conception of light is. For
fifteen years it has been common to hear heat spoken of as a mode of
molecular motion, and sometimes it has been characterized as
_vibratory_, and most persons have received the impression that the
vibratory motion was an actual change of position of the molecular in
space instead of a _change of form_. Make a ring of wire five or six
inches in diameter, and, holding it between the thumb and finger at the
twisted ends, pluck it with a finger of the other hand; the ring will
vibrate, have three nodes, and will give a good idea of the character of
the vibration that constitutes what we call heat. This vibratory motion
may have a greater or less amplitude, and the energy of the vibration
will be as the square of that amplitude. But the vibrating molecule
gives up its energy of vibration to the surrounding ether; that is to
say, it loses amplitude precisely as a vibrating tuning fork will lose
it. The ether transmits the energy it has received in every direction
with the velocity of 186,000 miles per second, whether the amplitude be
great or small, and whether the number of vibrations be many or few. It
is quite immaterial. The _form_ of this energy which the ether transmits
is _undulatory_; that is to say, not unlike that of the wave upon a
loose rope when one end of it is shaken by the hand. As every shake of
the hand starts a wave in the rope, so will every vibration of a part of
the molecule start a wave in the ether. Now we have several methods for
measuring the wave lengths in ether, and we also know the velocity of
movement. Let v = velocity, l = wave length, and n = number of
vibrations per second, then n = v/l, and by calculation the value of n
varies within wide limits, say from 1 x 10^{14} to 20 x 10^{14}. But all
vibrating bodies are capable of vibrating in several periods, the
longest period being called the fundamental, and the remainder, which
stand in some simple ratios to the fundamental, are called _harmonics_.
Each of these will give to the ether its own particular vibratory
movement, so that a single molecule may be constantly giving out rays of
many wave lengths precisely as a sounding bell gives out sounds of
various pitches at one and the same time.

Again, when these undulations in the ether fall upon other molecules the
latter may reflect them away or they may absorb them, in which case the
absorbing molecules are themselves made to vibrate with increased
amplitude, and