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And here I would ask you to make familiar to your
minds the idea that no chemical action can be produced by
a ray that does not involve the destruction of the ray. But
the term “ray” is unsatisfactory to us at present, when
our desire is to abolish all vagueness, and to affix a definite
physical significance to each of our terms. Abandoning
the term ray as loose and indefinite, we have to fix our
thoughts upon the waves of light; and to render clear to"
our minds that those waves which produce chemical action
do so by delivering up their own motion to the molecules-
which they decompose. We have here forestalled to some?
extent a question of great importance in molecular physics,
which, however, is worthy of being fixed more definitely in
your mind; it is this: When the waves of ether are in-
tercepted by a compound vapor, is the motion of the waves
transferred to the molecules of the vapor, or to the atoms
of the molecules? We have thus far leaned to the con-
clusion that the motion is communicated to the atoms; for
if not to these individually, why should they be shaken
asunder ? The question, however, is capable of, and is
worthy of, another test, the bearing and significance of
which you will immediately appreciate.

As already explained, the molecules are held in their
positions of equilibrium by their mutual repulsion on the
one side, and by an external pressure on the other. Their
rate of vibration, if they vibrate at all, must depend upon
the elastic force which they mutually exert. If this force
be changed, the rate of vibration must change along with
it; and after the change the molecules could no longer
absorb the waves which they absorbed prior to the change.
N ow, the elastic force between molecule and molecule is
utterly altered when a‘ vapor passes to the liquid state.
Hence, if the liquid absorbs waves of the same period as its
vapor, it is a proof that the absorption is not effected by
the molecules. Let us be perfectly clear on this important

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