186 



ELEMENTS OF GENERAL SCIENCE 



the wires by the currents of electricity. Notice that in all 

 these cases of lost energy the energy is not destroyed. It 

 changes into some form in which it is not useful to us (usually 

 into the form of heat which may warm the surrounding air), 

 but it has not gone out of existence. 



There is no way known by which we can destroy energy. 

 There is no way known by which we can create energy. 

 211. The source of energy. Since there is no way in which 

 we can create energy, where does the energy come from which 

 we are all the time using and allowing to escape from us ? 

 Perhaps this question may be best answered by taking 

 some energy-using machine as an ex- 

 ample and tracing back the energy as 

 far as we can toward its source. 



We will take a trolley car for ex- 

 ample. It is operated by electricity. 

 The current is gener- 

 ated by the dynamo, 

 which is run by a steam 

 engine. The engine 

 secures its energy from 

 coal. Plainly the coal 

 contains energy, and 

 we must find out where 

 this energy came from. 

 Those who have made it their business to examine into the 

 origin of coal tell us that it is formed of the remains of plants 

 of former ages. The plants grew and their dead bodies accu- 

 mulated in great abundance in some places, somewhat as plant 

 materials are accumulating now in our peat swamps. These 

 swamps were buried by sediments which later became rocks. 

 The plant material was buried far enough to be affected 

 by the heat in the interior of the earth, and it was under 

 great pressure from the rocks lying over it. All these things 

 caused it to condense into a solid, stone-like mass which does 



FIG. 92. The remains of ancient plants 



These fossils are found in rocks associated with 

 the coal beds 



