THE AVAILABILITY OF ENERGY. 245 



which is so dependent upon another for its existence, as it were, will, 

 in some way or other, mold itself to suit the form and convenience of 

 the other. It thus becomes a question of primary importance as to 

 how energy is connected with and dependent upon matter, and as to 

 how its form and availability are influenced by its connections. 



That the same amounts of different kinds of energy, or even of dif- 

 ferent forms of the same kind, are vastly different in their effects, is a 

 thing of every-day experience. A mass raised above the earth's sur- 

 face possesses in consequence a certain amount of energy, in virtue of 

 which it can do work ; but if it be allowed to fall to the ground under 

 the influence of the earth's attraction, then, although the amount of 

 energy on the earth is neither increased nor diminished, it may be abso- 

 lutely impossible to gain any work from it. In this instance energy 

 of position is at first transformed into energy of motion, but the mo- 

 ment the mass strikes the ground all motion, as far as we are able to 

 discover, is gone ; but we know that the motion of the mass as a whole 

 has only been transformed into motion of the particles among each 

 other, and of the particles of the body on which the mass impinges, 

 constituting the phenomenon of matter commonly called heat. The 

 first two states are available as sources of mechanical energy, but in 

 the third state the energy is scattered into an infinite number of in- 

 finitely small energies, as it were, and, in the case of small masses at 

 least, is lost forever as far as doing work is concerned ; and so much 

 energy is let down from a high to a low class, and the whole energy 

 of the universe is rendered less available by a corresponding amount. 



But not only do equal quantities of energies of different kinds 

 manifest themselves so very differently, but the same is true as to 

 equal amounts of the same kind. A boxer may receive a blow from 

 his antagonist which may stagger him, and perhaps throw him off his 

 balance, but yet do him no permanent injury ; while a rifle-ball with 

 half the energy, though it might not disturb his equilibrium, would in 

 all probability inflict instant death. Any number of examples of this 

 kind might be given to show to what an extent the form which a given 

 amount of energy assumes and the constancy of its effects are depen- 

 dent upon the matter with which it is associated. 



Now, the transf ormability of energy is a measure of its availability, 

 and, in fact, energy is of use to us only and solely because it may be 

 and is constantly transformed (consequently, whatever terms deter- 

 mine its transformability, the same hold good for the determination 

 of its availability). 



Since all forms of energy are essentially kinetic or potential, or the 

 energy of heat-motion, it is sufficient if we examine the laws with re- 

 gard to these. 



A body is said to possess potential energy when in virtue of its 

 position it can do work. A raised weight possesses potential energy, 

 which, by a simple contrivance, may be converted into work ; a bent 



