Giesel isolated from pitchblende a lanthanum-containing fraction which showed unique properties. He produced several compounds of the new element, and after studying for two years he was confident enough to give the new element the name emanium. He was aware of the discovery of actinium by André-Louis Debierne. In publications from  1899 and 1900 Debierne does not give the exact procedures to obtain actinium, but from the description of chemical properties which he described as similar to titanium (1899) or similar to thorium (1900) it was clear for Giesel that the two elements must be different. A later comparison of the two elements by Otto Hahn, Otto Sackur and others showed that the two elements were identical. Debierne's name was retained because it had seniority.
Giesel stated in one of his publications that the sample Debierne provided for the comparison was a lanthanum fraction obtained by the same process he used and not the titanium or thorium fractions from earlier publications. This fact never led to an open controversy but the history of the discovery stayed questionable, and publications from 1971 argue that the claims of André-Louis Debierne in 1904 conflict with the publications in 1899 and 1900, making Giesel the real discoverer of actinium.
A less confrontational vision of scientific discovery is proposed by Adloff. He suggests that hindsight criticism of the early publications should be mitigated by the nascent state of radiochemistry, highlights the prudence of Debiernes claims in the original papers, and notes that nobody can contend that Debiernes substance did not contain actinium. Debierne, who is now considered by the vast majority of historians as the discoverer, lost interest in the element and left the topic. Giesel, on the other hand, can rightfully be credited with the first preparation of radiochemically pure actinium and with the identification of its atomic number 89.