derable interest in a scientific point of
view, as well as of considerable importance in a dynamic one, for it
is a principle evidently antagonistic to the great prevailing
attribute of gravitation, so universally present in matter. The
comet's tail is the only substance known that is repelled instead of
being attracted by the sun.

The repulsive power to which the development of the comet's tail is
due, is one of extraordinary energy. The comet of 1680 shot out its
tail through something like 100 million miles in a couple of days.
Most probably, much of the matter that is thus thrown off from the
cometic nucleus is never collected again, but is dissipated into
space, and lost for ever to the comet. The tail of the comet of 1680
was seen in its greatest brilliancy soon before the solar approach;
this was, however, an exception to the general rule. Comets nearly
always have the finest tails, and present altogether the most
beautiful appearance, immediately _after_ they have been in the
closest proximity to the sun.

The comet's tail seems, in reality, to be a thin oblong case of
vapour, formed out of the cometic substance by the increasing
intensity of the sunshine, and enclosing the denser portion of that
substance at one end. The diverging streams which it displays upon the
sky are merely the retiring edges of the rounded case, where the
greatest depth of luminous matter comes into sight. As the comet nears
the sun, much of its substance is vaporised for the construction of
this envelope; but as it goes off again into remoteness, the vaporous
envelope is once more condensed. The tail may then be seen to flow
back towards the head, out of which it was originally derived.

But here, again, a difficulty presents itself. The comet's tail is
believed by most of the illustrious astronomers of the day, to be the
body converted into vapour by solar influence. If it be so, the
vaporising process must be a much more subtile one than any that could
be performed in our alembics, for the comet's substance is already all
vapour before the distillation commences. The faintest stars have been
seen shining through the densest parts of comets without the slightest
loss of light, although they would have been effectually concealed by
a trifling mist extending a few feet from the earth's surface. Most
comets appear to have bright centres--nuclei, as they are called; but
these nuclei are not solid bodies, for as soon as they are viewed