STRUCTURE. 



Fig. 83. 



,7 7t 



If we apply the edge of a knife to the surface of 

 any one of these cubic crystals of common salt, in a 

 direction parallel to an edge of the cube, as at a b, 

 fig. 83, the crystafmay, by a slight blow, be cleaved 

 parallel to one of its sides. 



If we apply a knife in the same manner successively 

 to the other lines c d, e f, g h, and to the other sur- 

 faces of the crystal, so that its edge be parallel in 

 each instance to the edge of the cube, we shall find 

 that there are cleavages parallel to all the planes of 

 the cube; and if the crystal be split with perfect 

 accuracy, a cubic solid may be extracted ; and the 

 rectangular plates which have been removed by 

 these cleavages, may be also subdivided into smaller 

 cubes. 



From these circumstances we infer that the mole- 

 cules which have successively covered the planes of 

 the small crystals, are cubes, and that they are so 

 arranged as to constitute a series of plates, as shewn 

 in p. 18. And we further conclude that the molecular 

 attraction is least, in common salt, between the sur- 

 faces of the molecules. 



This regular structure is supposed to belong to all 

 regularly crystallized bodies. 



It frequently happens that the regular crystal- 

 lization of bodies has been prevented by some dis- 



