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motion only, they must all consist in motion only, because 
they are many of them really convertible one into the other. 
Those motions in which the various forms of physical energy 
are exerted, are all of a vibratory character — that which is 
the case with regard to sound is also the case with regard to 
light. It has long been received as an axiom that light 
consists in vibrations, and from the strong analogy existing 
between light and heat — from the circumstance of their both 
being subject to the same laws of reflection and refraction, 
and even to what is called polarization, the different wave- 
lengths of heat being separated in the same way as the 
different prismatic colours of light — there can be no doubt 
that they are essentially of the same nature. But in order 
to meet the phenomenon of polarization, it is necessary to 
suppose that the vibrations of light and heat are transverse, 
while the vibrations of sound are longitudinal. There can 
be little doubt that electricity is motion also, and motion only, 
and that there is no such thing as an electric fluid. I am 
not aware that it has yet been determined what form 
electric motion assumes, but there are strong reasons for 
supposing it may be helical, like the form of a corkscrew. 
If motion of that kind takes place, that which would be right- 
handed motion if viewed in one direction, becomes left- 
handed motion if looked at in the other, and that would 
account for the phenomena of positive and negative in elec- 
tricity. Then there are strong reasons for supposing that 
magnetic energy consists of similar waves of motion, one of 
the strongest of those reasons being found in a fact, observed 
many years ago, that if a piece of copper be placed between 
the poles of a powerful magnet, and made to rotate, the 
rotation will continue so long as the magnet remains inert, 
but when a current of electricity excites the magnet, the 
rotating body is immediately arrested ; and if it be forcibly 
rotated, a considerable amount of heat is produced. If a 
piece of fusible metal be introduced into a copper cylinder 
between the poles, after a short period of rotation, it may be 
poured out in a melted state. It may here be remarked, that 
in this case there is a constant relation between the amount 
of energy expended in rotation and the amount of heat 
produced. 
I might give you many examples of the transformation 
of various forms of physical energy into one another — of 
dynamical energy into light and heat for instance, but I will 
only take one well-known fact as an illustration. A black- 
smith who wishes to light his fire, will often take a cold nail, 
and placing it upon his anvil, he will strike a succession of 
