MORSE. — STUDIES ON FLUORITE. 591 



any known substances, and the fact that the fluorescence spectrum 

 changes completely with a change in the exciting source immediately 

 excludes all ordinary substances from consideration. 



1 0. A change of this sort, i. e. a redistribution of sharp lines in a 

 spectrum, corresponding to a change in an exciting source, is something 

 wholly new in spectrum observations. 



11. The possibility of optical resonance has been examined with 

 wholly negative results. No relation between sharp spectral lines in the 

 exciting source (of wave-length between A 3000 and A 2000) and the 

 strong sharp lines of fluorescence has been found. 



12. There is a sharp strong line at about A 5735 in the fluorescence 

 spectra excited in the same crystal by the spark between the following 

 terminals : — iron, aluminium, cadmium, magnesium, and tin, and the 

 ultraviolet spectra of these metals are so very different as to exclude 

 any relation between a strong line in all of these spectra and a strong 

 line of the fluorescence spectrum. The possibility of optical resonance 

 is made still more improbable by the fact that the spectrum varies sharply 

 from crystal to crystal with the same exciting source. 



13. The possibility is suggested that these sharp lines of fluorescence 

 may be connected with an impurity in the crystal. The fluorites giving 

 sharp lines are colored, and the colored parts form layers or strata in 

 which the fluorescence appears stronger than in the other parts of the 

 crystal. These strata were noted by Stokes as being the seat of the "red 

 fluorescence " which he observed and which is here resolved into sharp 

 lines. 



14. Whatever the source of fluorescence, it is completely removed or 

 destroyed by heating the crystal to a temperature above 300°. 



15. A crystal which has been so heated may be so regenerated by 

 exposure to light that it will give the broad blue band of fluorescence, 

 but the sharp-line portion of the fluorescence light is not regenerated. 



16. Under the exciting influence of sunlight the fluorescence spectrum 

 of these crystals contains only the broad diffuse band from about A 4800 

 to about A 4000, without maxima or minima. 



The Absorption of Fluorite. 



The absorption and emission of fluorite of various colors have been 

 examined in the visible part of the spectrum by Nichols and Merritt. 8 

 Their results show that as far as their experiments reach the absorption 



8 Pliys. Rev., 19, 18 (1904). 



