Crystallization in the Mineral Kingdom. 133 



than it would be if it represented the derivation from \ J (viz. 

 by 6') . This obtuser pyramid, which I designate as P, is the espe- 

 cial primary form, while D, the rhombic dodecahedron, remains 

 as the general primary form of the garnets. I found a greater 

 difference in the inclination of the grossular garnet (Granatus 

 grossularis, B. M., p. 635), but, owing to the want of a perfect 

 crystal, I could not properly estimate the variation. 



I may ask, who has ever taken the trouble to centre and 

 measure a garnet crystal twenty-four times along the principal 

 edges ? Has not the labour of centring and measuring the 

 crystals of tourmaline three times according to their polar edges 

 been avoided ? 



Now that my measurements have so clearly proved those 

 newly discovered optical appearances which are above described, I 

 conclude that the tesseral symmetry of the garnet crystals above 

 described is also destroyed, and that as a tetragonal axis appears 

 to be the principal one, it must also be an optical axis, I gave 

 utterance to these views with the greatest confidence at the sit- 

 ting of the Freiberg Mining Union on the 17th of January last. 



As melanite is opake, I tried no experiments on it ; but as 

 M. Reich and I were examining some polished garnets on the 

 22nd of January in Werner's Museum, I was fortunate enough 

 to find a specimen which showed one optical axis perfectly and 

 beautifully. This specimen had a sp. gr. = 4*152 (nearly that 

 of the crystals that were measured). We convinced ourselves, 

 however, that there are red garnets which have no optical axis, 

 — for example, that garnet which has the sp. gr. = 4*20 to 4*27, 

 and which, according to Prof. Rammelsberg, is so rich in prot- 

 oxide of manganese. 



The beautiful hyacinth-red garnet from the granite druses 

 of the island of Elba belongs to this class, and has the same 

 optical character. This, the heaviest of the garnets, must be 

 separated from almandine, and called Granatus manganosus: 

 one must not be astonished that species of essentially different 

 character appear in one genus of an old system of crystallization. 

 We have only to remember the felspars, where adularia and peg- 

 matolite are monoclinohedric, and tetartine, Labrador, and peri- 

 cline are triclinohedric. Who would have believed that at least 

 one of the well-known topaz family is hemihedral ? and yet it is 

 so. I am much indebted to Dr. Krantz for sending me trans- 

 parent garnets from Piedmont and the East Indies. I found 

 one optical axis in Hessonite from Ala in Piedmont. 



It now remained to determine whether the optical coincided 

 with the tetragonal axis in those garnets which possessed one 

 optical axis. For this purpose I had a cube cut by M. Lingke 

 out of a crystal of Hessonite, having its planes parallel respect- 



