hard bodies, because their solidity does not seem to permit them to
receive movement except in their whole mass at the same time. This,
however, is not necessary because this solidity is not such as it
appears to us, it being probable rather that these bodies are composed
of particles merely placed close to one another and held 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