DUST AND ITS DANGERS. 



The dust particles of the air may be roughly 

 grouped in two classes first, those larger bodies 

 which are readily visible in-doors or out-doors, 

 and second, the smaller particles which are usu- 

 ally only seen when strongly illuminated. 



The coarser particles of dust, such as are 

 usually swept into our faces whenever we go 

 upon the streets in New York in dry and 

 windy weather, consist largely of small frag- 

 ments of sand, broken fibres of plants, pollen, 

 fine hairs, the pulverized excreta of various 

 domestic animals, ashes, fibres of clothing and 

 other fabrics, particles of lime or plaster or 

 soot, parts of seeds of plants, masses and clus- 

 ters of various kinds of micro-organisms, and 

 other partially ground up materials of kinds 

 too numerous to mention. 



The finer dust particles, whose presence, 

 when in considerable quantities, we may be 

 aware of by the choking sensation which they 

 cause when breathed in, even though we do 

 not see them, are most plainly visible as the 

 so-called " motes in the sunbeam," when sunlight 

 streams into more or less darkened places. 

 These are very light and consist of fragments 



