Mr. E. J. Chapman on the Notation of Crystals. 2>Tl 



the mineral, if not altogether hypothetical, I would select that 

 form most in keeping with the prevailing character of the 

 crystallization ; and therefore, sometimes an octahedron with 

 Weiss and his disciples, and sometimes a prism with Levy 

 and Dufreiioy. It would, however, be still better, as the pro- 

 posed notation, unlike those of the above crystallographers, 

 has no dependence upon the actual external shape of the fun- 

 damental form, to name merely the relative lengths of the 

 axes, thus: protaxialform =-756 X(idocrase); or '55^ ^'950, 

 &c., the value before the symbol to be considered the relative 

 length of the vertical axis, and that after the symbol to refer 

 to one of the lateral axes, the other axis being unity. The 

 protaxial forms of the different systems would be then as fol- 

 lows: — monometric system, X; dimetricand hexagonal systems, 

 ^X ; other sj'stems, xlLx. In the inclined systems the incli- 

 nations of the axes should of course also be given. 



Hexagonal System. 



Monaxial Planes : — 

 The basal planes, P. 



Biaxial Prismatic Planes : — 



Vertical planes of ordinary hexagonal 

 prisms, D. 



Triaxial Prismatic Planes : — 

 Vertical planes alternating in position 

 with the above, T. 



Triaxial Pyramidal Planes : — 



Ordinary hexagonal pyramids, O, 

 iO, mO. 



m 



Tetraxial Planes : — 



Hexagonal pyramids alternating in position with the above, 



Y, iY, mY. 

 Dihexagonal pyramids, xYm^ w = .rY4-2, —2. 



Apatite : Zillerthal. 



Hemihedral Forms'. 

 Rhombohedrons, 



O iO mO 



' 2' 2 

 xYmn 



——- ; or conventionally, R. 



Scalenohedrons, — - — ; or xrx^\ or, in general, simply 



•^S. In the conventional symbol of these forms, the sign r=: 

 Uie inscribed rhombohedron ; and as this is very generally 

 the same as the fundamental rhombohedron, the notation 

 becomes merely .rS, and is then, I think, more significative 



