What is Light?--notion of the ancients; requires Time to pass through Space; Roemer, Bradley, Fizeau; Emission Theory supported by Newton, opposed by Huyghens; the Wave Theory established by Young and Fresnel; Theory explained; nature of Sound; of Music; of Pitch; nature of Light; of Colour; two sounds may produce silence; two rays of light may produce darkness; two rays of heat may produce cold; Length and Number of waves of light; Liquid Waves; Interference; Diffraction; Colours of Thin Plates; applications of the foregoing to cloud iridescences, luminous trees, twinkling of stars, the Spirit of the Brocken, &c. 2.--Radiant Heat. 239 The Sun emits a multitude of Non-luminous Rays; Rays of Heat differ from rays of Light as one colour differs from another; the same ray may produce the sensations of light and heat 3.--Qualities of Heat. 241 Heat a kind of Motion; system of exchanges; Luminous and Obscure Heat; Absorption by Gases; gases may be transparent to light, but opaque to heat; Heat selected from luminous sources; the Atmosphere acts the part of a Ratchet-wheel; possible heat of a Distant Planet; causes of Cold in the upper strata of the Earth's Atmosphere 4.--Origin of Glaciers. 248 Application of principles; the Snow-line; its meaning; waters piled annually in a solid form on the summits of the hills; the Glaciers furnish the chief means of escape; superior and inferior snow-line 5. 249 Whiteness of snow; whiteness of ice; Round air-bubbles; melting and freezing; Conversion of snow into ice by Pressure 6.--Colour of Water and Ice. 253 Waves of Ether not entangled; they are separated in the prism; they are differently absorbed; Colour due to this; Water and Ice blue; water and ice opaque to radiant heat; Long Waves shivered on the molecules; Experiment; Grotto of Capri; the Laugs of Iceland 7.--Colours of the Sky. 257 Newton's idea; Goethe's Theory; Clausius and Bruecke; Suspended Particles; singular effect on a painting explained by Goethe; Light separated without Absorption; Reflected and Transmitted light; blueness of milk and juices; the Sun through London smoke; Experiments; Blue of the Eye; Colours of Steam; the Lake of Geneva 8.--The Moraines. 263