Delambre, on cosmic speculations, 14, 15 Democritus, mechanical theory of the world, 5, 22 Descartes, theory of vortices, 11-13 Dissipation of energy, 229 Double stars, genesis and development, 98-9, 106-7, 142-5, 226-7; occulting, 108-113; variable, 113-115, 143; 'dumb-bell' type, 115, 116, 143; frequency of occurrence, 144, 234 Earth, effective rigidity, 86; critical situation, 87; antique disruption, 89, 91; tidal effects upon, 90, 244; meteoric intakes, 119-120, 133; internal heat, 266 Easton, the galactic spiral, 214 Eclipses, stellar, 107-8; questionable occurrence, 109-113; criterion of reality, 114 Electricity, discharge through vacua, 154; relations with disintegrated matter, 155, 159-160, 163; gravity explained by, 177-8, 179; dual properties, 180-181; fundamental in nature, 182; ethereal transmission, 185 Electrons, copious production, 174; strain-centres, 189-190, 195; ultimates of matter, 231 Elements, ancient and mediæval ideas regarding, 5, 151; genesis, 152, 230-231 Empedocles, four elements, 5, 151 Ether, in Cartesian philosophy, 12; evasive character, 159, 183, 197-8; relation to gravity, 175, 185; properties, 184, 191, 193-4 Evolution, of solar system, 22, 241, 250; of stars, 35-6, 217-225; of stellar systems, 98-9, 106-7, 142-5, 226-7; of nebulæ, 145-6, 200-211; of the chemical elements, 151, 230-231 Faye, objections to Laplace's cosmogony, 25; modified scheme, 68-73, 81, 129; duration of the sun, 76 Fleming, nature of electrons, 195 Frost and Adams, spectroscopic binaries, 143, 221 Galileo, first law of motion, 12 Goldstein, cathode-rays, 155 Gravitation, mode of action, 168, 185; velocity of transmission, 169, 175, 177; explanatory hypotheses, 170-179 Hale, relationships of carbon stars, 225 Halley, nature of nebulæ, 36 Hansky, status of zodiacal light, 261 Harvey, life from life, 268 Helium, present in the sun, 55; origin by atomic decay, 158, 230; absorption in stars, 220, 222-3; a constituent of nebulæ, 230 Helmholtz, maintenance of solar heat, 31, 34, 35, 74-5 Helmont, spontaneous generation, 268 Heracleitus, elemental evolution, 151 Herodotus, omnipotence of time, 269 Herschel, Sir John, nature of atoms, 156; Saturn's satellites, 240 Herschel, Sir William, Uranian system, 29; sidereal evolution, 35-6, 37, 136; observations of nebulæ, 199; history of their growth, 200-201 Huggins, Sir William, discovery of gaseous nebulæ, 54, 153 Huggins, Sir William and Lady, stellar development, 226 Huxley, origin of life, 269, 271, 272 Hydrogen, molecular velocity, 55; the unit-atom, 152; tenuity, 153; absorption in stars, 221-2 Island-universes, 54 Jeans, figures of equilibrium, 104-5; nature of radio-activity, 196 Jupiter's system, 240-1