62 PRACTICAL LESSONS IN SCIENCE. 



ing its fall, potential energy is changed into kinetic energy. There 

 is no loss, but simply a change in the character of the energy. 

 The same thing is true in a swinging pendulum. The mechanical 

 energy of the hammer is not lost when the hammer stops. It 

 has been changed into molecular energy or heat. Whenever 

 energy disappears it has been changed into another form. 



One of the most interesting discoveries of modern science is 

 that all forms of energy are so related to one another that 

 energy of any kind may be changed into energy of any other 

 kind. That a definite quantity of mechanical work when trans- 

 formed without waste will yield a definite quantity of heat ; and 

 that this heat, if there were no waste, could perform the original 

 mechanical work. That when one form of energy disappears an 

 exact equivalent of another form takes its place, so that the sum 

 total of energy is unchanged. This principle is known as the law 

 of conservation of energy. 



