30 



SECTIONAL ADDRESSES. 



Catalan has remarked that, iu passing from Ca to Ni, the terms are 

 regular so long as the maximum multiplicity is increasing, and inverted 

 when it is decreasing, both classes of terms occurring at the turning-point 

 (Mn), as shown in Table IV. A similar alternation of even and odd 

 multiplicities is shown by the spark spectra of the elements Ca to Ni, as 

 will appear from Table VI, which is also taken from Meggers and Kiess. 

 The data here are less complete and for Co* and Ni* they are merely 

 conjectural. 



Table VI. 



Teem Systems in Spark Spectra, Ca'''-Ni"'". 



The spark spectra of only very few of the elements of the next row 

 (Rb-Pd) have at present been classified, but these are in accordance with 

 the above so far as they go. Thus 38.Sr"^=doublets ; 39.Y'^=singlets 

 and triplets ; 40.Zr^=doublets and quartets. 



It will be observed that the alternation law and restricted displacement 

 law are completely satisfied by the observational data includedinthe tables, 

 and the same appears to be true for other spectra which have been 

 sufficiently analysed. The tables, however, bring out several other points 

 of interest. It will be noted, for example, that not more than three 

 different systems of terms have yet been found in the same spectrum, and 

 it would seem probable that this is the actual maximum number which 

 can occur. 



A very striking relation, to which attention appears to have first been 

 directed by Hund,^* is that, as regards the ground term, the spark 

 spectrum of each of the elements Ca to Ni is more closely related to its 

 own arc spectrum than to the arc spectrum of the preceding element. 

 Thus, from Ca to Fe, the ground terms of the arc and spark spectra are of 

 the same type, and, apart from Cr, which is in certain other respects 

 exceptional, the multiplicity of the system to which the ground term 

 belongs is increased by unity in passing from the arc to the spark 

 spectrum. This may be shown as follows : — 



20.Ca 21.Sc 22.Ti 23.V 24.Cr 25.Mn 26.Fe 27.Co 28.Ni 



Arc 

 Spark 



IS 

 ^S 



2D 

 3D 



3F 



5F 



'S 



«s 



6S 



'S 



«D 

 «D 



4F 



(5F) 



3F 



(4F) 



" Zeif. f. Phys., vol. 33, p. 362 (1925). 



