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] Footnote 149: On these monographs see _Dox._ p. 103. Footnote 150: See the conspectus of extracts from Theophrastos given in _Dox._ p. 135. Footnote 151: “Felting” (πίλησις) is the regular term for this process with all the early cosmologists, from whom Plato has taken it (_Tim._ 58 b 4; 76 c 3). Footnote 152: A more condensed form of the same doxographical tradition is given by Ps.-Plut. _Strom._ fr. 3 (R. P. 25). [Sidenote: 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. Footnote 153: Simplicius, _Phys._ p. 149, 32 (R. P. 26 b), says, according to the MSS., that Theophrastos spoke of rarefaction and condensation in the case of Anaximenes _alone_. We must either suppose with Zeller (p. 193, n. 2) that this means “alone among the oldest Ionians” or read πρῶτου for μόνου with Usener. The regular terms are πύκνωσις and ἀραίωσις or μάνωσις. Plutarch, _de prim. frig._ 947 f (R. P. 27), says that Anaximenes used the term τὸ χαλαρόν for the rarefied air. [Sidenote: Air.]