the Foundations of Dynamics. 25 



utilised in the potential form ; as in a hydraulic-press or 

 steam-boiler. 



To say that all potential energy will turn out to be really 

 kinetic may be true enough, but it is not at present a specially 

 helpful truth, for even in cases (such as compressed air) when 

 we actually knoiv it to be kinetic, we are bound for all 

 practical purposes to treat it as potential, since the avail- 

 able working-power of a spring, or of compressed air, of 

 gunpowder, of Ley den jars, and of many other things, depends 

 on their potential energy alone; and such other energy as 

 they may possess is conveniently ignored, as not concerned in 

 any practical transformations or activities which we can bring 

 about : it remains as an untransferable, and therefore inactive 

 or useless residuum. 



This is the real meaning of available energy, it is the por- 

 tion concerned in transferences, the portion which can be 

 transformed, the portion which is able to transfer itself. 

 Potential energy, as commonly spoken of, is always of this 

 character. An unwound watch-spring has lost its potential 

 energy, it retains a quantity of untransferable heat-energy. 

 A hot body cooling is in the same case ; so long as it is hot, 

 some of its heat-energy is transferable ; and the transferable 

 portion may well be regarded as and styled potential. 



Available or potential heat-energy readily transfers itself 

 to space or to other bodies, just as compressed air readily 

 leaks out of its reservoir ; and in so doing becomes less 

 potential*. Even if it is all received by other bodies in the 

 room, the potential energy of the contents of the room is to 

 the extent of the leakage diminished. 



To say that heat-energy is constant in quantity is the same 

 as saying that the molecules of an escaping gas need not part 

 with their energy but may individually retain it. In the 

 abstract either proposition is true, but just as it was only 

 the compression part of gas energy that was potential or 

 available for us, so also it is only the high-temperature 

 part of heat-energy that is similarly available. 



It is customary and correct to say that the available part 

 of heat- energy is converted into other forms when allowed 

 to be active and do work. A distinction is thus apparently 

 drawn between heat-engines, on the one hand, and water- 

 engines, compressed-air engines, or electric engines, on the 

 other ; because in them the water, the air, and the electricity 

 flow away undiminished in quantity and only at a lower level, 

 pressure, or potential. 



* It is tempting thus to use the adjective " potential " in the sense of 

 available. I do not at present wish to justify this secondary usage of the 

 word, hut I let it stand as a suggestive and harmless eccentricity. 



