50 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 lime. And from the greater adhesion of the planes 

 of corundum, than of those of carbonate of lime, we 

 infer that the attraction is comparatively greater be- 

 tween the planes of the molecules of the corundum, 

 than between those of carbonate of lime.* 



This supposition of the existence of a greater or 

 less degree of molecular attraction in one direction of 

 the molecule than in another, appears to explain the 

 nature of the two sets of cleavages which occur in 

 Tungstat of lime : one of these sets is parallel to the 

 planes of an 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 attraction is 

 greatest in the direction of their prismat^p axis, and 

 nearly equal in the direction of their diagonal planes , 

 and of their oblique axes, the first set of cleavages may 

 be conceived to expose the edges of the molecules, and 

 the second set to expose their solid angles. 



* I am aware of an objection that may be made to this view of the 

 subject, by supposing all the cleavages which are 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 cohesion which frequently sub- 

 sists between the secondary planes of crystals and the plates of mole- 

 cules which successively cover them during the increase of the crystal 

 in size; but althpugh 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 



