14 HUMAN BIOLOGY 



a moist surface that will supply them with food, the spores 

 burst their thick envelope, assume once more their rod- 

 shaped or spiral form, and go on feeding, assimilating, and 

 reproducing their kind. 



II. Occurrence of Bacteria 



13. Are bacteria present in the air. — Laboratory 

 demonstration. 



Materials: The best method of cultivating bacteria is by the 

 use of a nutrient agar mixture in Petri dishes, which is prepared as 

 follows : — 



To prepare 1000 cc. (about a quart) of agar mixture, weigh out 

 10 grams of salt, 10 grams of peptone, 10 grams Liebig's beef ex- 

 tract, and 10 grams of agar. Measure into an agafe stewpan 

 1000 cc. of water, and stir in the salt, peptone, beef extract, and 

 agar (the latter having been cut into small pieces). Heat the 

 mixture in a double boiler until the agar is wholly melted. Slowly 

 stir in just enough baking soda to cause red litmus paper to turn 

 blue ; i.e. the mixture should be slightly alkahne. When the pieces 

 of sohd agar have aU disappeared, the hot hquid should be filtered 

 into flasks of 250 cc. capacity through several rather thick layers of 

 absorbent cotton placed in a funnel. This filtration might well be 

 done by placing the flasks in a steam steriUzer. If the fUtrate is 

 not clear, the hquid should be poured through the same layers of 

 cotton tiU it does become clear. Care should be taken to keep the 

 agar mixture hot during the filtering process, otherwise the agar 

 will not pass through the cotton. When the flasks are nearly full, 

 plug the mouth of each with a large wad of cotton batting, put 

 them into a steam sterilizer, and heat them at least thirty minutes 

 on each of three successive days to make sure that all germs and 

 their gpores are killed. The flasks of agar may then be kept as a 

 stock mixture until needed. 



Carefully clean and dry enough Petri dishes to supply, if pos- 

 sible, seventeen or more dishes for experiments with each division 



