and make their presence felt in that manner. We shall see
in the next chapter that there are both shorter and longer waves, which
may be detected in other ways. From these facts it can be readily seen,
that we have a common origin for both light and heat, and that they are
both due to periodic waves in the Aether, and therefore all the laws
that govern heat should also govern the phenomena of light. Further, if
heat possesses a dynamical value, and if there be such a truth as the
motive power of heat, then there ought equally to be a motive power of
light; and further, if heat possesses a repulsive motion, then because
of the identity of light and heat, light should equally possess this
repulsive power, because it is due to similar periodic wave motions in
the Aether. With regard to the same laws governing both light and heat,
we shall see that this fact also holds good. We have already seen (Art.
66) that the intensity of heat is inversely as the square of the
distance, and we shall also see in the succeeding chapter that the same
law holds good in relation to light. We have seen (Art. 65) that the
path of a ray of heat is that of a straight line; we shall see in the
succeeding chapter that the path of a ray of light is that of a straight
line also.

Indeed, there is no law applicable to heat which is not applicable to
light. The law of reflection and refraction of heat equally holds good
in relation to light; and further, Professor Forbes has shown that heat
can be polarized in a similar manner to the polarization of light. This
last fact is considered the most conclusive argument as to the identity
of light and heat, and proves that the only difference between the two
is simply the difference corresponding to the difference between a high
note and a low note in sound. That being so, I hope to be able to show
that as heat possesses a dynamical value, so light equally possesses a
dynamical value, and that as heat is a repulsive motion, then light must
equally possess a similar repulsive motion, that motion always being
directed from the central body, being caused by the same agency, viz.
the waves of the Aether, the common source of both light and heat. I
purpose to address myself to this subject in the following chapter,
which I have termed Light, a Mode of Motion.

CHAPTER VII

LIGHT, A MODE OF MOTION

ART. 70. _Light, a Mode of Motion._--No subject ha