.MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
31 
caesium salts as regards double refraction, the value being slightly nearer to that of 
the rubidium salt. 
Axial Ratios of the Optical Ellipsoid .—These are given in the next table, and 
compared with the corresponding values for rubidium and caesium manganese 
sulphates. 
Axial Ratios of 
Optical Indicatrix. 
Optical Velocity Ellipsoid. 
* . : P ■ 7 
a : t : r 
RbMn sulphate . 
. 00973 : I : I‘0067 
1*0027 : 1 : 00933 
NH 4 Mn „ . . 
. 09974 : I : I"0049 
f'0026 : 1 : 00951 
CsMn ,, . . 
. 0-9987 : 1 : U0039 
10013 : 1 : 00961 
The values when for the same salt = 1 show a similar arrangement as in the 
groups previously described, with the ammonium salt intermediate between the 
rubidium and caesium salts. The potassium salt not existing, the values cannot be 
calculated on the basis of /3 K saIt , = 1. 
Molecular Optical Constants .—These are given in the following three tables, and 
compared with the analogous values for the rubidium and caesium salts, using the 
newly determined values of the densities of the latter salts in the calculations for these 
salts. The specific refraction and dispersion of the ammonium salt again stand out 
prominently higher than those of the alkali-metal salts, and the molecular dispersion 
of ammonium manganese sulphate is slightly higher than that of the caesium salt. 
The molecular refraction, calculated by either formula, of the ammonium salt is just 
slightly higher than that of the rubidium salt, the relationship being very concisely 
expressed by the mean molecular refraction shown at the conclusion of the third 
table. 
Table of Specific Refraction and Dispersion (Lorenz). 
Sulphate. 
H 2 — 1 
Specific refraction, — ——= n. 
Specific dispel 
11G - 11 C . 
sion, 
For ray C (Hx). 
For 
’av Hy near G. 
a. 
A | ' 7- 
a. 
P- 1 7- 
a. 
P- 
7- 
Am Mn . 
RbMn . . 
CsMn . . . 
0 • 1545 
0-1143 
0-1059 
0-1556 ' 0-1576 
0-1151 0-1172 
0-1062 0-1073 
0-1582 
0-1167 
0-1082 
0-1593 0-1613 
0-1176 0-1198 
0-1085 0-1097 
0-0037 
0-0024 
0-0023 
0-0037 
0•0025 
0-0023 
0-0037 
0-0026 
0-0024 
