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