60 M. Levy's Observations on the preceding Paper. [Jan. 



M. Rose has adopted, as appears from a determination of Weiss, 

 an oblique rhombic prism for the primitive of felspar. I had 

 assumed the same form, from the observation of the crystals of 

 that substance 1 had an opportunity of examining in Mr. Tur- 

 ner's collection, as well as from the very figures given by Haiiy, 

 and the measurements given both by him and Mr. W. Phillips. 

 M. Rose has not stated the reasons which induced Weiss to alter 

 the determination of Haiiy ; and as I believe they are not 

 generally known, since Mr. Brooke and Mr. W. Phillips, in their 

 late publications, have adopted the primitive form of Haiiy, I 

 shall briefly explain by what considerations I was led to the 

 same result as Weiss.* 



On looking at the figures given by Haiiy in the last edition of 

 his treatise on mineralogy, as well as on looking at any crystal 

 of felspar, it will easily be seen that every one of them may be 

 derived from an oblique rhombic prism, the lateral planes of 

 which would be, for instance, the plane he has marked I, and 

 the face opposite and parallel to T, and the base the plane P. If 

 the primitive were not such an oblique rhombic prism as I have 

 just described, one would expect to meet with a crystal contain- 

 ing the face T without the face /, or the modification z without 

 the modification z', or s without s', or n without «', but the con- 

 stant simultaneous occurrence of these groups of modifications 

 perfectly symmetrical relative to the planes P, /, T, both in their 

 positions and incidences, is certainly decisive. Moreover, in 

 the form I have adopted, if a cleavage be found parallel to M, it 

 must be at right angles to the base P, because M is equally 

 inclined upon / and T, or because it is parallel to a plane through 

 the oblique diagonals of the bases. This cleavage, as it is well 

 known, exists in felspar, and is found perpendicular to P. This 

 angle of 90° would again be a very singular occurrence if the 

 primitive were a doubly oblique prism, fhe only argument in 

 favour of Haviy's determination is, his assertion that there is a 

 cleavage parallel to T, and none parallel to /, as should be 

 the case, if/, as I have assumed, was one of the lateral primitive 

 planes symmetrical to T. To this may be answered that even 

 the cleavage parallel to T is in most cases very difficult to obtain, 

 that this is not the only example of an oblique rhombic prism, 

 in which one of the lateral planes is more easily obtained by 

 cleavage than the other.f It is the case, for instance, in chro- 

 mate of lead. Moreover, in some of the flesh-coloured speci- 

 mens, I have succeeded in obtaining a cleavage parallel to /, by 

 detaching first a thin lamina parallel to P. Finally, Haiiy men- 

 tions the primitive he has adopted as one of the forms offered 

 by nature : this form I have never seen ; and I doubt very much 

 its existence, because it could not be derived from an oblique 

 rhombic prism froi:;vhich all the others are so obvioirsly deduced. 



* See Annals of Vhilosopliy for November. 



f See Brooke's Familiar Introduction to Crystallography, p. 189. 



