I. 



BACTERIA IX THE AIR. 



THE saprophytic bacteria are found wherever the organic material 

 which serves as their pabulum is exposed to the air under conditions 

 favorable to their growth. The essential conditions are presence of 

 moisture and a suitable temperature. The organic material may be 

 in solution in water or in the form of moist masses of animal or 

 vegetable origin, and the temperature may vary within considerable 

 limits to 70 C. But the species which takes the precedence will 

 depend largely upon special conditions. Thus certain species multi- 

 ply abundantly in water which contains comparatively little organic 

 pabulum, and others require a culture medium rich in albuminous 

 material or in carbohydrates ; some grow at a comparatively low or 

 high temperature, while others thrive only at a temperature of 20 to 

 40 C. or have a still more limited range ; some require an abun- 

 dant supply of oxygen, and others will not grow in the presence of 

 this gas. Our statement that saprophytic bacteria are found wherever 

 the organic material which serves as their pabulum is exposed to the 

 air under suitable conditions relates to the fact that it is through 

 the air that these bacteria are distributed and brought in contact 

 with exposed material. It is a matter of common laboratory experi- 

 ence that sterilized organic liquids quickly undergo putrefactive de- 

 composition when freely exposed to the air, and may be preserved in- 

 definitely when protected from the germs suspended in the air by 

 means of a cotton air filter. But the organic pabulum required for 

 the nourishment of these bacteria is not found in the air in any con- 

 siderable amount, and if they ever multiply in the atmosphere it 

 must be under very exceptional conditions. Their presence is due to 

 the fact that they are wafted from surfaces where they exist in a 

 desiccated condition, and, owing to their levity, are carried by the 

 wind to distant localities. But, under the law of gravitation, when 

 not exposed to the action of currents of air they constantly fall 

 again upon exposed surfaces, which, if moist, retain them, or from 

 which, if dry, they are again wafted by the next current of air. 

 Under these circumstances it is easy to understand why, as deter- 



