xiv INTRODUCTIOX. 



There are other physical characters which furnish extremely useful aids in 

 the identification of minerals. The most important of them will, therefore, 

 now be briefly noticed, nearly in the order in which they are alluded to in 

 the pages of the Glossary, that is as follows :— 



Colour. 



The colour of a mineral is not, in general, so much to be relied on as 

 some of the other characters. Certain peculiarities in the arrangement of 

 the colours are of importance, thus : — 



Play of Colours is said to take place when a mineral, on being turned, 

 presents the appearance of several prismatic colours in rapid succession. 

 Examples of this property,are afforded by the Diamond, and in a less degree 

 by the Precious Opal. 



A Change of Colours is of a somewhat similar nature to the play of 

 colours, only the succession of colours is less rapid, and each particular one 

 i.s spread over a larger surface. Labradorite furnishes a very good example 

 of this. 



Iridescence is when the prismatic colours appear to be reflected from the 

 interior of a crystal. 



Opalescence is when a milky or pearly reflection is displayed from the in- 

 terior of a mineral, as is the case in some kinds of Opal and Cat's-Eye. 



Tarnish signifies that the colour of the mineral is different from that ex- 

 hibited by a newly fractured surface. It is, consequently, merely superficial. 

 When the surface of a mineral (as, for example, Columbite) displays the 

 superficial blue colour of tempered steel, it is said to possess the sieel-tarnish ; 

 when, as in the Specular Iron Ore of Elba, it exhibits fixed prismatic 

 colours, it is said to be irised. 



Diaphaneity, or Transparency. 



The following terms are made use of to express the different degrees in 

 which minerals possess the capacity of transmitting light. 



]. Transparent: when the object seen through it appears perfectly dis- 

 tinct, as in Quartz and Gypsum. 



2„ Suhtransparent, or semitranspare7it : when the outlines of objects seen 

 through it do not appear distinct. 



3. Translucent : when only light is transmitted, and objects are not seen, 

 as in Oriental Alabaster. 



4. Sultranslucent : when light is only transmitted at the edges. 



5. Opaque : when no light is transmit tted. 



Lustre, 



The kinds of Lustre depending upon the nature of the reflecting surface 

 are six in number, viz. : — 



1. Metallic, or the lustre of metals ; Sub-metallic^ denoting that the 

 mineral only possesses the lustre imperfectly. 



In the determination of minerals it is very important to distinguish the 

 metallic from the non-metallic lustre. 



