22 DUST AND ITS DANGERS. 



It is very difficult to fix upon any definite 

 number of living micro-organisms in the out- 

 of-doors air which can be regarded as the usual 

 or normal number, because the number varies 

 so extremely under different conditions. Thus 

 on high mountains or deserts and on the sea 

 the unconfined air is practically free from 

 micro-organisms. In the winter months, when 

 snow is on the ground, during rain storms, and 

 when the air is still, the number may be very 

 small. On the other hand, a high wind blowing 

 across a region rich in dry and pulverized 

 germ-laden material, will for a time disseminate 

 large numbers of micro-organisms ; but at the 

 same time it tends, by the dilution which it 

 affords, and by carrying them off to other re- 

 gions, to speedily reduce the numbers in any 

 given place. A rainfall, to a certain extent, 

 tends to free the air of its germs by washing 

 them down, while during a snowstorm many 

 are caught in the snow crystals as they form. 



In wet weather mould-spores tend to pre- 

 dominate, partly because they then grow readily 

 and partly because they are very light, and not as 

 easily wetted and held down as are the bacteria. 



