66 PHOSPHORESCENCE. PART i. 



The fluorescent property of yellow uranite was dis- 

 covered by Professor Stokes some years ago, and now 

 he has added another fluorescent mineral in adularia or 

 moonstone ; from its natural faces and planes of cleavage 

 alike a beautiful blue fluorescence emanated under the 

 induction spark. As the same was observed in colour- 

 less felspars generally, Professor Stokes concluded that 

 fluorescence is an inherent property in the silicate of 

 alumina and potash constituting the crystal of moon- 

 stone. The blue fluorescence extended to a very sensible 

 though small depth within the substance.- 



A particular variety of fluor spar found at Alston 

 Moor in Cumberland, which is very pale by transmitted 

 light, shows a strong blue fluorescence, and is eminently 

 phosphorescent on exposure to the electric spark. It is 

 the same kind of crystal in which Sir David Brewster 

 originally discovered the property of fluorescence. On 

 presenting such a crystal to the spark passing between 

 aluminium terminals, besides the usual blue fluores- 

 cence, there was another of a reddish colour extending 

 not near so far into the crystal, produced by the rays 

 belonging to the strong lines of aluminium of extreme 

 refrangibility. 



The cube of fluor spar which showed these effects was 

 externally colourless to the depth of -$ of an inch ; then 

 came one or two strata parallel to the faces of the cube 

 showing the ruddy fluorescence, while the blue fluores- 

 cence extended to a much greater depth and had a 

 stratified appearance. This crystal was eminently phos- 

 phorescent, its blue phosphorescence being arranged in 

 strata parallel to the face of the cube like the blue 

 fluorescence, but it was not perceptible beyond a very 

 moderate distance below the surface at which the 

 exciting cause entered, that cause being the photo- 

 graphic rays of extremely high refrangibility of the 

 electric spark taken, as in all these experiments, between 



