distance from each other of about two-thirds the distance of the
moon from the earth, the twin comets meantime moved on tranquilly, so
far, at least, as their course through the heaven was concerned. Their
extreme _lightness_, or the small amount of matter contained in each,
could not have received a more signal illustration than by the fact that
their revolutions round the sun were performed independently; that is to
say, they travelled side by side without experiencing any appreciable
mutual disturbance, thus plainly showing that at an interval of only
157,250 miles their attractive power was virtually inoperative. Signs of
internal agitation, however, were not wanting. Each fragment threw out a
short tail in a direction perpendicular to the line joining their
centres, and each developed a bright nucleus, although the original
comet had exhibited neither of these signs of cometary vitality. A
singular interchange of brilliancy was, besides, observed to take place
between the coupled objects, each of which alternately outshone and was
outshone by the other, while an arc of light, apparently proceeding from
the more lustrous, at times bridged the intervening space. Obviously,
the gravitational tie, rendered powerless by exiguity of matter, was
here replaced by some other form of mutual action, the nature of which
can as yet be dealt with only by conjecture.

Once more, in August, 1852, the double comet returned to the
neighbourhood of the sun, but under circumstances not the most
advantageous for observation. Indeed, the companion was not detected
until September 16, when Father Secchi at Rome perceived it to have
increased its distance from the originating body to a million and a
quarter of miles, or about eight times the average interval at the
former appearance. Both vanished shortly afterwards, and have never
since been seen, notwithstanding the eager watch kept for objects of
such singular interest, and the accurate knowledge of their track
supplied by Santini's investigations. A dangerously near approach to
Jupiter in 1841 is believed to have occasioned their disruption, and the
disaggregating process thus started was likely to continue. We can
scarcely doubt that the fate has overtaken them which Newton assigned as
the end of all cometary existence. _Diffundi tandem et spargi per
coelos universos._[260]

Biela's is not the only vanished comet. Brorsen's, discovered at Kiel in
1846, and observed at four subsequen