After the nucleus has been formed it begins to throw out bright jets
directed toward the sun. A stream, and sometimes several streams, of
light also project sunward from the nucleus, occasionally appearing
like a stunted tail directed oppositely to the real tail. Symmetrical
envelopes which, seen in section, appear as half circles or parabolas,
rise sunward from the nucleus, forming a concentric series. The ends
of these stream backward into the tail, to which they seem to supply
material. Ordinarily the formation of these ejections and envelopes is
attended by intense agitation of the nucleus, which twists and turns,
swinging and gyrating with an appearance of the greatest violence.
Sometimes the nucleus is seen to break up into several parts. The
entire heads of some comets have been split asunder in passing close
around the sun; The comet of 1882 retreated into space after its
perihelion passage with five heads instead of the one that it had
originally, and each of these heads had its own tail!

The possession of the spectroscope has enabled astronomers during
later years to study the chemical composition of comets by analyzing
their light. At first the only substances thus discovered in them were
hydro-carbon compounds, due evidently to the gaseous envelopes in
which some combination of hydrogen with carbon existed. Behind this
gaseous spectrum was found a faint continuous spectrum ascribed to the
nucleus, which apparently both reflects the sunlight and gives forth
the light of a glowing solid or liquid. Subsequently sodium and iron
lines were found in cometary spectra. The presence of iron would seem
to indicate that some of these bodies may be much more massive than
observations on their attractive effects have indicated. In some
recent comets, such as Morehouse's, in 1908, several lines have been
found, the origin of which is unknown.