11 



SECTION I. 



. 19. CRYSTAL. 



The term crystal is applied to a body consisting of con- 

 tinuous, similar matter, which has been formed by nature, 

 within a regularly limited space. 



It is necessary to the idea of a crystal, that the entire space it oc- 

 cupies should be made up of particles, between which there is no 

 distinguishable difference, except indeed those variations which may 

 occasionally arise out of the relations of light. This limitation is 

 requisite in order to exclude certain minerals found under regular 

 shapes and composed of similar matter, but between whose particles 

 there is observed an evident distinction. Of this sort are what are 

 denominated pseudomorphous crystals, between which, and the 

 bodies intended to come under the above signification, there exist 

 too many dissimilarities to allow their being treated of together. 



The term crystal cannot be extended either to such minerals as 

 require, in order to present a regular shape, to have any of their 

 parts detached : since it is requisite by the above definition that a 

 crystal should be left by nature within a regularly limited space. 



The idea of transparency which* mankind in general attach to 

 crystals is of course incorrect, as the largest portion of these bodies 

 are opaque. 



. 20. OBJECT or CRYSTALLOGRAPHY. 



The object of Crystallography is to ascertain the form 

 of crystals, with a view to explain the relations and differ- 

 ences existing among them.* 



* History of Crystallography, The knowledge of the fact, that in- 

 organic substances affect a great variety of regular shapes, appears to 

 have been prevalent at an early period. Several of the precious stones, 



