Unlike electricity, magnetism was first known in the form of its
natural occurrence, namely as a property of certain minerals. If we
follow the same course which led us to start our study of electricity
with the primitive process of generating it, we shall turn now to the
basic phenomenon produced by a magnetic field already in existence.
(Only when we have learnt all we can from this, shall we proceed to ask
how magnetism comes into being.) Obviously, we shall find this basic
phenomenon in the effect of a magnet on a heap of iron filings.

Let us, to begin with, compare a mass of solid iron with the same
quantity of it in powdered form. The difference is that the powder
lacks the binding force which holds the solid piece together. Now lei
us expose the powdered iron to the influence of a magnet. At once a
certain ordering principle takes hold of the single particles. They no
longer lie at random and unrelated, apart from the inconspicuous
gravitational effect they exert on one another, but are drawn into a
coherent whole, thus acquiring properties resembling those of an
ordinary piece of solid matter.

Read thus, the phenomenon tells us that a part of space occupied by a
magnetic field has qualities which are otherwise found only where a
coherent solid mass is present. A magnetic piece of solid iron,
therefore, differs from a non-magnetic piece by giving rise in its
surroundings to dynamic conditions which would otherwise exist only in
its interior. This picture of the relatedness of magnetism to solidity
is confirmed by the fact that both are cancelled by heat, and increased
by cold.2

By its magnetic properties iron thus reveals itself as a substance