28 Prof. Oliver Lodge on 



That the available portion of heat should bear to the whole 

 heat the same ratio which the available drop of temperature 

 bears to the absolute temperature, is essentially but a defini- 

 tion of temperature ; it is an assertion that temperature is 

 best measured as proportional to heat, or that the zero of 

 temperature may conveniently be taken to correspond with 

 the zero of heat. 



An essential part of the second law of Thermodynamics 

 therefore proceeds to state itself in a very general and purely 

 commonsense form, thus ;--- 



The portion of energy which a body can automatically part 

 with is alone available for doing work; and only that portion 

 which is parted with reversibly is actually utilised. 



There is no need to mention " heat ; " it is equally true of 

 every form of energy. When a cell has run down, or a 

 reservoir leaked itself empty, as empty as it wants to, any 

 further energy it may have is useless ; and any portion which 

 flowed out in an uncontrolled or irreversible manner will have 

 been wasted. 



There is something specific to be said about each form of 

 energy in order to apply the above statement definitely to 

 that form ; and in the case of heat the supplementary state- 

 ment needed is that heat will not automatically leave a body 

 for others at higher temperature : if it goes to a hotter body 

 it must be carried by matter, or electricity, or something else, 

 so that it is not a pure and simple flow of heat. In other 

 words, heat will not flow "uphill" by pure conduction ; and 

 conduction is the only mode of automatic conveyance of heat 

 as heat*. "Water, air, or electricity can flow "uphill"" for a 

 time and can do work at the same time, by reason of their 

 property of inertia. Heat cannot : it has no inertia. None of 

 the uphill processes can go on continually or cyclically f. 

 The law of dissipation of energy states itself thus : — 

 If a body has any portion of energy in such condition that 

 it is able irreversibly to leave the body, that portion usually 

 does leave, sooner or later. This is only a rewording of the 

 customary statement that the potential energy of a system 

 tends towards a minimum ; or, really, except that circum- 

 stances often delay the consummation unpractically long, 

 towards zero. The universe will be stagnant, though by no 

 means stationary, when its potential energy is nothing. There 



* Radiation is not heat, but another quite distinct form of energy. 

 The phrase " radiant heat " is responsible for immense confusion. 



t Work and heat may be coaxed out of a body below the temperature 

 of surrounding objects, as, for instance, by letting air escape from a high- 

 pressure reservoir ; but such a process is not cyclical until the air is put 

 back again, and by that operation the heat has to be put back too. Heat 

 by itself cannot flow uphill at all. 



