44 



Supplement to '' Nature!' July 7, 1923 



of atomic structure discussed above, correspond 

 exactly to what one might expect from this theory. 

 At the foot of the figure the vertical lines indicate 

 where, according to the theory, we should first expect, 

 in the normal state of the atom, the occurrence of «* 

 orbits of the type designated. We see how it has 

 been possible to connect the occurrence of every 

 spectral term with the presence of an electron moving 

 in an orbit of a definite type, to the removal of which 

 this term is supposed to correspond. That in general 

 there corresponds more than one curve to each type 

 of orbit n^ is due to a complication in the spectra 

 which would lead us too far afield to enter into here, 

 and may be attributed to the deviation from the 

 previously described simple type of motion of the 

 electron arising from the interaction of the different 

 electrons within the same group. 



The intervals in the system of the elements, in which 

 a further development of an inner electron group 

 takes place because of the entrance into the normal 

 atom of electron orbits of a certain type, are designated 

 in the figure by the horizontal lines, which are drawn 

 between the vertical lines to which the quantum 

 symbols are affixed. It is clear that such a develop- 

 ment of an inner group is everywhere reflected in the 

 curves. Particularly the course of the N- and 0-curves 

 may be regarded as a direct indication of that stage 

 in the development of the electron groups with 

 4-quantum orbits of which the occurrence of the 

 rare-earths bears witness. Although the apparent 

 complete absence of a reflection in the X-ray spectra 

 of the complicated relationships exhibited by most 

 other properties of the elements was the typical and 

 important feature of Moseley's discovery, we can 

 recognise, nevertheless, in the light of the progress 

 of the last years, an intimate connexion between the 

 X-ray spectra and the general relationships between 

 the elements within the natural system. 



Before concluding this lecture I should like to mention 

 one further point in which X-ray investigations have 

 been of importance for the test of the theory. This 

 concerns the properties of the hitherto unknown 

 element with atomic number 72. On this question 

 opinion has been divided in respect to the conclusions 

 that could be drawn from the relationships within the 

 periodic table, and in many representations of the 

 table a place is left open for this element in the 

 rare-earth family. In Julius Thomsen's representation 

 of the natural system, however, this hypothetical 

 element was given a position homologous to titanium 

 and zirconium in much the same way as in our repre- 

 sentation in Fig. I. Such a relationship must be 

 considered as a necessary consequence of the theory 

 of atomic structure developed above, and is expressed 

 in the table (Fig. 9) by the fact that the electron 

 configurations for titanium and zirconium show the 

 same sort of resemblances and differences as the 

 electron configurations for zirconium and the element 

 with atomic number 72. A corresponding view was 

 proposed by Bury on the basis of his above-mentioned 

 systematic considerations of the connexion between 

 the grouping of the electrons in the atom and the 

 properties of the elements. 



Recently, however, a communication was published 

 by Dauvillier announcing the observation of some 



weak lines in the X-ray spectrum of a preparatitj 

 containing rare-earths. These were ascribed to a 

 element with atomic number 72 assumed to be identic ;; 

 with an element of the rare-earth family, the existen< > 

 of which in the preparation used had been presumed 

 by Urbain many years ago. This conclusion would, 

 however, if it could be maintained, place extni 

 ordinarily great, if not unsurmountable, difiiculti< 

 in the way of the theory, since it would claim a chanp' 

 in the strength of the binding of the electrons with 

 the atomic number which seems incompatible with 

 the conditions of the quantum theory. In the^i 

 circumstances Dr. Coster and Prof. Hevesy, who ar 

 both for the time working in Copenhagen, took u]> 

 a short time ago the problem of testing a preparation 

 of zircon-bearing minerals by X-ray spectroscopi 

 analysis. These investigators have been able t' 

 establish the existence in the minerals investigated 

 of appreciable quantities of an element with atomi< 

 number 72, the chemical properties of which sho\ 

 a great similarity to those of zirconium and a decided 

 difference from those of the rare-earths.^ 



I hope that I have succeeded in giving a summary 

 of some of the most important results that have been 

 attained in recent years in the field of atomic theory, 

 and I should like, in concluding, to add a few general 

 remarks concerning the view-point from which thes' 

 results may be judged, and particularly concernin:,, 

 the question of how far, with these results, it is possible 

 to speak of an explanation, in the ordinary sense of 

 the word. By a theoretical explanation of natural 

 phenomena we understand in general a classification 

 of the observations of a certain domain with the help 

 of analogies pertaining to other domains of observa- 

 tion, where one presumably has to do with simpler 

 phenomena. The most that one can demand of a 

 theory is that this classification can be pushed so 

 far that it can contribute to the development of the 

 field of observation by the prediction of new phenomena. 



When we consider the atomic theory, we are, how- 

 ever, in the peculiar position that there can be no 

 question of an explanation in this last sense, since 

 here we have to do with phenomena which from the 

 very nature of the case are simpler than in any other 

 field of observation, where the phenomena are always 

 conditioned by the combined action of a large number 

 of atoms. We are therefore obliged to be modest 

 in our demands and content ourselves with concepts 

 which are formal in the sense that they do not provide a 

 visual picture of the sort one is accustomed to require 

 of the explanations with which natural philosophy 

 deals. Bearing this in mind I have sought to convey 

 the impression that the results, on the other hand, 

 fulfil, at least in some degree, the expectations that 

 are entertained of any theory ; in fact, I have 

 attempted to show how the development of atomic 

 theory has contributed to the classification of extensive 

 fields of observation, and by its predictions has pointed 

 out the way to the completion of this classification. 

 It is scarcely necessary, however, to emphasise that the 

 theory is yet in a very preliminary stage, and many 

 fundamental questions still await solution. 



* For the result of the continued work of Coster and Hevesy with the new 

 element, for which they have proposed the name hafnium, the reader may be 

 referred to their letters in Nature of January 20, February 10 and 24, and 

 April 7. 



