all other
questions having to do with magnetism, they hold for the present
generation the double fascination of insoluble mystery. To be sure, one
may readily enough suggest that if magnetism be really a whirl in the
ether, this whirl is apparently interfered with by the waves of radiant
heat; or, again, that magnetism is presumably due to molecular motions
which are apparently interfered with by another kind of molecular
motions which we call heat vibrations; but there is a vagueness about
the terms of such guesses that leaves them clearly within the category
of explanations that do not explain.

When it comes to the phenomena of light, we can, as is fitting, see
our way a little more clearly, since, thanks to Thomas Young and his
successors, we know pretty definitely what light really is. So when
we learn that many substances change their color utterly at low
temperatures--red things becoming yellow and yellow things white,
for example--we can step easily and surely to at least a partial
explanation. We know that the color of any object depends simply
upon the particular ether waves of the spectrum which that particular
substance absorbs; and it does not seem anomalous that molecules packed
close together at--180 deg. of temperature should treat the ether waves
differently than when relatively wide apart at an ordinary temperature.
Yet, after all, that may not be the clew to the explanation. The packing
of the molecules may have nothing to do with it. The real explanation
may lie in the change of the ether waves sent out by the vibrating
molecule; indeed, the fact that the waves of radiant heat and those of
light differ only in amplitude lends color to this latter supposition.
So the explanation of the changed color of the cooled substance is at
best a dubious one.

Another interesting light phenomenon is found in the observed fact that
very many substances become markedly phosphorescent at low temperatures.
Thus, according to Professor Dewar, "gelatine, celluloid, paraffine,
ivory, horn, and india-rubber become distinctly luminous, with a bluish
or greenish phosphorescence, after cooling to--180 deg. and being stimulated
by the electric light." The same thing is true, in varying degrees,
of alcohol, nitric acid, glycerine, and of paper, leather, linen,
tortoise-shell, and sponge. Pure water is but slightly luminous, whereas
impure water glows brightly. On the other hand, alcohol loses its
phosphorescence when a tr