quicksilver and the water quitting the upper part of the glass tube,
it appears that it is immediately filled with ethereal matter, since
light passes across it. But here is another argument which proves this
ready penetrability, not only in transparent bodies but also in all
others.
When light passes across a hollow sphere of glass, closed on all
sides, it is certain that it is full of ethereal matter, as much as
the spaces outside the sphere. And this ethereal matter, as has been
shown above, consists of particles which just touch one another. If
then it were enclosed in the sphere in such a way that it could not
get out through the pores of the glass, it would be obliged to follow
the movement of the sphere when one changes its place: and it would
require consequently almost the same force to impress a certain
velocity on this sphere, when placed on a horizontal plane, as if it
were full of water or perhaps of quicksilver: because every body
resists the velocity of the motion which one would give to it, in
proportion to the quantity of matter which it contains, and which is
obliged to follow this motion. But on the contrary one finds that the
sphere resists the impress of movement only in proportion to the
quantity of matter of the glass of which it is made. Then it must be
that the ethereal matter which is inside is not shut up, but flows
through it with very great freedom. We shall demonstrate hereafter
that by this process the same penetrability may be inferred also as
relating to opaque bodies.
The second mode then of explaining transparency, and one which appears
more probably true, is by saying that the waves of light are carried
on in the ethereal matter, which continuously occupies the interstices
or pores of transparent bodies. For since it passes through them
continuously and freely, it follows that they are always full of it.
And one may even show that these interstices occupy much more space
than the coherent particles which constitute the bodies. For if what
we have just said is true: that force is required to impress a certain
horizontal velocity on bodies in proportion as they contain coherent
matter; and if the proportion of this force follows the law of
weights, as is confirmed by experiment, then the quantity of the
constituent matter of bodies also follows the proportion of their
weights. Now we see that water weighs only one fourteenth part as much
as an equal portion of quicksilver: therefore the matter of the water
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