NATURE-STUDY LESSONS. 137 



5. To discover the cause of the spongy appearance of 

 the loaf, perform the following experiments : — 



{a) Place a small quantity of compressed yeast, or 

 of home-made yeast, in a little flour mixed 

 with tepid water so as to form a paste. Keep 

 the whole in a moderately- warm place and 

 observe the results. Describe what takes 

 place. 



(b) Put some yeast in a bottle containing a spoonful 



of syrup and ten or twelve spoonfuls of tepid 

 water and observe the results. 



(c) Masticate some grains of wheat or some bread 



a long time to note whether it tastes sweeter 

 as a result of the chewing and mixing with 

 saliva. 



(d) Taste some wheat or other grain that is just 



beginning to germinate. 



(e) With a compound microscope, observe the yeast 



plants obtained from some of the fluid in 

 experiment {b). 



The white part of the grain of wheat consists largely of starch. The 

 starch is converted into sugar. 



Note. — With proper apparatus, the advanced pupils can discover that 

 the sugar in experiments (a) and (b) is converted into carbonic acid gas and 

 alcohol by the little yeast plants. 



6. Account for the swelling or rising of the mixture 

 in {a). 



7. What has become of the carbonic acid gas and 

 alcohol before the bread comes out of the oven ? 



Note. — Yeast plants live upon or grow upon the sugary substances and 

 give off carbonic acid gas and alcohol as waste products. The process is 

 called fermentation. 



