120 PHYSIOLOGY. 



and snowflakes 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 whiteness. But melt some of it, and you will 

 usually find a decided tinge darkening the water, showing 

 that as the flakes sifted down through the air they caught 

 myriads of particles of dust. 



The Sources 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. and pulverized may form 

 part of the common dust. The dust that you wipe from 

 your eye, or is caught by the mucus of the nasal passages, 

 may, instead of being made of clean soil, be from the 

 excreta of horses, decayed leaves, wood, grass, etc. In- 

 doors we are constantly making dust by wearing out our 

 clothes. Many of the tiny particles that we see floating in 

 the sunbeams are bits of cotton or woolen fibers. Shake 

 any garment in a beam of light to see how much, and how 

 easily, dust is given off. The worn-off particles of our 

 shoes, books, floors, all contribute to the ever-present 

 dust. 



The Effect of Dust on the Lungs. Now, this dust (so 

 far as it is mere dead, dry matter, not considering it as a 

 poison) is irritating to the lungs and respiratory passages. 

 There is provision, as we have seen, for catching and 

 getting rid of a good deal of it. 



But still much is taken into the lungs. Examination 



