A SCIEXTIFIC CAKEEK 21 



here faced with an "irreversible loosening" of the crystal structure. 

 This was a most modest beginning in a field tlial later proved to be of 

 great importance. 



Hafnium 



Bohr's first fundamental papers, published in 1913, in which the quan- 

 tum theory of the atomic structure was introduced, dealt only with the 

 structure of the atoms of hydrogen, helium, and lithium. In January, 1922, 

 I learned during a walk together with him that he now had extended his 

 theory to the entire periodic system, giving among other things an ex- 

 planation of the appearance of the rare-earth elements in that system. 

 Their number according to his theory was restricted to fourteen, from 

 which it followed that the unknown element 72 cannot be a rare earth, 

 it has to be a homologue of the titanium group. 



In the summer of that year I became interested in geochemical prob- 

 lems. Returning to Denmark, I proposed to Coster, who previously had 

 studied X-ray spectroscopy with Siegbahn in Lund, that he should 

 teach me the technique and that we ought at the same time to have a 

 look at zirconium minerals for the missing element 72. The first spectrum 

 obtained by him demonstrated the presence of the element in zirconium 

 minerals, and further studies revealed its presence in all commercial 

 zirconium samples, which indicated a very close kinship between zirco- 

 nium and the new element hafnium. By a very protracted fractional 

 crystallization of ammonium zirconium hexafluorides, hafnium could 

 be prepared in a pure state. 



The discovery of hafnium was not accepted without opposition. 

 Urbain, in Paris, a few years earlier crystallizing crude ytterbium salts, 

 observed twenty-six optical spectral lines not shown by the initial 

 sample. He ascribed these lines to the presence of the previously un- 

 known element 72 in his sample. After the discovery of hafnium, it was, 

 however, demonstrated that none of these lines is to be found in the 

 spectrum of hafnium. In spite of this fact, Urbain upheld his claim to 

 have discovered element 72. Rutherford took great interest in our 

 work— all our extensive correspondence with iVa^Mre passed through his 

 hands — and suggested that I should send a paper on the chemistry of 

 hafnium to the editor of Chemical News. He remarked in his letter that 

 the editors of this periodical were strongly pro-French and I should not 

 mind if they refused to publish my paper. In Sheffield, my friend the 

 physicist Lawson ("interned" as a prisoner of war in the Institute of 

 Radium Research of Vienna) handed my contribution over .to the editor 

 of Chemical News, Professor Wynne. He remarked that he was pleased 

 with the paper but they might have something to say about the name 

 "hafnium," adding : "We adhere to the original word Celtium given to 



