232 
DR. A. E. H. TUTTON ON THE 
The molecular refraction increases considerably more when caesium replaces rubidium 
than when the latter displaces potassium, the proportion being roughly as 5 to 3. 
The changes are nearly alike along the a and /3 axis, but are less along the y axis. 
These results are independent of the temperature, as both refractive power and 
density diminish (alter in the same direction) when the temperature is raised. 
The changes are obviously independent of the formulae employed, for the results are 
similar whether the values compared are calculated by the formula of Lorenz or by 
that of Gladstone and Dale. 
The position of ammonium nickel selenate with respect to molecular refraction is 
very close to that of the rubidium salt, in all cases less than one Lorenz or two 
Gladstone units higher. This result is very similar to that for the structural 
properties (dimensions), and again offers proof of the true isomorphism of the 
ammonium salt with the salts of the three alkali metals. 
It is interesting to compare these results for the nickel double selenate group with 
those for the corresponding nickel double sulphate group. A comparison of the 
Lorenz and Gladstone values for the molecular refraction shows that the selenate 
values are higher b} r 6'9 to 8'1 Lorenz units, or 12'7 to 14'4 Gladstone units. As 
there are two atoms of sulphur or selenium present, this gives as tire increase per 
atom, when sulphur is replaced by selenium, 3'5 to 4 Lorenz units or 6’3 to 7‘2 
Gladstone units. The following little table shows how these results for the 
replacement of sulphur by selenium in the nickel group compare with those obtained 
for the other groups already studied. 
Increase of Molecular Volume on Replacing S by Se. 
Salt group. 
Lorenz units. 
Gladstone units. 
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 values for the three groups of double salts are identical in the mean with 
those derived from the comparison of the simple rhombic alkali sulphates and selenates, 
which are therefore included in the table. This mean increase in molecular volume, 
when a sulphur atom is replaced by a selenium atom, in either series, is 3’6 Lorenz 
units, or 6’7 Gladstone units. 
Summary of Main Conclusions. 
The results now presented for the nickel group of double selenates, of the mono- 
clinic series R 2 M( ^ 0 4 ). 6LLO, are in line with those previously communicated for the 
“ VSe / 2 
