INFECTION. 97 



less, but the quantity of salts is the same. "Where we find salts 

 suspended in the air, there has been sufficient mechanical action not 

 to remove molecules of water, but to take it up in drops, as in a 

 storm at sea. With the cessation of the storm the salts drop of 

 their own weight ; but evaporation does not cease. Bacteria can 

 not be removed by simple processes of evaporation ; small as they 

 are, they are larger and heavier than the molecules of salt dissolved 

 in water. Infectious elements can only be taken up and transported 

 by the atmosphere when in a dried or dust form. 



Two important adjunct circumstances come also into considera- 

 tion in this regard : 



1. The degree of adhesion with which such elements cling to 

 their place of birth or lodgment. 



2. The mechanical means to which they are subjected. 



With reference to the latter, the most simple case is where the 

 dried mass, or the remnants of an evaporated fluid, are disturbed or 

 ground to a powder by some mechanical means which renders it 

 easy for the atmosphere to remove them. 



The formation of dust in our streets, which may frequently con- 

 tain bacteria, and its removal by the wind, is a fitting example. 



To this end it is essential that the material which contains the 

 germs does not contain anything of a mucilaginous or adhesive 

 nature, and that the particles of the same are sufficiently small. 



The dissemination of a gas takes place very rapidly in the at- 

 mosphere. Even though the movements of the latter be imper- 

 ceptible, a bad-smelling gas soon disappears if the supply be cut 

 off. 



The distribution of dust is dependent on its fineness and the vio- 

 lence or force with which the air moves ; but in no case is it capa- 

 ble of very extended dispersion. In a motionless atmosphere dust 

 molecules or germs soon fall to the ground. The smaller they are, 

 and the more rapid the movement of the air, the longer they are 

 kept in suspension. 



So many circumstances are necessary to the dispersion of in- 

 fectious elements by means of the atmosphere, it is evident that 

 they may be confined for a long time to an individual, a house, 

 street, or locality. 



An organism is, therefore, so much the more exposed to a given 

 infection, the more it is confined to a locality where infectious ele- 

 ments are or have been generated, or the more the air-currents come 

 from such a place. 



Infectious elements do not all possess the same degree of dis- 

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