MATTER AND ENERGY 59 



raging forest fire are no compensation to us for the valuable 

 timber destroyed. The sum total of matter has not been 

 changed, but the amount of matter which man can use has 

 been greatly lessened. 



The principle just stated embodies one of the fundamental 

 laws of science, called the law of the conservation of matter. 



A similar law holds for energy as well. We can transform 

 electric energy into the motion of trolley cars, or we can make 

 use of the energy of streams to turn the wheels of our mills, 

 but in all these cases we are transforming, not creating, 

 energy. 



When a ball is fired from a rifle, most of the energy of the 

 gunpowder is utilized in motion, but some is dissipated in pro- 

 ducing a flash and a report, and in heat. The energy of the 

 gunpowder has been scattered, but the sum of the various 

 forms of energy is equal to the energy originally stored away 

 in the powder. The better the gun is, the less will be the 

 energy dissipated in smoke and heat and noise. 



