the specific gravity of steel, we might, by making the magnets float
in it, realize this state of things, for in such a liquid the magnets
would neither sink nor swim. Now, the principle of gravitation
enunciated by Newton is that every particle of matter, of every kind,
attracts every other particle with a force varying inversely as the
square of the distance. In virtue of the attraction of gravity, then,
the magnets, if perfectly free to move, would slowly approach each
other.

But besides the unpolar force of gravity, which belongs to matter in
general, the magnets are endowed with the polar force of magnetism.
For a time, however, the polar forces do not come sensibly into play.
In this condition the magnets resemble our water-molecules at the
temperature say of 50°. But the magnets come at length sufficiently
near each other to enable their poles to interact. From this point the
action ceases to be solely a general attraction of the masses.
Attractions of special points of the masses and repulsions of other
points now come into play; and it is easy to see that the
rearrangement of the magnets consequent upon the introduction of these
new forces may be such as to require a greater amount of room. This, I
take it, is the case with our water-molecules. Like our ideal magnets,
they approach each other for a time _as wholes_. Previous to reaching
the temperature 39° Fahr., the polar forces had doubtless begun to
act, but it is at this temperature that their claim to more room
exactly balances the contraction due to cold. At lower temperatures,
as regards change of volume, the polar forces predominate. But they
carry on a struggle with the force of contraction until the freezing
temperature is attained. The molecules then close up to form solid
crystals, a considerable augmentation of volume being the immediate
consequence.