Prof. Drapei' on the Phosjjhorescence of Bodies. 99 



to the eethereal wave, the intensity or amphtude of the wave 

 becoming less and less as this abstraction of force is going on. 

 But, the sether being of uniform density and elasticity through- 

 out, 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 elementaiy considerations show how it is that a wave 

 of sound passes through the air, or of light through the fether, 

 and the particles of those media instantly come to rest ; but a 

 hot body, or a vibrating string, persists in its motions, which 

 only undergo a gradual decline. If the vibratoiy molecule was 

 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 

 gradually communicates its motion to the ?ether, so, in its turn, 

 does the sether to other systems of molecules. 



Upon these principles we may explain the phsenomena of 

 phosphorescence. From a shining body imdulations are pro- 

 pagated in the aether, and these impinging on a phosphorescent 

 sui-face, throw its molecules into a vibratoiy movement. These 

 in their turn impress on the sether undulations ; but by reason 

 of the difference of its density, compared with that of the mole- 

 cules, they do not lose their motion at once, but it continues for 

 a time, gradually declining away, and ceasing when the vis viva 

 of the molecules is exhausted. 



^Mien a phosphorescent surface is exposed to the luminous 

 source, it necessarily undergoes a rise of temperature, and the 

 cohesion of its parts is diminished ; but after its removal from 

 that source, as the temperature declines and irradiation goes on, 

 the cohesion increases and a restraint is put on those motions. 



Now let the phosphorus have its temperature raised, and the 

 cohesion of its molecules thereby weakened, and the restraint on 

 their motions abated. At once they resume their oscillations, 

 and continue them to an extent that belongs to the temperature 

 used, ^\^^en this has passed away, a still higher temperature 

 will release them once more, and the glowing will be again 

 resumed. 



What would be the result if we could cause the siu'face of a 

 mass of Water, on which circular waves are rising and falling, to 

 be instantaneously congealed':' It might be kept in that condi- 

 tion for a thousand years ; and then, if instantaneously thawed, 

 the WHVi.'s would resume their ancient motion from the point at 

 which it Mas arrested, and it would now go on to completion. 



So with these ])hos{)hori. Exposed to a light of suitable 

 intensity, their ])arts coinmence to vibrate, but the freedom of 

 those motions is interfered with by their cohesion. iVmplitude 

 of vibration must always be affected by cohesion ; and if the ray 



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