MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
23 
Determination of True Optic Axial Angle in Bromonaphthalene. 
Light. 
Li . 
C . 
Na. . . 
T1 . 
Cd . . 
F . . . .■ 
No. of plate 
perp. 1 M.L. 
Observed 
2H a . 
No. of plate 
perp. 2 M.L. 
Observed 
2H 0 . 
Calculated 
2V a . 
Mean 
2V a . 
1 
72 31 
la 
o / 
85 46 
o / 
81 594 
o / 
2 
72 53 
2 a 
85 51 
82 12 )■ 
82 1 
3 
72 29 
3a 
85 55 
81 53 J 
1 
72 30 
la 
85 43 
82 0 4 
2 
72 51 
2 a 
85 46 
82 13 } 
82 2 
3 
72 27 
3a 
85 50 
81 54 J 
1 
72 18 
la 
85 16 
82 6 4 
2 
72 43 
2 a 
85 22 
82 20 
82 9 
3 
72 16 
3 a 
85 22 
82 2 J 
1 
72 9 
la 
84 48 
82 164 
2 
72 35 
2 a 
84 58 
82 28 V 
82 17 
3 
72 4 
3 a 
84 57 
82 8 J 
1 
72 3 
la 
84 33 
82 20 4 
2 
72 29 
2a 
84 43 
82 32 y 
82 21 
3 
71 58 
3a 
84 42 
82 12 J 
1 
71 56 
la 
84 14 
82 26 | 
2 
72 22 
2a 
84 24 
82 38 } 
82 27 
3 
71 51 
3 a 
84. 23 
82 17 J 
Murmann and Rotter obtained 155° 12' for 2E, and 81° 39' for 2V a (no specific 
Wave-length). 
Dispersion of the Median Lines .—-The inclined dispersion is very minute, and is 
such that the first median line is nearer to the axis a by about eight minutes for red 
C-hydrogen light than for green thallium light. The determinations were carried 
out with Plates 1 , 2 and 3 immersed in toluene, the refractive index of which (l’4955) 
is almost identical with the mean refraction (l'4962) of ammonium cobalt sulphate. 
Optic Axial Angles 2V a of the Cobalt Group. 
KCo sulphate. 
BbCo sulphate. 
CsCo sulphate. 
AmCo sulphate. 
Li. 
° r 
■ 68 38 
o / 
75 15 
o / 
81 42 
o / 
82 1 
C. 
68 39 
75 14 
81 40 
82 2 
Na. 
68 41 
75 11 
81 34 
82 9 
T1. 
68 44 
75 8 
81 29 
82 17 
F. 
68 48 
75 3 
81 22 
82 27 
The last table comparing the optic axial angles of the four cobalt-containing 
salts shows that the angle of the ammonium salt is just slightly greater than that of 
the cresium salt. All four salts are positive. 
