306 FKAGMENTS OF SCIENCE. 



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 perfect- 

 ly 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 for- 

 mer position, the crystal cleaves again; and finally 

 placing the knife at right angles to the two former 

 positions, we find a third cleavage. Eocksalt cleaves 

 in three directions, and the resulting solid is this per- 

 fect cube, which may be broken up into any number 

 of smaller cubes. Iceland 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 exam- 

 ple 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 



