DUSTS, MOLDS, AND BACTERIA OF THE AIR 81 



These dust-like particles are called spores. The spores are easily 

 blown about by the wind, and any one of them, alighting upon 

 a piece of bread or in any other favorable place, is able to grow 

 into a new mold plant. Can you imagine how the mold got 

 started on the piece of bread ? 



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 found 

 to be 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 pres- 

 ent. A scum collects at the surface 

 of the liquid and probably at the bot- 

 tom also (fig. 42). The microscope 

 shows that this scum is made up of 

 minute living plants. These are the 

 yeast plants. 



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 may be taken to 

 prove that the yeast gets into the 

 solution from the air and that it is 

 the cause of fermentation. 



Another experiment may be per- 

 formed. Take several shallow dishes and into each pour a 

 thin layer of beef broth or similar substance. The broth or 

 other nutrient substance may be stiffened with gelatin or agar. 

 This must have been heated to destroy any living things that 

 may have been in it. One of these dishes should be uncovered 



FIG. 42. Fermentation by 

 yeasts 



In the sugar solution within 

 the jar, yeast plants were 

 placed. Bubbles of gas 

 formed by fermentation rise 

 through the solution, and 

 large numbers of yeast 

 plants settle as sediment 

 at the bottom 



