CRYSTALLISATION ULTIMATE MOLECULES. 



different species. There are several circumstances attending the 

 formation of these crystals which merit attention. 



If one of these be detached from the others, and the gradual 

 progress of its. formation be submitted to observation, it will be 

 found to grow large, always preserving its original figure. Now, 

 since its increase must be produced by the continual accession of 

 saline molecules, disengaged by the water evaporated, it follows 

 that these molecules, or atoms, must have such a shape, that, by 

 attaching themselves successively to the crystal, they will main- 

 tain the regularity of its bounding planes, and preserve the angles 

 which these planes form with each other unvaried. 



In fact, they must be so shaped, that the structure of the 

 crystal they form may be built up by their regular aggregation 

 into the form which it assumes. 



If one of these crystals be taken from the liquid during the 

 process of its formation, and be broken, so as to destroy the regu- 

 larity of its form, and then restored to the liquid, it will be 

 observed soon to recover its regular form, the atoms of salt, 

 successively dismissed by the evaporating water, filling up the 

 irregular cavities produced by the fracture. 



31. Two consequences obviously follow from this phenomenon. 

 First. That the atoms of the salt dismissed by the water 



evaporated have such a form, as enables them, by combination, 

 to give to the crystals the shape which they exhibit ; and, 



Secondly. That the atoms which are successively attached to 

 the crystals in the process of formation, attach themselves in a 

 particular position, to explain which it is necessary to suppose 

 that corresponding sides of the crystals have attractions for each 

 other, so that the atoms of salt not only attach themselves to the 

 sides of the crystals, but place themselves there in a particular 

 position. In a word, w.e must suppose that the walls of the crystal 

 are built with these atoms in the same manner, and with the same 

 regularity, as the walls of a building are formed with bricks. 



All these, and many similar details of the process of crystalliza- 

 tion, are, therefore, very evident indications of a determined figure 

 in the ultimate atoms of the substances which are crystallized. 



32. But besides these substances thus reduced by art to the 

 form of crystals, there are large classes of bodies which naturally 

 exist in this state. 



33. There are certain planes called planes of cleavage, in the 

 direction of which natural crystals are easily divided. In sub- 

 stances of the same kind, these planes have always the same rela- 

 tive position ; but they differ in different substances. 



The surfaces of the planes of cleavage are not always observable 

 before Hhe crystals are divided; but when the crystals are 



205 



