t
"no rational or even plausible account has yet been rendered of those
immensely luminous appendages which they bear about with them, and which
are known as their tails." Yet, he believes, and astronomers generally
believe, that a comet shines by reflected light. This theory of
reflexion is the incubus which clogs the question with such formidable
difficulties; for, it follows, that the reflecting matter must come
from the comet. But, what wonderful elements must a comet be made of, to
project themselves into space with such immense velocity, and in such
enormous quantities as to exceed in volume the body from which they
emanate many millions of times. This theory may be, therefore, safely
rejected.

From what we have already advanced concerning the coma or nebulosity of
the comet, we pass by an easy path to an explanation of the tail. In the
short-period comets, the density of the elementary atoms is too great to
be detached in the gross from the nucleus, or, rather, the density of
the atoms composing the nucleus is too great to permit the radiating
stream of the comet carrying them to a sufficient distance to be
detached by the radial stream of the sun. Hence, these comets exhibit
but very little tails. We may also conceive, that the continual siftings
which the nucleus undergoes at each successive perihelion passage, have
left but little of those lighter elements in comets whose mean distances
are so small. Yet, again, if by any chance the eccentricity is
increased, there are two causes--the density of the ether, and the heat
of the sun--which may make a comet assume quite an imposing appearance
when apparently reduced to the comparatively passive state above
mentioned.

According to our theory, then, the coma of a comet is due to the
elasticity of the ethereal medium within the nucleus, caused both by the
diminished pressure of the external ether near the sun, and also by the
increased temperature acting on the nucleus, and thus on the involved
ether. The tail, on the contrary, is caused by the lighter particles of
the comet's attenuated atmosphere being blown off by the electric blast
of the radial stream of the solar vortex, in sufficient quantities to
render its passage visible. It is not, therefore, reflected light, but
an ethereal stream rendered luminous by this detached matter still held
in check by the gravitating force of the sun, whose centre each
particle still respects, and endeavors to describe such