_MADAM,_
That _Rarefaction_ is onely a _change of figure_, according to your
_Authors_ opinion,[1] is in my reason very probable; but when he sayes,
that _in rarified bodies are little intervals or pores filled up with
some other subtil matter_, if he means that all rarified bodies are
porous, I dissent from him; for it is not necessary that all rarified
bodies should be porous, and all hard bodies without pores: but if
there were a probability of pores, I am of opinion, it would be more
in dense and hard, than in rare and soft bodies; as for example,
rarifying and dilating motions are plaining, smoothing, spreading and
making all parts even, which could not well be, if there were holes
or pores; Earth is dense and hard, and yet is porous, and flame is
rare and dilating, and yet is not porous; and certainly Water is not
so porous as Earth. Wherefore pores, in my opinion, are according to
the nature or form of the figure, and not according to the rarity or
thinness, and density or thickness of the substance. As for his thin
and subtil matter filling up the pores of porous bodies, I assent to
your _Author_ so far, that I meane, thin and thick, or rare and dense
substances are joyned and mixed together. As for plaining, smoothing
and spreading, I do not mean so much artificial plaining and spreading;
as for example, when a piece of gold is beaten into a thin plate, and
a board is made plain and smooth by a Joyners tool, or a napkin folded
up is spread plain and even, although, when you observe these arts, you
may judge somewhat of the nature of natural dilations; for a folded
cloth is fuller of creases then when plain, and the beating of a thin
plate is like to the motion of dilation, which is to spread out, and
the forme of rarifying is thinning and extending. I add onely this,
that I am not of your _Authors_ opinion, that Rest is the Cause or
Glue which keeps the parts of dense or hard bodies together, but it is
retentive motions. And so I conclude, resting,
Madam,
_Your Faithful Friend_
_and Servant._
[1] _Philos. part._ 2. _a._ 6, 7.