MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
423 
Molecular Optical Constants .—These work out as under :—■ 
Lorenz 
>1 
>> 
>> 
Gladstone 
Axis of optical indicatrix . . . 
a. 
A 
7 • 
Specific refraction, -AL —J_ = n . . {2 
1 (n 2 + 2) d \G 
Molecular refraction, A —- . ^ = m . { 9, 
n 2 + 2 d L Or 
Specific dispersion, n G - n 0 . 
Molecular dispersion, m G - m c . 
<72 | ^ 
Molecular refraction, —— M . . . . C 
a 
0-1009 
0-1036 
71-74 
73-64 
0-0027 
1-90 
122-99 
0-1017 
0-1043 
72-29 
74-16 
0-0026 
1-87 
124-11 
0-1027 
0-1054 
72-99 
74-91 
0-0027 
1-92 
125-56 
Mean molecular refraction (Gladstone), ^(a. + /3 + y) = 124 • 22. 
Ammonium Ferrous Selenate (NH 4 ) 3 Fe (Se0 4 ) 2 .6H 2 0. 
Morphology. 
An investigation of ammonium ferrous selenate, as regards goniometry and specific 
gravity only, was carried out by Haldor Tops0E # in the year 1870, and as regards 
the optics of the crystals by Tops0e and Christiansen f in 1874. 
The crystals employed in the present investigation were prepared by the method 
described in the introduction, the ferrous selenate being prepared by the action of 
selenic acid on ferrous sulphide. They were clear and transparent, and of a very 
pale yellowish green colour. Ten excellent little crystals were measured, belonging 
to eight different crops. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes. — a :b : c — 07433 : 1 : 0'5019. Values of Tops0e, 07405 : 1 : 0'5012. 
Axial Angle. — /3 = 106° 9'. Value of Tops0e, 106° 1 S^. 
Forms Observed.—b {010}, c {001}, p { 110}, q {Oil}, r'{20l}, and o'{Tll}. The 
same forms, and these only, were observed by ToPS0E. 
Habit. —From somewhat (thick) tabular parallel to c{00l}, to short prismatic 
parallel to jo {110} and the vertical axis c, and with r'{20l} more prominent than 
usual in the series. 
A typical crystal of the first type is represented in fig. 8, and one of the second type 
in fig. 9. 
* ‘ Kryst.-kem. Unders. o. d. selens. Salte, Dissertation,’ Copenhagen, 1870. 
t ‘Ann. Chim. Phys.’ (1874), series 5, vol. 1, p. 83. 
