APPENDIX 283 



55. Bacteria from Various Sources, (a) Into three tubes of melted 

 gelatin culture medium place a small drop of saliva. Mix thoroughly 

 with the gelatin and pour into petri dishes. (<) Place in other tubes 

 of melted gelatin, and also of melted agar, very small bits of decaying 

 meat or decaying egg. Mix thoroughly in the gelatin by rubbing 

 with a sterilized glass rod and pour out into a petri dish. (V) Into a 

 third set of tubes place small pieces of dirt swept up from the floor 

 or picked out of cracks in the floor. Mix with the gelatin and pour 

 into petri dishes. (W) Into a fourth set of tubes place a little dirt from 

 the street and proceed as before. Allow all plates to grow till the 

 colonies are visible. Note any differences between them. 



56. Bacteria on the Fingers. Pour gelatin into some petri dishes. 

 After it has hardened touch its surface with the fingers, replace the 

 cover, and set aside for bacterial growth. Wash the hands thoroughly 

 in clean water, wiping with a clean towel, and then proceed in the 

 same way with a second petri dish, touching the surface with the 

 fingers and setting aside for growth. 



57. Bacteria in the Air. Melt the contents of four tubes of gelatin 

 and four of agar. Pour each into a petri dish, replace the cover, and 

 allow the contents to harden without inoculation. Expose one gelatin 

 and one agar plate to the air of a schoolroom before the school ses- 

 sion, by removing the covers and leaving the plates uncovered for three 

 minutes. Expose two similar plates at the close of the school session 

 in the same way. Expose two in the hall at the time when many schol- 

 ars are passing through it. Expose two in a room after sweeping or 

 dusting. In all cases the plates are to be exposed the same length of 

 time, carefully labeled, and set aside for the bacteria to grow. The 

 relative number of bacteria is readily determined by an examination of 

 the plates. Molds will grow upon the surface of the plates, but a little 

 study will make it possible to distinguish them from bacteria. The 

 bacteria will commonly be more numerous than the molds. Similar 

 plates exposed in a variety of locations will be very instructive as 

 indicating the abundance of bacteria in the air. 



58. Bacteria in Milk. In an ordinary flask place one hundred 

 cubic centimeters of water and sterilize by steaming for two hours. 

 After cooling place one cubic centimeter of ordinary milk in the flask 

 and mix thoroughly by shaking. Melt three tubes of gelatin and three 



