A WONDERFUL X-RAY PHOTOGRAPH 255 Reproduced by permission of X-Rays Ltd. ELECTRIC DISCHARGE IN A VACUUM TUBE 258 THE RELATIVE SIZES OF ATOMS AND ELECTRONS 258 ELECTRONS STREAMING FROM THE SUN TO THE EARTH 259 PROFESSOR SIR J. J. THOMSON 262 ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR 262 From the Smithsonian Report, 1915. MAGNETIC DEFLECTION OF RADIUM RAYS 263 PROFESSOR R. A. MILLIKAN'S APPARATUS FOR COUNTING ELECTRONS 263 Reproduced by permission of _Scientific American_. MAKING THE INVISIBLE VISIBLE 266 THE THEORY OF ELECTRONS 267 ARRANGEMENTS OF ATOMS IN A DIAMOND 267 DISINTEGRATION OF ATOMS 270 SILK TASSEL ELECTRIFIED 270 Reproduced by permission from _The Interpretation of Radium_ (John Murray). SILK TASSEL DISCHARGED BY THE RAYS FROM RADIUM 270 A HUGE ELECTRIC SPARK 271 ELECTRICAL ATTRACTION BETWEEN COMMON OBJECTS 271 From _Scientific Ideas of To-day_. AN ELECTRIC SPARK 274 Photo: Leadbeater. AN ETHER DISTURBANCE AROUND AN ELECTRON CURRENT 275 From _Scientific Ideas of To-day_. LIGHTNING 278 Photo: H. J. Shepstone. LIGHT WAVES 279 THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT 279 THE MAGNET 279 ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS (_Coloured Illustration_) 280 WAVE SHAPES 282 THE POWER OF A MAGNET 282 THE SPEED OF LIGHT 283 Photo: The Locomotive Publishing Co., Ltd. ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS 283 NIAGARA FALLS 286 TRANSFORMATION OF ENERGY 287 Photo: Stephen Cribb. "BOILING" A KETTLE ON ICE 287 Photo: Underwood & Underwood. THE CAUSE OF TIDES 290 THE AEGIR ON THE TRENT 290 Photo: G. Brocklehurst. A BIG SPRING TIDE, THE AEGIR ON THE TRENT 291 Photo: G. Brocklehurst. The Outline of Science INTRODUCTION