PHYSIOGRAPHY. 253 



obtained by means of the Reflective Goniometer, or by the Common 

 Goniometer. 



The primary form being known, the series of forms to which it is 

 capable of giving rise, may, in general, be easily inferred from the 

 known laws of derivation (. 51). Still, it is necessary for us to 

 present in the general description, as far as possible, the forms that 

 actually do occur : since this information is peculiarly important and 

 interesting, and without it, the picture of the species would indeed 

 be deficient. Commencing, therefore, with the primary form, pro- 

 vided it exists unmodified among the crystals of the species, we pro- 

 ceed to enumerate the most frequent of the occurring forms. Next 

 to the primary, (or in the absence of this, the one most closely allied 

 to it,) we mention that modification which may be considered next 

 in remove from it, and so on in a series, terminating with the most 

 complicated and irregular forms ; illustrating such of them as are 

 conceived of with difficulty from mere description by diagrams, and 

 occasionally adding the mutual inclinations of particular faces. To 

 this enumeration will also be annexed, in many instances, remarks 

 to point out the localities of particular forms, as well as their com- 

 parative rarity or abundance. 



The phenomenon of cleavage being in the nearest relation to the 

 crystalline forms, the next place in the collective description will 

 be assigned to it. The forms of cleavage, so far as obtainable, are 

 described ; and particular notice bestowed upon the degree of per- 

 fection in the different faces of cleavage. 



Fracture, so far as it is contained in the collective description it- 

 self, refers only to simple minerals. It cannot be deemed however, 

 very important. Several varieties of fracture, if mentioned in one 

 and the same place, denote the limits between which the varieties 

 range, which occur in the species. Also, it is indicated whether the 

 fracture be obtained easily, or with difficulty. 



The physical quality of the surface of the crystals is far more im- 

 portant than fracture, since it is in close connexion with the crys- 

 talline forms themselves. These faces are expressed by the letters 

 appropriated to the faces of the different forms. 



The characters depending upon the presence of light, contribute 

 very much to enliven the image or representation of the species. 

 The kinds of lustre must every where be mentioned ; and if there 

 should be found a difference as to its occurrence upon different faces, 

 in which different kinds of lustre may be observed, this must be 

 pointed out, 



22 



