112 TERMINOLOGY. 



constancy, are not very serviceable in mineralogy, are nev- 

 ertheless, exceedingly interesting. They have been called 

 the Play of Colors, the Change of Colors, the Opales- 

 cence, the Iridescence, the Tarnish, and the Delineations of 

 Colors. 



The only use made of these properties is in the descriptive part of 

 Natural History. 



1. The Play of Colors is produced, if the mineral reflects, in 

 certain directions, other colors beside its own, and these, for the 

 most part, rainbow colors of very unusual brightness and intensity. 

 They are not steady, or always observable in the same place, but 

 alter with the position of the mineral, or with the direction of the 

 rays of light. It is witnessed in the highest perfection in the Dia- 

 mond when cut : and depends in this gem upon the reflexion of re- 

 fracted light, occasioned by the artificial facets. The precious Opal 

 presents the same phenomenon, even before being cut. Candle 

 light, or sunshine is much more advantageous for the display of 

 this effect than the ordinary light of day. 



2. The Change of Colors consists in the reflexion of bright hues 

 of color in certain directions, depending upon the structure of the 

 mineral. It covers larger spots than the play of colors, and they 

 do not disappear so rapidly on being moved. It is best seen in the 

 Labrador Feldspar. 



3. The Opalescence consists in a kind of milky light, which cer- 

 tain minerals reflect, either when cut or in their natural condition. 

 It occurs in the Cats-eye, where it depends upon composition, this 

 substance consisting of Quartz traversed by delicate fibres of As- 

 bestus : also, in Chrysoberyl and Feldspar, (var. Adularia) in both of 

 which it depends upon the crystalline structure ; as also in the Sap- 

 phire in which it appears in the six-rayed stars of light, and has re- 

 ceived the name of Asteria. 



4. The Iridescence shows the colors of the rainbow, similar to 

 those produced by the refraction of light, through a prisrn of glass. 

 It depends upon fissures in the interior of minerals. The cavities 

 present the phenomenon of the colored rings. It is most striking 

 in Quartz. 



Another remarkable property of certain minerals is, that they 

 show different colors, if examined by transmitted^ light in certain 

 directions. This property of minerals has been termed their Di- 



