The fire of which we speak above emits the purest light, and resides
in the highest region, by virtue of its nature. These celestial flames
are entirely distinct from the earthly flame, which after ascending
to a certain height, and meeting a greater quantity of air, becomes
extinguished. After ascending, it falls back on to the earth, because
(as a comet) it cannot rise any further; it stops in the sublunar
regions, though rendering the ambient air lighter. In those cases in
which it continues to subsist in higher regions, it becomes weaker,
gentler, and acquires a heatless glow, which is but a reflection of the
celestial light. The latter, on the other hand, is divided partly among
the stars in which it reveals great contrasts of magnitude and color,
and partly in the atmosphere. Its invisibility to our eyes is caused
both by its tenuity, and transparence, which causes it to become as
tangible as pure air, and also because of its distance from the earth.