MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
223 
than in potassium nickel selenate but not so large as in csesium nickel selenate. This 
will be clearly apparent from the typical crystal of the rubidium salt shown in fig. 3. 
The ammonium salt forms crystals of all three types, but the great majority affect 
a more or less intermediate habit akin to that of the rubidium salt, while at the same 
time showing some idiosyncracies. Fig. 7 shows this well. 
Crystal Elements and Angles. —The axial ratios and axial angles are compared in 
the following table. From an inspection of it the following facts become clear :— 
Comparison of the Axial Angles and Axial Ratios. 
Axial angle. 
Axial ratios. 
Potassium nickel selenate . . 
Rubidium „ „ . . 
Ammonium ,, ,, . . 
Csesium ,, „ . . 
A 
104° 27' 
105° 20' 
106° 17' 
106° 11' 
a : b : c 
0-7467 : 1 : 0-5059 
0-7395 : 1 : 0-5031 
0-7395 : 1 : 0-5048 
0-7288 : 1 :0-4993 
The morphological axial angle of rubidium nickel selenate is approximately the 
mean of the axial angles of potassium nickel and csesium nickel selenates, the differences 
being nearly identical, 53' and 51'. It will be remembered that the atomic weight of 
rubidium is almost exactly the mean of the atomic weights of potassium and csesium. 
The axial angle of the ammonium salt is nearly identical with that of the csesium salt. 
The morphological axial ratios of the rubidium salt are also intermediate between 
those for the potassium and csesium salts. Hence, both axial angles and axial ratios 
for the three alkali metallic salts follow the order of the atomic weights of the alkali 
metals, and in the case of the axial angle with simple proportionality. 
The axial ratios of the ammonium salt lie between those of the two extreme, the 
potassium and csesium, salts, thus indicating true isomorphism of the ammonium salt 
with the alkali metallic salts. 
The next table affords a comparison of the morphological angles of the four salts. 
Comparison of the Interfacial Angles. 
Angle. 
KNi selenate. 
RbNi selenate. 
CsNi selenate. 
AmNi selenate. 
fac =(100): (001) 
° J 
75 33 
o / 
74 40 
o / 
73 49 
o / 
73 43 
as =(100):(101) 
46 15 
45 36 
44 51 
44 56 
sc = (101): (001) 
29 18 
29 4 
28 58 
28 47 
< 
cr = (001): (201) 
63 13 
63 58 
64 54 
64 46 
cs' = (001) : (101) 
38 16 
38 38-5 
39 11 
39 0 
sY = (101):(201) 
24 57 
25 19-5 
25 43 
25 46 
[r'a = (201): (100) 
41 14 
41 22 
41 17 
41 31 
YOL. CCXVII.—-A. 2 I 
