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..