I. 
BACTERIA IN THE ATR. 
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—0° 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- 
