TRANSACTIONS OP THE SECTIONS. 



11 



the preceding layers. In the green fluor of Alston Moor there are 

 also different layers, some of which are pirik, and some of different 

 shades of green ; but the different shades of bine which they give out 

 under exposure to strong light, are not so strikingly contrasted as in 

 the Derbyshire specimens. As the blue colour now described is re- 

 flected from surfaces within the spar, and as it does not occur in all 



specimens, nor in every part of the same crystal, it must be produced 

 by extraneous matter of a different refractive power from the spar, in- 

 troduced between the molecules of the crystal during its formation. 

 That the blue colour is not produced by shallow cavities or minute 

 pores, as in some of the opals, is inferred from the perfect transparency 

 of the specimens in which it occurs, and from the fact that the same 

 reflected tints are found in fluids, particularly the juices of plants ex- 

 tracted by alcohol, and in several artificial glasses, particularly in those 

 of a, pink and orange colour, the former of which give a blue and the 

 latter a green colour. Having found that some of the dichroitic colours 

 in doubly refracting crystals were discharged by heat, it occurred to the 

 author that the blue tints in fluor spar might suffer a similar change, 

 and might even be connected with the phosphorescence of the mineral. 

 He therefore exposed two jiieces, one of the Derbyshire and one of the 

 Alston Moor fluor, to a considerable heat. Both of them gave out a 

 blue phosphorescence, similar to that of the reflected tint, and much of 

 the natural colour of the fragments was discharged by the heat. In 

 both specimens tlie blue reflected tint was greatly diminished. In an- 

 other specimen of the Alston Moor fluor, it appeared to be wholly re- 

 moved ; but in a third, taken from the solid angle of the cube, the 

 blue tint still appeared, though with an impaired brilliancy. It is pos- 



