200 The Story of the Bacteria 



of the dangerous elements by winds and air 

 currents is so vast as to reduce greatly the 

 chances of evil effects from swallowing or 

 inhaling them. Still, in large towns, whose 

 streets are not faithfully cared for, the prob- 

 ability of being obliged to pass through clouds 

 of dust whenever one goes upon the streets, 

 especially in the windy seasons, is very un- 

 pleasantly suggestive of danger, and more 

 than suggestive of filth. 



But after all, it is in living-rooms and in 

 places of assembly that we must look for the 

 most frequent sources of danger. Here the 

 dusty air is undiluted, is swept round and 

 round by interior currents, and breathed over 

 and over again. 



Among workers in very dusty trades such 

 as indoor stone cutters, miners, and steel 

 grinders, dust is a direct incitant of serious 

 disease of the lungs. Under the ordinary 

 conditions of life dust is rather a predisposing 

 factor, making the lungs and body at large 

 more susceptible than it should be, to the 

 incursions of disease-inducing germs. 



When much dust gets into the lungs the 



