28 DUST AND ITS DANGERS. 



mould spores, within one or two hours. Of 

 course violent currents of air, walking about, 

 etc., interfere with the very complete subsi- 

 dence of the bacteria-laden dust particles. 



Now it might be supposed that the frequent 

 renewal of the air of a room by such a system 

 of ventilation as would be effective in keeping 

 its gaseous ingredients pure would also suffice 

 to rapidly carry off dust particles, and bacteria 

 as well. But a long series of most carefully 

 conducted experiments by Stein has shown 

 that this is not the case. Even when the in- 

 troduction of fresh air is pushed to the com- 

 plete renewal of the air three times an hour, 

 the number of suspended micro-organisms 

 floating in the air is scarcely more diminished 

 than they would be by settling in still air. 

 Stein found that only when the ventilation 

 was carried to the degree of inducing marked 

 and disagreeable draughts in the room was 

 there a rapid diminution in the number of 

 micro-organisms which had been diffused arti- 

 ficially through the air for the purposes of the 

 test. Of course opening of the windows and 

 allowing large bodies of air to blow through 



