122 Prof. C. Bohn on Negative Fluorescence 



remained an hour in the heated space, so that equilibrium of 

 temperature had been reached, I threw small fragments of fluor- 

 spar upon the dish and upon the flat-iron, and shaded them by 

 means of a pill-box-lid from the heat radiated by the sides of the 

 stove. In from one minute to one and a half, the fluor-spar upon 

 the flat-iron began to be luminous ; but the fragments upon the 

 dish had not become luminous in five minutes, after which I was 

 obliged to interrupt the experiment. I wanted to ascertain di- 

 rectly by means of this arrangement whether fluor-spar was ca- 

 pable of becoming phosphorescent at the temperature which bodies 

 attain inside the oven, when they are not to any important extent in 

 conducting communication with the sides, without being exposed 

 to direct radiation. The experiment with the flat-iron answers 

 this question in the affirmative, and that with the iron dish gives 

 only an apparent negative; for the bright flat-iron, with its 

 greater mass, attains, chiefly through contact with the hot air, a 

 higher temperature than the small dish with its large surface, 

 rendered a good radiator by its covering of rust. If I raised 

 the temperature of the dish a little by laying it for a short time 

 directly upon the hot stove-plate, the little pieces of fluor-spar 

 which it contained at once became luminous. 



18. Large pieces of fluor-spar were brought to a state of very 

 vivid phosphorescence by being laid directly upon the hot stove- 

 plate. When taken off and laid upon any cold support, they con- 

 tinue to shine for a longer or shorter time, until, as I suppose, 

 their temperature has fallen below a certain limit. If the brightly 

 shining pieces are thrown into cold water, their light is extin- 

 guished instantly. 



19. According to M. Emsmann*, the difference between 

 phosphorescence and fluorescence is analogous to the difference 

 between the magnetic behaviour of steel and iron. Rays of light 

 are supposed to exert an influence upon the arrangement of the 

 atoms, and this influence is supposed to be capable of becoming 

 strong enough to produce chemical action. This idea is due to 

 Draper. The atomic motion which is superinduced by exposure 

 to light is further supposed to give rise to vibrations of the aether 

 and of the molecules of the substance, the result of which is the 

 luminosity exhibited. Each substance is supposed to be en- 

 dowed with a certain coercive force, which tends to keep its atoms 

 in the particular arrangement which corresponds to the circum- 

 stances under which it is placed. In phosphorescent bodies this 

 coercive force is regarded as strong, and in fluorescent bodies as 

 weak. It is thus explained why phosphorescence does not begin 

 until the exciting rays have acted for some time, and also why it 

 continues after these rays have ceased to act. The experiment 



* PoggendorfP s Annalen, vol. cxiv. p. 654. 



