"Radio-Active Substances" by Marie Curie is a scientific publication written in the early 20th century. This thesis, originally presented to the faculties of science in Paris, encompasses extensive research conducted on radio-active materials, including uranium and thorium, as well as the discovery and properties of radium. It sheds light on the experimental methods used to analyze radioactivity, the behaviors of these substances, and their significant implications in the field of chemistry and physics. The opening of the work introduces the objectives of Curie's research, which began with the study of phosphorescence in uranium and evolved into a deeper exploration of radio-active bodies. Curie highlights collaboration with her husband, Pierre Curie, in extracting new radioactive substances and examining their properties. The beginning focuses on the historical context of radioactivity, outlining earlier discoveries, particularly those of Henry Becquerel, which laid the groundwork for this groundbreaking study. Curie's personal involvement and gratitude towards mentors and collaborators set the tone for a rigorously detailed and methodologically sound investigation that endeavors to contribute to the knowledge of radioactivity and its applications. (This is an automatically generated summary.)