Formation of Compound or Twin Crystals. 281 



produce the same arrangement as in Fig. 2, as the same kind of for- 

 ces are in operation. 



It may be remarked here, that when the axes of a crystal are 

 hereafter referred to, those lines are understood which connect the 

 centres of the opposite faces. They may be called Crystallogenic 

 axes to distinguish them from the Crystallographic or those found 

 convenient in the descriptions of crystals and calculations connected 

 with them. Their nature is precisely expressed in the above ad- 

 jective, derived from xputfraXXoc: and yswaw, to make. 



In the Right Rectangular Prism, the axes being unequal, the mo- 

 lecule is an ellipsoid with unequal axes. Fig. 4, is a horizontal 

 section through the centre of the Prism and molecule. It will be 

 observed that the faces of the Prism in this and the preceding in- 

 stances are necessarily tangents to the curved surface of the molecule 

 at the poles of the axes. Such is also the case in all the primary 

 forms, and hence these molecules in their combinations will give rise 

 to the same solids that would proceed from the union of the Prisms 

 themselves. 



The molecule of the Right Rhombic Prism, is similar to that of 

 the last mentioned solid. The lateral axes are however the equal 

 conjugate diameters, (see Fig. 5.) instead of conjugate axes, and 

 have a mutual inclination equal to the lateral interfacial angles, (in- 

 clination of the lateral faces.) The vertical axis is still at right an- 

 gles with the lateral, and is therefore one of the conjugate axes of 

 the ellipsoid. This is also the fact in the Right Rhomboidal Prism, 

 which differs from the last in this only, that the lateral axes are any 

 conjugate diameters not equal, (see Fig. 6,) the axes of the crystal 

 being unequal. 



In the oblique prisms all the axes of the solid are conjugate diam- 

 eters (not axes) of the ellipsoid, their angles with each other being 

 oblique. ' 



The molecules of these prisms differ in the comparative lengths 

 of the crystallogenic axes and their angles of inclination. 



In the Rhombohedron the three axes (crystallogenic,) are equal 

 and incline to one another at equal angles, and consequently are the 

 equal conjugate diameters of an ellipsoid of revolution.* When the 



* The construction of tke Rhombohedron proposed by Wollaston, is quite 

 different from that produced by the axes here given. Influenced by these axes the 

 arrangement would be similar to that in the cube, Fig. 2, whilst this author adop- 



Vol. XXX.— No. 2. 36 



