e, or nearly the same, track; so that
identity of orbit can no longer be regarded as a sure test of individual
identity. Secondly, that at least the outer corona may be traversed by
such bodies with perfect apparent impunity. Finally, that their chemical
constitution is highly complex, and that they possess, in some cases at
least, a metallic core resembling the meteoric masses which occasionally
reach the earth from planetary space.

A group of five comets, including Halley's, own a sort of cliental
dependence upon the planet Neptune. They travel out from the sun just to
about his distance from it, as if to pay homage to a powerful protector,
who gets the credit of their establishment as periodical visitors to the
solar system. The second of these bodies to affect a looked-for return
was a comet--the sixteenth within ten years--discovered by Pons, July
20, 1812, and found by Encke to revolve in an elliptic orbit, with a
period of nearly 71 years. It was not, however, until September 1, 1883,
that Mr. Brooks caught its reappearance; it passed perihelion January
25, and was last seen June 2, 1884. At its brightest, it had the
appearance of a second magnitude star, furnished with a poorly developed
double tail, and was fairly conspicuous to the naked eye in Southern
Europe, from December to March. One exceptional feature distinguished
it. Its fluctuations in form and luminosity were unprecedented in
rapidity and extent. On September 21, Dr. Chandler[1339] observed it at
Harvard as a very faint, diffused nebulosity, with slight central
condensation. On the next night, there was found in its place a bright
star of the eighth magnitude, scarcely marked out, by a bare trace of
environing haze, from the genuine stars it counterfeited. The change was
attended by an eight-fold augmentation of light, and was proved by
Schiaparelli's confirmatory observations[1340] to have been accomplished
within a few hours. The stellar disguise was quickly cast aside. The
comet appeared on September 23 as a wide nebulous disc, and soon after
faded down to its original dimness. Its distance from the sun was then
no less than 200 million miles, and its spectrum showed nothing unusual.
These strange variations recurred slightly on October 15, and with
marked emphasis on January 1, when they were witnessed with amazement,
and photometrically studied by Mueller of Potsdam.[1341] The entire cycle
this time was run through in less than four hours--the