36 Mr. J. S. Ames on Relations between 



atomic weights of magnesium, zinc, and cadmium is -146. 

 Hence the variation in the increment of the wave-lengths is 

 •146 x 13 = 1*898. I have recently studied the spectra of zinc 

 and cadmium with much care ; and, as will be shown later, 

 each line in the one spectrum has an homologous one in the 

 other. Two of these lines have the wave-lengths 2800*9 and 

 2980*8 respectively. This law of M. de Boisbaudran should 

 enable us to find the corresponding line in the magnesium 

 spectrum. Calling it x, the variation in the increment is 



^-2621*0 

 2800-9 -#' 



But this must equal 1*898. Hence ^ = 2738*8. In reality 

 there is a line 2736*9 ; but its physical properties do not 

 correspond to those of the two zinc and cadmium lines. Two 

 other lines of zinc and cadmium, which are undoubtedly homo- 

 logous, are 3345*1 and 3610*6; and the hypothetical magnesium 

 line corresponding to these would have the wave-length 

 3253*6. There is, however, no line in this neighbourhood 

 which can possibly be homologous with the zinc and cadmium 

 lines. Therefore, while this law may apply roughly to certain 

 lines, it surely cannot be considered general. 



After M. de Boisbaudran several investigators, including 

 Ditte and Troost and Hauteville*, endeavoured to trace rela- 

 tions between atomic weights and " displacement " of the 

 spectra. But, as remarked above, it was impossible at this 

 time to make any accurate statements as to homologous lines 

 or their displacement. This fact is clearly shown by the dis- 

 agreement of observers of the same spectra in regard to homo- 

 logous lines. Cimiciant examined the spectra of a great 

 many substances in search of lines, and tried by a study of 

 them to prove the compound nature of the elements. The 

 dispersion of his instrument was, however, so feeble that he 

 could not properly decide upon similarities in different spectra. 

 He opposes M. de Boisbaudran's law of the shifting of the 

 spectra, and says that instead of increasing with the atomic 

 weight the wave-lengths increase with the intensity of the 

 " chemical kinetic energy.'"' (See Mohr, Mech. Th. der Chern. 

 Affinitdt, Wien, 1867.) The first really scientific work on 

 the relations between the lines of spectra was that of Hartley J. 

 In studying the spark- spectra of the elements, his attention 

 w T as attracted to the many points of resemblance between the 

 spectra of chemically similar elements, notably magnesium, 



* Comptes Rendus, lxxii. (1871). 

 t Wien. Sitzber. 1877, 1878, 1880. 

 % Journ. Chem. Soc. 1882, 1883. 



