But what is gravity? According to Newton: "Haec est qualitas omnium in
quibus experimenta instituere licet, et propterea per Reg. 3 de
universes affirmanda est." _Vide_ Prin. Lib. Ter. Cor. 2. Prop. vi.
Now the other primary qualities of matter are unaffected by
circumstances. The inertia of a particle of matter is the same at
Jupiter as on the earth, so also is its extension; but not so with
gravity. It depends on other matter, and on its distance from it; and
may be less or greater at different times, and in different places. It
is, therefore, not philosophical to say that all matter is necessarily
ponderous, inasmuch as it is a virtue not residing in itself alone, but
needs the existence of other matter to call it into action. If an atom
were isolated in space it would have no weight. If influenced by other
matter, there must be some physical medium to convey the influence, or
gravity is not in accordance with the laws of force and motion. Which
horn of the dilemma shall we take? Let us first admit that there is a
principle of gravitation, affecting all planetary or atomic matter, and
that there exists a highly elastic medium, pervading all space,
conveying to us the light of the most distant stars, and that this
medium is not affected by gravity. In this summary way, therefore, we
have arrived at the pivot on which this theory turns.
The prominent feature of the theory, therefore, is the necessity it will
show for the existence of an all-pervading medium, and that it possesses
inertia without ponderosity. That electricity is nothing more than the
effects of the condensation and rarefaction of this medium by force.
That it also pervades all atomic matter, whose motions necessarily move
the medium; and, consequently, that there can be no motion without some
degree of electricity. That no change can take place in bodies either by
chemical decomposition, by increase or decrease of temperature, by
friction or contact, without in some measure exciting electricity or
motion of the ether. That galvanism and magnetism are but ethereal
currents without condensation, induced by peculiar superficial and
internal molecular arrangement of the particles of certain substances.
That light and heat are effects of the vibrations of atoms, propagated
through this universal medium from body to body. That the atomic motion
of heat can be produced by the motion of translation or momentum of
bodies in the gross, that is, by friction, by compression, &c.; and can
be reconverted into momentum at our pleasure. Hence the latent heat or
specific atomic motion of combustibles, originally derived from the sun,
is transferred to atoms, which are capable of being inclosed in
cylinders, so as to make use of their force of expansion, which is thus
converted into momentum available for all the wants of man.