MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 



Now let the phosphorus have its temperature raised, 

 and the cohesion of its molecules be 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. When this has 

 passed away, a still higher temperature will release them 

 once more, and the glowing will again be resumed. 



What would be the result if we could cause the sur- 

 face of a mass of water on which circular waves are rising 

 and falling to be instantaneously congealed? It might 

 be kept in that condition for a thousand years, and then, 

 if instantaneously thawed, the waves would resume their 

 ancient motion from the point at which it was arrested, 

 and it would now go on to its completion. 



So with these phosphor! Exposed to light of a suit- 

 able intensity, their parts begin to vibrate ; but the free- 

 dom of those motions is interfered with by their cohe- 

 sion. Amplitude of vibration must always be affected 

 by cohesion, and if the ray be removed and the tem- 

 perature be permitted to decline, the restraint becomes 

 greater and greater, and they pass into a condition some- 

 what like that which has just been illustrated. It mat- 

 ters not how long a time may intervene, rise of tempera- 

 ture will enable them to resume their motions. 



These principles give an explanation of all the facts 

 we observe. We see how it is that as we advance from 

 one temperature to another the phosphorus will resume 

 its glow, and that there is, as it were, for every degree 

 a certain amount of vibratory movement that can be ac- 

 complished, or, to use a different phrase, a certain amount 

 of light that can be set free. It also necessarily follows 

 that different solids will display these motions with dif- 

 ferent degrees of facility, and hence shine for a longer or 

 shorter time, and with lights of different intensities. 



But in liquids and gases, which want that particular 



