§ 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.

§ 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

§ 1. The Cathode Rays: History of discovery--Crookes'
theory--Lenard rays--Perrin's proof of negative