ON CRYSTALLINE A *D SLAT7 CLEA VAGE. 333 



which after a time assume the visible shape of the crystal 

 now held in my hand. I may, like an ignorant meddler 

 wishing to hasten matters, introduce confusion into this 

 order. This may be done by plunging a glass rod into the 

 vessel; the consequent action is not the pure expression of 

 the crystalline forces; the molecules rush together with the 

 confusion of an unorganized mob, and not with the steady 

 accuracy of a disciplined host. In this mass of bismuth 

 also we have an example of confused crystallization; but 

 in the crucible behind me a slower process is going on: 

 here there is an architect at work " who makes no chips, 

 no din," and who is now building the particles into 

 crystals, similar in shape and structure to those beautiful 

 masses which we see upon the table. By permitting alum 

 to crystallize in this slow way, we obtain these perfect 

 octahedrons; by allowing carbonate of lime to crystallize, 

 nature produces these beautiful rhomboids; when silica 

 crystallizes, we have formed these hexagonal prisms capped 

 at the ends by pyramids; by allowing saltpeter to crystallize 

 we have these prismatic masses, and when carbon crystal- 

 lizes, we have the diamond. If we wish to obtain a per- 

 fect crystal we must allow the molecular forces free play; 

 if the crystallizing mass be permitted to rest upon a sur- 

 face 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. 



