34 
DR. V/. M. HICKS; A CRITICAL STUDY OF SPECTRAL SERIES 
course of that consideration, chiefly connected with the atomic weight term, and the 
constitution of certain spectra, in an ensuing paper. In the present one the 
constitutions of the sequences on which the Principal and Sharp series depend, and 
their relation to the atomic volume of the element, which were brought to light in 
Part I., are further considered in connection with the spectra of the second and third 
groups of elements. It was hoped to have included in the third group the high 
melting-point sub-group Sc, Y, La, Yb, and considerable progress had been made in 
reducing their spectra. They all show doublet series of S and D types, hut their 
complete consideration would have taken so much time, better devoted to the general 
constitutional points which had arisen, that it was decided to postpone it. The 
evident importance, however, of the comparative study of all the elements in the 
same group of the periodic system made it seem desirable to give some attention to 
those elements which might possibly fill the gaps. It is generally acknowdedged that 
these belong probably to elements associated with the rare earths, and their atomic 
weights give some indication of the places they should fill. Their spectra consist of a 
multitude of lines chiefly of medium or small intensity, and their complete discussion 
involves a great deal of time and close attention. All those, however, which the 
author has so far investigated sliow more or less analogous relationships, intimately 
connected, and irresistibly suggesting the debris, so to say, of strong lines. The 
connection is based on certain properties of the atomic weight term referred to above. 
The evidence of the spectrum of Eu leaves little doubt but that it fills the vacant 
space between Cd and Hg. Probably the gap between In and T1 is occupied by Gd. 
A superficial consideration of the spectrum of the latter shows a large number of 
doublets with a separation of 5000."^ This number agrees with its atomic weight and 
its position between In (2212) and T1 (7792). As is well known, the spectrum of Pa 
is quite analogous to those of the Ca group, whilst possibly the gap between it and 
Ba may he filled by another radio-active substance. An attempt has been made to 
allot the S and D series of Eu and Ba and the result is given in Appendix I. In 
response to suggestions made to me in respect of Part I., I have ventured to give in a 
second aj^pendix lists of the S and P series lines of the elements considered in the 
present communication. 
Tlie type of formula used here and in Part I. has been employed by Mogendorff 
in a still earlier paper read before the Boyal Society of Amsterdam in November, 
190G. He there showed that the formula reproduces the lines of Li, Na, Zn, and T1 in 
the diflerent series with great exactness, and stated that he had also obtained 
corresponding formulm for other elements. 
The fact that the series depends in some way on the atomic volume of the element 
has been pointed out by Beinganum. f He showed that if the atomic volumes of the 
2nd group of elements be divided by 4, and those of the 3rd by 6, and if the 
* There are 53 within -50 of 5000, or 76 within 1 A.U. of that value, 
t ‘ Rhys. Zeitsch.,’ 5, p. 302, 
