For the phrase τοῦ περὶ τὸν ἀέρα πάγου cf. Περὶ διαίτης, i. 10, 1, πρὸς τὸν περιέχοντα πάγον. _Et. M. s.v._ βηλὸς ... τὸν ἀνωτάτω πάγον καὶ περιέχοντα τὸν πάντα ἀέρα. This probably comes ultimately from Anaximenes. Cf. Chap. I. p. 82, _n._ 162. The first of the elements to be separated out by Strife, then, was Air, which took the outermost position surrounding the world (cf. fr. 38). We must not, however, take the statement that it surrounded the world “in a circle” too strictly. It appears that Empedokles regarded the heavens as shaped like an egg.[604] Here, probably, we have a trace of Orphic ideas. At any rate, the outer circle of the Air became solidified or frozen, and we thus get a crystalline vault as the boundary of the world. We note that it was Fire which solidified the Air and turned it to ice. Fire in general had a solidifying power.[605] Footnote 604: Aet. ii. 31, 4 (_Dox._ p. 363). Footnote 605: Aet. ii. 11, 2 (R. P. 170 c). In its upward rush Fire displaced a portion of the Air in the upper half of the concave sphere formed by the frozen sky. This air then sunk downwards, carrying with it a small portion of the fire. In this way, two hemispheres were produced: one, consisting entirely of fire, the diurnal hemisphere; the other, the nocturnal, consisting of air with a little fire. The accumulation of Fire in the upper hemisphere disturbs the equilibrium of the heavens and causes them to revolve; and this revolution not only produces the alternation of day and night, but by its rapidity keeps the heavens and the earth in their places. This was illustrated, Aristotle tells us, by the simile of a cup of water whirled round at the end of a string.[606] The verses which contained this remarkable account of so-called “centrifugal force” have been lost; but the experimental illustration is in the manner of Empedokles. [Sidenote: The sun, moon, stars, and earth.]