81 
Mr Levy’s Descriptian of Eudalite. 
(EjI . ^ ) 
(D) 
, cos ^ 
(n—1) 2 
2 cos I €f^ 
This formula gives the value of n, without knowing the inci- 
dences of the primitive faces, and by means only of the two 
angles {e „ . and {e„ : e„). When 
(^n • ^n) 
COS 
n=z3 
2 
cos 
{e„ : en) 
z= 1, consequently the two angles (e„.e„) and 
2 
(e„ : e„) are equal, and the dodecahedron is a right, double, six- 
sided pyramid. We may even find the value of the angle of the 
primitive by means of the same angles, without knowing the in- 
dex of the dodecahedron ; for, in estimating n between the equa- 
tions (C) and (D), we easily get 
cos 
(P. P) _ 
cos 
a 
-b cos 
(e„ : e„) 
2 
2 
a sin 
2 
( To be continued.) 
Art. VIII . — Description of Eudalite. By A. Levy, Esq. Mem- 
ber of the Academy of Paris. 
As no account of the forms of Eudalite has as yet been pub- 
lished, you will perhaps find room in your Journal for the fol- 
lowing short description of a very large and well defined crys- 
tal of this substance, belonging to Mr Heuland’s collection. I 
have also met with the same and simpler varieties in the collection 
of Mr Turner. 
The form is represented by Fig. 3. PI. Ill, and is obviously de- 
rived from a rhomboid. It is the combination of four rhomboids, 
with the faces of the two six-sided prisms, which are modifications 
of such a primitive form, and a plane, perpendicular to the axis. 
Although any one of these four rhomboids might be assumed 
as the primitive^, I have found that the simplest laws are ob- 
tained for the others, by assuming as such the faces which I have 
marked with the letter P. The incidence of P on P, I found 
Mr Brooke (Table at end of Introduction to (Crystallography) has given, for 
the primitive form of the same substance, an acute rhomboid of 74® 30'. 
VOL. XII. NO. 23. JANUARY 1825. E 
