1(54 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 



not for the recurrence of the continuing vibration of the 

 molecules. At each vibration the molecules of a lose a 

 part of their vis viva, by the quantity they have com- 

 municated to the ethereal wave, the intensity or ampli- 

 tude of the wave becoming less and less as this abstrac- 

 tion of force is going on. But the ether being of uniform 

 density and elasticity throughout, each of its particles 

 communicates the whole vis viva it has received to the 

 next adjacent, and would instantly come to rest were it 

 not again disturbed by the vibrations of the material 

 molecules. These elementary considerations show how 

 it is that a wave of sound passes through the air, or of 

 light through the ether, and the particles of those media 

 come instantly to rest; but a hot body or a vibrating 

 string persists in its motions, which only undergo a grad- 

 ual decline. If the vibrating molecule were in a medium 

 of the same density, it would impart to it all its motion 

 at once, and in the same way that a heavy molecule grad- 

 ually communicates its motion to the ether, so in its turn 

 does the ether to other systems of molecules. 



Upon these principles we may explain the phenomena 

 of phosphorescence. From a shining body undulations 

 are propagated in the ether, and these, impinging on a 

 phosphorescent surface, throw its molecules into a vi- 

 bratory movement. These in their turn impress on the 

 ether undulations ; but by reason of the difference of its 

 density compared with that of the molecules, they do 

 not lose their motion at once; it continues for a time, 

 gradually declining away and ceasing when the vis viva 

 of the molecules is exhausted. 



When a phosphorescent surface is exposed to the lu- 

 minous source, it necessarily undergoes a rise of temper- 

 ature, and the cohesion of its parts is diminished ; but 

 after its removal from that source, as the temperature 

 declines and radiation goes on, the cohesion increases, 

 and a restraint is put on those motions. 



