DUST AND ITS DANGERS. 31 



tween the atmospheric dust in closed rooms 

 and that out-of-doors is that in the former 

 there is no spontaneous mode of purification 

 of the air except that of settling, and that the 

 settled more or less bacteria-laden dust is 

 liable to frequent stirring-up by the ordinary 

 movements of people, while out-of-doors the 

 bacteria-laden air is constantly being swept off 

 by the wind. 



The effect of stirring about in rooms in 

 which micro-organisms are present is shown by 

 the analyses of Tucker in the wards of the 

 Boston City Hospital. He found that about 

 midnight after the wards had been quiet for a few 

 hours, the number of living bacteria in 10 litres 

 of air ranged from o to 13, while the number of 

 mould spores ranged from o to 4. The air had 

 practically freed itself from germs, by settling 

 to floors and beds. He found that in a long 

 series of hourly determinations in various wards 

 at all hours of the day, the average number of 

 bacteria in 10 litres of air was about 26 and of 

 moulds about 12, the number of bacteria rang- 

 ing from i to 477; of moulds from o to 227. 

 The germs were more abundant in the air in 



