. 205. OPTICAL PROPERTIES OF MINERALS. 

 . 205. DEGREES OF TRANSPARENCY". 



With regard to the transparency of minerals, we 

 have to observe the relative quantity of light which 

 is transmitted through their substance. The use of 

 the degrees of transparency is confined to the De- 

 scriptive part of Natural History. 



These degrees are, 



1, Transparent, if the light is transmitted in a sufficient 

 quantity to enable us to distinguish small objects placed 

 behind the mineral. 



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 outline. 



8, Translucent, if a small quantity of light only falls into 

 the mineral, but without allowing an object behind it to 

 be seen, except in so far as it in general may prevent the 

 Kght from falling upon the mineral. 



4, Translucent on the edges, if only the most acute edges 

 of a mineral receive some light, while the interior remains 

 perfectly dark. This degree of transparency has moreover 

 been distinguished into strongly and feebly translucent 

 on the edges ; and it is upon distinctions of this kind that 

 are founded the differences between the varieties of rhom- 

 bohedral Quartz, called flint, hornstone, jasper, &c. 



5, Opaque, if a mineral transmits no light at all. 



The species of the orders Metal, Glance, and Pyrites, 

 consequently most of those which possess a perfect metal- 

 lic lustre, are entirely opaque. This is, however, not quite 

 general for all the minerals of a metallic appearance, as 

 for instance the lamellar varieties of rhombohedral Iron-ore, 

 which transmit sometimes in the sun a very bright red 

 colour. 



Minerals of a non-metallic appearance, are not entirely 

 opaque, a few species of the order Ore, perhaps except- 

 ed. Yet accidental impurities influence so much their 



