.203. OPTICAL PROPERTIES OF MINERALS. 93 



It is deserving of notice, that very often all the species of 

 one genus possess nearly the same series of colours. Thus 

 the several species of the genus {Jarnet entirely agree with 

 each other ; the three most common species of the genus 

 Augite-spar, and more particularly the hemi-prismatic and 

 the paratomous one, almost entirely coincide in their series 

 of colours. 



The metallic colours do not form any series at ail, or at 

 least, the series which they form are very limited. This is 

 the reason why they are more applicable in the Character- 

 istic than the non-metallic colour, the employment of which 

 is almost entirely confined to the Descriptive part of Mine- 

 ralogy. There are series containing both metallic and non- 

 metallic colours, as, for instance, those of rhombohedral 

 lluby-blende, of rhombohedral Iron-ore, and others. 



The series of colours differ very essentially from the 

 series of homogeneous forms (. 85.) or from the series of 

 .crystallisation themselves (. 136.). The latter are deriv- 

 able from a single form, which is given or has been observ- 

 ed, and can be obtained in their whole extent between their 

 limits; whereas the former arise by the interpolation of 

 new members between known ones ; and these consequent- 

 ly cannot be produced with security beyond that extent, 

 which is given by immediate observation. 



. 203, SEVERAL OTHER PECULIARITIES IN THE 

 OCCURRENCE OF COLOURS. 



The Play of Colours, the Change of Colours, 

 the Opalescence, the Iridescence, the Tarnish, and 

 the Delineations of Colours, must be considered as 

 properties very remarkable in themselves, though 

 of comparatively little use in Natural History. 



The only use that can be made of these properties, is in 

 the Descriptive part of Natural History, and even here 

 it is very limited. 



