surface is now ready for inoc- 

 ulation. The boiling of the starch prepares a much better 

 culture medium than the surface of a raw potato. 

 B. Inoculation of the culture surface. Care should be taken to 



lift off the covers of Petri dishes gently, and replace at once 



after inoculation. 



1, 2. Lift the covers from two dishes and expose the potato 

 to the air for five minutes where it is likely to gather parti- 

 cles of dust. Place one dish in a cool situation (as an ice 

 box), and the other in a warm one (as near a radiator), 

 noting the temperatures with a thermometer. 



3. Draw the finger nail twice across the cut surface of a 

 potato in another dish, in two parallel lines half an inch 

 apart. 



4. Wash the edge of a public drinking cup or the outlet 

 of a faucet with a cupful of distilled or sterilized water. 

 Place a drop of the water on the surface of the fourth 

 slice, noting its position. 



5. Spread a very small quantity of milk over the surface of 

 another potato with the sterilized point of a knife. 



6. Leave the sixth potato slice untouched for comparison 

 with the inoculated ones. 



