440 



M. Levy on a new Mineral Substance. 



[Dec. 



The form of the crystals is represented by fig. 2, but the plane 

 marked g l is wanting in most of them. There is a distinct and 

 brilliant cleavage parallel to p, but I could not find any other. 

 The hardness of the substance is about the same as that of car- 

 bonate of lime. The faces a- are dull, and, as it were, hollowed 

 towards the middle : their determination has been deduced from 

 the parallelism of their intersections with the faces ft 1 . All the 

 other faces are sufficiently brilliant to obtain their incidences by 

 means of the reflecting goniometer. From these incidences, as 



4 



well as from the different characters of the faces a", e , and the 

 occurrence of the face g 1 , without the edge of intersection of the 

 faces a? being replaced, I was enabled to infer that Che primitive 

 form was not, as 1 had thought at first, an octohedron with a square 

 base, but might be supposed to be an octohedron with a rect- 

 angular base, or more simply a right rhombic prism. This last 

 hypothesis I have adopted, and determined the dimensions of 

 the prism by assuming that the faces ft 1 are the result of a decre- 

 ment by one row on the edges of the base of the primitive. 



In this supposition the primitive form, fig. ], is a right rhom- 

 bic prism of 125° 7', in which one side of the base is to the 

 height nearly in the ratio of 13 to 29. The face a 2 is on account 



of the parallelism already mentioned, the result of a decrement 



* 

 by two rows on the angle a of the primitive, and the face e J on 

 account of its incidence on p, the result of a decrement by four 

 rows in breadth and three in height on the angle e. 

 The incidences I have taken as data are, 



4 4 



ft 1 =± 109° 40' 



P, 



112° 30' ft' e 3 = 129° 



and I calculated the following, which very nearly agreed with 

 my observations. 



ft 1 , ft 1 = 114° 24' 



- ™° 15' 



a- = 113° 36'. 



ft 1 , ft 1 = 79 e 



m, m ss 125° 7'. 

 The specimen conies from Schneeberg, in Saxony, but must 

 of extreme scarcity, being the only one ever seen by Mr. 



