its place in the Classification of Elementary Substances. 9 



Professors Runge and Paschen, in their recent com- 

 munication to the Berlin Academy,* have pointed out that, 

 while the spectra of each vertical series of chemically- 

 related elements like Li, Na, K, Rb, Cs, shift towards the 

 less refrangible side of the spectrum with increasing atomic 

 weight, the spectra of elements in homologous positions in 

 each horizontal series like Na, Mg ; K, Ca ; Cu, Zn ; Rb, 

 Sr ; Ag, Cd, shift the opposite way, so that the spectrum 

 of the element of greater atomic weight is, as a whole, 

 situated on the more refrangible side. An examination of 

 the spectra of the different elements will show that this 

 generalisation holds fairly good for the first and second 

 series, and is also observable, but in a much less degree, 

 in members of the third series. So great, however, is the 

 difference of displacement between the spectra of the odd 

 and even series, taken horizontally, that the spectra of 

 members of the third series shift in the same relation to 

 the second as the spectra of Li, Na, K, Rb, Cs, i.e., to 

 the less refrangible side of the spectrum with increasing 

 atomic weights. The same inversions of displacement are 

 also observable in the spectra of the odd and even 

 members of the vertical series H», H2«, H37Z, as are 

 found in the horizontal series ; the spectra of the heavy 

 elements, like Cu, Ag ; Zn, Cd, shifting to the more 

 refrangible side of the spectrum in relation to the 

 alkaline and alkaline-earth metals above and below them. 



The spectra of members of the highest series of 

 elements, both vertically and horizontally, shift, on the 

 whole, towards the less refrangible side of the spectrum 

 with increase of atomic weights, like the spectra of the 

 alkaline metals. 



Mendeleeff and others have pointed out the greater 

 resemblance of chemical and other properties which odd 



* Sitzungsb. K. Preuss. Akad. zu Berlin, July n, 1895, p. 759. 

 Phil. Mag., September, 1895. Nature, September 26. 



