1824.] Mr. Levy on a new Mineral Substance. 135 



specific gravity and analysis I cannot give without spoiling the 

 specimen, because a sufficient quantity of the substance could 

 not be detached for the purpose of experiment. 



From figs. 2 and 3, it is obvious that the forms of the crystals 

 which they represent may be derived from an oblique rhombic 

 prism, and that by assuming the faces I have marked m as the 

 lateral planes of the primitive, and that marked P as the base, 

 all the others will be the result of simple laws of decrements. An 

 oblique rhombic prism may, therefore, be assumed as the primi- 

 tive form of this substance; but to the same class of prisms 

 belongs also the primitive form of pyroxene ; it remains, there- 

 fore, to examine whether those two primitive forms are connected 

 by any simple mode of derivation ; and first, in order to give 

 greater weight to the result of this investigation, it is necessary to 

 mention that although most of the crystals have their faces covered 

 with a greyish earthy substance, which render them unfit to be 

 measured by the reflecting goniometer, it is, however, possible 

 to measure some of the incidences by that means, by rubbing off 

 previously the earthy matter which covers the planes. 



If the crystal represented by fig. 2, is derived from the primi- 

 tive of pyroxene, the faces marked p and A 1 must be either the 

 results of decrements upon the angles a and o of the primitive, 

 or the one being the result of such a decrement, the other must 

 be the base of the primitive, or the face which replaces the edge 

 h, and is the result of a decrement by one row upon that edge. 

 Now let n be the law of decrements, which would produce upon 

 the angle a of the primitive of pyroxene a face corresponding to 

 one of the two faces p and h\ fig. 2, and n x the law which would 

 produce the other upon the angle o, let h be the lateral edge of 

 the primitive of pyroxene 1, being the oblique diagonal of the 

 base. Let a represent the angle of the base of the primitive of 

 pyroxene with the edge h, and let /3 equal the angle of the two 

 facesp and h\ fig. 2 ; that is to say, equal to (a n , o"'). Then the 

 following formula will easily be found : 



rp , , „, , „ h sin. a(n + n 1 ) 



Tang, (a", o nl ) = tang. |S = - 



+ (n — n 1 ) cos. « — 1 



Now, if in this formula the numerical values of h, a, /3, corre- 

 sponding to the particular case we are considering, are substi- 

 tuted, it will be found that no simple values of n and n l can 

 satisfy the equation, nor could any simple value be obtained for 

 the one if the other was supposed to be equal to nothing, or 

 infinity ; that is to say, if one of the faces p or h\ fig. 2, was 

 supposed to be parallel to the edge h of the primitive of pyrox- 

 ene, or to the base of the same form. It appears, therefore, that 

 this substance does not belong to pyroxene ; the only other mi- 

 neral to which it bears some analogy is amphibole, but this last 

 substance cleaves always very easily parallel to the lateral planes 

 of the primitive, and besides it might be proved that their forms 

 are incompatible in the same way as it has been done for pyroxene. 



