thought the lines of magnetic force had been rendered luminous
by the light rays; for, he announced his discovery in a paper entitled,
"Magnetization of Light and the Illumination of the Lines of Magnetic
Force." Indeed, this discovery was so far ahead of the times that it was
not until a later date that the results were more fully developed,
first by Kelvin, and subsequently by Clerk Maxwell. In 1865, two years
before Faraday's death, Maxwell proposed the electro-magnetic theory of
light, showing that light is an electro-magnetic disturbance. He pointed
out that optical as well as electro-magnetic phenomena required a medium
for their propagation, and that the properties of this medium appeared
to be the same for both. Moreover, the rate at which light travels is
known by actual measurement; the rate at which electro-magnetic waves
are propagated can be calculated from electrical measurements, and these
two velocities exactly agree. Faraday's original experiment as to the
relation between light and magnetism is thus again experimentally
demonstrated; and, Maxwell's electro-magnetic theory of light now
resting on experimental fact, optics becomes a branch of electricity. A
curious consequence was pointed out by Maxwell as a result of his
theory; namely, that a necessary relation exists between opacity and
conductivity, since, as he showed, electro-magnetic disturbances could
not be propagated in substances which are conductors of electricity. In
other words, if light is an electro-magnetic disturbance, all conducting
substances must be opaque, and all good insulators transparent. This we
know to be the fact: metallic substances, the best of conductors, are
opaque, while glass and crystals are transparent. Even such apparent
exceptions as vulcanite, an excellent insulator, fall into the law,
since, as Graham Bell has recently shown, this substance is remarkably
transparent to certain kinds of radiant energy.

In 1778, Brugmans of Leyden noticed that if a piece of bismuth was held
near either pole of a strong magnet, repulsion occurred. Other observers
noticed the same effect in the case of antimony. These facts appear to
have been unknown to Faraday, who, in 1845, by employing powerful
electro-magnets rediscovered them, and in addition showed that
practically all substances possess the power of being attracted or
repelled, when placed between the poles of sufficiently powerful
magnets. By placing slender needles of the s