a continuous atmosphere of elastic gas
of any sort, it seems probable that the gaseous molecules are only those
rising from the coma, possibly by ordinary evaporation, but more
probably by the action of the ultra-violet and other rays of the sun
giving rise to an ionization of disconnected gaseous molecules. The
matter cannot be wholly gaseous because in this case there could be no
central force sufficient to keep the parts of the comet together.

The facts also point to the conclusion that the solid matter of a comet
is formed of a swarm or cloud of small disconnected masses, probably
having much resemblance to the meteoric masses which are known to be
flying through the solar system and possibly of the same general kind as
these. The question whether there is any central solid of considerable
mass is still undecided; it can only be said that if so, it is probably
small relative to cosmic masses in general--more likely less than
greater than 100 m. in diameter. The light of the comet therefore
proceeds from two sources: one the incandescence of gases, the other the
sunlight reflected from the solid parts. No estimate can be formed of
the ratio between these two kinds of light until a bright comet shall be
spectroscopically observed during an entire apparition.

_Origin and Orbits of Comets._--The great difference which we have
pointed out between comets and the permanent bodies of the solar system
naturally suggested the idea that these bodies do not belong to that
system at all, but are nebulous masses, scattered through the stellar
spaces, and brought one by one into the sphere of the sun's attraction.
The results of this view are easily shown to be incompatible with the
observed facts. The sun, carrying the whole solar system with it, is
moving through space with a speed of about 10 m. per second. If it
approached a comet nearly at rest the result would be a relative motion
of this amount which, as the comet came nearer, would be constantly
increased, and would result in the comet describing relative to the sun
a markedly hyperbolic orbit, deviating too widely from a parabola to
leave any doubt, even in the most extreme cases. Moreover, a large
majority of comets would then have their aphelia in the direction of the
sun's motion, and therefore their perihelia in the opposite direction.
Neither of these results corresponds to the fact. The conclusion is that
if we regard a comet as a body not belonging to the solar s