Several solid bodies in which no such structure as we have supposed can be found, seem to possess a mechanical property of this kind*, and it seems probable that the same substances <s>when<\s> if dielectrics may possess the analogous electrical property, and if magnetic, may have corresponding properties relating to the acquisition retention, and loss of magnetic polarity. (15) It appears therefore that certain phenomena in electricity and magnetism lead to the same conclusion as those of optics namely that there is an aetherial medium pervading all bodies and modified only in degree <s>altered modified<\s> by their presence; that the parts of this medium are capable of being set in motion by electric currents and magnets; that this motion is communicated from one part of the medium to another by forces arising from the connexions of those parts; that under the action of these forces there is a certain yielding, depending on the elasticity of these connexions and that therefore energy in two different forms may exist in the medium, the one form being the actual energy of motion of its parts, and the other being the potential energy stored up in the connexions, in virtue of their elasticity. (16) Thus then we are led to the conception of a complicated mechanism, capable of a vast variety of motion, but at the same time so connected that the motion of one part depends, according to definite relations, on the motion of other parts, these motions being communicated by forces arising from the relative displacement of the connected parts, in virtue of their elasticity. Such a mechanism must be subject to the general laws of Dynamics, and we ought to be able to work out all the consequences of its motion, provided we know the form of the relation between the motions of the parts. *As for instance the composition of glue treacle &c of which small plastic figures are made, which after being distorted gradually recover their shape.
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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