228 THE POPULAR SCIENCE MONTHLY. 



a piece of iron or steel near to the magnet, and instantly it leaps 

 toward it, confirming our conviction. 



That a medium exists for the conveyance of such molecular mo- 

 tion, we are obliged to think ; but what it is, or how it is affected, 

 we cannot yet imagine. Thus much, however, is plain, that, as a me- 

 dium purely, it can be no source of motion, so that the mere fact of 

 distance between bodies can be no satisfactory explanation of attrac- 

 tion. 



Attraction, then, wherever it appears, I believe to be due to con- 

 tained motion in the particles of the bodies presenting it ; which 

 molecular motion is the equivalent of, and convertible into, the move- 

 ment of masses as wholes. 



Gravity, I venture to hold, is a force due to a distinct motion of 

 the ultimate parts of matter, which has not yet been formulated. 

 And, as the energy expended in sundering two united magnets reap- 

 pears in the increase of their attractive powers, or the acceleration of 

 such motion of their particles as constitutes magnetism, so the lifting 

 of a stone from the earth would imply that the force consumed in so 

 doing must take the form of a quickening in that motion of its parti- 

 cles which constitutes gravity. And we have seen how every succes- 

 sive act of obedience to both these forces of aggregation makes them 

 weaker and weaker, as it reasonably shoiild. 



Therefore, I conclude that the energy of a stone at a height is not 

 potential, but actual; that its value as a source of work, at any time, 

 is represented, by the swifter motion of its molecules as compared 

 with those of a stone on the earth's surface; and that, as a stone falls, 

 its internal motion takes the phase of mass motion. 



This theory would lead us to suppose that the onward momen- 

 tum of the planets, as they turn before the sun, is the expression of an 

 equal internal gravitive motion of their ultimate parts, which exactly 

 balances these mighty celestial revolutions. 



The next case of " potential " energy to be inquired into is that of 

 a coiled spring, wdiich, in unwinding, may yield the force it took to 

 coil it. 



This case may be intelligibly explained on the same principle of 

 the constancy of motion. The particles of a spring may be assumed 

 to have a definite plane of molecular motion, which motion we shall 

 presume is that due to temperature. In coiling, these planes are 

 changed, and energy is required to do it ; just as when a gyroscope- 

 top is rapidly revolving in a certain fixed plane, it may, by a measu- 

 rable effort, be made to revolve in a conical curve, with one end of its 

 axis stationary, and the other describing a circle in space. And, as 

 the gyroscope strongly tends to move in a uniform plane, and can do 

 work in resuming such a plane, so I assume that the particles in a 

 coiled spring have two similar motions, one of which is at any time 

 available for tangible work when the spring is freed. 



