332 Adeney — Photographs of Spark Spectra from the Rotvland Spectrometer. 



volatility, and oxidisability, and consequently they were correlated with the 

 periodic law.* A community of characteristics in spectra was recognised as being 

 due to a similarity in the properties of the elements belonging to the same 

 group. 



It was further shown that elements with properties in common likewise exhibit 

 spectra with similar groupings of lines ; but the dispersion of the lines and the 

 refrangibility of the strong lines in each group vary with the atomic mass of the 

 elements. 



Other observers had noted the significance of the constitution of spectra, as for 

 instance Lecocq de Boisbaudranf and Ciamician.J 



Liveing and Dewar§, at or about the same period, also noticed the similarity 

 in the constitution of the spectra of some metallic elements. 



A study of the facts observed]] led to the conclusion that groups of elements 

 which exhibit homologous spectra, are composed of the same kind of matter in 

 different states of condensation, the molecules having similar modes but different 

 rates of vibration. 



Ames thought there were difficulties in drawing inferences from spark 

 spectra, but proved the complete homology of the zinc and cadmium arc 

 spectra.^ 



Rydberg recognised the law independently in 1885.** 



Kayser and Runge first completely studied the subject. The formulae which 

 they have found for the spectra of certain groups, of which the atomic mass of the 

 elements is a function, show that the molecules of these elements vibrate according 

 to a general law. Their work is well known, and is of an exhaustive character, 

 being contained in a series of seven long memoirs, which are standard works of 

 reference, tt 



In determining the wave-lengths of the spark spectra of the elements, in colla- 

 boration with Professor Hartley, a small Rutherford plane grating was employed, 

 and it was found that many of the marked features of the lines in the different 

 spectra appeared with but little alteration, but only a small portion of each 



* " On Physical Characters of the Lines in the Spark Spectra of the Elements," Proc. Roy. See, 

 vol. xlix., p. 448, 1891 (Hartley). 



j- " Spectres LTimineux," and Wiirtz, " Dictionnaire de Chimie," new supplement, p. 859. 



{ Sitziingsherichte der K. Akademie (Wien), vol. Ixxvi., p. 499. 



§Proc. Royal Institution, March 9, 1883. 



II Loc. cit. 



II Phil. Mag., vol. xxx., p. 33, 1890. 



**Kongl. Svenska Vetenskaps-Akademiens Handlingar, Stockholm, vol. xxiii., No. 4, 1890. 



ft Konigl. Akademie der Wissenschaften. " Uber die Spectren der Elemente " 1889 to 1893. 



