A systematic physical account examines the properties and behaviors of naturally radioactive substances, presenting the disintegration theory as a unifying explanation for spontaneous atomic change and decay chains. It reviews the ionization of gases and the electric, magnetic, and velocity properties of emitted radiation, describes alpha, beta, and gamma rays and emanations, and outlines experimental methods and quantitative measurement techniques. Additional chapters analyze successive transformations among radio-elements, summarize recent results on rays and emanations, and include appendices on alpha-ray studies and the occurrence and chemistry of radioactive minerals, with brief comments on physiological effects.