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XVIII. On the Existence of a Relationship between tlte Spectra 

 of some Elements and the Squares of their Atomic Weights. 

 By W. Marshall Watts, D.Sc., F.LC* 



IT is well known that the spectra of some allied elements 

 exhibit certain resemblances ; so that in the spectrum of 

 one element we have, as it were, the spectrum of another 

 shifted through a certain distance. This has been made 

 clearer by the resolution of the lines of certain spectra into 

 groups by Kayser and Runge, who have shown that in many 

 cases the wave-lengths of the lines of a spectrum can be 

 calculated with considerable accuracy by means of a formula 

 based upon that applied with such striking success by Balmer to 

 the spectrum of hydrogen. In a general way it may be said 

 that an increase in atomic weight produces a shift towards 

 the red end of the spectrum, and the amount of the shift 

 seems in many cases to admit of simple expression in terms 

 of the squares of the atomic weights. 



There appear to be tw T o distinct kinds of connexion between 

 the spectra of allied elements. In one class of cases, of which 

 the family of zinc, cadmium, mercury, and that of gallium 

 and indium furnish the best examples, the differences between 

 the oscillation -frequencies of certain lines of the one element 

 are to the differences between the oscillation-frequencies of 

 the corresponding lines of the other element as the squares 

 of their atomic weights ; so that, if it be admitted that the 

 lines do correspond, it is possible to calculate the atomic 

 weight of the one element from that of the other by means 

 of the spectra. 



. In the other class of cases, of which the families potassium, 

 rubidium, and csesium, and calcium, strontium, and barium 

 offer the best examples, the element of greater atomic weight 

 has the smaller oscillation-frequency, and three elements arc 

 so related that the differences of oscillation-frequency between 

 the elements, in comparing corresponding lines in their spectra. 

 are proportional to the differences between the squares of the 

 atomic weights ; so that we can calculate the atomic weight 

 of one element from the atomic weights of two other elements 

 of the same family by means of their spectra. 



In the spectra of lithium, sodium, potassium, rubidium, ami 

 caesium Kayser and Runge distinguish a principal series and 

 two secondary series, the lines of each series being connected 



* Communicated by the Physical Society : read October 31, L902. 



