OPTICAL CHARACTERS OF MINERALS. 103 



6. The Slaty Fracture resembles imperfect faces of cleavage, 

 and partly arises from it. It is met with in the different kinds 

 of Slate, which, for the greater part, are compound minerals, or 

 even mixed, although they appear to be simple. The slaty frac- 

 ture keeps a constant direction, and is in this respect analogous to 

 cleavage. 



7. The Earthy Fracture is the same as the uneven fracture, ex- 

 cept that it occurs in decomposed minerals. 



SECTION III. 



THE NATURAL PROPERTIES, COMMON TO BOTH SIMPLE AND 

 COMPOUND MINERALS. 



. 86. DIVISION. 



Those Natural properties, which are common to both 

 the simple and the compound minerals, may be divided in- 

 to the Optical Properties, and into the Physical Properties 

 of minerals, or such as refer more particularly to their mass 

 or substance. 



Optical characters are such as depend upon light, and are not ob- 

 servable except in its presence. They include lustre, color, and 

 transparency. 



The physical properties of minerals, comprehend all those which 

 neither depend upon their form, and the space which they fill up, 

 nor upon the presence or absence of light. Of these are the follow- 

 ing, the state of aggregation, hardness, specific gravity, magne*> 

 tism t electricity, taste and odor. 



OF THE OPTICAL CHARACTERS OF MINERALS. 

 . 87. LUSTRE, COLOR, TRANSPARENCY. 



The phenomena observable in minerals, with respect to 

 reflected and transmitted light, are comprehended under 

 the heads of Lustre, Co/or, and Transparency. 



