440 DR. A. E. H. TUTTON ON MONOCLINIC DOUBLE SELENATES OF IRON GROUP. 
The mystery as to why this should be so, formerly surrounding the similar results 
derived from the three groups of double selenates and eight groups of double sulphates 
previously described by the author, has now been completely lifted, as shown in 
the addendum of February 7, 1917, to the author’s last communication, # by the 
brilliant discovery of Moseley, that the atomic sequence number is a fundamental 
constant governing the structure and complexity of the chemical atom. For as the 
atomic number is itself a measure of the positive charge on the atomic nucleus, and 
therefore correspondingly governs the number of negative electrons clustered around 
it, it must of necessity follow that a family group of chemical elements so vigorous as 
that of the monovalent alkali metals, each differing from the next by two whole 
rows (series) of elements in the periodic classification, must bring about by their 
successive replacement of each other a progressive change, corresponding to the 
increased atomic complexity, in the properties of the crystals of the salts in which 
they replace one another as dominating elements. As the atomic weight varies 
similarly to the atomic number, the differences between potassium and rubidium on 
the one hand and rubidium and caesium on the other being similarly related (equal) 
with respect to both constants, the law of progression may be stated with respect to 
either constant with like validity. To state it with respect to the atomic number is 
the better and more logical way, however, as the reason for the law is then at once 
apparent. 
One other important result is the further confirmation afforded, by the results for 
this iron group of double selenates, of the isostructural relationship of the ammonium 
and rubidium salts of any group of the series. The interesting -application of this 
conclusion as a test for the Pope-Barlow theory of valency volume to which special 
reference was made in a former communication,! is still further justified by the results 
now presented, which are clearly incompatible with that theory. 
In concluding this description of the ferrous double selenates satisfaction may, 
perhaps, be legitimately expressed that it has, after attempts lasting over several 
years, been at last found possible to isolate and investigate completely crystals so 
difficult to prepare, and so extremely fugitive when prepared, as those of potassium 
ferrous selenate. 
* ‘Phil. Trans.,’ A, vol. 217, p. 234 (1917). 
t ‘Roy. Soc. Proc.,’ A, vol. 93, p. 72 (1917). 
