The centenary of a controversial discovery: actinium

Andre Debierne, collaborator of Pierre and Marie Curie, announced in 1900 the discovery of actinium, a third radioelement in pitchblende with properties resembling those of thorium. Four years later, Friedrich Giesel found a substance with a strong emanating power following lanthanum and which he called emanium. It become progressively clear that actinium and emanium were the same element. The arguments developed in the ensuing controversy are reviewed. Eventually Debierne was recognized as the discoverer of actinium, element 89 in the periodic table.