332 ADENEY—Photographs of Spark Spectra from the Rowland 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 Boisbaudrant and Ciamician.t 
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 zine and cadmium are 
spectra.4] 
Rydberg recognised the law independently in 1885.** 
Kayser and Runge first completely studied the subject. The formulee 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 wayve-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 
* (Qn Physical Characters of the Lines in the Spark Spectra of the Elements,” Proc. Roy. Soc., 
vol. xlix., p. 448, 1891 (Hartley). 
” new supplement, p. 859. 
+ ‘“‘ Spectres Lumineux,”’ and Wurtz, ‘“‘ Dictionnaire de Chimie, 
+ Sitzungsberichte der K. Akademie (Wien), vol. lxxvi., p. 499. 
§ Proc. Royal Institution, March 9, 1883. 
|| Loe. ett. 
q Phil. Mag., vol. xxx., p. 33, 1890. 
** Konel. Svenska Vetenskaps-Akademiens Handlingar, Stockholm, vol. xxiii., No. 4, 1890. 
+} Konigl. Akademie der Wissenschaften, ‘‘ Uber die Spectren der Elemente.” 1889 to 1893, 
