160 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 



diamond be now put into water at 100, it shines again, 

 and again its light dies away. If next it be removed 

 from that water and suffered to cool, and then be re- 

 immersed, 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 

 disengaged and the temperature. We are not to con- 

 clude from the foregoing illustration that when the dia- 

 mond has its temperature 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 pressure, still more ; and for each degree of 

 pressure there will be a corresponding quantity of water 

 expelled. 



The connection between phosphorescence and temper- 

 ature may be instructively illustrated as follows : 



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

 simultaneously exposed to the sun, 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 li^ht, b shines more 



~ O > 



feebly, and c scarcely at all. This is what ought to be 

 expected from the principle laid down above ; for if at a 

 particular temperature a certain quantity of light is set 

 free, it is clear that a has the advantage of Z>, 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 connection between 

 temperature and phosphorescence which may be conven- 

 iently expressed in the following terms. The quantity of 

 light a substance can retain is inversely as its temperature. 



