598 REPORT— 1903. 



The prosecution of researches in connection with the constitution of spectra was 

 initiated by Johnstone Stoney, by Balmer with respect to hydrogen, and continued 

 by Rydberg, Deslandres, Ames, and, above all, by Kayser and Runge, who by an 

 elaborate and exhaustive investigation of the arc spectra of the elements have 

 given us formulte by which the wave-lengths of lines in the spectra of different 

 elements in certain definite groups may be calculated from their atomic masses. 

 They also showed the spectra to be constituted of three series of lines, the principal 

 series and two subordinate series, one sharp and the other difi'use. Lately, Ramage 

 has given us a simpler formula, which applies to several groups, and he has 

 co-ordinated the spectra of several of the elements with the squares of their atomic 

 masses, and also their atomic masses with others of their physical properties. 



It may here be remarked that the homology of the spark spectra in the magne- 

 sium, zinc, and cadmium series was at first called in question by Ames, though he 

 proved the arc spectra of zinc and cadmium to be stiictly homologous. 



Preston decided the question by demonstrating by means of beautiful photo- 

 graphs that corresponding lines such as the pairs, triplets, and the quadruple 

 groups in the spark spectra of the three metals when under the influence of a very 

 powerful magnetic field underwent the same kind of change ; for instance, each 

 quadruple group changed to sextuple, the second and fourth lines in each group 

 becoming double. Lines in spectra which have not the same constitution 

 behave differentl}'. Recently Runge and Paschen have arrived at the same 

 conclusion ; and, furthermore, have established homology in the spectra of 

 sodium, copper, and silver ; also between aluminium and thallium. Indium 

 is almost certainly homologous with aluminium and thallium, but it was 

 probably not investigated on account of its rarity. Marshall Watts has pointed 

 out that a relationship exists between the lines in the spectra of some elements and 

 the squares of their atomic weights, from which it is possible to calculate the 

 atomic weight of an element if that of another in the same homologous series is 

 known, and the oscillation frequencies of corresponding lines are known. This 

 enables the determination of atomic weights to be controlled with quite as much 

 efficiency and certainty in many instances as by specific heat or vapour-density 

 determinations of the metals. 



The first application of the observed homology in spectra was directed towards 

 the question of the atomic mass of beryllium, for which purpose the lines in the 

 ultra-violet spark spectrum of this element were first photographed and measured. 

 The nature of the evidence on the subject adduced at the time was in outline as 

 follows: — 



' If, as Nilson and Petterson suggest, the position of bei^llium is at the head 

 of a fceries of triad rare earth metals, the element scandium (at. wt. 44) and 

 yttrium (at. wt. 89) must be members of the same group. If this be the case 

 the spectra of the three elements must have certain characters in common, for the 

 series of which aluminium and indium are the first and third terms yield strictly 

 homologous spectra. As a matter of fact no two spectra could be more dissimilar 

 than those of beryllium and scandium.' 



Having compared the photographs and wave-length measurements of a large 

 number of spectra of the elements, I felt justified in making the following 

 remarks: — 



' The spectrum of beryllium exhibits no marked analogy with the calcium, the 

 magnesium, or the aluminium spectra, all of which are members of well-defined 

 homologous series. There is nothing similar in it to the boron, silicon, or carbon 

 spectra, nor to those of the scandium, yttrium, or cerium. The spectrum of 

 lithium is most closely analogous to that of beryllium in the number, relative posi- 

 tions, and intensities of the lines. This leads to the conclusion that beryllium is the 

 first member of a dyad series of metals, to which in all probability calcium, stron- 

 tium, and barium, as a sub-group, are homologous, its atomic mass being 9-2, its 

 place is above magnesium.' 



Subsequently Nilson, and also Humpidge, by chemical evidence and from 

 vapour-density determinations of certain compounds, substantiated the conclusion 



