70 A YEAR IN SCIENCE 



dough in bread making. As a result of the action of 

 yeast, the sugar in. bread is broken down into carbon 

 dioxide and small quantities of alcohol. The firm 

 dough then swells because the gas imprisoned within 

 it forms air spaces. In baking the dough, the spaces 

 are enlarged as the gas escapes, and at the same time 

 the alcohol is evaporated. 



Yeast is not always used to produce this carbon 

 dioxide. Sometimes, as in soda biscuits, baking soda 

 (bicarbonate of soda) is used and to it is added cream 

 of tartar or sour milk. In either case, by the action 

 of the cream of tartar or the sour milk upon the soda, 

 carbon dioxide is given off. If baking powder is used, 

 as in baking powder biscuits, cakes, and many other 

 foods, the action of the substances within the baking 

 powder is such as to liberate carbon dioxide. This 

 escapes through the dough and makes it light and 

 porous. 



Questions 



1. Chemically speaking, what is charcoal? 



2. How is charcoal made? 



3. To what various uses is it put? 



4. How is lampblack produced? 



5. Has lampblack a commercial value ? What is it ? 



6. What is coal? 



7. What are the successive stages in its formation ? 



8. Look up all the material you can on coal forma- 

 tion and write an essay on it. 



9. How is coke made from coal? 



