have so much work if there were no change of figures: and since her onely action is change of motion, change of motion must needs make change of figures: and thus natural parts of matter may change from lines to points, and from points to lines, from squares to circles, and so forth, infinite ways, according to the change of motions; but though they change their figures, yet they cannot change their matter; for matter as it has been, so it remaines constantly in each degree, as the Rational, Sensitive and Inanimate, none becomes purer, none grosser then ever it was, notwithstanding the infinite changes of motions, which their figures undergo; for Motion changes onely the figure, not the matter it self, which continues still the same in its nature, and cannot be altered without a confusion or destruction of Nature. And this is the constant opinion of, Madam, _Your faithful Friend_ _and humble Servant._ [1] _Philos. part._ 3. _a._ 40. XXXIV. _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_