282 ME. A. E. TUTTOX OX A COMPAEATIVE CEYSTALLOGEAPHICAL STUDY 
Molecular Kefraction (Gladstone), ^ M. 
a. A 
7- 
KMgsel. . . . 
. . 103-G3 104-09 
107-13 
5-G8 
5-G3 
4-8G 
BbMg sel. . . 
. . 109-31 109-72 
111-99 
10-33 
9-97 
9-00 
GsMg sel. . . . 
. 119-G4 119-G9 
120-99 
The A^’hole of tlie .‘Specific and molecular optical constants of rubidium magnesium 
selenate are intermediate between those of potassium magnesium seleuate and of 
csesium magnesium selenate. The molecular refraction increases considerably more 
when rul)idium is replaced by cresium than when potassium is replaced by rubidium, 
the proportion being as 5:3. The amount of change is less along the axis y than 
along the two other directions, along which the amounts are nearly identical. 
These results are independent of the temperature, for it has been shown that the 
refraction diminishes when the temperature is raised, and the density naturally varies 
in the same direction. They are also independent of the formula employed, whether 
that of Loeenz or of Gladstone and Dale. 
On comparing the molecular refraction constants of the magnesium double selenates 
with those previously given by the author for the magnesium double sulphates, in 
order to arrive at the effect of replacing sulphur by selenium, it is found that this 
replacement is accompanied by an increase of G'9 to 7T Lorenz units or 12'“ to 13‘3 
Gladstone units. Allowing for the })resence of two atoms, this gives for the 
increase per atom an aveiuge of 3'5 Lorenz and G'5 Gladstone units. Both values 
refer to ligTit of the wave-length of the 0 hydrogen line. 
The specific and molecular dispersions of the magnesium double selenates are 
higher than those of the magnesium double sulphates, as was also observed with 
regard to the tvu) zinc groups. 
Cviupai'ison of the Optic Axial Angles .—The magnitudes of the optic axial angles 
of the three magnesium doidjle selenates are not strictly com|)arable, on account of the 
extraordinary phenomena j'l'esented l)y the c;esium salt, involving the cro.ssing of the 
})lane of the optic axes. It lias already been shown, in discussing the refraction 
}>henomena, that this is the result of the operation of the I'ule of progression of the 
doidde refraction with tlie atomic v^eiuht of the alkali metal. 
O 
Tlie closeness of the similarity between the optical behaviour of this .salt and 
cmsium magnesium sul})hate, already alluded to as regards the I'efraction, is very 
apparent as regards the o})tic axial angle phenomena. Giesium magnesium sulphate 
and selenate exhibit I'espectively an angle for lithium light in the plane of symmetry 
of 18° 10' and 20° 49'. lii both cases the ano-le dimini.shes with diminishiiiQ- wave- 
length imtii the axes unite, for wave-lengths 450 and 4GG respectively, in the centre 
