114 INORGANIC EVOLUTION. [CHAP. 



lines show a considerable resolution in the magnetic field, other lines 

 of nearly the same wave-length, in the same substance, are scarcely 

 affected at all. This deviation is most interesting to those who con- 

 cern themselves with the ultimate structure of matter, for it shows 

 that the mechanism which produces the spectral lines of any given 

 substance is not of the simplicity postulated in the elementary theory 

 of this magnetic effect. 



" According to the prediction of the simple theory, the separation 

 8A. should be proportional to A 2 , and although this law is not at all 

 obeyed if we take all the lines of the spectrum as a single group, yet 

 we find that it is obeyed for the different groups if we divide the lines 

 into a series of groups. In other words, if the lines of a given 

 spectrum be arranged in a series of groups, the lines of the first 

 group being denoted by the letters AI, BI, Ci, . . . , those of the second 

 group by A 2 , Bo, C 2 , . . ., and so on, then the corresponding lines A b 

 Ao, A 3 , &c., have the same value for the quantity ejm, or, as we may 

 say, they are produced by the motion of the same ion. The other 

 corresponding lines, B b B^ B 3 , &c., have another common value for 

 e/m-, and are produced therefore by a different ion, and so on. We. are 

 thus led by this magnetic effect to arrange the lines of a given spectrum 

 into natural groups, and from the nature of the effect we are led to 

 suspect that the corresponding lines of these groups are produced by 

 the same ion, and therefore that the atom of any given substance is 

 really a complex consisting of several different ions, each of which 

 gives rise to certain spectral lines, and these ions are associated to form 

 an atom in some peculiar way which stamps the substance with its 

 own peculiar properties." 



The general law announced by Preston states the further remark- 

 able fact that if we consider a group of chemically related metals such, 

 for example, as magnesium, zinc, and cadmium, then the sets of lines 

 into which the spectrum of any one of these may be divided as 

 above, correspond set for set with those into which the lines of any 

 other of these metals are divided, in such a way that the magnetic 

 change of frequency (or e/m) for any one set is the same as that for the 

 corresponding set in each of the other metals. This seems to point to 

 the conclusion that the metals of the. same chemical group are built up, 

 in part at least, of ions which are the same in all the metals of the 

 group. 



It will be abundantly seen, then, that these new inquiries have 

 presented exactly the same difficulties as the old ones, and that they 

 have been met in exactly the same* way, by establishing the fact that 



