358 FRAGMENTS OF SCIENCE. 



and when carbon crystallises, we have the diamond. If 

 we wish to obtain a perfect crystal we must allow the 

 molecular forces free play ; if the crystallising mass be 

 permitted to rest upon a surface it will be flattened, 

 and to prevent this a small crystal must be so suspended 

 as to be surrounded on all sides by the liquid, or, if it 

 rest upon the surface, it must be turned daily so as to 

 present all its faces in succession to the working builder. 



In building up crystals these little atomic bricks 

 often arrange themselves into layers which are perfectly 

 parallel to each other, and which can be separated by 

 mechanical means ; this is called the cleavage of the 

 crystal. The crystal of sugar I hold in my hand has 

 thus far escaped the solvent and abrading forces which 

 sooner or later determine the fate of sugar-candy. I 

 readily discover that it cleaves with peculiar facility in 

 one direction. Again I lay my knife upon this piece of 

 rocksalt, and with a blow cleave it in one direction. 

 Laying the knife at right angles to its former position, 

 the crystal cleaves again ; and finally placing the knife 

 at right angles to the two former positions, we find a 

 third cleavage. Rocksalt cleaves in three directions, 

 and the resulting solid is this perfect cube, which may 

 be broken up into any number of smaller cubes. Ice- 

 land spar also cleaves in three directions, not at right 

 angles, but oblique to each other, the resulting solid 

 being a rhomboid. In each of these cases the mass 

 cleaves with equal facility in all three directions. For 

 the sake of completeness I may say that many crystals 

 cleave with unequal facility in different directions : 

 heavy spar presents an example of this kind of cleavage. 



Turn we now to the consideration of some other 

 phenomena to which the term cleavage may be applied. 

 Beech, deal, and other woods cleave with facility along 

 the fibre, and this cleavage is most perfect when the 



