

SECTION III. 

 STRUCTURE. 



THE structure of crystals, or the order in which 

 their molecules are arranged, may be inferred from 

 an experiment with common salt. If we dissolve a 

 portion of this salt in water, and then suffer the water 

 to evaporate slowly, crystals of salt will be deposited 

 on the sides and bottom of the vessel. These will at 

 first be very minute, but they will increase in size as 

 the evaporation proceeds; and if the quantity of salt 

 dissolved be sufficient, they will at length attain a 

 considerable bulk. If the forms of the small crystals 

 be examined, they may be found to consist of en- 

 tire, or modified cubes. If we continue to observe 

 any of these cubic or modified crystals during their 

 increase in bulk, we may find that the forms of some 

 of them undergo a change, by the addition of new 

 planes, or the extinction of some that had previously 

 existed. But we shall also frequently find that both 

 the cube, and the modified crystal, when enlarged, 

 preserve their respective forms. The increase of a 

 crystal in she appears therefore to be occasioned by the 

 addition of molecules to some, or all. of the planes of 

 the smaller crystal, whether these planes be primary or 

 secondary. 



