Sec. 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

Sec. 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.

Sec. 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.

Sec. 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.

Sec. 4.  Electrons in Metals:  Movement of electrons in metals foreshadowed by Weber—­Giese’s, Riecke’s, Drude’s, and J.J.  Thomson’s researches—­Path of ions in metals and conduction of heat—­Theory of Lorentz—­Hesehus’ explanation of electrification by contact—­Emission of electrons by charged body—­ Thomson’s measurement of positive ions.

CHAPTER IX

CATHODE RAYS AND RADIOACTIVE BODIES

Sec. 1.  The Cathode Rays:  History of discovery—­Crookes’ theory—­Lenard rays—­Perrin’s proof of negative charge—­Cathode rays give rise to X-rays—­The canal rays—­Villard’s researches and magneto-cathode rays—­ Ionoplasty—­Thomson’s measurements of speed of rays—­ All atoms can be dissociated.

Sec. 2.  Radioactive Substances:  Uranic rays of Niepce de St Victor and Becquerel—­General radioactivity of matter—­Le Bon’s and Rutherford’s comparison of uranic with X rays—­Pierre and Mme. Curie’s discovery of polonium and radium—­Their characteristics—­Debierne discovers actinium.

Sec. 3.  Radiations and Emanations of Radioactive Bodies:  Giesel’s, Becquerel’s, and Rutherford’s Researches—­Alpha, beta, and gamma rays—­Sagnac’s secondary rays—­Crookes’ spinthariscope—­The emanation —­Ramsay and Soddy’s researches upon it—­Transformations of radioactive bodies—­Their order.

Sec. 4.  Disaggregation of Matter and Atomic Energy:  Actual transformations of matter in radioactive bodies —­Helium or lead final product—­Ultimate disappearance of radium from earth—­Energy liberated by radium:  its amount and source—­Suggested models of radioactive atoms—­Generalization from radioactive phenomena -Le Bon’s theories—­Ballistic hypothesis generally admitted—­Does energy come from without—­Sagnac’s experiments—­Elster and Geitel’s contra.