been the comet's chief
stock-in-trade as an earth-threatener, is in reality composed of
the most filmy vapors, repelled from the cometary body by the sun,
presumably through electrical action, with a velocity comparable to that
of light. This luminous suggestion was held more or less in abeyance for
half a century. Then it was elaborated by Zollner, and particularly by
Bredichin, of the Moscow observatory, into what has since been regarded
as the most plausible of cometary theories. It is held that comets
and the sun are similarly electrified, and hence mutually repulsive.
Gravitation vastly outmatches this repulsion in the body of the comet,
but yields to it in the case of gases, because electrical force varies
with the surface, while gravitation varies only with the mass. From
study of atomic weights and estimates of the velocity of thrust of
cometary tails, Bredichin concluded that the chief components of the
various kinds of tails are hydrogen, hydrocarbons, and the vapor of
iron; and spectroscopic analysis goes far towards sustaining these
assumptions.

But, theories aside, the unsubstantialness of the comet's tail has been
put to a conclusive test. Twice during the nineteenth century the
earth has actually plunged directly through one of these threatening
appendages--in 1819, and again in 1861, once being immersed to a depth
of some three hundred thousand miles in its substance. Yet nothing
dreadful happened to us. There was a peculiar glow in the atmosphere,
so the more imaginative observers thought, and that was all. After such
fiascos the cometary train could never again pose as a world-destroyer.

But the full measure of the comet's humiliation is not yet told. The
pyrotechnic tail, composed as it is of portions of the comet's actual
substance, is tribute paid the sun, and can never be recovered. Should
the obeisance to the sun be many times repeated, the train-forming
material will be exhausted, and the comet's chiefest glory will have
departed. Such a fate has actually befallen a multitude of comets which
Jupiter and the other outlying planets have dragged into our system and
helped the sun to hold captive here. Many of these tailless comets were
known to the eighteenth-century astronomers, but no one at that time
suspected the true meaning of their condition. It was not even known how
closely some of them are enchained until the German astronomer Encke,
in 1822, showed that one which he had rediscovered,