• Absorption of light, 148
• Æther, the, 24
• Æther waves, 96, 131, 133, 137, 146, 148, 163
• Alpha rays from radium, 190
• Alternating electric current, 121
• Amber electrified, 32, 34 to 37, 201
• Artificial light, 140, 142
• Atoms breaking up, 188, 190
• Atoms co-operating with electrons, 108, 123
• Atom's internal energy, 187, 191
• Atoms of matter, 52, 54, 78, 128, 180, 184
• Attraction between atoms, 180
• Attraction, electrical, 35, 202
• Attraction, magnetic, 78, 205
• Aurora, 132
• Automatic telegraph transmitter, 91
• Battery, electric, 70, 203
• Beginning of the world, 53
• Beta rays from radium, 189
• Birth of the moon, 52, 54
• Bricks of the universe, 180, 195
• Chemical affinity, 206
• Chemical combinations, 56, 182
• Chemistry of the stars, 52, 55, 144, 153
• Chlorine atoms, 56, 182
• Cloud formation, 56
• Circuit, earth, 72
• Coherer, tube, 98
• Cohesive force, 183
• Colour, 136
• Compass needle, 77
• Complete electric circuit, 71
• Conductors, 37, 68
• Connecting link between Æther and matter, 118, 127
• Corpuscles, 66
• Crookes, Sir William, 205
• Current of electricity, 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

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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"