Who announced the discovery of actinium in 1899 from pitchblende residues?

André-Louis Debierne announced the discovery of actinium in 1899 from pitchblende residues.

Debierne was a French chemist who worked with residues left after Marie and Pierre Curie had extracted radium from pitchblende. In 1899 he reported a new radioactive substance and later called it actinium, from a Greek word associated with a ray or beam. The element is now identified by the symbol Ac and atomic number 89.

The discovery has a complicated history. Friedrich Oskar Giesel independently found a similar radioactive substance in 1902 and called it emanium. The name actinium ultimately prevailed because Debierne’s claim had priority, although later historians and chemists have questioned whether his earliest samples and descriptions corresponded perfectly to modern actinium.

This makes Giesel an important neighboring name, but not the answer to the question’s 1899 announcement. Debierne is the conventional historical answer, while actinium belongs to the actinide series and occurs naturally only in trace amounts in uranium and thorium ores.