specularly, or polarised, that is to say, light coming
from the sun; and the experiment was
considered a proof that comets are not luminous
of themselves, at least in part. Porro holds
(rather, it would appear, on theoretical than
experimental grounds), that matter, in the
state in which it exists in the comets, is
incompetent to reflect the solar light, but that
its atoms are capable of making luminous
vibrations under the sun's influence; comets
would therefore shine like light itself. Matter,
in the two states in which it is observed in
the comets, is distinguished from matter in
the ethereal state mainly in this, namely, that
when acted on by a centre of attraction
around which it tends to agglomerate, it
assumes a form with a definite outline and
boundary.

In open space, and far away from the
sun's attraction, a comet would necessarily
be spherical: its nucleus, if it had one, would
be in the centre. But, under the influence of
the sun's attraction, and in consequence of the
resistance of the ether (now admitted by
several astronomers), this sphere would
necessarily become a very long ellipsoid, in which
the nucleus would occupy one of the foci.
The form of comets is taken to be an evident
proof of the resistance of the ether. If that
resistance makes itself felt by the comets and
not by the planets, the reason is that its
density is an infinitesimal of the second order
in respect to the density of the planets;
whilst, although relatively very small, it is
nevertheless comparable to that of the
comets.

The outline of Donati's comet has always
appeared perfectly clean and round, and the
light of the nucleus has always preserved its
planetary aspect. No ebullition, scattering
of sparks, nor currents of cosmic matter,
have been observed in any part of the star.
Neither has anything been seen which allows
us to conclude that either the nucleus or its
ellipsoidal atmosphere revolve on any axis
whatever. Lastly, the passage of the comet
in front of Arcturus showed that the ring
round the nucleus actually existed, and was
not an optical deception.

Monsieur Porro's estimate of the excessive,
the unimaginable lightness of the comets,
goes even further than the opinions of Sir
John Herschel and Monsieur Babinet, and
quite relieves us from participating in
Laplace's uneasiness lest a comet should
possibly dash against the earth. It has been
already proved that comets can pay a visit
to planets and take their departure quite
inoffensively. In seventeen hundred and
seventy, Lexell's comet (which came nearer
to the earth than any other has done) passed
amidst Jupiter's satellites without in the
least disturbing their motions, although two
of them are interior in volume to our moon.
Biot calculated that, if the mass of the comet
had been equal to two ten-thousandth parts of
the terrestrial mass, it would have produced,
by passing at the distance at which it did
pass, an alteration of three seconds of time in
the duration of our year. As it has not
produced any such alteration, the conclusion is
that the density of this comet does not
amount to the required two ten-thousandth
parts of that of the earth; which is a very
reassuring figure for those who believe in
the possibility of an encounter with our
globe.

If comets are thus constituted, it is
scarcely possible for one of them even to
enter the atmosphere of the earth. It is
believed that the air, at an elevation of
from thirty to forty miles, is in as rarefied
a state as that in the receiver of an air-pump
in which we have made what we call
a vacuum; and yet, according to Herschel's
opinion, the density of such atmospheric air
would be thousands of times greater than
that of the nebulous matter composing a
comet. The inference from these facts is,
that a comet which should dash full-butt
against a planetary atmosphere, would no
more be able to traverse it than the water
from a syringe would be competent to force
its way through the compact mass of a sea of
mercury. As soon as the nebulous matter of
a comet reached the upper stratum of a
planetary atmosphere, it would slide along
the surface of the stratum, and then fly off at
a tangent, completely changing the form of
its primitive orbit. From this we may
derive some explanation of the enormous
perturbations which the very same comet
experiences between two of its successive
appearances. The perturbation is sometimes
so great, that its altered form and disturbed
orbit render it no longer recognisable at the
end of a very few years. There is no reason,
therefore, to fear—at least, on the part of the
nucleusless comets—either shock or suffocation
by means of the deleterious gases which
might enter into the composition of their
nebulosity.

Up to the present time, there is nothing to
give us certain proof that comets are
furnished with a solid, and as it were, planetary
nucleus: unless, indeed, we consider aërolites
(some of which have attained proportions not
less colossal than those of our ancient Gothic
cathedrals) as cometary nuclei, which have
been stripped of their nebulosity by being
plunged into our atmosphere, and which,
then obeying the laws of attraction, take fire
by the friction of their passage through the
air, and finish their course by falling to the
ground in the shape of stony masses. This
theory, in spite of its novelty, is not more
improbable than other theories, its
predecessors. A member of the British Association
argued three years ago, with strong reasons,
that the greater part of the phenomena which
we call meteors are not engendered in our
atmosphere, but that all their characters
tend to connect them with the comets. There
are luminous appearances, such as shooting