gal force of their primary. The hypothesis
also points out the time of revolution of the planets in their orbits,
and of the satellites in theirs; it furnishes a reason for the genesis
and existence of Saturn's rings, which are indeed its remaining
witnesses--their position and movements answering to its requirements.
It accounts for the physical state of the sun, and also for the physical
state of the earth and moon as indicated by their geology. It is also
not without furnishing reasons for the existence of comets as integrant
members of our system; for their singular physical state; for the
eccentric, almost parabolic orbits of so many of them; for the fact that
there are as many of them with a retrograde as with a direct motion; for
their more frequent occurrence about the axis of the solar system than
in its plane; and for their general antithetical relations to planets.

[Sidenote: Whether nebulae actually exist.] If these and very many other
apparently disconnected facts follow as the mechanical necessities of
the admission of a gravitating nebula--a very simple postulate--it
becomes important to ascertain whether, by actual observation, the
existence of such material forms may be demonstrated in any part of the
universe. It was the actual telescopic observation of such objects that
led Herschel to the nebular hypothesis. He concluded that there are two
distinct kinds of nebulae, one consisting of clusters of stars so remote
that they could not be discerned individually, but that these may be
discerned by sufficient telescopic power; the other being of a hazy
nature, and incapable of resolution. Nebulae do not occur at random in
the heavens: the regions poorest in stars are richest in them; they are
few in the plane of our sidereal system, but numerous about its poles,
in that respect answering to the occurrence of comets in the solar
system. The resolution of many of these hazy patches of light into stars
by no means disproves the truly nebulous condition of many others.

Fortunately, however, other means than telescopic observation for the
settlement of this question are available. In 1846, it was discovered by
the author of this book that the spectrum of an ignited solid is
continuous, that is, has neither dark nor bright fixed lines. Fraunhofer
had previously made known that the spectrum of ignited gases is
discontinuous. Here, then, is the means of determining whether the light
emitted by a given nebula come