DUST AND ITS DANGERS. 75 



against danger, even in very dirty and very 

 infectious places. Many disease-producing bac- 

 teria die, soon after they are expelled from the 

 bodies of sick persons ; many are swept away 

 by the winds into uninhabited regions ; many 

 are killed by sunlight ; many fail to come in 

 contact under favorable conditions with sus- 

 ceptible human beings. But these natural safe- 

 guards cannot be implicitly relied upon for 

 safety by any one at all times, nor can any 

 one with impunity overtask their capacities 

 by unnecessary and constant exposure of his 

 person to infective dust. 



It is certain that in the out-of-doors air in the 

 country, and also in cities whose streets are 

 kept decently clean, there is little danger of 

 harm from the inhalation of germs of con- 

 sumption or of any other disease, because the 

 constant purifying agency of wind and air 

 currents will either soon sweep away the dust 

 or so largely dilute it that it will be practically 

 free from disease germs, the sources of which 

 are so comparatively limited. If, however, the 

 streets of cities be or are allowed to remain 

 filthy, so that abundant and pretty constant 



