4 DUST AND ITS DANGERS. 



of fine vegetable or animal fibres, such as cot- 

 ton or woollen or other light material, and of 

 the greatest variety of micro-organisms, either 

 singly or in masses, such as bacteria and mould 

 spores. Furthermore, these micro-organisms 

 are very apt to be found clinging singly or 

 in clusters to the larger or smaller inorganic 

 particles of one kind or another which usually 

 make up the bulk of visible or invisible dust in 

 inhabited regions. 



It is not necessary for our purposes here to 

 enter in detail into those conditions of soil and 

 climate and human occupation which favor the 

 presence of dust in the air. That dry air and 

 dry-ground surfaces and winds favor the distri- 

 bution of the fine particles which we call dust, 

 and that still air and moist ground tend to hold 

 it in check, are facts which every one's observa- 

 tion teaches. 



It is well known that there are certain occu- 

 pations which confine persons to closed rooms 

 or places in which dust particles of one kind 

 or another are very abundant. Thus day after 

 day persons confined in air charged with coal- 

 dust or stone-dust or metallic-dust or cotton- 



