120 BACTERIA, YEASTS, AND MOLDS 



particularly milk. So ubiquitous are they that it is an 

 absolute impossibility for the housewife, by any means 

 at her command, to keep her pantry and food free from 

 them. 



These facts forcibly emphasize the futility of the com- 

 mon method of sweeping and dusting rooms. Bacteria 

 are heavier than the air and if undisturbed settle and lie 

 quietly upon floors, tables, etc. Every sweeping the room 

 receives stirs them up. A dustbrush sends them flying 



FIG. 59. Petri dishes exposed, one, a, before, and the other, 3, 

 after a class has occupied a schoolroom. 



through the room only to settle down again later. On the 

 other hand, wiping with damp cloths removes the bacteria 

 and is the only proper method of cleaning. This is espe- 

 cially true for kitchens and pantries where food is exposed 

 to the air, and for schoolrooms where there is likely to be 

 a collection of numerous kinds of bacteria, including some 

 disease germs brought by the many children. Fig. 59 

 shows two plates, one exposed to the air before and the 

 other after a school session. The relative abundance of 

 bacteria floating in the air is clearly shown. 



