. 161. OF THE STRUCTURE OF MINERALS. 



tallisation. It has been observed, that certain crystals, 

 part of which is differently formed from another one, which 

 is similarly situated, likewise present differences in their 

 electric action, on being exposed to an elevated temperature. 

 Prismatic Zinc-baryte, prismatic Topaz, rhombohedral 

 Tourmaline, and tetrahedral Boracite, may be quoted as 

 examples of this peculiarity. Could this observation be 

 established as a general law, it might prove useful in com- 

 pleting crystals thus imperfectly formed, though it would 

 not indicate what faces are to be added on that termination 

 which is opposite to the observed one. 



Another case, in which the two opposite terminations of 

 crystals are differently formed, does not refer to the pre- 

 sent place, in as much as it is found only in compound mi- 

 nerals. It will be treated of more at large in . 179. 



The preceding ones are the most simple modes of the 

 occurrences of minerals in nature. 



CHAPTER II. 



OF THE STRUCTURE OF MINERALS. 

 . 161. EXPLANATION OF STRUCTURE. 



Structure represents the mechanical connexion 

 among the particles of a simple mineral. It may 

 be observed, if we destroy this connexion, or sepa- 

 rate the particles from each other. 



We have to distinguish here between the regular and the 

 irregular structure. 



If we break a crystal of hexahedral Lead-glance, or of 

 rhombohedral Lime-haloide, we observe particles detach- 

 ed which are contained under even, smooth, and shining 

 faces. The property of allowing these particles to be se 



