PRIMARY FORMS. 23 



. 39. THE OCTAHEDRON WITH A RHOMBIC BASE. 



The Octahedron with a rhombic base is contained under 

 eight equal scalene triangles. 



The crystals of this form are in position, when pjg. 27 



the rhombic base is horizontal. Fig. 27 is drawn 

 with the greater diagonal* of the base horizontal. 

 The faces which meet at the edge B, form a more 

 acute, angle than those which meet at the edge C. 

 The edge B is therefore denominated the acute 

 edge of the pyramid, and the edge C the obtuse 

 edge of the pyramid. The solid angle at E is 

 termed the acute lateral solid angle, and that at I 

 the obtuse lateral solid angle. 



The individuals belonging to this class will differ from each other, 

 in the inclinations of P on P', and of P on P". 



This form is seen in the crystals of Sulphur. 



. 40. THE RIGHT-)- SQUARE PRISM. 



The right square Prism is a quadrangular J prism, whose 

 bases are equal squares, and whose sides are equal rect- 

 angles. 



* A line connecting the opposite angles of any parallelogram is termed 

 a diagonal of that figure. 



t Those prisms which stand perpendicularly when resting on one of 

 their bases, are called right prisms. Those which incline from the per- 

 pendicular, are called oblique prisms. 



t By the use of the expression " quadrangular prism," it is apparent 

 that two faces of the solid are chosen as bases. It may be asked, how 

 are these to be distinguished ? Those crystals which come within the 

 definition of that general form, the parallelepiped (the character of which, 

 is, that it is contained under three pairs of parallel planes,) with the ex- 

 ception of the Cube and Rhomboid present themselves to us, un- 

 der an uniform appearance, as respects the modifications they undergo. 

 These are- ordered in a symmetrical manner, in relation to an imaginary 



