The rapid advance of our knowledge of radio-activity has been dependent on the information already gained by research into the electric properties of gases. The action possessed by the radiations from radio-active bodies of producing charged carriers or ions in the gas, has formed the basis of an accurate quantitative method of examination of the properties of the radiations and of radio-active processes, and also allows us to determine with considerable certainty the order of magnitude of the different quantities involved. For these reasons, it has been thought advisable to give a brief account of the electric properties of gases, to the extent that is necessary for the interpretation of the results of measurements in radio-activity by the electric method. The chapter on the ionization theory of gases was written before the publication of J. J. Thomson’s recent book on “Conduction of Electricity through Gases,” in which the whole subject is treated in a complete and connected manner. A short chapter has been added, in which an account is given of the methods of measurement which, in the experience of the writer and others, are most suitable for accurate work in radio-activity. It is hoped that such an account may be of some service to those who may wish to obtain a practical acquaintance with the methods employed in radio-active measurements. My thanks are due to Mr W. C. Dampier Whetham, F.R.S., one of the editors of the Cambridge Physical Series, for many valuable suggestions, and for the great care and trouble he has taken in revising the proof sheets. I am also much indebted to my wife and Miss H. Brooks for their kind assistance in correcting the proofs, and to Mr R. K. McClung for revising the index. E. R. MACDONALD PHYSICS BUILDING, MONTREAL, _February, 1904_. PREFACE TO THE SECOND EDITION. I 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.