1 88 THE VITAL PROCESSES 



bodies fall. As they fall, the stored-up (potential) energy 

 becomes kinetic and can be made to do work. 



2. Storing of Energy through Elasticity. Energy is 

 stored also by doing work in opposition to elasticity, as in 

 bending a bow or in winding a clock spring. The bending, 

 twisting, stretching, or compressing of elastic substances 

 puts them in a condition of strain which causes them to 

 exert a pressure (called elastic force) that tends to restore 

 them to their former condition. Energy stored by this 

 means becomes active as the distorted or compressed sub- 

 stance returns to its former shape or volume. 



These simple methods of storing energy will serve to 

 illustrate the general principles upon which such storage 

 depends: 



1. To store energy, energy must be expended, or work 

 done. 



2. The work must be against some force, such as gravity 

 or elasticity, which can undo the work, i.e., bring about an 

 effect opposite to that of the work. 



3. The stored energy becomes active (kinetic) as the 

 force through which the energy was stored undoes the 

 work, or puts the substance upon which the work was 

 done into its former condition (gravity causing bodies to 

 fall, etc.). 



These principles are further illustrated by the 

 Storing of Energy through Chemical Means. A good 

 example of storing energy by chemical means is that of 

 decomposing water with electricity. If a current of elec- 

 tricity is passed through acidulated water in a suitable 

 apparatus (Fig. 82), the water separates into its component 

 gases, oxygen and hydrogen. These gases now have 

 power (energy) which they did not possess before they 

 were separated. The hydrogen will burn in the oxygen, 



