446 Note on the employment of Right Rhomboidal Prisms. 



To analyse this mineral, or even to determine its specific 

 gravity, with any approach to accuracy would be next to im- 

 possible, owing to the difficulty of separating a sufficient 

 quantity from the lava, nepheline, &c. with which it is asso- 

 ciated. 



LX. Note on the employment of Right Rhomboidal Prisms 

 in Crystallography. By E, J. Chapman, Professor of Mi- 

 neralogy in University College, London*. 



IN the conventional position of the axes of monoclinic or 

 oblique prismatic crystals, the three monaxial forms, id est, 

 such as cut but one axis, P, M and L, necessarily constitute 

 a rectangular prism with oblique base ; P and L, and M and 

 L, being at right angles to each other, whilst the inclination 

 of P on M is variable. In oblique rhombic prisms the basal 

 plane P remains the same, M and L giving place to the di- 

 axial form D ; and when P, M, L and D are in combination, 

 as in certain augite crystals, for example, an eight-sided prism 

 is produced, with an oblique base P. The accompanying 

 section (No. 1) through the secondary axes, exhibits the rela- 

 tive positions of these forms f. Fig. 2 is a vertical section 

 through the principal axis and the orthodiagonal ; fig. I, a 

 vertical section through the clinodiagonal. In the latter, 

 therefore, the point of the orthodiagonal, or the position of a 

 side plane L, fronts the observer. 



Fig. 1. Fig. 2. Fig. 3. 



OS & 



JX£ 



JML 







, 



. . , 



M 



A- 



m. 



This premised, it is evident that if we place an oblique 

 rectangular prism so that one of the planes L shall constitute 

 the base, a right rhomboidal prism will be produced — a form 

 frequently adopted by certain mineralogists to the exclusion 

 of the former solid. Here, however, it is obvious that the 

 positions of the axes can no longer be the same as in the 

 oblique rhombic prisms; so that, in adopting this form, we 

 must also adopt two positions for the axes of monoclinic cry- 

 Communicated by the Author, 

 t In the notation of Naumann,P=OP- M, ooPoo; L(ooPoo); D,ooP. 



