478 
PROFESSOR HENRY A. MIERS: AN ENQUIRY INTO 
These examples indicate that the angle of the vicinal faces is generally of the same 
order of magnitude upon the different faces at any given tune, but they do not 
indicate any cause for the differences, or any special regularity. 
In K-alum I., 11., opposite faces seem to show nearly the same form, which differs 
from that on the adjacent faces; but in K-alum IV., and Am.-alum I., the forms 
which most resemble each other are on adjacent faces. 
It is quite clear, at any rate, that there is no constancy of angle in the vicinal 
faces either of K-alum or of Am.-alum. 
Several attempts were made to determine whether, during the growth of a crystal, 
the changes are progressively in the same direction; whether the flat tnakis-. 
octahedron becomes more flat or more acute; also whether the angle varies m any 
way with the temperature or the barometric pressure. 
The following observations on a crystal of ammonia-alum form a continuous and 
fairly lengthy series 
o and oj are two a 
yielded two images, 
horizontal plane. 
djacent octahedron faces and C is a cube face, o and to both 
a/3, in the same vertical plane; C two images, a/3, neail\ in a 
Fehruary 4, 1894.—The crystal was adjusted for a point halfway between and 
and a point halfway between co^ and co^. 
0,3 = 139° 18', 
C, = 84°31i', . 
all the images were very good 
co„, = 29° 54', 
_o', = 319° 25f', 
At 5 r.M. it was immersed in solution containing loose cr^^stals. 
Barometer 769'6 millim. ; t — 12|-°. 
At 7.30 P.M. 
"o., 139° 20', i = 12f°, 
_co^p 29° 56'. 
At 10.0 P.M., t = 12-j-°, o yielded two images. 
n39° 201', 
Li39° 19 V. 
The crystal having been withdrawn, the solution was then warmed to 35 and the 
cry.stal re-immersed at 11.15 when the temperature was 25 . 
