He is often considered the discoverer of the element actinium, though H. W. Kirby disputed this in 1971 and gave credit instead to German chemist Friedrich Oskar Giesel.

[2] He was a student of Charles Friedel, was a close friend of Pierre and Marie Curie and was associated with their work.

In 1899, he discovered the radioactive element actinium, as a result of continuing the work with pitchblende that the Curies had initiated.

In 1911, he and Marie Curie prepared radium in metallic form in visible amounts.

Having demonstrated the metal's existence as a matter of scientific curiosity, they reconverted it into compounds with which they might continue their researches.