100 Prof. Draper on the Phosphorescence of Bodies. 



be removed, and the temperature permitted to decline, tlie re- 

 straint becomes greater and greater, and tliey pass into a con- 

 dition somewhat Hke that which has just been illustrated. It 

 matters not how long a time may intervene, rise of temperature 

 will enable them to resume their motions. 



These principles give us an explanation of all the facts we 

 observe. We see how it is, that, as we advance from one tem- 

 perature 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 accomplished, or, to use a differ- 

 ent phrase, a certain amount of light which can be set fi-ee. It 

 also necessarily follows, that different solids will display these 

 motions Mith different degrees of facility, and hence shine for a 

 longer or shorter time, and with light of different intensities. 



But in liquids and gases, which want that particular condition 

 of cohesion which is characteristic of the solid state, and whose 

 parts move freely among each other, phosphorescence cannot 

 take place, for it depends on the influence that cohesion has had 

 in restraining the vibratory movements. 



Further, the condition of opacity does not permit the ph?e- 

 nomenon to be established. The provoking ray cannot find 

 access to disturb the interior layers of the mass ; and even if it 

 did, and phosphorescence ensued, how could we expect to be 

 able to discern it through the impervious veil of the superficial 

 layers ? The light of the most brilliant phosphorus cannot be 

 seen through the thinnest gold-leaf. Its intensity is vastly too 

 small. And these, therefore, are the reasons that no one has 

 ever yet succeeded in detecting phosphorescence in metals or 

 black bodies. 



It will be gathered from this explanation, that I am led to 

 believe that all the facts of phosphorescence can be fully ex- 

 plained on the principles of the communication of vibratory 

 motion through the iether; that, as upon that theory, an in- 

 candescent body, maintained at incandescence, would eventually 

 compel a cold body in its presence to come up to its own tem- 

 perature, by making its particles execute movements like those 

 of its own, so the sunshine, or tlie flash of an electric spark, 

 compels a vibratory movement in the bodies on which its rays 

 fall ; that these movements are interfered with by cohesion in 

 the case of solids, but that they are instantly established and 

 almost as instantly cease in the case of gases and liquids ; that 

 reducing the cohesion of a solid by raising its temperature per- 

 mits a resumption of the movement ; and that the condition of 

 opacity, either melantic, or otherwise, is a bar to the whole phte- 

 nomenon. 



Universitv, New York, 

 December 1.^), 1850. 



