Mendeleev and the Rare-Earth Crisis

Despite Brauner s belief in the validity of the Mendeleev methodology, he also had to admit that he had not yet succeeded in resolving the rare-earth crisis. Thus Brauner wrote in 1901 with reference to praseodymium that its maximum valency was tetravalent, like that of cerium but that no place had been found in the periodic table for an element possessing the physical and chemical properties of praseodymium and its compounds (Brauner, 1901b). He also admitted that the difficulties of finding a place for neodymium in the periodic table were even greater than in the case of praseodymium. 

Bohuslav Brauner had approached the rare-earth crisis from a fradifional chemical poinf of view. Bofh the search for higher valencies and complicated atomic weight determinations had been central to his research during the last quarter of fhe nineteenfh century, but neither of fhese methodologies allowed to prove the validity of the homologous accommodation. Brauner s adherence to the Mendeleev method had led him to a... 

A few years ago, C. H. Evans noticed that historically the rare-earth elements [had] created theoretical, as well as practical, headaches for chemists , but he admitted that none [had been] greater than their proper position in the periodic table / According to Evans opinion, the interplay between Mendeleev s periodic system and the rare-earth elements is an interesting one, worfliy of deeper analysis by future scholars. Following this suggestion, the substance of this paper will focus on Mendeleev s attitude towards the problematic accommodation of the rare-earth elements in the periodic system during the period 1869-1871. Henceforth, the term rare-earth crisis will be used to denote this accommodation issue. 

There has been relatively little mention of the rare-earth crisis in the scholarly literature on Mendeleev and his periodic system. One notable exception is provided by the work of the Russian historian, D. N. Trifonov, who wrote two books on this subject-matter in Russian. Within the English scholarly literature, however, not much material is to be found. And yet, as will be argued in this paper, the rare-earth crisis embodied one of the greatest threats to the periodic system—endangering some of the most cherished beliefs of Mendeleev. 

It thus appears that seven little studied elements remained outside of the table This septuplet of homeless elements consisted of indium (In), thorium (Th) and the five rare earths—erbium (Er), yttrium (Yt), cerium (Ce), lanthanum (La), and didymium (Di). Question marks and wrong atomic weights reigned in the last rows of Mendeleev s system, and a new place had to be found for the homeless septuplet We will call this problematic accommodation issue the rare-earth crisis. 

The dual sense of the epistemological concept of chemical elements has been the focus of much philosophical debate and research by contemporary philosophers of chemistry. Although this debate goes back to the 1931 paper by Paneth, the main idea was clearly already present in the Mendeleev corpus." It is therefore interesting to investigate to what extent the rare-earth crisis forced Mendeleev to change his points of view with regard to the nature and concept of elements. 

Mendeleev said, more than elsewhere in the system of elements, that new invesligatiOTis are to be desired and for which the periodic law provides guidance. About 6 years later, the yoimg Czechoslovakian chemist, Bohuslav Brauner, discovered Mendeleev s wonderful communication . It made such a profound impressimi on him that he fixed his life s aim at that very moment it was the experimental research of the solution of the following problems What is the position of the so called rare elements and especially those of the rare earths in Mendeleev s system Brauner would become the main defender of the periodic system in the late nineteenth century, and his rare-earth research became of the utmost importance for the further resolution of the rare-earth crisis. But that is another story. ... 

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