TERMINOLOGY. 



. 37. THE OCTAHEDRON WITH A SQUARE BASE. 



The Octahedron with a square base is contained under 

 eight equal, isosceles triangular faces. 



Fig. 25. 



The parallelogram formed by the unequal side in each of the tri- 

 angles, is the base of the Octahedron, which, from the equality of 

 the triangles, is a square. It is obvious from the 

 nature of an isosceles triangle, that in the Octahe- 

 dron with a square base, the angle at a, Fig. 25. 

 may be less, or greater than 60 ; when it is less, 

 the Octahedron is called acute, when greater, 

 obtuse. 



This form is capable of presenting a variety of 

 angles in its individuals, as respects the inclination 

 of P on P", and consequently of P on P 7 . 



Its axes will be the same in number as in the 

 regular Octahedron. When the base is horizontally situated, this 

 solid is in position, and its upper and lower extremities are termed 

 its summits. 



Crystals of the present form are seen in the species Zircon, An- 

 atase, &c. 



. 38. THE OCTAHEDRON WITH A RECTANGULAR BASE. 



Tnis Octahedron is contained under eight triangular 

 planes, and possesses one rectangular base. 



The planes are generally isosceles triangles in this form, though 

 it may happen that four of them may be equilateral, without de- 

 stroying its rectangular base. It is described, or said to be in pO' 

 sition,' with its rectangular base horizontally situated. The broad 

 planes P, P', Fig. 26. meet at the edge of 

 the rectangular base, at a more obtuse angle 

 than the narrow ones M, M'. The edge D 

 may therefore be termed the greater, and the 

 edge F the lesser edge of the base. 



Like the Octahedron with a square base, 

 the individuals belonging to this form will 

 differ from each other, in the inclination of P 

 on P', or of M on M'. Its axes and summits 

 are distinguished also, as in the case of that form. 



Crystals of this form are found in the species Areeniate of Copper, 



Fig. 26. 



