THE BACTERIA. 137 



IV. BACTERIA IN THE AIR. 



Material. 30 c.c.. of nutrient gelatin. 

 Apparatus. 3 sterilized Petri dishes. 



a. Pour 10 c.c. of the warm nutrient gelatin into a Petri 

 dish, and allow it to stand for two minutes uncovered and 

 exposed to the air of the room. Then cover and place 

 with the other tests. Study the colonies as they appear. 



b. Test in the same way the air of a room before and 

 after sweeping and compare your results. 



1. Why are hardwood floors more healthful than 



carpets ? 



2. Which is better, a cloth or a feather duster? 



Why? 



V. BACTERIA IN VINEGAR. 



Materials. A few c.c. of mother-of-vinegar. 



Apparatus. A compound microscope, glass slide, and cover-glass. 



Mount a few drops of mother-of-vinegar on a glass slide, 

 examine with the microscope, .and study the bacteria. 



Notice : 



a. Their size, shape, and motion. 



6. In what are the bacteria embedded? This mass of 

 jelly is called a zooglea. It is formed by many individuals 

 whose cell-walls have become mucilaginous. If the mass 

 is membranous, as in the mother-of-vinegar, it is called 

 mycoderma, or fungus-skin. 



VI. THE MICROSCOPICAL EXAMINATION OP BACTERIA. 



Materials. Colonies of bacteria obtained from samples of water 

 and milk. 



Apparatus. A compound microscope. 



