274 BACTERIA, YEASTS, AND MOLDS 



a mass of mold spores, preferably the common blue mold. Put the 

 apple aside in a jar and examine carefully until it decays. Note that 

 the decay begins rather quickly and starts at the point of the cut 

 where the spores were inoculated. 



20. Molds in Decaying Fruit. Obtain some thoroughly decayed 

 fruit, several different kinds if possible. Remove a bit of the 

 decayed material with a knife blade and plant it in gelatin in a 

 petri dish. Replace the cover and set aside until the molds begin to 

 germinate. Allow them to grow for a number of days and then study 

 with a microscope, determining if possible the method of forming 

 spores and comparing them with the figures of molds given in the 

 previous pages. Is the species found similar to any described in 

 this work? 



EXPERIMENTS ILLUSTRATING YEASTS 



21. Fermentation of Molasses. Into a common test tube or any 

 glass vial place a solution made by mixing one spoonful of molasses 

 with ten spoonfuls of water. Rub up a little compressed yeast in 

 water and put a few drops into the tube of molasses water. Set aside 

 in a warm place and let it stand for about twenty-four hours. At the 

 end of this time a vigorous fermentation will be seen. The liquid will 

 have become somewhat cloudy, numerous bubbles can be seen rising 

 through it, a froth forms on top, and a mass of sediment soon 

 collects at the bottom. The bubbles are the carbon dioxide which 

 is escaping into the air, the sediment at the bottom is the growing 

 mass of yeast, and the alcohol, which looks just like water, is 

 dissolved in the liquid and is of course invisible. 



22. Proof of the Nature of the Gas. Prepare two tubes, as shown 

 in Fig. 31. In tube a place molasses and water inoculated with sev- 

 eral drops of yeast, as in the last experiment. Put the cork in place 

 and insert the other end of the tube into a second tube underneath 

 the surface of some clear limewater, as shown in Fig. 31. Set aside 

 in a warm place until vigorous fermentation occurs. Note the bubbles 

 of gas that arise from the fermenting tube and bubble up through 

 the limewater. The limewater soon becomes turbid, showing that the 

 gas contains carbon dioxide (CO 2 ). 



