erge perpendicularly by a contrary
Situation of the Coast of unusual Refraction at the second Surface; the
Crystal acting upon the Rays after they have pass'd through it, and are
emerging into the Air; or, if you please, into a _Vacuum_. And since the
Crystal by this Disposition or Virtue does not act upon the Rays, unless
when one of their Sides of unusual Refraction looks towards that Coast,
this argues a Virtue or Disposition in those Sides of the Rays, which
answers to, and sympathizes with that Virtue or Disposition of the
Crystal, as the Poles of two Magnets answer to one another. And as
Magnetism may be intended and remitted, and is found only in the Magnet
and in Iron: So this Virtue of refracting the perpendicular Rays is
greater in Island-Crystal, less in Crystal of the Rock, and is not yet
found in other Bodies. I do not say that this Virtue is magnetical: It
seems to be of another kind. I only say, that whatever it be, it's
difficult to conceive how the Rays of Light, unless they be Bodies, can
have a permanent Virtue in two of their Sides which is not in their
other Sides, and this without any regard to their Position to the Space
or Medium through which they pass.

What I mean in this Question by a _Vacuum_, and by the Attractions of
the Rays of Light towards Glass or Crystal, may be understood by what
was said in the 18th, 19th, and 20th Questions.

_Quest._ 30. Are not gross Bodies and Light convertible into one
another, and may not Bodies receive much of their Activity from the
Particles of Light which enter their Composition? For all fix'd Bodies
being heated emit Light so long as they continue sufficiently hot, and
Light mutually stops in Bodies as often as its Rays strike upon their
Parts, as we shew'd above. I know no Body less apt to shine than Water;
and yet Water by frequent Distillations changes into fix'd Earth, as Mr.
_Boyle_ has try'd; and then this Earth being enabled to endure a
sufficient Heat, shines by Heat like other Bodies.

The changing of Bodies into Light, and Light into Bodies, is very
conformable to the Course of Nature, which seems delighted with
Transmutations. Water, which is a very fluid tasteless Salt, she changes
by Heat into Vapour, which is a sort of Air, and by Cold into Ice, which
is a hard, pellucid, brittle, fusible Stone; and this Stone returns into
Water by Heat, and Vapour returns into Water by Cold. Earth by Heat
becomes Fire, and by Cold returns into Earth. Dens