a process in the sun which will be intensified until it falls into the
state of such a star as Mira. Stars very far advanced in evolution,
without showing variability, also exhibit similar spectra; so that
there is much reason for regarding sunspots as emblems of advancing
age.

The association of the corona with sun-spots is less evident than that
of the eruptive prominences; still such an association exists, for the
form and extent of the corona vary with the sun-spot period of which
we shall presently speak. The constitution of the corona remains to be
discovered. It is evidently in part gaseous, but it also probably
contains matter in the form of dust and small meteors. It includes one
substance altogether mysterious -- ``coronium.'' There are reasons for
thinking that this may be the lightest of all the elements, and
Professor Young, its discoverer, said that it was ``absolutely unique
in nature; utterly distinct from any other known form of matter,
terrestial, solar, or cosmical.'' The enormous extent of the corona is
one of its riddles. Since the development of the curious subject of
the ``pressure of light'' it has been proposed to account for the
sustentation of the corona by supposing that it is borne upon the
billows of light continually poured out from the sun. Experiment has
proved, what mathematical considerations had previously pointed out as
probable, that the waves of light exert a pressure or driving force,
which becomes evident in its effects if the body acted upon is
sufficiently small. In that case the light pressure will prevail over
the attraction of gravitation, and propel the attenuated matter away
from the sun in the teeth of its attraction. The earth itself would be
driven away if, instead of consisting of a solid globe of immense
aggregate mass, it were a cloud of microscopic particles. The reason
is that the pressure varies in proportion to the surface of the body