SLATES. 385 



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 succes- 

 sion 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 crys- 

 tal of sugar I hold in my hand 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 rock-salt, 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. Rock-salt cleaves in three di- 

 rections, 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 rhom- 

 boid. In each of these cases the mass cleaves with equal 

 facility in all three directions. For the sake of complete- 

 ness I may say that many crystals cleave with unequal fa- 

 cility in different directions: heavy spar presents an ex- 

 ample of this kind of cleavage. 



Turn we now to the consideration of some other phe- 

 nomena 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 edge of the axe 

 is laid across the rings which mark the growth of the tree. 

 If you look at this bundle of hay severed from a rick, you 

 will see a sort of cleavage in it also ; the stalks lie in paral- 

 lel planes, and only a small force is required to separate 

 them laterally. But we cannot regard the cleavage of the 

 tree as the same in character as that of the hay-rick. In 

 17 



