MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
13 
A comparison of the optic axial angles of the four salts containing nickel, instituted 
in the next table, shows that the angle for the ammonium salt is close to that of the 
caesium salt, being slightly lower than the latter. It has to be remembered that 
caesium nickel sulphate is (by exception in the series) of negative double refraction, 
so that the first median line is, in this case, the a axis of the indicatrix ellipsoid, 
instead of (as usual in the series) the y axis. 
Optic Axial Angles 2Y a of the Nickel Group. 
KNi sulphate. 
RbNi sulphate. 
AmNi sulphate. 
CsNi sulphate. 
Li. 
o / 
75 21 
o / 
82 5 
o / 
86 27 
o / 
87 15 
C. 
75 19 
82 4 
86 28 
87 17 
Na. 
75 16 
82 0 
86 33 
87 21 
■ T1. 
75 13 
81 56 
86 37 
87 29 
F. 
75 9 
81 48 
86 43 
87 40 
Ammonium Cobalt Sulphate (NH 4 ) 2 Co (S0 4 ) 2 .6H 2 0. 
Morphology. 
The only crystallographic investigation of this salt hitherto made is that of 
Marignac ( loc. cit.). The values of the angles observed by him are quoted in the 
last column of the table of angles. 
Crystal System .—Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes .—- 
a : b : c = 07386 : 1 : (F4975. Values of Marignac, 07392 : 1 : (F4985. 
Axial Angle .— (3 = 107° 2'. Value of Marignac, 106° 56'. 
Forms Observed.—a {100}, {010}, c{001},y>{ll0}, q {Oil}, r'{20l}, o {ill}, 
o'{Tll}. 
Habit .—Short prismatic parallel p {l 10}, often with narrower c faces and larger 
q faces than in the nickel salt ; or more or less tabular parallel to a pair of faces of 
this prism p {110} predominatingly grown, or to c {001}. 
This is one of the finest salts of the series, the crystals being of a ruby or red-currant 
red colour. The crystals of several of the numerous crops 
obtained were composed of completely transparent well- 
developed individuals, having absolutely plane faces yielding 
brilliant single images of the signal of unusual perfection. 
Ten superb crystals were measured, selected from five different 
crops. The results are shown in the table of angles. A typical 
crystal is shown in fig. 4, and fig. 2, representing a crystal 
of ammonium, nickel sulphate, is equally typical of many crops 
of the cobalt analogue. Both are of the short prismatic type, but not infrequen 
