132 Mr. Levy on a neiv Mineral. [Feb. 



principles of classification generally adopted, these crystals 

 should form a distinct mineral species. 



The face marked p in fig. 4, is always very small ; and in the 

 crystals I have measured, it is entirely obliterated. The angles 

 which I could measure with the greatest accuracy, were the 



incidences of m on;», that off/ 2 on m, and that of e' on the simi- 

 lar face below. I could, besides, deduce from my measurements, 

 that, supposing the faces m, m, to correspond to the lateral 

 faces of the primitive, the plane p to the base of it, and 

 e l to be the result of a decrement bv one row on the an cries 



e of the base, then d might be considered as produced by a 

 decrement by two rows in height on the edges d. These were 

 the data from which I had to determine the dimensions of the 

 primitive form, as the other incidences that I measured could 

 not be relied upon. For this purpose I made use of the follow- 

 ing formulae relative to an oblique rhombic prism ; the investiga- 

 tion of which presents no difficulty. 



h sin. (p, m) . sin'. (r,'<\ m) 

 sin. (m, fit) . sin. v rf*, p) 

 h sin. (p, m) 



i (m, m) . tang. (e», p) 



h is the length of the lateral edge, one side of the base being- 

 supposed to be equal to one, and {in, m), (p } m), (d x , m), (e v , p), 

 denote, respectively, the incidences of m on m 3 of p on m, of a 

 face produced by x rows in breadth on the edge d of the base 

 on m, and of a face produced by y rows in breadth on the angle 

 t of the base on p*. To apply these formulae to the present case, 

 it is sufficient to observe, that here X = -i- and y =? 1. 



Substituting these values in the two preceding formulae, and 

 dividing the one by the other, there will arise 



tang, (e 1 , p), sin. (d , m), = cos. 4- (»h »0> &*• (^ > p)- 

 The only unknown quantity in this equation is the angle 



(d , p), as the others are precisely those which I could measure 

 with accuracy. The angle (e',p), is not the immediate result of 

 observation, but is evidently the supplement of half the incidence 



of e l on the similar face below it. The angle (d , p), will, 

 therefore, be easily calculated, and by subtracting 180° from the 



sum of the two angles (d , p), (d , m), the value of the angle 

 \p, m), will be obtained. This last angle being known, the first 



* I think that this notation, which consists in denoting the incidence of two faces, by 

 placing their crystallographical signs, separated by a comma, between two brackets, if 

 generally used, would considerably abridge the language of crystallography. I believe 

 it will also be found very convenient to write, as I have done in figs. 2 and 4, on each 

 face of the drawing of a crystal, its crystallographical sign. It is certainly preferable to 

 the method used by Ha'uy and the Count de Bournon ; in which each new modification 

 h designated by a new sign which has no reference to the decrement that produces it. 





