What is the English title of Marie Curie's 1903 doctoral thesis?

The English title of Marie Curie’s 1903 doctoral thesis is Research on Radioactive Substances.

The original French title was Recherches sur les substances radioactives. Curie defended the thesis at the University of Paris in June 1903, presenting a systematic account of the newly observed radiation from uranium, thorium, and other substances. The work grew from Henri Becquerel’s 1896 discovery of uranium radiation.

Her research showed that radioactivity was associated with atoms themselves rather than merely with a chemical reaction or an external source of energy. She measured the radiation from minerals such as pitchblende and found that some were far more active than uranium alone, leading to the search for previously unknown elements.

The thesis also described the Curies’ investigations of polonium and radium, discovered in 1898, and helped establish Marie Curie as a leading experimental physicist. Readers sometimes confuse the thesis title with a title about X-rays or atomic spectra, but its central subject was radioactivity and the radioactive substances that produced it. Curie received her doctorate several months before she shared the 1903 Nobel Prize in Physics with Pierre Curie and Becquerel.