BACTERIA 



227 



currents of air in the room. (3) Upon opening a culture 

 medium for inoculation, pass the mouth of the tube through 

 the flame (flaming) ; if it has stood for some time, flame the 

 cotton before opening the tube. (4) Never allow the tube 

 end of a plug to come in contact with anything while re- 

 moved from the tube. (5) If a plate culture is to be made, 

 melt the gelatin in a test tube (placed 

 in warm water) and pour into a ster- 

 ile Petri dish. If Petri dishes are not 

 available, test tubes may be substi- 

 tuted, provided the gelatin in them 

 is allowed to cool while they are lying 

 in a nearly horizontal position. (6) In- 

 oculation should not take place before 

 the gelatin hardens, unless germs from 

 a liquid are to be grown. In this case 

 the gelatin is inoculated in the test 

 tube and then poured into the Petri 

 dish. (7) Unless otherwise directed, 

 all cultures that have been inoculated 

 should be kept in the dark, or in dif- 

 fused light and at room temperature. 

 (8) If possible, duplicate each experi- 

 ment, using both potato and gelatin 

 media. Note appearance of growth in 

 each case. Label and keep careful records of each experi- 

 ment. (9) After your experiments are finished, do not allow 

 the media to dry; place all dishes in water and boil for 

 fifteen or twenty minutes before cleaning them. 



Experiments for bacteria of the air. 1 (1) Expose a Petri 

 dish of gelatin for five minutes in the laboratory before the 

 class enters. (2) Expose another for the same length of time 



FIG. 106. Exposing Petri 

 dishes 



Photograph by the author 



1 Each member, or group of members, of the class should perform one or 

 more of these experiments. 



