DUST, MOLDS, AND BACTERIA OF THE AIR 89 



within a few days it will be covered with a growth of tine, 

 white threadlike bodies and will exhibit the appearance of the 

 growth to which we apply the name mold. Mold is a living 

 plant, but in appearance it is quite unlike ordinary plants. 



In a few days the mold produces small black heads which 

 yield a fine, dustlike black powder. 

 These dustlike particles are called 

 spores. Spores are easily blown about 

 by the wind, and if one of them 

 alights upon a piece of bread or in 

 any other favorable place, it is able to 

 grow into a new mold plant. 



Again, if a weak solution of sugar 

 in water is exposed to the dust of the 

 air and allowed to stand for several 

 days, it ferments ; that is, bubbles of 

 gas rise through the liquid. This gas 

 is carbon dioxide. The sugar can no 

 longer be found in the solution, but 

 in its place there is a small amount of 

 alcohol, and a peculiar odor is present. 

 A scum collects at the surface of the 

 liquid and probably at the bottom also 

 (fig. 44). The microscope shows that 

 this scum is made up of minute, living 

 yeast plants. 



102. Tests for living materials. If 

 a sugar solution is boiled in order to 

 kill any yeast that may be in it, and is then inclosed so that 

 no yeast can enter, fermentation will not occur; but if the 

 solution remains exposed to the air after boiling, fermentation 

 will follow. This shows that the yeast gets into the solution 

 from the air and that it is one cause of fermentation. 



If a thin layer of beef broth or similar substance which is 

 stiffened with gelatin or agar is poured into shallow dishes, 



Fi<;.44. 



i bv 



Yeast plants were placed 

 in the sugar solution within 

 the jar. Bubbles of gas 

 formed by fermentation rise 

 through the solution, and 

 large numbers of yeast 

 plants settle as sediment 

 at the bottom 



