MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
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arrive at the effect of replacing sulphur by selenium, it is found that the selenate 
values are higher by 6'94 to 8'06 Lorenz units or 1276 to 14'23 Gladstone units. 
As there are two atoms present of the sulphur or selenium, this affords as the increase 
per atom on replacing sulphur by selenium 3‘47 to 4'03 Lorenz units or 6'38 to 7 12 
Gladstone units. The following, therefore, is the table for the effect of the replacement 
of sulphur by selenium in the monoclinic hexahydrated double salt series brought up 
to date as regards the groups of double selenates now completely investigated. 
It will be remembered that the double sulphate series has been quite completed, as 
regards all the eight known groups. 
Increase of Molecular Refraction on Replacing S by Se. 
Salt group. 
Lorenz units. 
Gladstone units. 
Iron. 
3-5-4-0 
6•4-7•1 
Nickel. 
3•5-4•0 
6-3-7-2 
Magnesium. 
3 • 4-3 • 6 
6 • 3-6 • 7 
Zinc. 
3-5-3-7 
6'5-6 - 9 
Simple salts. 
3•4-3•8 
6•2-7•2 
The table also includes the effect of replacing sulphur in the simple alkali sulphates 
by selenium, and the result is seen to be almost (and in the mean exactly) identically 
the same. The effect of replacing sulphur by selenium as regards molecular refraction, 
is thus found to be on the average 3'6 or 67 units, according as the formula of 
Lorenz or that of Gladstone and Dale is employed. 
Summary of Main Conclusions. 
The outstanding result of this investigation of the iron group of double selenates 
of this grand series of isomorphous salts R 2 M ( ^ 0 4 j . 6H 2 0, is to add further evidence 
of the truth of the general law of progression of the crystallographic, structural, and 
optical properties with the atomic weight and atomic number of the interchangeable 
alkali metals (represented by R) potassium, rubidium, and caesium. The change in 
the principal interfacial and axial angle, the monoclinic angle /3, the mean change of 
all the 38 different interfacial angles, and the maximum change of angle, when one 
alkali metal is replaced by another, are all directly proportional to the change in the 
atomic weight or number, to a degree of precision which is surprising. The change 
in the dimensions of the structural unit cell of the space-lattice, as indicated by the 
molecular volumes and the topic axial ratios, and the corresponding optical change as 
indicated by the molecular refraction, are instances and expressions of the accelerating 
progressive effect of the increase of the atomic weight or number of the alkali metal. 
