“Just as,” he said, “our soul, being air, holds us together, so do breath and air encompass the whole world.”—Aet. i. 3, 4 (R. P. 24).
And the form of the air is as follows. Where it is most even, it is invisible to our sight; but cold and heat, moisture and motion, make it visible. It is always in motion; for, if it were not, it would not change so much as it does.—Hipp. Ref. i. 7 (R. P. 28).
It differs in different substances in virtue of its rarefaction and condensation.—Phys. Op. fr. 2 (R. P. 26).
When it is dilated so as to be rarer, it becomes fire; while winds, on the other hand, are condensed Air. Cloud is formed from Air by felting;[[151]] and this, still further condensed, becomes water. Water, condensed still more, turns to earth; and when condensed as much as it can be, to stones.—Hipp. Ref. i. 7 (R. P. 28).[[152]]
Rarefaction and condensation.
26. At the first glance, this undoubtedly looks like a falling off from the more refined doctrine of Anaximander to a cruder view; but a moment’s reflexion will show that this is not altogether the case. On the contrary, the introduction of rarefaction and condensation into the theory is a notable advance.[[153]] In fact, it makes the Milesian cosmology thoroughly consistent for the first time; since it is clear that a theory which explains everything by the transformations of a single substance is bound to regard all differences as purely quantitative. The infinite substance of Anaximander, from which the opposites “in it” are “separated out,” cannot, strictly speaking, be thought of as homogeneous, and the only way to save the unity of the primary substance is to say that all diversities are due to the presence of more or less of it in a given space. And when once this important step has been taken, it is no longer necessary to make the primary substance something “distinct from the elements,” to use Aristotle’s inaccurate but convenient phrase; it may just as well be one of them.
Air.
27. The air that Anaximenes speaks of includes a good deal that we should not call by that name. In its normal condition, when most evenly distributed, it is invisible, and it then corresponds to our “air”; it is identical with the breath we inhale and the wind that blows. That is why he called it πνεῦμα. On the other hand, the old idea, familiar to us in Homer, that mist or vapour is condensed air, is still accepted without question. In other words, we may say that Anaximenes supposed it to be a good deal easier to get liquid air than it has since proved to be. It was Empedokles, we shall see, who first discovered that what we call air was a distinct corporeal substance, and was not identical either with vapour or with empty space. In the earlier cosmologists “air” is always a form of vapour, and even darkness is a form of it. It was Empedokles who cleared up this point too by showing that darkness is a shadow.[[154]]
It was natural for Anaximenes to fix upon Air in this sense as the primary substance; for, in the system of Anaximander, it occupied an intermediate place between the two fundamental opposites, the sphere of flame and the cold, moist mass within it ([§ 19]). We know from Plutarch that he fancied air became warmer when rarefied, and colder when condensed. Of this he satisfied himself by a curious experimental proof. When we breathe with our mouths open, the air is warm; when we breathe with our lips closed, it is cold.[[155]]
The world breathes.