Energ'y 15 



differences in essence. Animals familiar with 

 heat and light are unable to produce fire, 

 and man was familiar with electricity before 

 he could produce magnetism. 



Except in theory, energy does not exist 

 separate from matter. The amount of energy 

 existing in a body may be increased or de- 

 creased, but we cannot, by available means, 

 take away from a body the total energy 

 existing within it. That part of the energy 

 in a body which may be used to perform 

 work, is called available energy. How much 

 energy in any particular body is available to 

 perform work is not known in any case, but 

 by experience we have learned that certain 

 amounts are available and how we can 

 determine those amounts. Thus, in an ele- 

 vated weight, the available energy is the 

 amount of work it can perform in falling to 

 the surface of the earth. In the case of a 

 man or a horse, the available energy is that 

 which can be applied to perform work before 

 exhaustion or weariness puts an end to the 

 expenditure. Different inorganic bodies of 



