70 PHYSICAL PEOPEPwTTES OF MDTEBALS. 



5. DIAPHANEITY, LUSTRE, COLOR, 

 1. Diaphaneity. 



Diaphaneity is the property •which many objects possess oi 

 transmitting light ; or, in other words, of permitting more or 

 less light to pass through them. This property is often called 

 transparency, but transparency is properly one of the degrees 

 cf diaphaneity. The following terms are used to express the 

 different degrees of this property : 



Transparent — a mineral is said to be transparent when the 

 outlines of objects, viewed through it, are distinct. Example, 

 glass, crystals of quartz. 



/Subtransparenf, or semitransparent — when objects are seen 

 but their outlines are indistinct. 



Translucent — when light is transmitted, but objects are not 

 seen. Loaf sugar is a good example ; also Carrara marble. 



/Subtranslucent — when merely the edges transmit light faintly. 



When no light is transmitted the mineral is described as 

 opaque. 



2. Lustre. 



The lustre of minerals depends on the nature of their surfaces, 

 which causes more or less light to be reflected. There are dif- 

 ferent degrees of intensity of lustre, and also different kinds of 

 lustre. 



a. The kinds of lustre are six, and are named from some 

 familiar object or class of objects. 



1. Metallic — the usual lustre of metals. Imperfect metallic 

 aistre is expressed by the term submetaUic. 



2. Vitreous — the lustre of broken glass. An imperfect 

 vitreous lustre is termed subvitreous. Both the vitreous and 

 sub vitreous lustres are common. Quartz possesses the former 

 in an eminent degree ; calcareous spar often the latter. Thia 

 kind of lustre may be exhibited by minerals of any color. 



3. liesinous — lustre of the yellow resins. Example, seme 

 opal, zinc blende. 



4. Pearly — like pearl. Example, talc, native magnesia, etil- 

 bite, etc. When united with submetallic lustre the term 

 metallic-pearly is applied. 



5. Silky — like silk ; it is the result of a fibrous structure. 



