ENERGY IN GENERAL. 93 



iron on an anvil may bring it to a red heat. A shell 

 which passes through an armour plate is heated, and 

 melts and volatilizes the metal all round the hole it 

 has made. By utilizing mechanical action under the 

 form of friction all energy can be converted into heat. 



The inverse transformation of heat into work, on 

 the contrary, cannot be complete. The best motor 

 that we can think of, and a fortiori the best we can, 

 realize, can only transform a third or a fourth of the 

 heat with which it is supplied. 



This is an extremely important fact. It is of in- 

 calculable importance to natural philosophy, and may 

 be ranked among the greatest discoveries. 



Higher and Degraded Forms of Energy. — Of these 

 we may give an account by distinguishing among the 

 forms of universal energy higher forms, and lozver or 

 degraded forms. Here we have the principle of the 

 degradation of energy on its trial, and it may be 

 regarded as a particular aspect of the second principle 

 of energetics, or Carnot's principle. Mechanical 

 energy is a higher form. Thermal energy is a lower 

 form, a degraded form, and one which has degrees in 

 its degradation. Higher energy, in general, may be 

 completely converted into lower energy; for example, 

 work into heat : the slope is easy to descend, but it is 

 diflficult to retrace our steps ; lower energy can be 

 only partially transformed into higher energy, and the 

 fraction thus utilizable depends upon certain con- 

 ditions on which Carnot's principle has thrown con- 

 siderable light. 



Thus, although in theory the thermal energy of a 

 body may have its equivalent in mechanical energy, 

 the complete transformation is only realizable from 

 the latter to the former, and not from the former to 



