INTRODUCTIOX. xv 



2. Vitreous, or the lustre of broken glass, of which the lustre of Eock 

 Crystal is a good example ; Calc Spar, on the other hand, presenting a s^lb~ 

 vitreous or imperfectly vitreous lustre. 



3. Resinous, or the lustre of common rosin ; of which Opal, and some 

 kinds of Blende are examples. 



4. Pearly, or like the lustre of a pearl ; as in Talc, Steatite, Brucite, &c. 

 The term metallic-pearly is used to denote when the pearly and sub- 

 metallic lustre are displayed in the same mineral, as in Hypersthene. 



5. Silky, or like silk. It is generally the result of a fibrous structure, as 

 is apparent in fibrous Gypsum and Satin Spar. 



6. Adamantine, or like Diamond. When combined in the same mineral 

 with sub-metallic it is called metallic -adamantine, of which Cerusite and 

 Pyrargyrite are examples. 



The different degrees of intensity of lustre produced by a variation in the 

 quantity of light reflected from the surface are four in number :— 



(1.) Splendent: when the surface of the mineral reflects with sufficient 

 brilliancy to give well-defined images, as is the case with Oxide of Tin and 

 Specular Iron. 



(2.) Shining : when the image produced by reflection from the surface is 

 not well defined, as in Celestine. 



(3.) Glistening : when the surface reflects the light, but without pro- 

 ducing an image, as in Talc, Copper Pyrites, &c. 



(4.) Glimmering : when the reflection of the light is imperfect, and appa- 

 rently proceeding from points on the surface, as in Flint, Chalcedony, &c. 



Optical and Physical Properties. 



The former of these belong, properly, to the science of Optics, and can be 

 only alluded to here. 



The principal properties dependent on light, besides those already noticed, 

 employed in the determination of minerals are Pefraction, Polarization, and 

 Dichroism. 



1. Refraction. — It is frequently of importance to know the index of re- 

 fraction, or the ratio between the sine of the angle of incidence, and that of 

 the angle of refraction ; for although there is often some variation in the 

 ratio in the same species (frequently corresponding to a change of colour), 

 yet, as a general rule, each mineral refracting the light in an equal degree 

 has its own index of refraction. Those minerals which refract light most 

 powerfully, or in which the rays passing through them deviate the most from 

 their straight path, afford the most brilliant gems. It is to its high refract- 

 ing power (2-439 to 2-755) that the Diamond owes its brilliancy. 



Double Refraction. — Calc Spar and some other minerals present a double 

 image of a point or line seen through them, in every position but one. This 

 is called double-refraction, and a knowledge of whether a mineral possesses 

 this property will enable the observer to refer it at once to its proper crys- 

 tallographic system. All forms exhibit double refraction, except those be- 

 longing to the Cubical System, which have three axes equal to one another. 

 In the Pyramidal and Hexagonal (or Rhombohedral) Systems, in which the 



