215 



CR YSTALLOGRAPHY. 



CHAPTER I. 

 PRELUDE TO THE EPOCH OF DE LISLE AND HAUY. 



OF all the physical properties of bodies, there is 

 none so fixed, and in every way so remark- 

 able, as this; that the same chemical compound 

 always assumes, with the utmost precision, the 

 same geometrical form. This identity, however, is 

 not immediately obvious; it is often obscured by 

 various mixtures and imperfections in the sub- 

 stance ; and even when it is complete, it is not 

 immediately recognized by a common eye, since it 

 consists, not in the equality of the sides or faces of 

 the figures, but in the equality of their angles. 

 Hence it is not surprizing that the constancy of 

 form was not detected by the early observers. 

 Pliny says 1 , "Why crystal is generated in a hexa- 

 gonal form, it is difficult to assign a reason; and 

 the more so, since, while its faces are smoother than 

 any art could make them, the pyramidal points are 

 not all of the same kind." The quartz crystals of 

 the Alps, to which he refers, are, in some specimens, 

 very regular, while in others, one side of the pyra- 

 mid becomes much the largest; yet the angles 

 1 Nat. Hist, xxvii. 2. 



