This, then, is the state of the case: Matter attracts matter directly as
the mass, and inversely as the squares of the distances. This law is
derived from the planetary motions; space is, consequently, a void; and,
therefore, the power which gives mechanical momentum to matter, is
transferred from one end of creation to the other, without any physical
medium to convey the impulse. At the present day the doctrines of
Descartes are considered absurd; yet here is an absurdity of a far
deeper dye, without we resort to the miraculous, which at once
obliterates the connection between cause and effect, which it is the
peculiar province of physical science to develop. Let us take another
view. The present doctrine of light teaches that light is an undulation
of an elastic medium necessarily filling all space; and this branch of
science probably rests on higher and surer grounds than any other. Every
test applied to it by the refinements of modern skill, strengthens its
claims. Here then the Newtonian vacuum is no longer a void. If we get
over this difficulty, by attributing to this medium a degree of tenuity
almost spiritual, we shall run upon Scylla while endeavoring to shun
Charybdis. Light and heat come bound together from the sun, by the same
path, and with the same velocity. Heat is therefore due also to an
excitement of this attenuated medium. Yet this heat puts our atmosphere
in motion, impels onward the waves of the sea, wafts our ships to
distant climes, grinds our corn, and in various ways does the work of
man. If we expose a mass of metal to the sun's rays for a single hour
the temperature will be raised. To do the same by an artificial fire,
would consume fuel, and this fuel would generate the strength or force
of a horse. Estimate, therefore, the amount of force received from the
sun in a single day for the whole globe, and we shall find that nothing
but a material medium will suffice to convey this force.
Let us appeal to analogy. The undulations of our atmosphere produce
sound; that is, convey to the ear a part of a mechanical force imparted
to a solid body--a bell for instance. Let us suppose this force to equal
one pound. On account of the elasticity of the bell, the whole of the
force is not instantaneously imparted to the surrounding air; but the
denser the air the sooner it loses its motion. In a dense fluid like
water, the motion is imparted quickly, and the sound is not a ring but a
click. If we diminish the density of the air, the loss of motion is
retarded; so that we might conceive it possible, provided the bell could
be suspended in a _perfect vacuum_, without a mechanical tie, and there
was no friction to overcome from the rigidity of its particles, that the
bell would vibrate forever, although its sound could never reach the
ear. We see, therefore, that the mechanical effect in a given time, is
owing to the density of the medium. But can we resort to such an
analogy? Every discovery in the science confirms more and more the
analogy between the motions of air and the medium of space; the angle of
reflexion and incidence follows the same law in both; the law of
radiation and interference; and if experiments were instituted, there
can be but little doubt that sound has also got its spectrum.