562 Introduction to the Study of Science 



After the yeast mixture has stood for two days, you may test it by 

 tasting and smelling it. Record the results and compare them with 

 the results of similar tests made at the beginning. For the next step 

 you need a flask equipped as described on page 305. Pour a quantity 

 of the yeast mixture into the flask, and heat, collecting and condensing 

 the vapor in a clean bottle. Keep delivery tube cool with wet cloth. 

 When you have three or four ounces of condensed vapor, redistill it 

 until you have about one eighth of the first quantity of the distilled 

 liquor. This latter distillation should be done slowly and very carefully. 

 Examine the product by tasting and smelling. Try burning a small 

 quantity of it in a spoon. Does it produce a flame? What flame do 

 you know which this resembles ? 



The observed facts indicate several changes in the mixture. 

 The sweetened substance produced after a short time a gas 

 which is shown by the test to be carbon dioxid. The process 

 continued until the sugary substance became an alcoholic 

 mixture from which alcohol is distilled, as is shown by the 

 tests. The process is known as alcoholic fermentation. It 

 might be continued longer with different results, as later 

 experiments will show. 



In making bread the process of alcoholic fermentation is 

 started promptly by adding some sugar or molasses to the 

 yeast mixture. The result is that carbon dioxid and alcohol 

 are quickly produced and the dough is made porous and light. 

 The fermentation is stopped by baking, which also causes the 

 gas to expand and be driven out with the alcohol, producing a 

 bread that is light and digestible. 



It is now evident that the sponge or dough rises because of 

 the carbon dioxid gas which is produced in the fermentation. 

 As the dough imprisons the gas, it is filled with small cavities 

 full of gas and thus made to increase in volume, or rise. There 

 is, however, another question which demands an answer. 

 What is it that produces the carbon dioxid and alcohol? Are 

 these products the result of merely chemical changes in the 

 dough or of changes of a different nature? We have carried 

 the problem along to a new difficulty which must now be at- 



