198 TECHNOLOGY OF BACTERIA. 



enumeration of atmospheric bacteria. He finds, 

 as might have been expected, that they are more 

 abundant during the summer months; and that 

 they are less numerous immediately after a heavy 

 rain, which has the effect of purifying the atmos- 

 phere, by washing out suspended particles. 



Kain-water will always be found fertile in germs ; 

 and it is evident that when collected with care it 

 represents the bacterial flora of the atmosphere at 

 the time of its fall. We may therefore study this 

 by means of culture-experiments, in which a variety 

 of sterilized organic infusions are inoculated with 

 one or more drops of rain-water which has just 

 fallen. It is necessary to use many different 

 culture-fluids, because various organisms require 

 special media for their development. 



Again, we may expose our sterilized organic in- 

 fusions to the air, and thus permit them to become 

 fertilized by the deposition of air-borne germs, the 

 development of which is subsequently studied as 

 they germinate upon the surface, or in the interior, 

 of these infusions. 



Solid culture-media are especially useful for this 

 mode of investigation, and we may employ organic 

 infusions to which three to five per cent of gelatine 

 has been added, as recommended by Koch ; and 

 also a variety of cooked alimentary substances, 

 such as moist bread, slices of boiled potato, turnip, 

 onion, etc., various fruits (cooked or uncooked), 

 meats of different kinds, etc. Upon the surface 

 of these, if they are kept moist, and are placed in 



