MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
235 
single positive unit, the positive charge carried by the nucleus being in all cases an 
integral multiple of the charge on the hydrogen nucleus ; thus, for instance, while 
hydrogen has one nuclear charge helium has two such charges, and lithium three. 
The decisive work of Moseley, in his latest (unhappily last) research, # has been to 
show (a) that the X-ray spectra of the line series known as K are derived from the 
innermost ring of electrons, while those of the L series are derived from electrons 
further from the centre of the atom; (h) that the frequency of the spectra is 
proportional to (N — a) 2 where a is a constant dependent on the line series used, its 
value being 1 for the K series and 7'4 for the L series (for the a line in each case); 
and (c) that N itself, the nuclear charge, is none other than the atomic number. 
The atomic number, or element sequence number, is thus a very important factor 
in the constitution of the atom, and it is therefore very likely to be a correspondingly 
governing factor in determining the relative properties of an isomorphous series of 
salts, formed by the interchange of atoms of elements of the same family group of 
the periodic classification, such as the three alkali metals, which differ regularly 
from one another in position by two whole horizontal rows of elements at a time 
(18 elements). Indeed, the work of Moseley shows clearly that the properties of 
the atom are better defined by the atomic number than by the atomic weight. It 
has always been difficult to see why mere atomic weight should wield the influence 
which has been pointed out by the author, and the author has been careful to 
emphasise that he regarded atomic weight as only another progressively changing 
property, very convenient, however, for purposes of reference as regards the chemical 
position of the element, but equally with all the other progressing properties dependent 
on some hitherto unknown fundamental difference in the chemical elementary atoms. 
This fundamental difference is now proved by Moseley to be the positive electric 
charge on the atomic nucleus, and as this determines the atomic electronic com¬ 
plexity and the atomic number is its measure, there is a logical reason why 
increase in the atomic number, in the case of a regularly ascending series of family 
group elements, should be effective in similarly progressively modifying the crystallo¬ 
graphic and physical properties of isomorphous salts containing those elements. 
Moreover, the fact that both atomic weight and atomic number have been shown 
above to be similarly related, in the case of the alkali metals, rubidium standing 
exactly midway between potassium and csesium in both respects, affords full 
explanation for the progression according to atomic weight which has been pointed 
out by the author to occur in the eutropically isomorphous series, both the rhombic 
simple salts and the monoclinic double salts, which have been described and 
compared in this and the author’s former communications. 
* ‘ Phil. Mag.,’ series 6, vol. 26, p. 1024 (1913), and vol. 27, p. 703 (1914); see also “ Obituary Notice,” 
by Sir E. Rutherford, ‘Roy. Soc. Proc.,’ A, vol. 93, p. xxv. (1917). 
