in the Magnetic Field. 177 



latter class are shown in my communication to this Journal 

 (April 1898, p. 335), where the increasing character of the 

 magnetic effect is well exhibited in the natural triplets 5086, 

 4800, 4678 of cadmium, and 4811, 4722, 4680 of zinc. 

 Further examples of this, 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 electromagnetic theory the separation, S\, 

 of the side lines of a magnetic triplet should, under the same 

 conditions, vary directly as X 2 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 hX 

 varies as A. 2 , 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 in- 

 vestigation." 



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 2 3 

 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 mag- 

 nesium, cadmium, and zinc, which are substances possessing 

 homologous spectra and belonging to the same chemical 

 group (MendelejefFs second group). The spectra of these 

 metals consist of 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 consists of the lines 5086, 4800, 4678, and 



Phil. Mag., April 1898, p. 337. 



