HOW MATTER CHANGES 145 



used to cover the flames and keep out the air. If the clothing 

 of a person takes fire, the quickest and safest method of ex- 

 tinguishing the flame is to wrap the body closely in a woolen 

 rug or blanket. This excludes the air. If the accident 

 occurs out of doors, the victim may be rolled in soft earth 

 or covered with earth. 



157. Chemical Engines. Chemical engines and hand 

 extinguishers furnish a spray of water and carbon dioxide, 

 a gas in which combustion will not take place. Carbon di- 

 oxide is a heavy gas and spreads over the fire in much the 

 same way that a wet blanket might, thus shutting out the 

 air and preventing combustion until the combustible sub- 

 stance has cooled. It would take a great quantity of water 

 to produce the same effect, and much injury to goods would 

 be caused by the water. 



If kerosene is spilled and burning, carbon dioxide is a better 

 extinguisher than a stream of water. As kerosene is lighter 

 than water, the water would go under the burning kerosene, 

 leaving it still exposed to the oxygen in the air. Carbon 

 dioxide, however, would cover the kerosene and extinguish 

 the flames. 



EXERCISES 



1. Arrange the following occurrences under the proper heading, as 

 Physical Change or Chemical Change: melting of glass, burning of paper, 

 magnetizing of iron, boiling of water, rusting of iron, dissolving of salt, 

 drying of clothes, explosion of a torpedo, formation of ice, tarnishing 

 of silver, lighting the gas, extinguishing a fire, evaporation of water. 



2. What is meant by the statement that the kindling point of coal 

 is higher than that of wood? 



3. Which gives a higher temperature, slow or rapid burning? 



4. A given weight of carbon unites with 2f times its own weight of 

 oxygen, in case of complete combustion, (a) How much oxygen will 

 15 g. of carbon require? (6) What weight of carbon dioxide is made 

 from this combination? 



5. 1,000 cu. cm. of carbon dioxide weigh 2 g. In Ex. 4, how many 

 thousand cubic centimeters of gas would be made? 



6. How does water act in putting out a fire? 



