DEAD DUST. 143 



by the sticky mucus that covers all the walls of this passage- 

 way. If we breathe through the mouth (especially out-of- 

 doors in cold weather), the air may not be sufficiently warmed 

 before entering the lungs, and much more dust would be car- 

 ried into the lungs. Then, too, the air has a drying effect on 

 the throat, whereas the mucus of the nasal passages will 

 moisten the air as it enters. The cilia, which extend from 

 most of the cells lining the respiratory passages, are con- 

 stantly causing the mucus to slowly flow toward the external 

 opening, so a good share of the dust is gotten rid of. A 

 further advantage of breathing through the nose is that we 

 detect odors, and can thus judge of the quality of the air. 



DEAD DUST. 



Every one will recall how delightfully refreshing the air is 

 after a rain or a snowstorm. This is not due merely to the 

 fact that the air is cool. It is clean because it has been 

 washed. The rain and snow absorb a considerable amount of 

 the various impure gases that are in the air. But raindrops 

 and sriowflakes bring down with them many particles of dust 

 that were floating in the air. Take some of the snow that has 

 fallen in a town. It looks pure in its almost dazzling white- 

 ness. But melt some of it, and you will usually find a de- 

 cided tinge darkening the water, showing that as the flakes 

 sifted down through the air they caught myriads of particles 

 of dust. Where soft coal is used to any large extent it is one 

 abundant source of this dust. In summer dust has many 

 sources. The dust that blows into your face, and perhaps 

 into your mouth, may be made of dry soil. Take a dry clod 

 and drop it ; it falls quickly to the ground. Pulverize it in 

 your hand before dropping it, and considerable of it floats in 

 the air for some time. Any substance that is easily dried 



