Thorium is radioactive

Was uranium unique? To find out, Marie Curie gathered samples of all the known elements: uranium from Henri Moissan, pure oxides from Eugène Demarçay, and minerals of known origin from the Natural History Museum. She tested each in her ionisation chamber.

Her note of 12 April 1898 was presented to the Académie des sciences by her former professor Gabriel Lippmann, because she was not a member. It reported that thorium compounds are strongly active: a thick layer of thorium oxide gave more than twice the current of metallic uranium. A few weeks earlier Gerhard Carl Schmidt had published the same finding in Germany, so she does not hold sole priority.

The same short note held her most important sentence. Two minerals, pitchblende and chalcolite, were more active than uranium itself, while chalcolite she made from pure chemicals was not. "This fact is most remarkable," she wrote, "and suggests that these minerals may contain an element much more active than uranium." The hunt for polonium and radium began here.