A concise popular account introduces radioactivity and describes how radium emits penetrating rays, explains its effects on living tissue and its use in medicine and industry, surveys experiments and anecdotal claims about therapeutic springs and radiotherapy for cancer, and outlines the physical mechanisms by which radiation ionizes matter. It discusses the discovery and isolation of radium, the difficulties and expense of production, the element's role in estimating the Earth's age, procedures for conversion and decay chains, and the geological sources and extraction methods that supply radioactive materials.