CHAPTER V. 



THE MICRO-ORGANISMS OF IN-DOORS DUST. 



WHEN we consider the comportment of 

 dust particles in closed rooms, we see 

 at once that the great renovating and cleansing 

 agency which is so efficient out-of-doors is, ex- 

 cept on special occasions, absent, namely, the 

 winds and strong air currents and the more or 

 less frequent and prolonged wettings. Once 

 in a closed room dust is very apt, as every 

 housekeeper knows, to stay there, unless 

 special means are resorted to to get rid of it. 

 But although the dust remains in the room, 

 those heavier parts of it which contain most of 

 the bacteria gradually sink to the lowest 

 available levels, floors, shelves, furniture, etc., 

 so that it has been found that the still air of a 

 room may almost completely free itself from 

 micro-organisms, except some of the lighter 



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