only their bulk but their density also may be changed,) or to divide
them into smaller Corpuscles, (whereby a colour'd Liquor may become
transparent,) or to make many of them associate into one cluster,
whereby two transparent Liquors may compose a colour'd one. For we see
how apt those saline Menstruums are to penetrate and dissolve Substances
to which they are applied, and some of them to precipitate what others
dissolve. In like manner, if we consider the various Ph nomena of the
Atmosphere, we may observe, that when Vapours are first raised, they
hinder not the transparency of the Air, being divided into parts too
small to cause any Reflexion in their Superficies. But when in order to
compose drops of Rain they begin to coalesce and constitute Globules of
all intermediate sizes, those Globules, when they become of convenient
size to reflect some Colours and transmit others, may constitute Clouds
of various Colours according to their sizes. And I see not what can be
rationally conceived in so transparent a Substance as Water for the
production of these Colours, besides the various sizes of its fluid and
globular Parcels.
PROP. VI.
_The parts of Bodies on which their Colours depend, are denser than the
Medium which pervades their Interstices._
This will appear by considering, that the Colour of a Body depends not
only on the Rays which are incident perpendicularly on its parts, but on
those also which are incident at all other Angles. And that according to
the 7th Observation, a very little variation of obliquity will change
the reflected Colour, where the thin Body or small Particles is rarer
than the ambient Medium, insomuch that such a small Particle will at
diversly oblique Incidences reflect all sorts of Colours, in so great a
variety that the Colour resulting from them all, confusedly reflected
from a heap of such Particles, must rather be a white or grey than any
other Colour, or at best it must be but a very imperfect and dirty
Colour. Whereas if the thin Body or small Particle be much denser than
the ambient Medium, the Colours, according to the 19th Observation, are
so little changed by the variation of obliquity, that the Rays which
are reflected least obliquely may predominate over the rest, so much as
to cause a heap of such Particles to appear very intensely of their
Colour.
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