BUILDIKG MATEEIAL 61 



and the result is the production of a form of energy which 

 we call heat. This heat in turn acts upon water inclosed 

 in a boiler, and changes it to steam. Steam has the 

 power of expansion; that is to say, the heat energy has 

 been transformed into expansive power, another form 

 of energy. If the steam is admitted to the cylinder of the 

 engine and allowed to expand against the piston, its form 

 of energy (expansion) is transformed into still another 

 form, thrusting movement. Further, if we attach the 

 piston to a wheel this form of energy is changed once 

 more into rotary movement, and that in turn may be 

 applied ,to a saw, and thus the engine may be made to 

 saw wood. In all these steps the process has simply con- 

 sisted in transforming the energy derived from the burning 

 fuel into first one form and then another form of energy. 

 In other words, the steam engine is simply an apparatus 

 for changing the form of energy derived from combustion. 

 We may summarize what this example tells us as follows : 



First. In the coal we see that energy may exist with- 

 out manifesting itself. Coal, in other words, has the 

 power to burn and produce heat even when it is not bin-n- 

 ing. This power becomes manifest as soon as the coal 

 actually begins to burn or oxidize. We express this fact 

 by saying that energy is of two forms, potential energy 

 and kinetic energy. Energy is potential when, owing to 

 its position or composition, a body possesses the power to 

 do work without manifesting that power. When the 

 energy manifests itself in the actual doing of work we have 

 an example of kinetic energy. 



Second. Potential energy may be transformed into 

 kinetic energy, and kinetic energy into potential energy. 

 Thus the potential energy of the coal to produce heat is 



