SECTION III. 



THE NATURAL-HISTORICAL PROPERTIES, COMMON 

 TO BOTH, SIMPLE AND COMPOUND MINERALS. 



. 193. DIVISION. 



Those natural-historical properties, which are 

 common to both, the simple and the compound mi- 

 nerals, may be divided into the Optical Properties, 

 and into the Physical Properties of minerals, or such 

 as refer more particularly to their Mass or Substance. 



CHAPTER I. 



OF THE OPTICAL PROPERTIES OF MINERALS. 

 . 194. EXPLANATION. 



Optical properties are such as depend upon light, 

 and are not observable except in its presence. 



The consideration of the natural-historical properties in 

 general, presupposes the presence of light. Yet all of them 

 do not depend upon its presence. This is the case, however, 

 with the properties to be considered in this chapter. "We 

 cannot maintain that a mineral possesses in the dark that 

 same colour, lustre, or transparency, which it exhibits 

 when observed under the influence of light. This charac- 

 ter, therefore, is sufficient for distinguishing the optical 

 properties from all other properties of the minerals. 



