THE VARIATION OF ANGLES OBSERVED IN CRYSTALS. 
477 
Potash-alum. 
. 
Ammonia-alum. 
y/x. 
oy. 
oy. 
2 15 
1 30 
1 
1 lb 
3 15 
2 10 
3 0 
2 0 
4 45 
3 10 
8 30 
5 40 
6 0 
4 0 
16 0 
10 40 
7 30 
5 0 
17 0 
11 20 
8 15 
5 30 
21 0 
14 0 
9 0 
6 0 
22 30 
15 0 
12 0 
8 0 
24 30 
16 20 
13 0 
8 40 
26 30 
17 40 
15 0 
10 0 
27 15 
18 10 
19 30 
13 0 
28 30 
19 0 
20 0 
13 20 
31 0 
20 40 
21 0 
14 0 
_ 
_ 
22 30 
15 0 
- _ 
__ 
24 30 
16 20 
— 
_ 
25 15 
16 50 
— 
_ 
28 0 

18 40 
— 
and a number of intermediate values were obtained not quite so perfect. Tlie 
largest values observed for y/x were 0° 33' on potash-alum, and 0'^ 3G' on ammonia- 
alum, corresponding to inclinations of 0° 22' and 0° 24', re.spectively, to the true 
octahedron face. 
The following examples illustrate the differences in vicinal planes replacing the four 
octahedron faces of one and the same crystal of potasli-alum at the same time : — 
Crystal I. 
Crystal 11. 
Crystal III. 
Crystal IV. 
Oy : . 
n 
28^- 
ojy ; . 
1 o q 
lo| 
91 
234 
o’y : o’^ . 
n 
251 
. 
12f 
Si 
28 
Similarly on ammonia-alum : — 
Crystal I. 
Crystal II. 
Crystal III. 
o.y : 0 . 
17 
22 
314 
My : . 
24| 
31 
36 
"'y : "b. 
24 
274 
w'y: . 
164 
— 
