114 TERMINOLOGY. 



color ; of the latter are ores of the metals, for the most part, as Hae- 

 matite, which changes from reddish brown to red, and Columbite, 

 which changes from black to reddish brown. The former are said 

 to be unchanged in the streak ; of the latter, the alteration of the 

 color in the streak is indicated. A white or grey streak of minerals 

 is said to be uncolored. 



. 96. DEGREES OF TRANSPARENCY. 



The relative quantity of light which is transmitted through 

 the substance of minerals constitutes the degrees of transpa- 

 rency. 



The use of the degrees of transparency is limited to the descrip- 

 tive part of Mineralogy. 



These degrees are, 



1. Transparent, if the light is transmitted in sufficient quantity 

 to enable us to distinguish small objects placed behind the mineral. 

 Ex. Quartz crystals. 



. 2. Semi-transparent, if it is possible to see an object behind the 

 mineral, without, however, being able to distinguish more of it 

 than its general figure. Ex. Smoky Quartz. 



3. Translucent, if the light every where pervades the mineral, 

 so as to give it an uniform milky appearance, without, however, 

 permitting objects to be perceived through it. Ex. Chalcedony. 



4. Translucent on the edges, when the above is only true of th'e 

 sharp edges of the mineral ; the main mass remaining perfectly 

 dark. Ex, Hornstone and Jasper. 



5. Opaque, if a mineral transmits no light at all. Ex. The na- 

 tive metals. 



Minerals of a non-metallic character are but very rarely entirely 

 opaque. Yet accidental impurities influence so much their trans^ 

 parency, that this property becomes almost entirely useless for the 

 determination of minerals. The best employment to be made of it, 

 seems to be, in the distinction of compound varieties from simple 

 ones, where the minuteness of the particles of composition prevents 

 them from being observed immediately. Commonly, in the same 

 species, the compound varieties possess a less degree of transparen- 

 cy than the simple ones. This is well exemplified in the varieties 

 of Quartz. Almost all its single individuals, provided they are not 

 impure, shew higher degrees of transparency than Flint, Hornstone 

 and Chalcedony, and other compound varieties. 



