Based on a series of lectures given in 1926-27 by this famous radiochemist, this book opens with a brief history of alchemy, and then in three parts discusses radioactive elements as indicators, volatile hydrides, and the natural system of chemical elements. Throughout his life, the author worked with famous scientists such as Soddy, Rutherford, and Hevesy in researching radium D and designing methods for discovering trace amounts of helium in rocks. He explains in Part One how radioactive elements are used as indicators in both analytical as well as electro-chemistry. In Part Two, he classifies and compares volatile hydrides. Lastly, the author defines concepts like “chemical element,” “simple element,” and “complex element” while tracing the natural system of chemical elements and isotopes. This is one of the first books written about the subject in the period after the existence of isotopes was understood, and after the technique of using radioactive traces was developed.