_Between the parts of opake and colour'd Bodies are many Spaces, either
empty, or replenish'd with Mediums of other Densities; as Water between
the tinging Corpuscles wherewith any Liquor is impregnated, Air between
the aqueous Globules that constitute Clouds or Mists; and for the most
part Spaces void of both Air and Water, but yet perhaps not wholly void
of all Substance, between the parts of hard Bodies._