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so long as the weight remains suspended, the energy which 
that weight has acquired by the employment of the string 
and my arm is potential only. But whenever that weight is 
allowed to fall, supposing it is so circumstanced as to exert 
its whole energy upon the nail, it will produce exactly the 
same effect in driving the nail into the wood which would be 
produced by the hammer when struck upon the nail by my 
hand. That is a simple illustration to show that energy may 
be so acquired as to remain in a potential form, capable of 
being applied at any subsequent time ; but whenever it is so 
applied, the potential becomes actual energy, exactly the same 
in amount as that energy which was employed in producing it. 
In order to form a clear conception on this subject, it is 
necessary to say something about the diffusion of energy. 
What becomes of energy when it is employed ? That is a 
point which it is not very difficult to answer. It is not de- 
stroyed. Take an example : when a stone falls to the ground, 
it strikes the ground with a certain amount of energy. Now 
what becomes of that energy ? It is occupied in producing 
disturbance — molecular motion — at that point of the earth's 
surface on which it falls, and that molecular disturbance pro- 
duces a minute degree of heat, and the energy is expended 
in producing that heat, which, however small it may be, 
becomes diffused by radiation, and is lost sight of. It is 
diffused, but not destroyed. That heat is the result of the 
application of dynamic energy, is evident by observing what 
happens when a rifle-ball strikes an iron target against which 
it is directed. What becomes of the energy of the ball ? It is 
expended in the production of heat ; for the ball, which was a 
solid mass when it reached the target, becomes fused, and is 
scattered in splashes as though it were liquid. 
It is a remarkable fact that, in the earlier days of scientific 
investigation, material forms were ascribed to all the various 
forms of physical energy. Light was supposed to consist of 
material particles, thrown off from a luminous source, and 
producing an impression upon the organs of vision. In the 
same way heat was supposed to be a material substance — 
caloric — which was in some way thrown off from the heated 
body. Electricity and magnetism were supposed to be fluids. 
In electricity there were supposed to be two fluids combined, 
and it was thought that the manifestation of electrical effects 
consisted in the separation of those two fluids. Magnetism 
also was supposed to consist of two fluids, which were sup- 
posed in the same way to co-exist in magnetic bodies, 
and it was thought that when they were disturbed or 
separated the effecs of magnetism were manifested. As 
