Air: 2.6.1 ff., 2.11.2, 6.21.1; its unity, 2.6.2, 2.6.6, 2.7.1, 2.7.2; its tension, 2.6.2–4, 2.8.1, 2.9.1–4; its spontaneous movement, 5.5.2, 5.6.1; its variations, 2.10.1–4, 2.11.1–2, 2.14.2; its function, 2.4.1, 2.6.1; sound-producing, 2.29.1: its capacity for self-movement, 5.5.1–5.6.1;; catches fire under ground, 5.14.4 emitted from the earth (spiritus), 6.16.1–4; underground air becomes water, 3.9.1–3; at its thickest where closest to earth, 4.10.1; as a cause of earthquakes, 6.12.1–6.19.2, 6.20.6–7, 6.25.1–4, 6.31.1–2; poisonous air, arising from earthquakes, 6.27.1–6.28.3; Epigenes (on comets), 7.6.2, 7.8.1–4; condensed, giving rise to comets, 7.21.1; see wind
clouds: their colliding produces lightning, 1.1.6; their part in the explanation of rainbows, 1.3.1, 1.3.12, 1.8.6–7; the source of lightning, 2.12.1–2.17.1; 2.22.2, 2.32.2; cp. 1.1.6; lightning passing through them, 2.20.2, 2.58.1; fire produced in, 2.26.1–9; the source of thunder, 2.27.2–2.29.1, 2.54.1–2.55.3; cp. 1.1.6; their colour foreshadowing hail, 4.6.2; the origin of winds, 5.12.1–5; underground, 5.14.2–3, 6.9.1; always in motion, 7.22.1
columns (meteors), 7.20.2
comets: their characters, 7.11.1–3; their diversity, 7.6.1, 7.17.3; meteors or planets?, 7.2.1–2, 7.4.1; as meteors (Stoics), 7.19.1, 7.20.1–7.22.2, 7.30.2; fixed (Epigenes), 7.6.1–3, 7.7.1; wandering (Epigenes), 7.7.2–3, 7.8.1–4, 7.9.1–4, 7.10.1; not produced from two or several planets being joined, 7.12.1 ff., 7.14.4, 7.15.1–2; not broken into two stars or planets, 7.16.2–3; early theories of, 7.3.1–3, 7.4.1, 7.5.3–5, 7.19.1–2; as planets (Apollonius of Myndos), 7.17.1–7, 7.18.2; as stars (Seneca), 7.22.1, 7.24.3, 7.25.7, 7.26.1–2, 7.27.2–6, 7.29.2–3; of Claudius and Nero, 7.17.2, 7.21.3–4, 7.29.2–3