and Phosphorescence. Ill 



convinced that the emission of rays of greater refrangibility in 

 the cases mentioned above was simply a consequence of the ele- 

 vation of temperature which the various bodies underwent, and 

 that it was entirely immaterial whether they were heated by 

 dark rays or by luminous rays of heat, or by conduction, or in 

 any other way. So far as I am aware, there is not a single phe- 

 nomenon dependent on temperature in which the mode in which 

 the temperature is raised, or the origin and particular character 

 of the heat which causes it, exerts any recognizable influence. 



In the following pages I will indicate the essential results of 

 my experiments, and endeavour to establish the opinion I have 

 expressed above, that no case of negative fluorescence, such as 

 M. Emsmann originally supposed to be possible, has as yet been 

 shown to exist. 



2. I placed a piece of fluor-spar in the oven of a common stove, 

 as I had done in my first experiments, supporting it so that 

 it was not in conducting connexion -with the stove, but received 

 the radiation of the five black-leaded iron sides of the oven, 

 which were heated to a point below redness, and thus gave out 

 chiefly, if not entirely, invisible rays. Sometimes the perforated 

 door of the oven was closed, so that a sixth heated surface 

 radiated heat upon the crystal. 



The first result was that different-sized pieces of the same 

 crystal required to be suspended for different lengths of time in 

 the heated space before they began to be luminous. Large 

 pieces were always longer in becoming luminous than small 

 pieces. A piece of from 7 to 8 cubic centims. in bulk did not 

 become luminous even in half an hour, although its fragments 

 very soon became luminous under the same circumstances. 



If the rays underwent a change similar to that which consti- 

 tutes fluorescence, it is obvious that pieces of the same substance 

 must become luminous after being exposed for equal lengths of 

 time to the action of the rays. Fluorescence is a process which 

 takes place essentially at the surface of bodies ; and, in the expe- 

 riments that have just been described, equal surfaces were ex- 

 posed to an equal number of equally intense rays. If the ap- 

 pearance of phosphorescence depends only upon temperature, 

 the retarded luminosity of the larger pieces becomes very intel- 

 ligible, since they are more slowly heated both superficially and 

 throughout their mass. 



3. In my next experiments I placed three pieces of the same 

 crystal of fluor-spar, of nearly the same size and shape, in the 

 heated space. One of them was freely suspended in the air by 

 thin threads, the second hung freely in an enclosure constructed 

 with plates of rock-salt, and the third in a similar envelope made of 

 plates of glass. The " salt cell " and the " glass cell " were each 



