[ 477 ] 
LXXI. On the Form o/Eudialyte. Bj/ Professor Miller^. 
T he angle between normals to two adjacent faces p, of one 
of the rhombohedrons of eudialyte, is stated to be 106° 20' 
in some mineralogical treatises, and 106° 36' in others. A 
very accurate determination of its form is difficult on account 
of the unevenness and dullness of its faces. In order if pos- 
sible to render it a little less uncertain, I measured some very 
perfect crystals, for which I am indebted to Dr. A. Smith of 
Dublin, and also some of the best crystals in Mr. Brockets 
collection. The resulting values of the angle between nor- 
mals to the faces o and jo, either given immediately by ob- 
servation or computed from the angles between other pairs of 
faces, after excluding all the observations that were unsatis-. 
factory on account of the largeness of their difference from 
the mean, or the indistinctness of the reflected images, are 
67° 42'-- -40'— 44'— 48'— 41'— 39'— 45'-~4l'- 35'—43'~4l' 
-—47' — 41' — 36' — 40'— 47' — 42' — 47'. The angles betvreen 
normals to the faces calculated from the mean of the pre- 
ceding values of o p, are 
0 u 
90' 
' 0' 
c u 
30' 
° 0' 
0 c 
90 
0 
zz^ 
53 
35 
0 z 
31 
22 
X 
84 
4 
0 X 
50 
38 
pp' 
106 
30 
op 
67 
42 
5 s' 
116 
4 
0 s 
78 
25 
U t 
13 
59 
ot 
81 
11 
p t 
22 
46 
The symbols of the forms to which the different faces be- 
long are, in the notation which I have adopted in my treatise 
on crystallography. 
o {ill}, c {211}, u {101}, p (lOOjj ^{110}, 
^ {211}, 5 {111}, t {201}. 
St. John’s College, Cambridge, 
May 6, 1840. 
* Communicated by the Author. 
