ad partly
grasped the meaning of the doctrine of conservation. When, late in
the fifties, that marvellous young Scotchman, James Clerk-Maxwell,
formulating in other words an idea of Faraday's, expressed his belief
that electricity and magnetism are but manifestations of various
conditions of stress and motion in the ethereal medium (electricity a
displacement of strain, magnetism a whirl in the ether), the idea met
with no immediate popularity. And even less cordial was the reception
given the same thinker's theory, put forward in 1863, that the ethereal
undulations producing the phenomenon we call light differ in no respect
except in their wave-length from the pulsations of electro-magnetism.

At about the same time Helmholtz formulated a somewhat similar
electro-magnetic theory of light; but even the weight of this combined
authority could not give the doctrine vogue until very recently, when
the experiments of Heinrich Hertz, the pupil of Helmholtz, have shown
that a condition of electrical strain may be developed into a wave
system by recurrent interruptions of the electric state in the
generator, and that such waves travel through the ether with the
rapidity of light. Since then the electro-magnetic theory of light has
been enthusiastically referred to as the greatest generalization of
the century; but the sober thinker must see that it is really only
what Hertz himself called it--one pier beneath the great arch of
conservation. It is an interesting detail of the architecture, but the
part cannot equal the size of the whole.

More than that, this particular pier is as yet by no means a very firm
one. It has, indeed, been demonstrated that waves of electro-magnetism
pass through space with the speed of light, but as yet no one has
developed electric waves even remotely approximating the shortness of
the visual rays. The most that can positively be asserted, therefore,
is that all the known forms of radiant energy-heat, light,
electro-magnetism--travel through space at the same rate of speed, and
consist of traverse vibrations--"lateral quivers," as Fresnel said of
light--known to differ in length, and not positively known to differ
otherwise. It has, indeed, been suggested that the newest form of
radiant energy, the famous X-ray of Professor Roentgen's discovery, is
a longitudinal vibration, but this is a mere surmise. Be that as it
may, there is no one now to question that all forms of radiant energy,
whatever th