PART II.

[1.]—Light and Heat. [223]

What is Light?—notion of the ancients; requires Time to pass through Space; Römer, 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