56 



HABIT AND INTELLIGENCE. 



[chap. 



desires. In a broader scientific sense/motive power is to 

 be defined as energy that is capahle of transformation into 

 some different form of energy ; ^ and in this sense, energy- 

 is not always motive power. Potential energy, as we have 

 seen, is capable of transformation into actual, and the 

 actual energies of motion and electricity are capable of 

 transformation into heat. But the converse is not neces- 

 sarily true. All other forms of energy are capable of 

 transformation into heat, but heat is not always capable of 

 transformation back again into other forms of energy. The 

 heat in the furnace of a steam-engine is capable of being 

 used as motive power; not because the furnace is hot, 

 but because it is hotter than the surrounding air. But if 

 the surrounding air were as hot as the furnace, and no 

 supply of cold air or cold water could be brought in, no 

 part of the heat could be converted into motive power. 

 Heat is Heat is capahU of becoming motive poxoer only in the act of 

 motrvr*^^^ attaining to equilibrium: and consequently, no motive jpoiver 

 is due to heat in perfect equilibrium. 



Suppose the case of two bodies of unequal tempera- 

 tures, which for the sake of a practical illustration let us 

 call the boiler and the condenser of a steam-engine ; and 

 suppose that the heat of the boiler is flowing towards the 

 condenser, so as to establish equilibrium. If the con- 

 denser is perfectly cold, all the heat of the boiler is theo- 

 retically capable of being used as motive power ; but if 

 the condenser, though colder than the boiler, is not per- 

 fectly cold, only a portion of the heat of the boiler can be 

 so used. This last condition is the one that always occurs. 

 Perfect cold is never found, nor can it be obtained by 

 any known process : perfect cold — the absolute zero of 

 temperature, or the total absence of heat — would be 

 492° Fahrenheit below the temperature of freezing water. 

 Consequently, under the most favourable circumstances 



power. 



Illustra- 

 tion from 

 a steam- 

 eugine. 



1 To be rigidly accurate, we should say that motive power is energy that 

 is capable of transformation into some different form of energy, other than 

 radiance. Heat is motive power in being transformed into motion, but it 

 would be absurd to say that heat is motive power in being transformed 

 into radiance — that is to say, in being radiated away. 



