the violent agitation of
the particles of the metal or of the wood; those of them which are on
the surface striking similarly against the ethereal matter. The
agitation, moreover, of the particles which engender the light ought
to be much more prompt and more rapid than is that of the bodies which
cause sound, since we do not see that the tremors of a body which is
giving out a sound are capable of giving rise to Light, even as the
movement of the hand in the air is not capable of producing Sound.

Now if one examines what this matter may be in which the movement
coming from the luminous body is propagated, which I call Ethereal
matter, one will see that it is not the same that serves for the
propagation of Sound. For one finds that the latter is really that
which we feel and which we breathe, and which being removed from any
place still leaves there the other kind of matter that serves to
convey Light. This may be proved by shutting up a sounding body in a
glass vessel from which the air is withdrawn by the machine which Mr.
Boyle has given us, and with which he has performed so many beautiful
experiments. But in doing this of which I speak, care must be taken to
place the sounding body on cotton or on feathers, in such a way that
it cannot communicate its tremors either to the glass vessel which
encloses it, or to the machine; a precaution which has hitherto been
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which the quicksilver has withdrawn itself, remaining void of air,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.