neglected. For then after having exhausted all the air one hears no
Sound from the metal, though it is struck.
One sees here not only that our air, which does not penetrate through
glass, is the matter by which Sound spreads; but also that it is not
the same air but another kind of matter in which Light spreads; since
if the air is removed from the vessel the Light does not cease to
traverse it as before.
And this last point is demonstrated even more clearly by the
celebrated experiment of Torricelli, in which the tube of glass from
which the quicksilver has withdrawn itself, remaining void of air,
transmits Light just the same as when air is in it. For this proves
that a matter different from air exists in this tube, and that this
matter must have penetrated the glass or the quicksilver, either one
or the other, though they are both impenetrable to the air. And when,
in the same experiment, one makes the vacuum after putting a little
water above the quicksilver, one concludes equally that the said
matter passes through glass or water, or through both.
As regards the different modes in which I have said the movements of
Sound and of Light are communicated, one may sufficiently comprehend
how this occurs in the case of Sound if one considers that the air is
of such a nature that it can be compressed and reduced to a much
smaller space than that which it ordinarily occupies. And in
proportion as it is compressed the more does it exert an effort to
regain its volume; for this property along with its penetrability,
which remains notwithstanding its compression, seems to prove that it
is made up of small bodies which float about and which are agitated
very rapidly in the ethereal matter composed of much smaller parts. So
that the cause of the spreading of Sound is the effort which these
little bodies make in collisions with one another, to regain freedom
when they are a little more squeezed together in the circuit of these
waves than elsewhere.
But the extreme velocity of Light, and other properties which it has,
cannot admit of such a propagation of motion, and I am about to show
here the way in which I conceive it must occur. For this, it is
needful to explain the property which hard bodies must possess to
transmit movement from one to another.
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