Stability of the solar system.—Losses and possible gains of heat by the sun.—Theses of Mayer and of Helmholtz.—Temperatures of the white, yellow, and reddish stars, and of the sun.—Sun-spots and sun faculæ.—Prominences.—Spectra of the parts of the sun.—Temperature of the sun.—The interior of the sun.—Its composition according to the mechanical theory of heat.—The losses of heat by the sun probably covered by the enormous solar energy. IV. THE RADIATION PRESSURE 94 Newton’s law of gravitation.—Kepler’s observation of comets’ tails.—The thesis of Euler.—Proof of Maxwell.—The radiation pressure.—Electric charges and condensation.—Comets’ tails and radiation pressure.—Constituents and properties of comets’ tails.—Weight of the solar corona.—Loss and gain of matter by the sun.—Nature of meteorites.—Electric charge of the sun.—Electrons drawn into the sun.—Magnetic properties of the sun and appearance of the corona.—Constituents of the meteors.—Nebulæ and their heat and light. V. THE SOLAR DUST IN THE ATMOSPHERE. POLAR LIGHTS AND THE VARIATIONS OF TERRESTRIAL MAGNETISM 118 The supply of dust from the sun rather insignificant.—Polarization of the light of the sky.—The upper clouds.—Different kinds of auroræ.—Their connection with the corona of the sun.—Polar lights and sun-spots.—Periodicity of polar lights.—Polar lights and magnetic disturbances.—Velocity of solar dust.—Fixation of atmospheric nitrogen.—The Zodiacal Light. VI. END OF THE SUN.—ORIGIN OF NEBULÆ 148 The extinction of the sun.—Collision between two celestial bodies.—The new star in Perseus.—Formation of nebulæ.—The appearance of nebulæ.—The nebulæ catch wandering meteors and comets.—The ring nebula in Lyra.—Variable stars.—Eta in Argus.—Mira Ceti.—Lyra and Algol stars.—Evolution of the stars. VII. THE NEBULAR AND THE SOLAR STATES 191 The energy of the universe.—The entropy of the universe.—The entropy increases in the suns, but decreases in the nebulæ.—Temperature and constitution of the nebulæ.—Schuster’s calculations of the condition of a celestial body consisting of gases.—Action of the loss of heat on nebulæ and on suns.—Development of a rotating nebula into a planetary system.—The hypothesis of Kant-Laplace.—Objections to it.—The views of Chamberlin and Moulton.—The radiation pressure balances the effect of Newtonian gravitation.—The emission of gases from the nebulæ balances the waste of heat characteristic to the solar systems. VIII. THE SPREADING OF LIFE THROUGH THE UNIVERSE 212