ld convert that vast mass into steam as fast as it entered his
atmosphere without cooling its surface in the least degree. "The great
mystery, however, is to conceive how so enormous a conflagration (if
such it be) can be kept up. Every discovery in chemical science here
leaves us completely at a loss, or rather seems to remove farther the
prospect of probable explanation."[212] Yet, the sun is the nearest of
the fixed stars, and by far the best known, and most nearly related to
us. In fact, we are dependent on his influences for life and health. But
if the theorist _can not tell his substance, or the nature and cause of
the light and heat he sends us_, how can he presume so far on the
world's credulity as to present a theory of his formation?

"Astronomical problems accumulate unsolved upon our hands, because we
can not, as mechanicians, chemists, or physiologists, experiment on the
stars. Are they built of the same material as our planet? Are Saturn's
rings solid, or liquid? Has the moon an atmosphere? Are the atmospheres
of the planets like ours? Are the light and heat of the sun begotten of
combustion? And what is the fuel which feeds these unquenchable fires?
These are questions, which we ask, and variously answer, _but leave
unanswered after all_."[213] But, till he can answer these, and a
thousand questions like these, let no man presume to describe the
formation of these unknown orbs.

Comets constitute by far the greatest number of the bodies of our solar
system. Arago says seven millions frequent it, within the orbit of
Uranus.[214] They are the largest bodies known to us, stretching across
hundreds of millions of miles. They approach nearer to this earth than
any other bodies, sometimes even involving it in their tails, and
generally exciting great alarm among its inhabitants. But the nature of
the transparent luminous matter of which they are composed is utterly
unknown. As they approach the sun, they come under an influence directly
the opposite of attraction. The tail streams away from the sun, over a
distance of millions of miles, _and yet the rate of the comet's motion
toward the sun is quickened_, as though it were an immense rocket,
driven forward by its own explosion.

Further, while the body of the comet travels toward the sun, sometimes
with a velocity nearly one-third of that of light, the tail sends forth
coruscations in the opposite direction, with a much greater velocity.
The greatest velocity