ENERGY IN GENERAL. 95 



ajain, that it is connected only with bodies im- 

 permeable to heat. 



This is Carnot's principle in one of its concrete 

 forms. 



A machine which realizes this condition, that the 

 agent (steam, alcohol, ether) is in relation, at all 

 phases of its function, with boidies which can neither ' 

 take heat from it nor give heat to it, is a reversible 

 machine. Such a machine is perfect. The fraction 

 of heat that it transforms into motion is constant; it 

 is a maximum ; it is independent of the motor, of its 

 organs, of the agent : it accurately expresses the 

 transformability of the heat agent into a mechanical 

 agent under the given conditions. 



The Degradation and Restoration of Energy. — The 

 fraction not utilized, that which is carried to the 

 condenser at a lower temperature, is degraded. It 

 can only be used by a new agent, in a new machine 

 in which the boiler has exactly the same temperature 

 as the condenser in the first machine, and the new 

 condenser has a lower temperature, and so on. The 

 proportion of utilizable energy thus goes on diminish- 

 ing. Its utilization requires conditions more and 

 more difficult to realize. The thermal energy loses its 

 potential and its value, and is further and further de- 

 graded as its temperature approaches that of the 

 surrounding medium. 



The degraded energy, theoretically, has kept its 

 equivalent value but, practically, it is incapable of 

 conversion. However, it is shown in physics that it 

 can be raised and re-established at its initial level. 

 But for that purpose another energy must be utilized 

 and degraded for its benefit. 



TJie End of the Universe. — What we have just 



