hat is to say, of portions
of the substance so small that we cannot by any mechanical method cut
them in two, so as to obtain a north pole separate from the south pole.
We have arrived at no explanation, however, of the nature of a magnetic
molecule, and we have therefore to consider the hypothesis of
Ampere--that the magnetism of the molecule is due to an electric current
constantly circulating in some closed path within it.

Ampere concluded that if magnetism is to be explained by means of
electric currents, these currents must circulate within the molecules of
the magnet, and cannot flow from one molecule to another. As we cannot
experimentally measure the magnetic action at a point within the
molecule, this hypothesis cannot be disproved in the same way that we
can disprove the hypothesis of sensible currents within the magnet. In
spite of its apparent complexity, Ampere's theory greatly extends our
mathematical vision into the interior of the molecules.

_III.--The Electro-Magnetic Theory of Light_

We explain electro-magnetic phenomena by means of mechanical action
transmitted from one body to another by means of a medium occupying the
space between them. The undulatory theory of light also assumes the
existence of a medium. We have to show that the properties of the
electro-magnetic medium are identical with those of the luminiferous
medium.

To fill all space with a new medium whenever any new phenomena are to be
explained is by no means philosophical, but if the study of two
different branches of science has independently suggested the idea of a
medium; and if the properties which must be attributed to the medium in
order to account for electro-magnetic phenomena are of the same kind as
those which we attribute to the luminiferous medium in order to account
for the phenomena of light, the evidence for the physical existence of
the medium is considerably strengthened.

According to the theory of emission, the transmission of light energy is
effected by the actual transference of light-corpuscles from the
luminous to the illuminated body. According to the theory of undulation
there is a material medium which fills the space between the two bodies,
and it is by the action of contiguous parts of this medium that the
energy is passed on, from one portion to the next, till it reaches the
illuminated body. The luminiferous medium is therefore, during the
passage of light through it, a receptacle of energy. This