commonly call fire. It is not really fire, for fire is an excess of
heat and a sort of ebullition; but in reality, of what we call air,
the part surrounding the earth is moist and warm, because it
contains both vapour and a dry exhalation from the earth. But the next
part, above that, is warm and dry. For vapour is naturally moist and
cold, but the exhalation warm and dry; and vapour is potentially
like water, the exhalation potentially like fire. So we must take
the reason why clouds are not formed in the upper region to be this:
that it is filled not with mere air but rather with a sort of fire.
However, it may well be that the formation of clouds in that upper
region is also prevented by the circular motion. For the air round the
earth is necessarily all of it in motion, except that which is cut off
inside the circumference which makes the earth a complete sphere. In
the case of winds it is actually observable that they originate in
marshy districts of the earth; and they do not seem to blow above
the level of the highest mountains. It is the revolution of the heaven
which carries the air with it and causes its circular motion, fire
being continuous with the upper element and air with fire. Thus its
motion is a second reason why that air is not condensed into water.
But whenever a particle of air grows heavy, the warmth in it is
squeezed out into the upper region and it sinks, and other particles
in turn are carried up together with the fiery exhalation. Thus the
one region is always full of air and the other of fire, and each of
them is perpetually in a state of change.
So much to explain why clouds are not formed and why the air is
not condensed into water, and what account must be given of the
space between the stars and the earth, and what is the body that fills
As for the heat derived from the sun, the right place for a
special and scientific account of it is in the treatise about sense,
since heat is an affection of sense, but we may now explain how it can
be produced by the heavenly bodies which are not themselves hot.