1823.] Mr, Brooke's Introduction to Crystallography, 145 



but it is generally found in the form of a cuhcy and sometimes as 

 a rhombic dodecahedron, and it has a cleavage in the direction of 

 its primary planes. 



" Galenay whose primary form is a cabe, is also found under 

 the forms of an octahedron, and rhombic dodecahedron, with 

 a cleavage parallel to its cubic planes. 



" Grey copper, whose primary form is a tetrahedron, occurs 

 under the forms of the cube, octahedron, and rhombic dodecahe^ 

 dron. 



*^ Blotde is found sometimes, though rarely, crystaUized in 

 cubes, sometimes in octahedrons, tetrahedrons, and rhombic dode- 

 cahedrons. 



" Having thus observed that the cube, the regular tetrahedron 

 and octahedron, and the rhombic dodecahedron, are common as 

 primary or secondary forms to different crystallized substances, we 

 may reasonably infer that they are produced in each instance by 

 molecules of a form which is common to all ; and let us suppose 

 this common molecule to be a cube.'' 



Mr. Brooke here gives four diagrams, showing the arrange- 

 ment of the cubic molecules in each of these forms : their 

 arrangement in the cube may readily be conceived, without 

 explanation ; in the tetrahedron they are so arranged that the 

 true mathematical edges of the solid are described by the diago- 

 nals of the cubic molecules which form the rude edges in such a 

 merely approximative representation of the subject as can be 

 presented by a diagram ; the axes of the octahedron consist of 

 the prismatic axes of its cubic molecules ; the arrangement in 

 the rhombic dodecahedron is precisely that which is commonly 

 represented in figures showing the formation of that solid, by 

 decrement, from a primary cube. 



" These arrangements of cubic molecules," continues Mr. B. 

 " cannot be objected to on account of any supposed imperfection 

 of surface which would be occasioned by the laces of all the pri- 

 mary forms, except the cube, being constituted of the edges, or 

 solid angles, of the molecules. For as we observe that the octahe- 

 dral and dodecahedral planes of some of the secondary crystals of 

 galena, which are obviously composed of the solid angles, or edges, 

 of the cubic molecules, are capable of reflecting objects with great 

 distinctness, it is evident that the size of the molecules of galena 

 is less than the smallest perceptible inequality of the splendent 

 surface of those planes, and hence we infer generally, that there 

 zvill be no observable difference in brilliancy between the surfaces of 

 the planes obtained by cleavage parallel to the sides of molecules, 

 ana of those which would expose their edges or solid angles. 



" This theory may be reconciled with the cleavages which are 

 found to take place parallel to the primary planes of the tetrahe- 

 dron, the octahedron, and the rhombic dodecahedron, as well as to 

 those of the cube, if we suppose the cubic molecules capable of being 



New Series, \oL, \i, l 



