AIR. 



393 



duced into liquefied gelatin or agar, and plates are made. 

 The amount of air is measured by means of a gas-meter. 



3. If a precise quantitative result is not desired, gelatin- 

 plates may simply be exposed to the air for a certain period 

 of time, and the germs deposited may be permitted to de- 

 velop into colonies. 



Bacteria of the Air. One cubic meter of air contains, 

 on the average, from 500 to 1000 germs, including from 

 100 to 200 bacteria. Molds constitute by far the majority 

 of the germs ; next in frequency are yeast-fungi, and last, 



Fig. 85. Hesse's apparatus for collecting 

 bacteria from the air. 



Fig. 86. Petri's sand-filter for 

 air-examination. 



bacteria, which are usually represented only by micrococci 

 and sarcinae. 



The number of germs present in the air is in marked 

 degree dependent upon local and temporal variations. In 

 inhabited localities, where dust is being constantly created 

 and blown about, the air contains a larger number of micro- 

 organisms than in deserts. On uninhabited mountains the 

 air is almost entirely free from germs, as it is also at sea. 

 At a short distance from land sea-air contains germs carried 

 thither from the land by currents of air. After a heavy 



