here all his efforts to produce any sensible action upon the
polarized beam were ineffectual. He then passed from magnets to
currents, enclosing bars of heavy glass, and tubes containing
liquids and aqueous solutions within an electro-magnetic helix.
A current sent through the helix caused the plane of polarization to
rotate, and always in the direction of the current. The rotation
was reversed when the current was reversed. In the case of magnets,
he observed a gradual, though quick, ascent of the transmitted beam
from a state of darkness to its maximum brilliancy, when the magnet
was excited. In the case of currents, the beam attained at once its
maximum. This he showed to be due to the time required by the iron
of the electro-magnet to assume its full magnetic power, which time
vanishes when a current, without iron, is employed. 'In this
experiment,' he says, 'we may, I think, justly say that a ray of
light is electrified, and the electric forces illuminated.' In the
helix, as with the magnets, he submitted air to magnetic influence
'carefully and anxiously,' but could not discover any trace of
action on the polarized ray.

Many substances possess the power of turning the plane of polarization
without the intervention of magnetism. Oil of turpentine and quartz
are examples; but Faraday showed that, while in one direction,
that is, across the lines of magnetic force, his rotation is zero,
augmenting gradually from this until it attains its maximum, when
the direction of the ray is parallel to the lines of force; in the
oil of turpentine the rotation is independent of the direction of
the ray. But he showed that a still more profound distinction
exists between the magnetic rotation and the natural one. I will
try to explain how. Suppose a tube with glass ends containing oil
of turpentine to be placed north and south. Fixing the eye at the