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be considered as actual or existing, or as potential; that is 
to say, as placed under circumstances under which they may 
be called into action at any time. Now, as an illustration of 
this point, the term “ force ” has been indiscriminately applied 
to different things. I may remind you that it is a very common 
thing to speak of the force of a charge of gunpowder, and 
also of the force of the shot propelled by it. It is true that 
the gunpowder has force, but the shot has only energy. The 
shot has energy communicated to it by the force of gunpowder, 
and there is clearly a misapplication in the same term being 
applied to those two totally distinct things. Again, the force 
of gunpowder may be considered as potential when the gun- 
powder is stored away in a magazine. It is a force capable 
of being called into action at any moment ; and the moment 
the gunpowder is ignited the force becomes actual. So again, 
energy may be either actual or potential. It may be actually 
in existence, or it may only be capable of being called into 
existence. Thus a body, in falling to the ground, acquires 
actual energy. Energy may be defined to be the power of 
doing work — of effecting some change — the putting of matter 
in motion, and producing some effect. If a body be raised to 
a higher position than it originally occupied, and there main- 
tained, it possesses potential energy. It has the power of 
doing some work when it it is allowed to fall, and acquires 
energy in the act of falling. I would here remark that both 
matter and energy are alike indestructible : — it is not within 
human power to create or to destroy either matter or energy. 
The indestructibility of matter is a fact so well known that 
perhaps I need not say a word on the subject; but at the 
same time, in order to make it familiar to those who are not 
acquainted with scientific details, I may give one illustration. 
Suppose I take a bit of gun-cotton and ignite it, you see a 
flash, hear a noise, and the whole of it disappears. But the 
matter of that gun-cotton is not destroyed. It is converted, 
by chemical agency, into invisible gaseous matter ; but it has 
just as much an existence as it had before. And now I will 
explain, by an illustration, what is meant by the conservation 
of energy. I may take a hammer, and by the force of my 
arm exerted upon that hammer I may drive a nail into a 
block of wood. Or, instead of employing the force of my arm 
to urge the hammer upon the nail, I may use the same 
amount of muscular force in raising the hammer, by means 
of a string passing over a pulley. In the one case the energy 
that is exercised by my arm is actual, being applied in giving 
a blow with the hammer upon the nail. In the other case the 
force of my muscles is employed in raising the weight ; and 
