there is such a
Virtue, seems to follow from the Reflexions and Inflexions of the Rays
of Light. For the Rays are repelled by Bodies in both these Cases,
without the immediate Contact of the reflecting or inflecting Body. It
seems also to follow from the Emission of Light; the Ray so soon as it
is shaken off from a shining Body by the vibrating Motion of the Parts
of the Body, and gets beyond the reach of Attraction, being driven away
with exceeding great Velocity. For that Force which is sufficient to
turn it back in Reflexion, may be sufficient to emit it. It seems also
to follow from the Production of Air and Vapour. The Particles when they
are shaken off from Bodies by Heat or Fermentation, so soon as they are
beyond the reach of the Attraction of the Body, receding from it, and
also from one another with great Strength, and keeping at a distance,
so as sometimes to take up above a Million of Times more space than they
did before in the form of a dense Body. Which vast Contraction and
Expansion seems unintelligible, by feigning the Particles of Air to be
springy and ramous, or rolled up like Hoops, or by any other means than
a repulsive Power. The Particles of Fluids which do not cohere too
strongly, and are of such a Smallness as renders them most susceptible
of those Agitations which keep Liquors in a Fluor, are most easily
separated and rarified into Vapour, and in the Language of the Chymists,
they are volatile, rarifying with an easy Heat, and condensing with
Cold. But those which are grosser, and so less susceptible of Agitation,
or cohere by a stronger Attraction, are not separated without a stronger
Heat, or perhaps not without Fermentation. And these last are the Bodies
which Chymists call fix'd, and being rarified by Fermentation, become
true permanent Air; those Particles receding from one another with the
greatest Force, and being most difficultly brought together, which upon
Contact cohere most strongly. And because the Particles of permanent Air
are grosser, and arise from denser Substances than those of Vapours,
thence it is that true Air is more ponderous than Vapour, and that a
moist Atmosphere is lighter than a dry one, quantity for quantity. From
the same repelling Power it seems to be that Flies walk upon the Water
without wetting their Feet; and that the Object-glasses of long
Telescopes lie upon one another without touching; and that dry Powders
are difficultly made to touch one another so as to