157 



A more general form of the rule first tested, has been proposed 

 by RuNGE^j: in ewery group of chemically related elements the atomic 

 weight is proportional to a power of the distance of the lines of 

 the doublet. Or expressed differently, when taking the logarithms 

 of atomic weight and of distance as coordinates, the corresponding 

 points in a gi'oup of chemically related elements are in a straight line. 



Our graphical representation shows that for Na, K, Rh, Cs the 

 mentioned rule is beautifully fulfilled, but that lithium forms an 

 exception. 



log. atomic weight. 



loe. line distance 



Hence we must conclude that the law connecting the frequency 

 difference of the com[)onents of pair lines and the atomic weight 

 is yet unknown to us. 



One observation concerning the intensities of the lithium pair 

 6708 seems worthy of record. With very small vapour density the 

 violet component is the most intense. With increased vapour density, 

 a case to which Fig. 1 of the Plate relates, the ratio of the intensi- 

 ties is reversed and no more corresponds to that of the sodium 

 lines. With still greater density new absorption lines appear in the 

 vicinity of the lithium pair. They merit a separate investigation, but 

 they are probably of the same nature as the line groups observed 

 by Wood -) in the neighbourhood of several lines of the principal 

 series of sodium. 



1) RuKGE and Precht. Pliysik. Zeilschr. 4. 285, 1903. 



2) Wood. Phil. Mag. 18, 530, 1900. 



