MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
217 
Ammonium Nickel Selenate (NH 4 ) 2 Ni (Se0 4 ) 2 . 6H 2 0. 
Morphology. 
The crystals of ammonium nickel selenate were goniometrically investigated by 
Tops0E # in the year 1870, and as regards their optics were included, like the 
potassium salt, by Topsce and Christiansen f in their well-known research of 1874. 
The crystals are bright green in colour, very like the potassium salt, but more 
generally perfectly transparent. 
Crystal System. — Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes.—a : h : c — 07395 : 1 : 0'5048. Values of Tops0e, 07378 : 1 : 
07042. 
Axial Angle. — /3 — 106° 17'. Value of Tops0e, 106° 19'. 
Forms Observed.—b {010}, c {001}, {110}, p'{l20}, g{011}, r'{20l}, and 
(/{Til}. The same forms, and no others, were observed by Tops0e. 
Habit. —More or less tabular parallel to c {001}, as shown in fig. 7. 
Eleven small crystals belonging to five different crops were measured, the crops 
being selected from the thirty or more prepared for their greater freedom from 
striation. The results are given in the accompanying table of angles. 
A large proportion of the crystals showed no other forms than c {001}, g {Oil}, 
p {110}, and r r {20l}. Certain crops, however, exhibited characteristically in 
addition excellent little faces of the clinopinakoid b {010}, with occasional develop¬ 
ment of the prism p'{l20} as a fairly broad face, and of the hemipyramid o'{Ill} 
as narrow faces blunting the pc edges. The faces of q {011} were particularly well 
formed, usually affording single sharp images of the signal. The b faces and r' faces 
were also similarly excellent when present. The faces of c {001} and p {110}, 
however, were generally much striated. The measured crystals were selected on 
account of their exceptional freedom from this defect. 
* ‘Kryst.-kem. Unders. o. d. selens. Salte, Dissertation,’ Copenhagen, 1870, p. 39. 
t ‘Ann. Chim. Phys.’ (1874), series 5, vol. 1, p. 81. 
2 h 2 
