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RADIANT HEAT AND ITS RELATIONS. 217

therein powerful undulations; while others are unable
thus to communicate their motion, but glide through the
ether without materially disturbing its repose. Recent
experiments have proved that elementary bodies, except
under certain anomalous conditions, belong to the class of
bad radiators. An atom vibrating in the ether resembles
this naked tuning-fork vibrating in the air. The amount
of motion communicated to the air by these thin prongs is
too small to evoke at any distance the sensation of sound.
But if we permit the atoms to combine chemically and
form molecules, the result in many cases is an enormous
change in the power of radiation. The amount of ethereal
disturbance produced by the combined atoms of a body
may be many thousand times that produced by its constitu-
ent atoms when uncombined. The effect is roughly typi-
fied by this tuning-fork when connected with its resonant
case. The fork and its case now swing as a compound
system, and the vibrations which were before inaudible, are
now the source of a musical sound so powerful that it
might be plainly heard by thousands at once. The fork
and its case combined may be roughly regarded as a good
radiator of sound. .

The pitch of a musical note depends upon the rapidity
of its vibrations, or, in other words, on the length of its
waves. Now, the pitch of a note answers to the color of
light. Taking a slice. of white light from the beam of an
electric lamp, I cause that light to pass through an arrange-
ment of prisms. It is decomposed, and we have the effect
obtained by Newton, who first unrolled the solar beam into
the splendors of the solar spectrum. At one end of this
spectrum we have red light, at the other violet, and be-
tween those extremes lie the other prismatic colors. As
we advance along the spectrum from the red to the violet,
the pitch of the light—if I may use the expression—height-
ens, the sensation of violet being produced by a more rapid

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