Chemical Change and Energy 345 



drive wheel of the automobile, and when the piston is 

 pushed down it gives the automobile a push forward. 

 If it were not for the expansion of a gas in the cylinder, 

 this gas being confined to a small space, the piston would 

 not be pushed down. 



An explosion is simply the sudden pushing of a con- 

 fined gas expanding on its way to freedom. The gasoline 

 vapor and air were the confined gas. Their chemical 

 combining made them expand ; by pushing the piston 

 out of its way the newly formed gas suddenly freed itself. 

 This was an explosion, and it gave the automobile one 

 forward push. But the automobile engine is so ar- 

 ranged that the piston goes up into the cylinder again, 

 and is pulled down again, drawing a spray of gasoline 

 and air into the cylinder after it. Then it goes up a 

 second time, an electric spark explodes the gasoline, 

 the piston is forced down violently once more, and so it 

 goes on. There are several cylinders, of course, and the 

 explosions take place within them one after the other 

 so as to keep the automobile going steadily. 



How a gun shoots. Pulling a trigger makes a gun 

 shoot by causing an explosion. There is a spring on 

 the hammer of a gun. This drives the hammer down 

 suddenly when you release the spring by pulling the 

 trigger. The hammer jars the chemicals in the cap and 

 causes them to explode. The heat and flame then cause 

 the oxygen in the gunpowder to combine with some of 

 the other elements in the powder to make a gas. The 

 gas requires more room than the powder and is further 

 expanded by the heat released by the chemical change. 

 The expanding gas frees itself by pushing the bullet 



