144 FIRST YEAR COURSE IN GENERAL SCIENCE 



Stoves and furnaces have an opening below the firebox. 

 Air entering there mixes with the hot fuel and helps to pro- 

 duce rapid combustion. If the opening is closed, the current 

 of air does not pass through the fuel, combustion is less rapid, 

 and the fire is "checked." 



156. Extinguishing Fires. Two conditions are necessary 

 to produce a fire: a temperature at or above the ignition 

 point, and the presence of oxygen. There are consequently 



TST 



J_^_ _ LOOSE 



STOPPER 



SODIUM 



CARBONATE 



FIG. 73. A CHEMICAL FIRE EXTINGUISHER 



When the cylinder is turned "upside down," according to directions, 

 acid runs out of a loosely stoppered bottle inside, into a strong solution of 

 sodium carbonate or sal soda. This causes a large quantity of carbon dioxide 

 to be made in a small space. Under its own pressure, the gas is pushed 

 out of the small hose attached, bringing some liquid with it. 



two methods of extinguishing fires: first, by cooling the 

 burning material below its ignition point; and second, by 

 preventing air from coming in contact with the burning ma- 

 terial. In the first method water is used. The second 

 method may be successful when the fire is confined to a small 

 area; for instance, blankets or a suffocating gas may be 



