This is encompassed by the highest region of the heavens, which is called the sky, which is joined to the extremity of the air, but retains its own heat pure and unmixed.
XLVI.

The stars have their revolutions in the sky, and are continued by the tendency of all parts towards the centre.

Their duration is perpetuated by their form and figure, for they are round; which form, as I think has been before observed, is the least liable to injury; and as they are composed of fire, they are fed by the vapors which are exhaled by the sun from the earth, the sea, and other waters; but when these vapors have nourished and refreshed the stars, and the whole sky, they are sent back to be exhaled again; so that very little is lost or consumed by the fire of the stars and the flame of the sky.

Hence we Stoics conclude--which Panaetius[221] is said to have doubted of--that the whole world at last would be consumed by a general conflagration, when, all moisture being exhausted, neither the earth could have any nourishment, nor the air return again, since water, of which it is formed, would then be all consumed; so that only fire would subsist; and from this fire, which is an animating power and a Deity, a new world would arise and be re-established in the same beauty.
I should be sorry to appear to you to dwell too long upon this subject of the stars, and more especially upon that of the planets, whose motions, though different, make a very just agreement.

Saturn, the highest, chills; Mars, placed in the middle, burns; while Jupiter, interposing, moderates their excess, both of light and heat.