^4 M. Levy on the Determination of Secondary Faces in 
the Red Oxide of Copper and the Oxide of Tin, which Mr 
Phillips has given in the 1st and Sd volumes of the Transactions 
of the Geological Society. 
Mr Phillips, in his paper on the Red Oxide of Copper, has 
described six modifications of that substance;, the incidences of 
the planes of which he could not measure. Out of these six, 
five may, however, be determined without knowing any angle, 
supposing the parallelisms which exist in Mr Phillips’ drawings 
to be correct. ^ 
For this investigation, it will be more simple to consider the 
cube, instead of the octohedron, as the primitive form of the 
substance. Once the indices relative to the cube are known, 
those relative to the octohedron may be obtained without diffi- 
culty. 
Let oa, oh^ oc^ Fig. 13., be the edges of the cube, to which 
all the secondary faces are to be referred. 
It is evident, Jirst^ That the faces of the octohedron result 
from a decrement by one row upon the angles of the cube, and, 
consequently, that the three indices of that face which replaces 
the angle o, will be equal to each other. 
Second Modification. — The planes of this modification being 
those of the rhomboidal dodecahedron, result obviously from a 
decrement by one row upon the edges of the cube ; so that, of 
the three indices relative to any one of them replacing one of the 
edges oa, o6, or, one will be infinite, and the other two equal. 
Fifth Modification. — The Fig. 81. (Plates to the 1st vol. of 
the Trans, of the Geol. Society), shews that one plane of this 
modification is parallel to the diagonal, ac^ Fig. 13., and to the 
intersection of the two planes of the second modification which 
replace the edges o«, oc. The new face being parallel to a dia- 
gonal, the second of the preceding formulae must be used. In 
the present case, = 1, = 1, — go, = 00 , n^ = 1, 
=1. These substitutions being made, the value of ^ is 
found to be equal to 2. Therefore, the fifth modification re- 
sults from a decrement by two rows in breadth upon the angles 
of the cube. 
Third Modfication. — The planes of this modification are 
evidently parallel to the edges of the cube, and one of them is 
