Hydrogen atoms, 55, 182 Insulators (non-conductors), 37, 47 Iron atoms, 77 Iron wires discarded, 88 Lamp, electric, 140 Leyden jar, 42, 202 Light, 23, 60, 64, 133 Light absorbed, 148 Light, artificial, 140, 142 Light, reflected, 148 Lightning, 42, 48 Lines in the spectrum, 152, 154, 160, 162 Lorentz's declaration, 206 Magnetic attraction, 78, 205 Magnetic field, 68, 76, 118 Magnetic repulsion, 205 Magnetism, 73, 76, 203, 204 Magnetism and electricity, 73 Magnets, electro-, 76, 81, 83, 205 Magnets, permanent, 83 Mariner's compass, 77 Matter, 52, 54 Metal electrified, 37, 38 Molecules of matter, 181, 183 Moon's birth, 52, 54 Morse telegraph, 88 Motion in line of sight, 162 Motor, electric, 116, 122 Negative electricity, 23, 32, 39 Oersted's discovery, 204 Oxygen atoms, 182 Permanent magnets, 82 Positive electricity, 23, 32, 39, 52, 180 Prism of glass, 147, 152 Radiant heat, 131, 133, 142 Radiant matter, 63, 205 Radium, 188 Rainbow, 147 Rays from radium, 189, 190 Reflection of light, 148 Repulsion, electrical, 202 Repulsion, magnetic, 205 Roentgen rays, 167 Roentgen's discovery, 168, 206 Sea, cause of saltness, 56 Shock, electric, 47 Silk, electrified, 38 Sodium atoms, 56, 182 Spark, electric, 44 Spectroscope, 152, 154, 207 Spectrum, 144, 147, 152, 154 Speed of electrons in conductor, 70 Stars approaching the earth, 162 Stars, constituents of the, 52, 55, 146 Stoney, Dr. Johnstone, 206 Sun, constituents of the, 154 Sun's heat, 128, 131 Telegraph signals, 90 Telegraphy, wireless, 95 Telephone, 109 Telephony, wireless, 110 Temperature, 180 Tramway, electric, 117, 118, 122 Transparent substances, 149 Vacuum tubes, 60, 61, 132, 205 Velocity of electrons, 70 Volta's discovery, 203 Waves in the æther, 96, 133, 137, 146, 148, 163 Wireless messages from the stars, 162 Wireless telegraphy, 95 Wireless telephony, 110 X-rays, 166, 206 X-rays from radium, 189 X-ray photography, 173 Zeeman proves existence of electrons, 161, 206 Printed by BALLANTYNE, HANSON & CO. Edinburgh & London * * * * * Transcriber's Note The following changes have been made to the original text: Page xi: "always necessary, How" changed to "always necessary. How" Page 205: "vacuum tubes, When" changed to "vacuum tubes. When" Page 214: "Negative electricity, 23, 32, 9" changed to "Negative electricity, 23, 32, 39"