f is
in itself a perfect magnet, possessing two poles. This may be proved
by breaking something of less value than the magnet--the steel of a
lady's stays, for example, hardened and magnetized. It acts like the
magnet. When broken, each half acts like the whole; and when these
parts are again broken, we have still the perfect magnet, possessing,
as in the first instance, two poles. Push your breaking to its utmost
sensible limit--you cannot stop there. The bias derived from
observation will infallibly carry you beyond the bourne of the senses,
and compel you to regard this thing that we call magnetic polarity as
resident in the ultimate particles of the steel. You come to the
conclusion that each molecule of the magnet is endowed with this polar
force.

Like all other forces, this force of magnetism is amenable to
mechanical laws; and, knowing the direction and magnitude of the
force, we can predict its action. Placing a small magnetic needle near
a bar magnet, it takes a determinate position. That position might be
deduced theoretically from the mutual action of the poles. Moving the
needle round the magnet, for each point of the surrounding space there
is a definite direction of the needle and no other. A needle of iron
will answer as well as the magnetic needle; for the needle of iron is
magnetized by the magnet, and acts exactly like a steel needle
independently magnetized.

If we place two or more needles of iron near the magnet, the action
becomes more complex, for then the needles are not only acted on by
the magnet, but they act upon each other. And if we pass to smaller
masses of iron--to iron filings, for example--we find that they act
substantially as the needles, arranging themselves in definite forms,
in obedience to the magnetic action.

Placing a sheet of paper or glass over a bar magnet and showering iron
filings upon the paper, I notice a tendency of the filings to arrange
themselves in determinate lines. They cannot freely follow this
tendency, for they are hampered by the friction against the paper.
They are helped by tapping the paper; each tap releasing them for a
moment, and enabling them to follow their tendencies. But this is an
experiment which can only be seen by myself. To enable you all to see
it, I take a pair of small magnets and by a simple optical arrangement
throw the magnified images of the magnets upon the screen. Scattering
iron filings over the glass plate to which the small magn