96 Prof. Draper on the Phosphorescence of Bodies. 



1st, phosphorescence produced by insolation; 2nd, by heat. 

 An example of this is offered in the standard M^ork on chemistry 

 by L. Gmeliu. 



A di\ision of this kind brings the whole subject into confusion. 

 It assigns different causes for things that are essentially allied. 

 It leads to the inference, that, as under certain circumstances 

 the sunlight or an electric spark can make bodies glow, so under 

 other circumstances heat will produce the same effect, and this 

 wholly independent of incandescence. 



But what are the facts ? If a yellow diamond, placed upon 

 ice, is submitted to the sun and then brought into a dark room 

 the temperature of which is 60°, for a time there is a glow, but 

 presently the light declines and dies out. Let the diamond now 

 be put in water at 100° ; again it shines, and again its light dies 

 away. If it next be removed from that water and suffered to 

 cool, and then be reimmersed, it will not shine again ; but if the 

 water be heated to 200°, and the diamond be dropped into it, 

 again it glows and again its light dies away. 



There is, therefore, a correspondence between the light disen- 

 gaged and the temjierature. ^Ye are not to conclude from the 

 foregoing illustration, that when the diamond has its tempera- 

 ture raised from 100° to 200°, the light is due to the heat. On 

 the contrary, the light is unquestionably due to the primitive 

 exposure to the sun : just as in Lemery's illustration of the 

 sponge, if we exert a little pressure, a portion of the water flows 

 out ; if a stronger pressm-e, still more ; and for each degree of 

 pressure there will be a corresponding quantity of water expelled. 



Tlie connexion between phosphorescence and temperature may 

 be established in another way, and readily verified by experiment. 



Suppose that three diamonds, a, b, c, have been simultaneously 

 exposed to the smi, a being kept at 32°, b at 60°, c at 100° ; 

 and that they are then simultaneously removed to a bath of 

 water at 100° in a dark room ; it will be found that a emits 

 a bright light, b shines more feebly, and c scarcely at all. This 

 is what ought to be expected from the principle laid downa above ; 

 for if at a particular temperature a certain quantity of light is 

 set free, it is clear that a has the advantage of b, so that it will 

 disengage all the light to be set free between 32° and 60°. 



From such experiments and considerations it is to be inferred, 

 that there is an intimate connexion between temperature and 

 phosphorescence, which may be conveniently expressed in the 

 following terms : — 



The quantity of light a substance can retain is inversely as its 

 temperature. 



This principle furnishes the explanation of a multitude of ob- 

 servations made at various times. Thus Dufay discovered that 

 the Bolognian stone shines brighter when exposed to the sky 



