10 DUST AND ITS DANGERS. 



most stifling density, and again in very small 

 numbers, collecting in whirls and eddies, and 

 finally, always sooner or later, settling down to 

 the lowest available resting-place, as soon as 

 the buoyancy of air currents gives way to the 

 ever acting attraction of gravitation. Since 

 the bacteria of dust are very apt to be in little 

 groups or clusters or to cling to other dust par- 

 ticles, most of them readily settle, so that a 

 very considerable part, in fact, of the finer 

 dust the " motes in the sunbeam " is not 

 made up of bacteria or germs but of other 

 forms of lifeless matter. 



