§ 5. The X-Rays: Röntgen's discovery--Properties of X-rays--Not homogeneous--Rutherford and M'Clung's experiments on energy corresponding to--Barkla's experiments on polarisation of--Their speed that of light--Are they merely ultra-violet?--Stokes and Wiechert's theory of independent pulsations generally preferred--J.J. Thomson's idea of their formation-- Sutherland's and Le Bon's theories--The N-Rays-- Blondlot's discovery--Experiments cannot be repeated outside France--Gutton and Mascart's confirmation-- Negative experiments prove nothing--Supposed wave-length of N-rays. § 6. The Ether and Gravitation: Descartes' and Newton's ideas on gravitation--Its speed and other extraordinary characteristics--Lesage's hypothesis--Crémieux' experiments with drops of liquids--Hypothesis of ether insufficient. CHAPTER VII WIRELESS TELEGRAPHY § 1. Histories of wireless telegraphy already written, and difficulties of the subject. § 2. Two systems: that which uses the material media (earth, air, or water), and that which employs ether only. § 3. Use of earth as return wire by Steinheil --Morse's experiments with water of canal--Seine used as return wire during siege of Paris--Johnson and Melhuish's Indian experiments--Preece's telegraph over Bristol Channel--He welcomes Marconi. § 4. Early attempts at transmission of messages through ether--Experiments of Rathenau and others. § 5. Forerunners of ether telegraphy: Clerk Maxwell and Hertz--Dolbear, Hughes, and Graham Bell. § 6. Telegraphy by Hertzian waves first suggested by Threlfall--Crookes', Tesla's, Lodge's, Rutherford's, and Popoff's contributions--Marconi first makes it practicable. § 7. The receiver in wireless telegraphy--Varley's, Calzecchi--Onesti's, and Branly's researches-- Explanation of coherer still obscure. § 8. Wireless telegraphy enters the commercial stage-- Defect of Marconi's system--Braun's, Armstrong's, Lee de Forest's, and Fessenden's systems make use of earth-- Hertz and Marconi entitled to foremost place among discoverers. CHAPTER VIII THE CONDUCTIVITY OF GASES AND THE IONS § 1. The Conductivity of Gases: Relations of matter to ether cardinal problem--Conductivity of gases at first misapprehended--Erman's forgotten researches--Giese first notices phenomenon--Experiment with X-rays-- J.J. Thomson's interpretation--Ionized gas not obedient to Ohm's law--Discharge of charged conductors by ionized gas. § 2. The Condensation of water-vapour by Ions: Vapour will not condense without nucleus--Wilson's experiments on electrical condensation--Wilson and Thomson's counting experiment--Twenty million ions per c.cm. of gas--Estimate of charge borne by ion-- Speed of charges--Zeleny's and Langevin's experiments--Negative ions 1/1000 of size of atoms--Natural unit of electricity or electrons. § 3. How Ions are Produced: Various causes of ionization--Moreau's experiments with alkaline salts--Barus and Bloch on ionization by phosphorus vapours--Ionization always result of shock.