48 



MOLECULES. 



gether with different degrees of attractive force in dif- 

 ferent directions. * 



I shall call this force molecular attraction. 



Fig. 79. 



When this attraction is least between the planes of 

 the molecules, they will be more easily separated by 

 cleavage in the direction of their planes, than in any 

 other direction, as shewn in fig. 79, and a cubic solid 

 will be obtained. 



Fig. 80. 



When the attraction is least in the direction of the 

 axis of the molecules, they will be the most easily se- 

 parated in that direction, as in fig. 80, and the octa- 

 hedron or tetrahedron will be the result of cleavage. 



* It is possible to conceive that the nature, the number, and the par- 

 ticular forms, of the elementary particles which enter, respectively, into 

 the composition of these three species of cubic molecules, may vary so 

 much as to produce the variety of character which I have supposed to 

 exist. 



