[ 04 ] 



A^III. Oh the Atomic Weiaht of Eadiiun. 

 By W. Marshall Watts'. JJ.ScF.l.C.'^ 



IX a short paper read before the Physical Society f I have 

 recently called attention to some relationships between 

 the spectra of certain allied elements, and the squares of their 

 atomic weights, which do not seem to be o^enerally known. 



There appear to be two distinct kinds of connexion between 

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

 the family of zinc, cadmium, and mercnry, and that of 

 gallium and indium, furnish the best examples, the differ- 

 ences 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 ad- 

 mitted 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, ca?sium, and calcium, strontium^ and barium offer 

 the best examples, the element of greater atomic weight has 

 the smaller oscillation-frequency, and three elements are 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. 



These relationships are most easily seen by plotting squares 

 of atomic weights as ordinates_, and oscillation-frequencies as 

 abscissae : it is then seen that — in the first case — the straight 

 lines joining corresponding points in the homologous spectra 

 intersect on the line of zero atomic weight : and. in the second 

 case, that the corresponding points in the three spectra com- 

 pared lie on straight lines. It is further observed that these 

 straight lines are very nearly parallel, although they seem 

 to convero'e sliohtlv. 



As an example, the following lines in the a re- spectra of 

 barium, strontium, and calcium lie on such (nearly) parallel 



straight lines : — 















Barium. 





Strontium 





Calcium. 





(«) 



Io387-0 



(6;-) 



19:387-7 



(10) 



21616-6 



(In) 



(^) 



17872-7 



i4n) 



22061-0 



(6) 



243900 



(4^0 



(c) 



21951 -2 



(10;-) 



26975-9 



(6.0 



29735-8 



(8;0 



{d) 



229790 



(8;-) 



28176-9 



(670 



30991-1 



(47?) 



(0 



24200-7 



(8r) 



29399-4 



[In) 



32228-6 



{In) 



Communicated bv tlie Author. 

 Phil. Maff. Feb. 1903. 



