ndation of a mathematical theory
of cometary emanations. Professor W. A. Norton, of Yale College,
considerably improved this by inquiries begun in 1844, and resumed on
the apparition of Donati's comet; and Dr. C. F. Pape at Altona[1267]
gave numerical values for the impulses outward from the sun, which must
have actuated the materials respectively of the curved and straight
tails adorning the same beautiful and surprising object.

The _physical_ theory of repulsion, however, was, it might be said,
still in the air. Nor did it even begin to assume consistency until
Zoellner took it in hand in 1871.[1268] It is perfectly well ascertained
that the energy of the push or pull produced by electricity depends
(other things being the same) upon the _surface_ of the body acted on;
that of gravity upon its _mass_. The efficacy of solar electrical
repulsion relatively to solar gravitational attraction grows,
consequently, as the size of the particle diminishes. Make this small
enough, and it will virtually cease to gravitate, and will
unconditionally obey the impulse to recession.

This principle Zoellner was the first to realise in its application to
comets. It gives the key to their constitution. Admitting that the sun
and they are similarly electrified, their more substantially aggregated
parts will still follow the solicitations of his gravity, while the
finely divided particles escaping from them will, simply by reason of
their minuteness, fall under the sway of his repellent electric power.
They will, in other words, form "tails." Nor is any extravagant
assumption called for as to the intensity of the electrical charge
concerned in producing these effects. Zoellner, in fact, showed[1269]
that it need not be higher than that attributed by the best authorities
to the terrestrial surface.

Forty years have elapsed since M. Bredikhine, director successively of
the Moscow and of the Pulkowa Observatories, turned his attention to
these curious phenomena. His persistent inquiries on the subject,
however, date from the appearance of Coggia's comet in 1874. On
computing the value of the repulsive force exerted in the formation of
its tail, and comparing it with values of the same force arrived at by
him in 1862 for some other conspicuous comets, it struck him that the
numbers representing them fell into three well-defined classes. "I
suspect," he wrote in 1877, "that comets are divisible into groups, for
each of which the repulsiv