14ft Amly$et of Books, [Aug. 



held together with different degrees of attractive force in different 

 directions, I shall call this force molecular attraction,* 



" When this attraction is least between the planes of the mole- 

 ctdes, they will be more easily separated by cleavage in the direc- 

 tion of their planes, than in any other direction, and a cubic solid 

 will be obtained. 



** When the attraction is least in the direction of the axis of 

 the moleculeSf they will be the most easily separated in that direc- 

 tion, and the octahedron or tetrahedron will be the result of 

 cleavage. 



" And if the attraction be least in the direction of its diagonal 

 planesj the edges will be most easily separated, and a rhombic 

 dodecahedron viill he the solid produced by cleavage. 



" This supposition of greater or less degree of molecular attrac- 

 tion in one direction of the molecule than in another, is consist- 

 ent with many well-known facts in crystallography. 



*t The primary form both of corundum, and of carbonate of 

 lime, is a rhomboid ; and the crystals of these substances may 

 be cleaved parallel to their primary planes, the carbonate of lime 

 cleaving much more readily than the corundum. But the 

 corundum may also be cleaved in a direction perpendicular to its 

 axis, which carbonate of hme cannot be. 



" This cleavage would either divide the rhombic molecules in 

 half, or, the cleavage planes would expose the terminal solid 

 angles of the contiguous molecules, 



" But it is contrary to the nature of molecules that they 

 should be thus divided, and we may therefore infer from this 

 transverse cleavage that the molecular attraction is comparatively 

 less in the direction of the perpendicular axis of the molecules of 

 corundum, than it is in the same direction of those of carbonate 

 of hme. And from the greater adhesion of the planes of corun- 

 dum, than of those of carbonate of lime, weinfer that the attrac- 

 tion is comparatively greater between the planes of the molecules 

 of the corundum, than between those of carbonate of hme.f 



" This supposition of the existence of a greater or less degree 



• '* It is possible to conceive that the nature, the number, and the particular forms, of 

 the elementary particles which enter, respectively, into the composition of these three 

 species of cubic moleailes, may vary so much as to produce the variety of character 

 which I have supposed to exist." 



+ " I am aware of an objection that may be made to this view of the subject, by sup- 

 posing all the cleavages which arc not parallel to the primary planes of a crystal, to be 

 parallel to some secondary plane, and to be occasioned by the slight degree of adhesion 

 which frequently subsists between the secondary planes of crystals and the i)late8 of 

 molecules which successively cover them during tlie increase of the crystal in size ; but 

 although the second set of cleavages may sometimes be connected with the previous 

 existence of a secondary plane, it may also be explained according to the theory I have 

 assumed." 



" Those cleavage planes which would not expose the planes, edges, or solid angles of 

 the molecules, must be considered to belong always to the class of planes ofcmnposiiion, 

 a term which Mr. W. Phillips has applied to those cleavage planes which result from 

 cleavages parallel to secondary planes only." 



