58 
DR. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 
on 9 times its volume constant, and Hg on G times its corresponding volume constant. 
On the other hand, Zn and Hg may depend on their volume constants in the same 
way, but the centres of the Zn molecules be packed 9 times more closely together 
tlian in the normal case, and the centres of the Hg molecules 6 times. On the first 
view the multiples depend on the internal configuration of the atom, on the second 
on the configurations of the molecules in gross matter, in other words, on the value of 
the ratio atomic weight to density. If the first interpretation is correct, we might 
expect to find other series in an element depending on other, multiples. I am not 
sure that I have not oliserved indications of such, but it will require further search 
liefore a definite reply can be given. On the second interpretation no such additional 
lines can be expected. 
2. Additional evidence to that obtained in Part I. has been afforded of the 
existence of a universal constant, approximately '21520, which gives the ratio of a//^ 
in the j^-sequence fx + ajm, and also in the 5-sequence if certain relations indicated 
lietween the s- and p-sequences are found to be exact. This constant must be a pure 
number* in contradistinction to N, which is of the dimension (length)~h 
3. A comparison of the denominators of VPj (m), VP 2 (m), VP 3 (m) has indicated 
that whilst in the alkalies they differ by the same amount in the same element for all 
orders, in the 2nd and 3rd groups the difference proceeds in two steps, viz., that after 
the first order the remaining differ liy the same amount, and that this is about '7 
times the difference for m — 1. Further, a comparison of the above with VS(7n) 
shows that a similar law holds between the denominators of VS (771) and VP(w). 
These laws are applied as a criterion to the lines allotted by Paschen for HgP, MgP, 
and CaP, and, in consequence, ceifain modifications are suggested. 
4. It is found in the 2nd and 3rd groups, in opposition to the rule in the first, that 
the S series follows the j7-sequence and the P series the 5-sequence. 
5. An attempt is made to arrange the S and D series for Europium and Iladium. 
Eu is found to fit the gap between Cd and Hg and a density 12'58 deduced for it. 
The density of Ea should be l'02a;, where x is probably 5 or 6. 
APPENDIX I. 
The S ciJid D Series of Europium. 
Tlie only published spectra of Europium are tlie arc and spark spectra of the ultra 
violet lines by Exneu and Haschek. CROOKEsf has observed, l)ut not published, its 
spectrum, but remarks that the Europium used by Exner and Haschek must have 
contained seveml impurities. Exner and Has(JHEk’s list of arc lines contains over 
* 2/(37r) = -21511. 
t “On Europium and its Ultra-Violet Spectrum,” ‘Roy. Soc. Froc.,’ 74, p. 550 (1905). 
