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.

When one takes a number of spheres of equal size, made of some very
hard substance, and arranges them in a straight line, so that they
touch one another, one finds, on striking with a similar sphere
against the first of these spheres, that the motion passes as in an
instant to the last of them, which separates itself from the row,
without one's being able to perceive that the others have been
stirred. And even that one which was used to strike remains motionless
with them. Whence one sees that the movement passes with an extreme
velocity which is the greater, the greater the hardness of the
substance of the spheres.

But it is still certain that this progression of motion is not
instantaneous, but successive, and therefore must take time. For if
the movement, or the disposition to movement, if you will have it so,
did not pass successively through all these spheres, they would all
acquire the movement at the same time, and hence would all advance
together; which does not happen. For the last one leaves the whole row
and acquires the speed of the one which was pushed. Moreover there are
experiments which demonstrate that all the bodies which we reckon of
the hardest kind, such as quenched steel, glass, and agate, act as
sp