360 Prof. W. N. Hartley on Relations 



communications in which these matters were dealt with were 

 illustrated by photographs of spectra enlarged to 36 x 28 

 inches. In the group of metals comprising magnesium, zinc, 

 cadmium, each spectrum consists of (1) single lines, (2) pairs 

 of lines, (3) a series of triplets, (4) a quadruple group, and 

 (5) a quintuple group. When oscillation-frequencies are cal- 

 culated from the wave-lengths of the lines in each spectrum, 

 the differences between the oscillation-frequencies in each 

 triplet in any one series is a constant quantity ; the law 

 applies also to the series of doublets. The differences between 

 the oscillation-frequencies of individual rays in the series of 

 triplets increases with the atomic mass of the elements to 

 which the triplets belong. 



A survey of the facts led me to the conclusion that these 

 elements are composed of the same kind of matter in different 

 states of condensation, the molecules having similar modes, 

 but different rates of vibration. This law of homology in 

 spectra has been elaborately studied by J. R. Rydberg, who 

 it appears recognized it independently in 1885 *. 



The spectra studied by me were spark- spectra, and con* 

 cerning this point Mr. Ames writes : — " But it is hardly safe 

 to draw inferences from spark-spectra ; for, although they are 

 undoubtedly characteristic, yet the state of affairs in the spark- 

 discharge is so unnatural, if 1 may use the term, that similar 

 molecules could not be expected to act alike. " There are, of 

 course, good reasons for studying both arc- and spark-spectra 

 and there are certain features in the latter which render them 

 of particular interest ; for instance, their physical characters 

 are more strongly marked than those of arc-spectra, their 

 lines are sharper and very seldom diffuse as in the arc, and 

 the same element frequently gives lines common to both arc 

 and spark. The disadvantage is that they are more compli- 

 cated. But there does not appear to be any definite reason 

 for rejecting observations made upon spark-spectra, or re- 

 garding the inferences drawn therefrom as unsatisfactory. 

 The conditions which are not specified, but which are referred 

 to as influencing the mode of vibration of the molecules in 

 vapours resulting from the spark-discbarge, have never varied 

 the spectrum of any of the metals which have come under 

 my observation. The only element which has exhibited vari- 

 ations in its spectrum within my experience, is carbon in the 

 form of graphite. This substance has been observed to yield 

 a spectrum from which some of the lines belonging to carbon 



* "Recherches sur la Constitution des spectres demission des elements 

 Ch uniques. 1 ' Kongl. Svenska Vetenskaps-akademiens Handlingar : Stock- 

 holm, Bandet 23, No. 4, 1890. 



