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MATTER AND FORCE. 77

a straight line; but its manifestations are various, and some-
times so complex as entirely to disguise its elementary con-
stituents. Its different manifestations have received differ-
ent names. Here, for example, is a magnet freely suspended.
I bring the end of a second magnet near one of the. ends
of the suspended one—attraction is the consequence. I re-
verse the position of one of the magnets—repulsion follows.
This display of power is called magnetic force. In the
case of gravitation we have a simple attraction, in the case
of magnetism attraction and repulsion always go together.
Thus magnetism is a double force, or, as it is usually called,
a polar force. I present a bit of common iron to the magnet,
the iron itself becomes a temporary magnet,'and it now
possesses the power of attracting other iron. And if sev-
eral pieces of iron be presented at the same time, not only
will the magnet act on them, but they Will also act upon
each other.

This leads me to an experiment which will give you
some idea of how bodies arrange themselves under the
operation of a polar force. Underneath this plate of glass
is placed a small magnet, and by an optical arrangement
comprising a powerful lamp, a magnified image of the mag-
net is now cast upon the screen before you. I scatter iron
filings over the glass. You already notice a certain arrange-
ment of the particles of iron. Their free action is, how-
ever, hampered by friction. I therefore tap the glass,
liberate the particles, which, as I tap, arrange themselves in
these beautiful curves. This experiment is intended to
make clear to you how a definite arrangement of particles
——a kind of incipient structure—may result from the oper-
ation of a polar force. We shall by-andéby see far more
wonderful exhibitions of the same structural action when
we come to deal with the force of crystallization.

The magnetic force has here acted upon particles of
matter visible to the eye. But, as already stated, there are

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