together by
some pressure from without of some other matter, and by the
irregularity of their shapes. For primarily their rarity is shown by
the facility with which there passes through them the matter of the
vortices of the magnet, and that which causes gravity. Further, one
cannot say that these bodies are of a texture similar to that of a
sponge or of light bread, because the heat of the fire makes them flow
and thereby changes the situation of the particles amongst themselves.
It remains then that they are, as has been said, assemblages of
particles which touch one another without constituting a continuous
solid. This being so, the movement which these particles receive to
carry on the waves of light, being merely communicated from some of
them to others, without their going for that purpose out of their
places or without derangement, it may very well produce its effect
without prejudicing in any way the apparent solidity of the compound.

By pressure from without, of which I have spoken, must not be
understood that of the air, which would not be sufficient, but that of
some other more subtle matter, a pressure which I chanced upon by
experiment long ago, namely in the case of water freed from air, which
remains suspended in a tube open at its lower end, notwithstanding
that the air has been removed from the vessel in which this tube is
enclosed.

One can then in this way conceive of transparency in a solid without
any necessity that the ethereal matter which serves for light should
pass through it, or that it should find pores in which to insinuate
itself. But the truth is that this matter not only passes through
solids, but does so even with great facility; of which the experiment
of Torricelli, above cited, is already a proof. Because on the
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 obl