PRIMARY FORMS. 



19 



the oblique rhombic Prism; 13, the doubly oblique Prism; 

 14, the Rhomboid; 15, the regular hexagonal Prism. 



The student must not suppose that the primary form is altogether 

 an imaginary one ; on the contrary, it is often an actua Iform among 

 the crystals of a mineral species, and when this is not the case, its 

 planes frequently form a conspicuous part of crystals, which are 

 only slightly altered at their angles or edges. This remark may be 

 exemplified in the instance of the species Uranite, three of whose 

 forms are figured in . 30. Fig. 17 is the primary form which is 

 still observable in Fig. 18 and 19, if we make an abstraction of their 

 edges and angles. There do exist, however, crystals in which it is 

 wholly out of sight ; in such cases, it is developed by mechanical 

 means, or inferred to exist according to rules hereafter to be ex- 

 plained. 



. 33. THE CUBE. 

 The Cube is contained under six square faces. 



All the angles of the Cube are right angles. I ig. 20. 



An axis* passes through the centre and 

 through two opposite solid angles ; as a, b, 

 Fig. 20. From its equality in length, breadth, 

 and altitude, the Cube has a similar axis in 

 four directions, or passing through its centre 

 and through each pair of opposite solid angles. 



It is a form very often seen among crystals, 



as in Iron Pyrites, Fluor, Galena, &c. 



<. 34. THE REGULAR TETRAHEDRON. 

 The regular Tetrahedron is contained under four equi- 

 lateral, triangular faces. 



The mutual inclination of its faces, as P on 

 P', Fig. 21, = 70 31' 43". Its plane an- 

 gles =60. 



The axis A& passes through the centres of 

 the summit and base, and in consequence of 

 its symmetrical form, it possesses a similar 

 axis in four directions. 



* An axis is a line, passing through the centre of a solid, drawn 

 from any angular point formed by the meeting of equal plane angles to 

 the opposite angle or face. 



