1823.] Mr, Brooke's Introduction to Crystallography. 147 



qf molecular attraction in one direction of the molecule than in 

 another, appears to explain the nature of the two sets of cleav- 

 ages which occur in tungstate of lime : one of these sets is parallel 

 to the planes of ow acute octahedron with a square base, which we 

 will call the primary crystal ; the other set would produce tangent, 

 planes upon the terminal edges of that crystal. If we suppose 

 the molecules to consist of square prisms whose molecular attrac- 

 tion is greatest in the direction oi tliQiv prismatic axis, and nearly 

 equal in the direction of their diagonal planes, and of their 

 oblique axes, the/irst set of cleavages may be conceived to expose 

 the edges of the molecules, and the second set to expose their solid 

 angles. 



" This theory may, by analogy, be extended to the foriQ of 

 molecules of every class of octahedron. ;^ 



" For we may conceive the molecules of all the irregular Qctan. 

 hedrons to be parallelopipeds, whose least molecular attraction is 

 in the direction of their diagonal planes. 



" Thus the molecules of octahedrons with a square, a rectangular, 

 and a rhombic base, would be square, rectangular, and rhombic 

 prisms respectively ; the dimensions of such rnolecules being propor^ 

 tional respedtively to the edges of th^ base, c^nd to the ajpis of each 

 particular octahedron. 



" According to the view here taken, the following table will 

 exhibit the form of the molecules belonging to each of th^ 

 classes of primary forms." 



The cube • • • *| 



re2:ular tetrahedron . . . I i i , 



-^— octahedron .... >»»olepuIe, a cube, 



rhombic dodecahedron. J 

 All quadrangular prisms .... molecules, similar prisms 



octahedron with a "1 molecule, a square 

 square base . . . . / prism 



with a") molecule, a rectan- 



rectungular base/ gular prism 



with a X molecule, a rhom- 



Proportional in 

 dimensions to 

 the edges of the 

 base, and to the 

 " axis of each par- 

 ticular octahe- 

 dron, respect- 

 ively. 



rhombic base. . . j bic prism 



rhomboid ........ molecule, a similar rhomboid. 



hexagonal prism.. I '^''^^j^g^^' ^'' equilateral triangular 



" Having thus advanced a new theory of molecules in opposi- 

 tion to one that had been long estabhshed, and possibly without 

 a much better claim to general reception than the former theory 

 possessed, I cannot avoid observing that the lohole theory of 

 molecules and decrements is to be regarded as little else than 

 a series of symbolic characters, by whose assistance we are ena- 

 bled to investigate and to demonstrate with greater facility th^ 



h 4 



