_The Parts of Bodies and their Interstices must not be less than of some
definite bigness, to render them opake and colour'd._
For the opakest Bodies, if their parts be subtilly divided, (as Metals,
by being dissolved in acid Menstruums, &c.) become perfectly
transparent. And you may also remember, that in the eighth Observation
there was no sensible reflexion at the Superficies of the
Object-glasses, where they were very near one another, though they did
not absolutely touch. And in the 17th Observation the Reflexion of the
Water-bubble where it became thinnest was almost insensible, so as to
cause very black Spots to appear on the top of the Bubble, by the want
of reflected Light.
On these grounds I perceive it is that Water, Salt, Glass, Stones, and
such like Substances, are transparent. For, upon divers Considerations,
they seem to be as full of Pores or Interstices between their parts as
other Bodies are, but yet their Parts and Interstices to be too small to
cause Reflexions in their common Surfaces.