gorgeous when it is near the sun, and as soon as it gets a reasonable
distance away from him it is perfectly invisible.

The matter evaporated from the comet by the sun's heat does not
return--it is lost to the comet; and hence, after a few such journeys,
its volatile matter gets appreciably diminished, and so old-established
periodic comets have no tails to speak of. But the new visitants, coming
from the depths of space for the first time--these have great supplies
of volatile matter, and these are they which show the most magnificent
tails.

[Illustration: FIG. 101.--Head of Donati's comet of 1858.]

The tail of a comet is always directed away from the sun as if it were
repelled. To this rule there is no exception. It is suggested, and held
as most probable, that the tail and sun are similarly electrified, and
that the repulsion of the tail is electrical repulsion. Some great force
is obviously at work to account for the enormous distance to which the
tail is shot in a few hours. The pressure of the sun's light can do
something, and is a force that must not be ignored when small particles
are being dealt with. (Cf. _Modern Views of Electricity_, 2nd edition,
p. 363.)

Now just think what analogies there are between comets and meteors. Both
are bodies travelling in orbits round the sun, and both are mostly
invisible, but both become visible to us under certain circumstances.
Meteors become visible when they plunge into the extreme limits of our
atmosphere. Comets become visible when they approach the sun. Is it
possible that comets are large meteors which dip into the solar
atmosphere, and are thus rendered conspicuously luminous? Certainly they
do not dip into the actual main atmosphere of the sun, else they would
be utterly destroyed; but it is possible that the sun has a faint trace
of atmosphere extending far beyond this, and into this perhaps these
meteors dip, and glow with the friction. The particles thrown off might
be, also by friction, electrified; and the vaporous tail might be thus
accounted for.

[Illustration: FIG. 102.--Halley's Comet.]

Let us make this hypothesis provisionally--that comets are large
meteors, or a compact swarm of meteors, which, coming near the sun, find
a highly rarefied sort of atmosphere, in which they get heated and
partly vaporized, just as ordinary meteorites do when they dip into the
atmosphere of the earth. And let us see whether any facts bear out the
analogy and jus