Mineralogy and Geology. A431 
If we conceive of an ellipsoid of revolution tangent at its sides to the 
several faces of the prism, as the molecule of each form, it is plain 
that the angles alluded to afford a correct exhibition of the relations of 
such molecules as respects their dimensions, and therefore a true repre- 
sentation of the relations of the forms. ' 
There are blanks in the Tables, which are easily filled by calcula- 
groups, and exhibit their relations. The Corundum group was first 
brought to its present extent by the investigations of G. Rose, who 
added to it the rhombohedral metals, with tetradymite and red zinc ore. 
The earliest recognition of the relations of corundum, specular iron 
and ilmenite, was made by the distinguished chemist and mineralogist 
of Munich, Prof. von Kobell, in an important paper in the Jahrbuch 
der Chemie und Physik of Dr. F. W. Schweigger-Seidel for 1832,* in 
which he was also the first to point out the homa@omorphism of apophyl- 
lite and anatase; of copper pyrites, braunite, scheelite, scheeletine, 
wulfenite, cerasine, idocrase, uranite and mellite, besides other facts of 
interest. , 
The following important conclusions flow from the Tables. 
A. Hexaconat Sysrem.—(1.) ‘The vertical axis in Section IT is to 
that of Section las 14:1. The species of the two Sections may be re- 
garded as closely related in form if not homa@omorphous. 
2.) The vertical axis of Section IV is to that of Section I nearly as 
3:1, showing again a simple mathematical relation, if not pointing to 
actual equality. 
3.) The fundamental pyramid of Section V, is nearly identical in 
angle with the intermediate pyramid 2-2 of Section I, 4-2 of Section II, 
in 2.2 of Section IV. 
(4.) The Rhombohedral angle of Section IV, is near 90°—or mostly 
between 84° and 88°; and this approximation to the angle of a cube is 
its characteristic. The pyramid 1(1P) should consequently approach 
28’, (the angle of a monometric octahedron,) in its basal angle ; 
it actually varies for the species from Corundum to Iridosmine from 
113° to 117°. 
(5) In Section V, the basal angle of pyramid 1 is near 90°, and 
this appears to be the characteristic of the species. Hence the Rhom- 
bohedral angle of the Calcite series is near 105°. Hence this rhombo- 
hedral angle is a common one among rhombohedral! species. , 
6.) Section IIL in its rhombohedral angle approaches 90° as nearly 
‘as Section IV, but differs in having the angle greater than 90° instead 
of Ides. Its vertical axis is most nearly related to that of Section I, it 
being approximately twice as long as in that Section. 
5 d E 
iv, Heft.7, 1832; paper entitled, Beitrag zur isometrischer und hombometrischer 
ihen. he writer's attention has been directed to this paper since the re- 
i plished; but he has not yet been able to 
+ Neues Jahrbuch der Chemie und Physik, of Dr. F. W. Schweigger-Seidel, Band 
‘ honk 
reihen. 
sults on page 210 of this volume were pu 
consult it, except at second hand. 
