ount, and, perhaps, have robbed other planets of their captures. His
family at present numbers about thirty. Of the other planets, so far as
we know, Saturn possesses a comet family of two, Uranus three, and
Neptune six. There are, indeed, a few comets which appear as if under
the influence of some force situated outside the known bounds of the
solar system, a circumstance which goes to strengthen the idea that
other planets may revolve beyond the orbit of Neptune. The terrestrial
planets, on the other hand, cannot have comet families; because the
enormous gravitative action of the sun in their vicinity entirely
overpowers the attractive force which they exert upon those comets which
pass close to them. Besides this, a comet, when in the inner regions of
the solar system, moves with such rapidity, that the gravitational pull
of the planets there situated is not powerful enough to deflect it to
any extent. It must not be presumed, however, that a comet once captured
should always remain a prisoner. Further disturbing causes might
unsettle its newly acquired orbit, and send it out again into the
celestial spaces.

With regard to the matter of which comets are composed, the spectroscope
shows the presence in them of hydrocarbon compounds (a notable
characteristic of these bodies), and at times, also, of sodium and iron.
Some of the light which we get from comets is, however, merely reflected
sunlight.

The fact that the tails of comets are always directed away from the sun,
has given rise to the idea that this is caused by some repelling action
emanating from the sun itself, which is continually driving off the
smallest particles. Two leading theories have been formulated to account
for the tails themselves upon the above assumption. One of these, first
suggested by Olbers in 1812, and now associated with the name of the
Russian astronomer, the late Professor Bredikhine, who carefully worked
it out, presumes an electrical action emanating from the sun; the other,
that of Arrhenius, supposes a pressure exerted by the solar light in its
radiation outwards into space. It is possible, indeed, that repelling
forces of both these kinds may be at work together. Minute particles are
probably being continually produced by friction and collisions among the
more solid parts in the heads of comets. Supposing that such particles
are driven off altogether, one may therefore assume that the so-called
captured comets are disintegrating a