A collected account of radio-active phenomena entitled Radio activity. This was followed a year later by a revised and enlarged edition. In the seven years that have elapsed since the latter publication there has been a steady and rapid growth of our knowledge of the properties of the radiations from active sub stances, and of the remarkable series of transformations that occur in them. In the present work I have endeavoured to give an accurate and concise account of the whole subject as it stands to-day within the compass of a Single volume. A few pages from the earlier book have been utilised, but, otherwise, the present.