DUST AND BACTERIA. 121 



shows that the lungs have many black specks from parti- 

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

 no benefit, to say the least. 



LIVE DUST. 



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 ate 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 

 right kind of a surface, 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 

 off a cloud of dust when they are crushed. This dust is 

 composed of live spores that will grow in suitable places 

 and conditions. So, too, from a patch of mold, when 

 brushed, we often see a little cloud of dust. These are a 

 few instances of kinds of living dust that simply act on us 

 like so much dead matter. 



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 

 yeast that has gotten into it. Boil the cider to kill any 

 yeast that is already in it, and cork it securely so that air 

 cannot get at it, and it will not ferment. Dried yeast 

 germs float in the air, settle on the fruit or in the cider, 

 and cause it to ferment. Cider is a good soil for yeast. 



