58 GENERAL SCIENCE 



and it keeps the oxygen of the air away, so the fire dies 

 because it cannot get sufficient oxygen to make it burn. 

 The liquid also helps to lower the temperature of the 

 burning material below the kindling point, thus giving 

 the extinguisher or "chemical wagon" a double effect. 

 It is only used, however, when the fire is small or 

 is just starting. The way water extinguishes fire is 

 by cooling the burning material below the kindling 

 temperature, and the steam that is formed keeps the 

 air away. 



The carbon dioxide which comes from the breath of 

 all animals and from fires used for heating, lighting, and 

 power production would soon make the air unfit for us 

 to breathe if it were not for a natural provision for the 

 consumption of this gas. Nearly all plants have the 

 power to make starch out of water and carbon dioxide, 

 when they grow in the presence of sunlight. This starch 

 is used by the plant to make heat and wood fiber or 

 the body of the plant. When the plants make starch 

 they give off pure oxygen. So the plants use what the 

 animals give off as waste and the animals use what the 

 plants give off as waste. 



QUESTIONS AND EXERCISES 



1. Examine some hard and soft coal and charcoal. What differ- 

 ences do you find? Of what are they chiefly composed? 



2. Name all of the things produced when carbon is oxidized. 

 Which one is most useful? 



3. What effect has carbon dioxide upon fire? Why? 



4. What effect has water upon fire? Why? 



5. What commercial uses are made of carbon dioxide? 



6. What causes bread dough to rise? 



