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.