134 



Mr. Levy on the Ciystalline 



one he has ever seen. It was sent to him still adhering to the 

 matrix, but unfortunately by some accident has been detached 

 from it. As no determination has been hitherto given, as far 

 as I know, of the primitive form of this substance, the de- 

 scription of this crystal may not be uninteresting. 



The form it presents is rather complicated, being the com- 

 bination of fourteen modifications, and containing when com- 

 plete fifty planes. Fig. 1. is a representation of it, but differs 

 in appearance from the crystal, which offers only one of its 

 summits, and in which the planes belonging to the same mo- 

 dification ai'e of unequal extent. The simplest primitive form, 

 from which 1 find fig. 1. may be derived, is an oblique rhombic 

 prism, (fig. 2.), the lateral planes of which would correspond 

 to the faces in, and the base to the face p of fig. 1., the ratio 

 between one side of the base and the lateral edge being deter- 

 mined on the supposition that the planes c^ result from a decre- 

 ment by one row on the angles e of the base. Referred to 

 such a primitive form the crystallographical sign of fig. 1. is 



P mg^ h^ h" e^ e\ ah bi bi a^ a, e, {¥ di g-). 



All the planes of the crystal are sufiiciently brilliant to al- 

 low the use of the reflective goniometer to measure their in- 



Fig. 1. 



Fie. 2 



cidences, but the most brilliant are those of the modification 

 ^ and of the modification (6* di g'^) ; and it is from the mea- 

 sures of their incidences that the dimensions of the primitive 

 and the other angles have been calculated. The agreement 

 between the observations and the calculation has been in every 



instance 



