Dust and Bacteria. 121 



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 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. This dust 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. Exam- 

 ination shows that the lungs have many black specks from 

 particles of carbon, etc., that have become lodged, and are 

 of no benefit, to say the least. 



Composition of Live Dust. Bad as this dead dust is, the 

 injury from it is slight compared to that from live dust. 

 We know that certain seeds float in the air, carried along 

 by the wind. But these are comparatively heavy, and soon 

 sink to the ground. 



We all know pollen. At certain seasons it forms, in the 

 vicinity of cornfields, for instance, a considerable part of 

 the dust. This is alive. It will grow if it falls on the 

 stigma of the right plant at the right time. Such dust will 

 not grow in our bodies. We do not furnish a soil in which 

 it can grow. It merely adds to the amount of irritating 

 dust. 



Puffballs and Molds. We have seen puffballs give oft 

 a cloud of dust when they are crushed. So, too, from a 

 patch of mold, when brushed, we often see a little cloud of 

 dust. This dust is composed of live spores that will grow 

 in suitable places and conditions. 



Yeast. If we set a tumbler of cider on a table in a 

 warm room, in a few days it ferments. This is due to 



