Part II. — Magnetic Perturbations of the Spectral Lines. 19 



subsidiary series of Kayser and Runge), and when the action of the magnetic 

 field is recorded, it is found that the magnetic efEect increases with the 

 refrangibility for the members of a ti-iplet of the second series (fig. 8), whereas 

 the reverse is the case for a triplet of the first series (fig. 9). Examples of the 

 former class occur in the natural triplets of cadmium, zinc, and magnesium, 

 already mentioned ; and further examples of these and other peculiarities I hope 

 to give in the near future, as soon as I have fully examined and verified them. 



General Law. 



The first general survey of the magnetic effect on the spectral lines of any 

 given substance did not appear to favour the view that the phenomena are 

 subject to any simple law. According to the electro-magnetic theory the 

 separation 8X, of the side lines of a magnetic triplet, should, under the same 

 conditions, vary directly as X", as we pass from line to line of the same spectrum. 

 The possibility of such a law as this seemed to be refuted by the fact that some 

 lines are largely affected in the magnetic field, while others, of nearly the same 

 wave-length in the same spectrum, are not appreciably affected under the same 

 circumstances. In this connexion, however, I pointed out * that " it is possible 

 that the lines of any one substance may be thrown into groups for each of 

 which 8X varies as X", and each of these groups might be produced by the 

 motion of a single ion. The number of such groups in a given spectrum would 

 then determine the number of different kinds of ions in the atom or molecule. 



" Homologous relations may also exist between the groups of different spectra, 

 but all this remains for complete investigation." 



Although the investigation referred to in the foregoing is still far from 

 complete, yet the measurements so far made uniformly tend to confirm the above 

 speculation. For the corresponding lines of the natural groups into which a 

 given spectrum resolves itself possess the same value of e/m or 8X/X- ; and further, 

 this value is the same for corresponding lines in homologous spectra of different 

 substances. 



To illustrate the meaning of this, take the case of magnesium, cadmium, and 

 zinc, which are substances possessing homologous spectra, and belonging to the 

 same chemical group (Mendelejeff's second group). The spectra of these metals 

 consist of a series of natural triplets. The first triplet of the series in magnesium 

 is the green b group, consisting of the wave-lengths 5183*8, 5172"8, 5167"5, 

 while the first cadmium triplet consist of the lines 5086, 4800, 4678, and the first 

 zinc triplet consists of the lines 4810-7, 4722, 4680. Each of these triplets 



*■ Phil. Mag., April, 1898, p. 337. 



