93 Prof. Draper on the PhospJwrescence of Bodies. 



rescent by exposure to the sun ; but this remark does not apply- 

 to all varieties of it, as several specimens used in these experi- 

 ments prove. Thus, some chlorophane which had been ignited 

 in a glass tube till it had ceased to shine, was pulverized and 

 again ignited in a platinum crucible. It emitted an emerald 

 light. A piece of wood was now put on it to screen a part of 

 its surface, and it was exposed to the sun for a few minutes. On 

 ignition it shone again huely with a green light, the shadow of 

 the wood being beautifidly depicted. The same result having 

 been repeated a great many times, it appeared that the phos- 

 phorescence at last began to decrease, perhaps by frequent igni- 

 tion causing a change. 



A screen of yellow glass intervening between the sun and 

 some powdered chlorophane prevented phosphorescence, but it 

 took place through a plate of polished fluor-spar. But when 

 the light of an electric spark was used, instead of the sunshine, 

 in this experiment, the fluor-spar prevented phosphorescence. 



General Conclusions. 



The results to which the foregoing experiments bring us are, 

 therefore, — 



1st. That the methods employed in these experiments are not 

 si^fficiently delicate to detect any increase of the dimensions of a 

 phosphorus while it is in a glowing state. 



2nd. No structural change can be discovered by resorting to 

 polarized light ; but there is reason to believe, from the change 

 of colour which certain bodies exhibit when the quality of shining 

 is communicated to them, and from the manner that vapours 

 condense on their surfaces, that such has actually taken place. 



3rd. That phosphorescence is attended with a minute rise of 

 temperatiu'e. 



4th. Tliat it is not necessarily connected with any electrical 

 disturbance. 



On comparing these conclusions, it is obvious that if the 

 third be correct, there must necessarily be a change of volume ; 

 and that the reason the dilatation is not discovered by direct 

 experiment, is owing to the insuflicieucy of the means employed. 



The general definition given of phosphorescence is, that it is 

 the extrication of light without heat (Gmelin) . But these results 

 . show that that definition is essentially incorrect ; for if the ex- 

 periment be made with due care, a rise of temperature can be 

 detected, though its absolute amoiint may be very small. 



