MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 



This principle furnishes the explanation of a multitude 

 of facts. Thus Du Fay discovered that the Bolognian 

 stone shines brighter when exposed to the sky than to 

 the sun. In the latter case the temperature rises, and 

 the quantity of light retained is less. Under violet 

 and other glasses, stained with such colors as impede 

 the warming effect, phosphorescence is even more vivid 

 than when no glass has intervened. On the same prin- 

 ciple we have an explanation of Du Fay's apparently 

 successful attempt to prevent the escape of light from 

 diamonds by putting them in ink or covering them with 

 black wax. When removed from the ink and brought 

 into the air, they became somewhat warmer perhaps 

 the touch of the finger aided the effect and a corre- 

 sponding quantity of light was set free. 



But though temperature is a controlling, it is not the 

 only condition involved. If it were, phosphorescence 

 after insolation should occur only after a rise of tempera- 

 ture. The fundamental fact of the whole inquiry proves 

 that a glowing body can retain more light in presence 

 of a lucid surface than it can in the dark. 



Is not this fact analogous to what we meet with in 

 the exchanges of heat? A substance can retain more 

 heat in presence of a hot body than a cold one. The 

 brilliancy and quantity of light to which a phosphorus 

 is exposed goes very far to determine the intensity of 

 the subsequent glow. Thus I found that a piece of 

 chlorophane exposed to one spark of a contact-breaker 

 shone feebly, but if it had received one hundred sparks, 

 its light was very vivid ; and it has long been known 

 that in delicate phosphori a certain degree of luminosity 

 can be communicated by the moonbeams, a more intense 

 one by lamp-light, and one still more brilliant by the 

 sunshine or a Leyden spark. This, therefore, leads to 

 the conclusion that the quantity of light a phosphorus 



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