and Phosphorescence. 123 



described in § 18, which most likely has been long ago made by- 

 others, is contrary to the assumption that any such coercive force 

 exists. The fact that the intensity of the phosphorescence gra- 

 dually decreases and at last becomes nothing, although the ex- 

 ternal conditions remain unchanged, and can be renewed only 

 after a certain interval of repose, also speaks against M. Ems- 

 mann's view. Moreover the production of light at all, without 

 the necessity for exposing the phosphorescent bodies to light, is 

 opposed to a literal acceptation of M. Emsmann's view, although 

 the statement of this view might easily be modified so as to be 

 in harmony with this fact. But the single circumstance that 

 many substances, including fluor-spar itself, are both fluorescent 

 and phosphorescent, and therefore, according to this theory, 

 possess at the same time a weak and a strong coercive force, is 

 sufficient to show its inadmissibility. 



20. We may still mention some phenomena of phosphores- 

 cence in which no coercive force is manifested. If a piece of 

 fluor-spar is drawn along a hot plate without much pressure, a 

 luminous streak appears immediately, and disappears again after 

 some time, just as when apiece of phosphorus is drawn along the 

 surface of a plate at a lower temperature. Chalk-marks also, 

 when they are made upon a hot plate, show a phosphorescent 

 light, as was first pointed out by Mr. Napier*. If a piece of 

 fluor-spar is allowed to fall, even from a very small elevation, 

 upon a hot plate, there is an immediate burst of light, which ge- 

 nerally dies away again very quickly, especially if the tempera- 

 ture of the plate is rather high. It is evident that when a streak 

 is made with fluor-spar, or when it is let fall, very minute par- 

 ticles are detached from it, which, in consequence of their small- 

 ness, rapidly acquire the temperature necessary to make them 

 luminous. To this category also belongs a very beautiful phe- 

 nomenon which I have repeatedly had occasion to observe. If a 

 cold piece of fluor-spar is put into a heated oven, either freely 

 suspended or laid upon the bottom, splendid luminous points 

 not unfrequently shoot out like lightning from the crystal very 

 soon after it is put in, and generally disappear again very quickly. 

 This is a decrepitation of the crystal ; small particles break away 

 and then comport themselves like those separated by friction or 

 by a blow. If coarsely powdered fluor-spar is thrown upon the 

 hot stove-plate, the decrepitation is very brisk; there is a di- 

 stinctly audible crackling, and small brightly luminous particles 

 are scattered around. 



21. The more attention I devote to the phosphorescence of 

 fluor-spar, the more strongly am I impressed with the idea of 

 some kind of chemical process taking place at the surface, pos- 



* Phil. Mag. S. 4. vol. i. p. 432. 



