ON CRYSTALLINE AND SLAT7 CLEA VAQE. 233 
which after a time assume the visible shape of the crystal 
now held iu my hand. I may, like au 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 bs 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. 
