chel, in his 4th Reflection, states that "a
considerable part of the vapour actually produced remains in the
neighbourhood of the nucleus forming the head or coma." So that the head
of a comet is simply the vaporised part of the nucleus which is produced
by the increased heat of the sun, in the same way that water would be
vaporised by the addition of heat, the vapour in that case being thrown
off in the form of steam.

This formation of the head is but a continuation of the reversal of the
process of condensation, which originally gave existence to the mass of
matter termed the comet. The diameter of this head or coma often extends
to thousands of miles. The head of the 1811 comet was 540,000 miles in
diameter, while that of the 1843 was 112,000 miles. As the nucleus is
formed of a series of envelopes, so the head also consists of a series
of envelopes.

The comet of 1858 constantly threw off these envelopes, which were first
expelled _towards_ the sun, and then repelled away _from_ the sun,
forming the tail. The matter forming the head and the nucleus is
perfectly transparent, as stars have been seen through the matter which
forms those parts. Herschel,[42] paragraph 558, states "that whenever
powerful telescopes have been turned on these bodies, they have not
failed to dispel the illusion which attributes solidity to that more
condensed part of the head which appears to the naked eye, though it is
true that in some a very minute stellar point has been seen indicating
the existence of a stellar body."

_Tails._--The tail of a comet is that part which flows from the head,
and is afterwards repelled by the repulsive power of the sun into space.
We shall deal with this repulsive power, whose existence we have already
demonstrated, and the part which it plays in the formation of a comet's
tail, in the next article. The tail of a comet is oftentimes considered
to be the comet itself, rather than a part of the same, but as the tail
is the most distinctive feature of a comet, and is the part most visible
to the naked eye, there has arisen the popular but mistaken idea of
identity between the tail and the comet itself.

Tails are of all kinds. There are some which are short, while others are
long. Then we have comets with single tails, or double, and in some
cases even multiple tails. Occasionally comets appear which have no
tails at all. The comet of 1744 had six tails, which spread out in the
shape of a large fan.

On