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FEEL that some apology is due to my readers for bringing out at such an early date a new edition which includes so much new material, and in which the rearrangement is so extensive as to constitute almost a new work. Though only a year has passed since the book first made its appearance, the researches that have been carried out in that time have been too numerous and of too important a character to permit the publishing of a mere reprint, unless the author were to relinquish his purpose of presenting the subject as it stands at the present moment.

The three new chapters which have been added possibly constitute the most important change in the work. These chapters include a detailed account of the theory of successive changes and of its application to the analysis of the series of transformations which occur in radium, thorium, and actinium.

The disintegration theory, which was put forward in the first edition as an explanation of radio-active phenomena, has in these later researches proved to be a most powerful and valuable method of analysing the connection between the series of substances which arise from the transformation of the radio-elements. It has disclosed the origin of radium, of polonium and radio-tellurium, and of radio-lead, and now binds together in one coherent whole the large mass of apparently heterogeneous experimental facts in radio-activity which have been accumulating since 1896. The theory has received a remarkable measure of verification in the past year, and, in many cases, has offered a quantitative as well