3O DUST AND ITS DANGERS. 



laden air from out-of-doors to pass through 

 them we are actually, so far as micro-organisms 

 are concerned, cleansing the air and sending it 

 out much freer from germs than when it en- 

 tered, these having slowly settled as the air 

 made its way from the entrance to the exit of 

 the ventilating openings. The same of course 

 applies, though in a less striking way, to the 

 so-called natural mode of ventilation that is 

 a ventilation system which has for its exit a 

 warm air-shaft or chimney, and " trusts to 

 luck " for channels of air entrance through 

 loose joints in windows, doors, and walls. 



Now, although in rooms through which for 

 purposes of ventilation large volumes of dusty 

 out-of-doors air are pumped, day and night, 

 there will be in the aggregate a considerable 

 accumulation of more or less bacteria-laden 

 dust, it is, after all, the ground-up dirt which 

 we bring in from the streets upon our shoes 

 and garments, and the accumulations of waste 

 material, which in dwelling-houses and places 

 of assembly are so abundant, which furnish the 

 larger proportion of the bacterial ingredients 

 of in-doors air. The marked difference be- 

 3 



