ON CRYSTALLINE AND SLATY CLEAVAGE 823 



when silica crystallizes, we have formed these hexagonal 

 prisms capped at the ends by pyramids; by allowing salt- 

 petre to crystallize we have these prismatic masses, and 

 when carbon crystallizes we have the diamond. If we 

 wish to obtain a perfect crystal we must allow the molec- 

 ular forces free play; if the crystallizing mass be per- 

 mitted 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, es- 

 caped the solvent and abrading forces which sooner or 

 later determine the fate of sugar-candy. I readily dis- 

 cover that it cleaves with peculiar facility in one direc- 

 tion. 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. 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 differ- 



