APPENDIX 309 



to be stained brilliant red. It is instructive to examine a number of 

 decaying fluids in this way. 



50. Bacteria from the Teeth. Scrape a little tartar from the teeth, 

 spread upon a cover glass, and stain in a similar manner. 



Further microscopic study of bacteria requires higher-power objec- 

 tives and more apparatus than can be found in ordinary schools. 



Culture Experiments with Bacteria 



Nearly all experiments in bacteriology involve the use of culture 

 media prepared for the purpose. Such culture media may be made 

 by any one who has at his command a laboratory with proper appa- 

 ratus for sterilizing. If a teacher does not have facilities for making 

 culture media, they may be bought from the dealers in bacteriological 

 apparatus. The following is easy to prepare. 



Gelatin Culture Medium 

 Mix together in a common stew pan the following : 



I liter of water. 



5 grams of Liebig's extract of beef. 

 10 grams of peptone. 

 100 grams of gelatin. 



Carefully weigh the mixture in the dish in which it is to be boiled. 

 Heat the mixture at about 140° until the gelatin is thoroughly melted, 

 and then boil briskly for a few moments. Test with litmus paper. 

 It will be found to be acid. Add to it, drop by drop, a solution of 

 caustic soda (NaOH) until it is slightly alkaline to litmus paper. 

 Boil briskly for half an hour. Weigh once more, add enough water 

 to bring it up to the original weight, and test again with litmus 

 paper. If the reaction is still slightly alkaline, the material is ready 

 for filtering. Filter through absorbent cotton, as already described, 

 and collect the clear liquid in a sterilized liter flask. Fill with the 

 material as many sterilized test tubes as it is desired to use, putting 

 about 10 cc. in each, which should fill them about two inches deep. 

 Replace the plugs and then steam all of the gelatin in a steamer for 



