1901.] 



and Melting Points to Atomic Mass. 



15 



evident in the lines of the metals of the alkalies, particularly 

 of potassium, rubidium, and caesium, and in the strongest lines 

 of calcium, strontium, and barium. 



In Plate 1 the lines joining the diffuse subordinate series of the 

 potassium group approach to straight lines in the more refrangible 

 members of the series, and the lines joining the principal series are 

 nearly parabolic. The latter are nearly straight in the more refrangi- 

 ble members in Plate 2. The shift of the subordinate lines is ap- 

 proximately proportional to the atomic mass, and the shift of the 

 principal lines to the square of the atomic mass. 



The fundamental lines of calcium, strontium, and barium are given 

 by the equation n = 24170 - 0-3232 W 2 , in which n= 10 s A -1 and 

 W = atomic mass. The differences from the observed values are — 

 calcium 0, strontium - 7, and barium + 9. 



The atomic mass is not the only cause which determines the shift of 

 the lines. There are, doubtless, other causes at work, and it is prob- 

 ably in these we must seek the explanation of the abnormal shifts 

 which produce the breaks in the connecting lines referred to above. 

 In passing from group to group, for instance, an increase in the valency 

 with but slight increase in the atomic mass results in the whole of a 

 series being displaced to a considerable extent. The following figures 

 have been taken to illustrate this point from Eydberg's " La Distribu- 

 tion des Eaies Spectrales," read to the Congres International de 

 Physique de 1900 :— 



Element. 



Atomic 

 Mass. 





23-06 



Magnesium . . 



24-38 



Aluminium . . 



27-08 





107 -94 



Cadmium .... 



112 -08 





113-7 



Difference 





Limit of Series. 



Difference 



in Atomic 

 Mass. 



Valency. 



Oscillation 

 Frequencies. 



in limit 

 of series. 



1- 32 



2- 70 



I 

 II 

 III 



24470 

 39755 

 48156 



15285 

 8401 



4-14 

 1*62 



£ 

 in 



30648 

 40717 

 44448 



10069 

 3731 I 



| 



The figures in the fifth and sixth columns refer to the subordinate 

 series of lines, 



Changes of valency therefore produce much greater displacements of 

 the spectral lines than changes in the atomic masses. 



Another cause is doubtless at work in the elements — copper, silver, 

 and gold. The connecting lines here take a remarkable form, and this 

 form is seen also in the lines selected as corresponding to the H and K 

 solar lines of calcium in the spectra of calcium, strontium, and barium. 



