xvi INTRODUCTION. 



horizontal axes are equal, there is one axis of double refraction, or one direc- 

 tion in which double refraction is not observable ; but in the Rhombic and 

 Anorthic Systems, in which the horizontal axes are unequal, there are two 

 axes of double refraction. 



2. Polarization has the same relation to crystalline form as double refrac- 

 tion, and is displayed by many minerals, of which Tourmaline is a well-known 

 example. 



3. Dichroism is when crystals present different colours when viewed by 

 transmitted light in two different directions, of which examples are afforded 

 by lolite. and Mica. Pleochroism is when the above property is exhibited in 

 more than two directions. 



Phosphorescence. 



When minerals appear more or less luminous they are said to be phos- 

 phorescent. That property may be produced either, 1st, ly friction^ as in 

 Quartz ; 2nd, hy heat^ as in Fluor Spar ; 3rd, hy electricity, as in Diamond, 

 Calc Spar, Apatite, and some other kinds of Fluor Spar ; and 4th, as in the 

 case of some Diamonds, hy exposure to the light of the sun. 



Fluorescence. 



This name has been given to the peculiar phenomenon exhibited by 

 Fluor Spar, of transmitting one colour and reflecting another (according to 

 Sir J. Herschel) from a stratum of small but finite thickness, adjacent to 

 the surface by which the light enters. 



After passing through this stratum, the incident light, though not sensibly 

 enfeebled or coloured, has lost the power of producing the same effect, and 

 therefore may be considered as in some way or other qualitatively different 

 from the original light. 



This dispersion of the rays, which takes place near the surface, has been 

 called, by Professor Stokes, Fluorescence. It is exhibited by Green and 

 Yellow Uranite and by Chalcolite ; as well as by certain specimens of 

 Apatite, Aragonite, Chrysoberyl, Kyanite and Top,az, but in these latter 

 cases (as in Fluor Spar) the phenomenon is due to the presence of some 

 substance accidentally present in small quantity.* 



Streak. 



This is a test of considerable importance, as the colour of the powder of a 

 mineral is more constant and to be depended on than the colour of the 

 mineral itself, which is liable to be altered by the accidental admixture of 

 foreign substances. The streak is produced either by scratching the mineral 

 or by drawing it across a piece of white unglazed porcelain ; and observing 

 the colour of the powder or of the trace it leaves behind. 



Stain. 



This character consists in leaving a mark on paper or linen, and is con- 

 fined to a few soft minerals. Graphite may be distinguished from sulphide 

 of Molybdenum, which it much resembles in other respects, by the mark 

 which it leaves behind when drawn across paper. 



* Phil. Trans. 1852 ; part ii, 1853 - part i. 



