eliminary part of his
enquiry, which possesses a value that cannot be affected even if the
ulterior portion of his labours should be found to require
qualification. In the examination of the various tails, he observed that
the curvilinear shapes of the outlines fall into one or other of three
special types. In the first we have the straightest tails, which point
almost directly away from the sun. In the second are classed tails
which, after starting away from the sun, are curved backwards from the
direction towards which the comet is moving. In the third we find the
appendage still more curved in towards the comet's path. It can be shown
that the tails of comets can almost invariably be identified with one or
other of these three types; and in cases where the comet exhibits two
tails, as has sometimes happened, then they will be found to belong to
two of the types.

The adjoining diagram (Fig. 72) gives a sketch of an imaginary comet
furnished with tails of the three different types. The direction in
which the comet is moving is shown by the arrow-head on the line passing
through the nucleus. Bredichin concludes that the straightest of the
three tails, marked as Type I., is most probably due to the element
hydrogen; the tails of the second form are due to the presence of some
of the hydrocarbons in the body of the comet; while the small tails of
the third type may be due to iron or to some other element with a high
atomic weight. It will, of course, be understood that this diagram does
not represent any actual comet.

[Illustration: Fig. 72.--Bredichin's Theory of Comets' Tails.]

[Illustration: Fig. 73.--Tails of the Comet of 1858.]

An interesting illustration of this theory is afforded in the case of
the celebrated comet of 1858 already referred to, of which a drawing is
shown in Fig. 73. We find here, besides the great tail, which is the
characteristic feature of the body, two other faint streaks of light.
These are the edges of the hollow cone which forms a tail of Type I.
When we look through the central regions it will be easily understood
that the light is not sufficiently intense to be visible; at the edges,
however, a sufficient thickness of the cometary matter is presented, and
thus we have the appearance shown in this figure. It would seem that
Donati's comet possessed one tail due to hydrogen, and another due to
some of the compounds of carbon. The carbon compounds involved appear to
be of considerable