For the convenience of those who have read the first edition, a list of the sections and chapters which contain the most important additions and alterations is added below the table of contents. The writing of a complete account of a subject like radio-activity, in which so much new work is constantly appearing, has been a matter of no little difficulty. Among other things it has involved a continuous revision of the work while the volume was passing through the press. I wish to express my thanks to my colleague Professor Harkness for the care and trouble he has taken in revising the proofs and for many useful suggestions; also to Mr R. K. McClung for his assistance in correcting some of the proofs and in preparing the index. E. R. MCGILL UNIVERSITY, MONTREAL, _9 May, 1905_. Transcriber’s Note: The plain text version of this ebook includes complex mathematical formulas in TeX notation. These formulas are enclosed between double dollar signs. The HTML version includes the same formulas rendered in their original form. ERRATA. Transcriber’s Note: These corrections have been applied to the text in the book. page 48, line 24 section 218 should read section 284 „ 77, last line „ 263 „ „ „ 270 „ 123, 5th line from bottom „ 254 „ „ „ 261 „ 124, 10th „ „ „ „ 246 „ „ „ 253 „ 151, line 3 „ 228 „ „ „ 229 „ 156, 13th line from bottom „ 261 „ „ „ 268 „ 200, line 9 „ 246 „ „ „ 253 „ 216, line 3 „ 260 „ „ „ 267 „ 184, at the top of 5th column of table the letter γ should be inserted. TABLE OF CONTENTS. I. Radio-active Substances 1 II. Ionization Theory of Gases 31 III. Methods of Measurement 82 IV. Nature of the Radiations 108 V. Properties of the Radiations 201 VI. Continuous Production of Radio-active Matter 218 VII. Radio-active Emanations 238 VIII. Excited Radio-activity 295 IX. Theory of Successive Changes 325 X. Transformation Products of Uranium, Thorium and Actinium 346 XI. Transformation Products of Radium 371 XII. Rate of Emission of Energy 418 XIII. Radio-active Processes 437 XIV. Radio-activity of the Atmosphere and of Ordinary Materials 501 Appendix A. Properties of the α Rays 543 Appendix B. Radio-active Minerals 554 Index 559 Plate (Fig. 46A: Spectrum of Radium Bromide) _to face p._ 206 * * * * * For the convenience of the reader, the sections and chapters which contain mostly new matter, or have been either partly or wholly rewritten, are appended below. Chap. I. Sections 18, 20–23. „ II. „ 48–52. „ III. „ 69. „ IV. „ 83–85, 92, 93, 103, 104, 106–108, 111, 112. „ V. „ 115, 117, 119, 122. „ VII. „ 171–173. „ VIII. „ 182–184, 190. „ IX-XIV. Mostly rewritten.