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A LECTURE ON MAGNETISM. 371

brought near or withdrawn. Fixing a definite pole in your
imagination, you must picture the precise arrangement of
the two fluids with reference to this pole. And you must
not only be well drilled in the use of this mental imagery
yourself, but you must be able to arouse the same pictures
in the minds of your pupils, and satisfy yourself that they
possess this power of placing actually before themselves
magnets and iron in various positions, and describing the
exact magnetic state of the iron in each particular case.
The mere facts of magnetism will have their interest im-
mensely augmented by an acquaintance with those hidden
principles whereon the facts depend. Still, while you use
this theory of magnetic fluids to track out the phenomena
and link them together, be sure to tell your pupils that it
is to be regarded as a symbol merely—a symbol, more-
over, which is incompetent to cover all the facts,1 but which
does good practical service while we are waiting for the
actual truth.

This state of excitement into which the soft iron is
thrown by the influence of the magnet, is sometimes called
“ magnetization by influence.” More commonly, however,
the magnetism is said to be “induced” in the soft iron,
and hence this way of magnetizing is called “ magnetic
induction.” N ow, there is nothing theoretically perfect in
Nature : there is no iron so soft as not to possess a certain
amount of coercive force, and no steel so hard as not to be
capable, in some degree, of magnetic induction. The
quality of steel is in some measure possessed by iron, and-
the quality of iron is shared in some degree by steel. It is
in virtue of this latter fact that the unmagnetized darning—

1 This theory breaks down when applied to diamagnetic bodies, which
are repelled by magnets. Like soft iron, such bodies are thrown into a
state of temporary excitement in virtue of which they are repelled, but
any attempt to explain such a repulsion by the decomposition of a fluid
will demonstrate its own futility.

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