ery 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 deg.. 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 deg. 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.

Sec. 3. _Ordinary Refraction of Light explained by the Wave Theory_.

We have now to exhibit the bearings of this act of crystallization
upon optical phenomena. According to the undulatory theory, the
velocity of light in water and glass is less than in air. Consider,
then, a small portion of a wave issuing from a point of light so
distant that the minute area may be regarded as practically plane.
Moving vertically downwards, and impinging on a horizontal surface of
glass or water, the wave would go through the medium without change of
direction. As, however, the velocity in glass or water is less than
the velocity in air, the wave would be retarded on passing into the
denser medium.

[Illustration: Fig. 25.]

But suppose the wave, before reaching the glass, to be _oblique_ to
the surface; that end of the wave which first reaches the medium